heliusmct users guide ansys

7/28/2019 HeliusMCT Users Guide Ansys

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Helius:MCT Users Guide - ANSY S Page 2 of 75

Table of Contents1 INTRODUCTION TO HELIUS:MCT ................................................................................................... 4

1.1 A NOTE ON THE HELIUS :MCT- LINEAR VERSION ............................................................................ 4 1.2 HELIUS :MCT INTERACTION WITH ANSYS ........................................................................................ 4 1.3 HELIUS :MCT S UPPORT DOCUMENTATION ..................................................................................... 7

2 GENERAL REQUIREMENTS FOR ANSYS INPUT FILES ............................................................... 9

2.1 IDENTIFICATION AND DEFINITION OF HELIUS :MCT MATERIALS ......................................................... 9 2.2 NONLINEAR SOLUTION CONTROL P ARAMETERS FOR HELIUS :MCT .................................................. 9 2.3 REQUESTING OUTPUT OF S OLUTION VARIABLES THAT ARE UNIQUE TO HELIUS :MCT ...................... 10

3 USING THE ANSYS MECHANICAL APDL TO CREATE ANSYS INPUT FILES FOR USE WITHHELIUS:MCT .................................................................................................................................. 11

3.1 CREATING USER -DEFINED MATERIALS WITH THE HELIUS :MCT GUI .............................................. 11 3.2 MODELING ISSUES FOR IMPOSING T EMPERATURE CHANGES ......................................................... 17 3.3 NONLINEAR SOLUTION CONTROL P ARAMETERS ........................................................................... 19 3.4 REQUESTING MCT STATE VARIABLE OUTPUT ............................................................................... 22

4 USING A TEXT EDITOR TO CONVERT PRE-EXISTING ANSYS INPUT FILES FOR USE WITHHELIUS:MCT .................................................................................................................................. 24

4.1 DEFINING A HELIUS :MCT MATERIAL ............................................................................................ 24 4.2 MODELING ISSUES FOR IMPOSING T EMPERATURE CHANGES ......................................................... 33 4.3 NONLINEAR SOLUTION CONTROL P ARAMETERS FOR HELIUS :MCT ................................................ 33 4.4 REQUESTING OUTPUT OF THE MCT S TATE VARIABLES ................................................................. 34 4.5 MODELING DAMAGE TOLERANCE ................................................................................................ 35

5 EXAMINING HELIUS:MCT RESULTS WITH ANSYS MECHANICAL APDL ................................ 36

5.1 USING CONTOUR PLOTS TO VIEW THE MCT STATE VARIABLES ....................................................... 36 5.2 DETECTION OF GLOBAL STRUCTURAL FAILURE .............................................................................. 41

6 RUNNING ANSYS WITH HELIUS:MCT .......................................................................................... 45

6.1 BATCH MODE ............................................................................................................................. 45 6.2 GUI MODE ................................................................................................................................ 45

APPENDIX A HELIUSMCT COMMAND ARGUMENTS ....................................................................... 48

APPENDIX A.1 ARGUMENT #1: MATERIAL REFERENCE NUMBER ........................................................ 50 APPENDIX A.2 ARGUMENT #2: NUMBER OF S TATE VARIABLES (SVARS) ........................................... 52 APPENDIX A.3 ARGUMENT #3: S YSTEM OF UNITS ............................................................................. 53 APPENDIX A.4 ARGUMENT #4: P RINCIPAL MATERIAL COORDINATE S YSTEM ........................................ 55 APPENDIX A.5 ARGUMENT #5: P ROGRESSIVE FAILURE ANALYSIS ...................................................... 57 APPENDIX A.6 ARGUMENT #6: PRE -FAILURE NONLINEARITY ................................................................ 59 APPENDIX A.7 ARGUMENT #7: POST -FAILURE NONLINEARITY .............................................................. 61 APPENDIX A.8 ARGUMENT #8: H YDROSTATIC S TRENGTHENING ........................................................... 63 APPENDIX A.9 ARGUMENT #9: RESIDUAL S TRESSES FROM S TRESS -FREE T EMPERATURE ..................... 64 APPENDIX A.10 ARGUMENT #14: MATRIX P OST -FAILURE S TIFFNESS FRACTION ...................................... 66 APPENDIX A.11 ARGUMENT #15: FIBER POST -FAILURE S TIFFNESS FRACTION ......................................... 67

APPENDIX B MCT STATE VARIABLES (SVARS) .............................................................................. 68


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Table of Figures

FIGURE 1: S CHEMATIC DIAGRAM OF THE INDIVIDUAL COMPONENTS OF THE HELIUS:MCT SOFTWARE

AND THEIR INTERACTION WITH THE ANSYS SOFTWARE COMPONENTS .......................................... 6FIGURE 2: ANSYS CLASSICAL GUI AT START -UP ................................................................................... 11FIGURE 3: THE HELIUS:MCT GRAPHICAL USER INTERFACE (GUI) ...................................................... 12FIGURE 4: REFERENCE TEMPERATURE DIALOG BOX ACCESSED FROM WITHIN THE ANSYS GUI. ....... 18FIGURE 5: UNIFORM TEMPERATURE DIALOG BOX ACCESSED FROM WITHIN THE ANSYS GUI. ............ 18FIGURE 6: EXAMPLE OF NROPT COMMAND ENTERED INTO THE ANSYS COMMAND PROMPT . ............ 19FIGURE 7: ACCESS OF PRED COMMAND VIA ANSYS MECHANICAL APDL. ......................................... 20FIGURE 8: WHERE TO SPE CIFY THE NUMBER OF SUBSTEPS IN THE ANSYS MECHANICAL APDL. ....... 21FIGURE 9: ACCESS OF CNVTOL COMMAND VIA THE ANSYS MECHANICAL APDL. ............................. 21FIGURE 10: ACCESS OF THE NEQIT COMMAND VIA THE ANSYS MECHANICAL APDL. ....................... 22FIGURE 11: NON-UNIFORM CONTOURS DIALOG BOX ............................................................................. 37FIGURE 12: COMPARISON OF A PLNSOL CONTOUR PLOT AND A PLESOL CONTOUR PLOT USING

THREE DISCRETE COLOR CONTOURS TO REPRESENT DISTRIBUTION OF SVAR1=1,2,3 .............. 38FIGURE 13: OPTIONS FOR OUTPUT DIALOG BOX ................................................................................... 39FIGURE 14: PLESOL CONTOUR PLOTS OF SVAR1 AT SEVERAL DIFFERENT POINTS IN TIME DURING A

PROGRESSIVE FAILURE ANALYSIS ................................................................................................... 40FIGURE 15: 8- PLY COMPOSITE PLATE UNDER IMPOSED AXIAL DISPLACEMENT .................................... 42FIGURE 16: THE GLOBAL STRUCTURAL FORCE IS OBTAINED BY SUMMING THE VERTICAL REACTION

FORCES AT ALL NODES ALONG THE TOP EDGE OF THE COMPOSITE P LATE ................................... 42FIGURE 17: GLOBAL STRUCTURAL FORCE VS . GLOBAL STRUCTURAL DEFORMATION .......................... 43FIGURE 18. HELIUS:MCT FOR ANSYS EXAMPLE BATCH RUN ............................................................... 45FIGURE 19. EXAMPLE OF OPENING THE HELIUS:MCT FOR ANSYS LAUNCHER .................................. 46FIGURE 20. HELIUS:MCT FOR ANSYS LAUNCHER ............................................................................... 46

FIGURE 21. EXAMPLE OF SELECTING THE CUSTOM ANSYS EXECUTABLE IN THE ANSYS P RODUCTLAUNCHER ....................................................................................................................................... 47FIGURE A22: HELIUS:MCT SOLUTION FOR FAILURE PROPAGATION IN THE 0 PLIES OF A COMPOSITE

LAMINATE LOADED IN TENSION ........................................................................................................ 58FIGURE A23: COMPARISON OF PREDICTED VS . MEASURED LONGITUDINAL SHEAR RESP ONSE FOR A

TYPICAL FIBER -REINFORCED COMPOSITE LAMINA .......................................................................... 60FIGURE A24: HELIUS:MCT STRESS -STRAIN SOLUTIONS FOR THE CENTRAL 90 PLY WITHIN A (0/90/0)

LAMINATE UNDER AXIAL TENSION , SHOWING THE EFFECT OF INCLUDING THE POST -FAILURENONLINEARITY FEATURE .................................................................................................................. 62


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Functionality not available inHelius:MCT Linear

1 Introduction to Helius:MCTHelius:MCT is composed of a set of two software modules and a composite material database that

integrate seamlessly with the Ansys finite element analysis system, providing the user with state-of-the-art material modeling capability for fiber-reinforced composite materials. Helius:MCT utilizes a form of

multiscale material modeling that is based on Multicontinuum Theory (or MCT) which has been under continuous joint development by the University of Wyoming and Firehole Composites for the past 15years. The MCT modeling methodology provides an unsurpassed combination of accuracy and efficiencyin predicting damage evolution and material failure in composite materials.

In sharp contrast to traditional continuum mechanics, where physical quantities of interest (e.g.,stress and strain) are averaged over the entire heterogeneous microstructure of the composite material,Multicontinuum Theory retains the identities of the distinct material constituents within themicrostructure. Consequently, physical quantities of interest (e.g., stress and strain) are averaged over each individual constituent material. These constituent average quantities provide much deeper insightinto the thermo-mechanical behavior of the composite material than the traditional composite averagequantities. To briefly summarize, Multicontinuum Theory focuses on two concepts: 1) the development

of relationships between the various constituent average quantities of interest, and 2) the development of relationships that link the composite average quantities to the constituent average quantities. For acomplete discussion of Multicontinuum Theory and the advantages that it provides in the analysis of composite materials, see the Helius:MCT Theory Manual.

1.1 A Note on the Helius:MCT- Linear Version

Starting with Helius:MCT version 3.0, Firehole Composites began offering a limited-functionalityversion of Helius:MCT. This version is often referred to as Helius:MCT-Linear. Helius:MCT-Linear

provides users access to advanced multi-scale analysis, constituent level stress and strain values and multi-scale failure indices when running linear elastic finite element simulations.

Helius:MCT Linear does not provide access to many of the advanced, nonlinear functionality thatis available in the full version of Helius:MCT such as progressive failure modeling, material non-linearity, and many other advanced features.

In an effort to communicate the features that are not available in the Linear version of Helius:MCT,the following graphic will be displayed in this document when describing a feature that requires a licenseto the full featured version of Helius:MCT.

1.2 Helius:MCT Interaction with Ansys

In an Ansys structural-level finite element analysis of a composite structure, Helius:MCT quicklyand accurately decomposes the composite average stress/strain field into constituent average stress/strainfields. The constituent average stress states are then used by Helius:MCT to predict damage evolution and material failure for each constituent material (fiber and matrix). Subsequently, Helius:MCT homogenizesthe current damaged microstructure in order to provide an accurate assessment of the current compositeaverage stiffness for use in the structural-level finite element analysis. Helius:MCT is designed to providethis enhanced composite modeling capability without significantly increasing the time required to run the


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structural-level finite element analysis. For example, using Helius:MCT in conjunction with a structural-level finite element analysis usually increases the overall solution time by two to three percent (a verysmall price to pay for the increased solution accuracy provided by Helius:MCT).

Figure 1 shows a schematic diagram of the individual components of the Helius:MCT software and their interaction with the Ansys software components. In Figure 1, bold rectangles indicate thecomponents of the Ansys finite element modeling package, while ovals indicate the individualcomponents of the Helius:MCT software. In Figure 1, the Helius:MCT Graphical User Interface (GUI) isaccessed from within the Ansys Mechanical APDL and assists the user in defining the Ansys input filecommands that are required during a finite element analysis that employs the use of Helius:MCT.

The Helius:MCT User Programmable Feature (see Figure 1) calculates composite constitutiverelations and stresses for use within the Ansys finite element code. The Helius:MCT User ProgrammableFeature contains all of the MCT constitutive relations for the individual constituent materials (fiber and matrix) and the homogenized composite material. In addition, the Helius:MCT User ProgrammableFeature contains the constituent damage and failure criteria and the algorithms to degrade the stiffnessesof the constituents and the homogenized composite material to reflect the current damaged state of eachconstituent material. The Ansys finite element code calls the Helius:MCT User Programmable Feature ateach Gaussian integration point in the model where constitutive relations or stresses are needed.

In Figure 1, the Helius:MCT Composite Material Database is used to store all of the materialcoefficients that are needed to completely define the MCT multiscale material model for variouscomposite materials. Before a particular composite material can be used in a Helius:MCT-enhanced finiteelement model, the composite material must undergo MCT characterization, and its coefficients must beentered into the Helius:MCT Composite Material Database. As shown in Figure 1, the Helius:MCT User Programmable Feature opens and reads the Helius:MCT Composite Material Database to extract thenecessary material coefficients for any composite materials that are used in the finite element model.


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Figure 1: Schematic diagram of the individual components of the Helius:MCT software andtheir interaction with the ANSYS software components .

In addition to the software modules depicted in Figure 1, Helius:MCT contains one additionalauxiliary program: Helius Material Manager. Helius:Material Manager is a stand-alone program thatallows the user to characterize new composite materials and add them to the Helius:MCT CompositeMaterial Library.

ANSYS Solver Helius:MCT

Use Programmable Feature

Helius:MCT Composite Material

Database

ANSYS Mechanical APDL

Helius:MCT Graphical User

Interface (GUI)

ANSYS input file

ANSYS Mechanical APDL

ANSYS results file


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1.3 Helius:MCT Suppor t Documentation

This Users Guide assumes that the reader is familiar with the basic use of the Ansys finite elementmodeling system, including the following three processes:

1) Creating input files for the Ansys solver,2) Running Ansys finite element analyses,3) Viewing the results from an Ansys finite element analysis.

Given this assumption, the purpose of this document is to describe those aspects of creating an Ansysinput file that are unique to finite element analyses that utilize Helius:MCT for enhanced multiscalemodeling of fiber-reinforced composite structures. In addition, this document discusses appropriatemethods for viewing the enhanced results that are available in the output file when Helius:MCT is used inthe finite element analysis.

The remainder of this document is organized as follows:

Section 2 This section identifies the different groups of Ansys commands that should be present in an Ansys input file to achieve compatibility with Helius:MCT and takefull advantage of its superior convergence characteristics for nonlinear problems.

Section 3 This section describes the use of the Ansys Mechanical APDL to create Ansysinput files that are compatible with Helius:MCT. More specifically, Section 3describes the use of the Helius:MCT Graphical User Interface (GUI) that isaccessed from within the Ansys Mechanical APDL.

Section 4 For users who choose to employ a text editor to manually create their Ansysinput files or for users who use a general pre-processor to create an Ansys inputfile, Section 4 describes the process of manually converting existing Ansys inputfiles to achieve compatibility with Helius:MCT.

Section 5 Finally, this section describes the enhanced solution variables that are computed by Helius:MCT during a finite element simulation, and describes the use of theAnsys Mechanical APDL to view the enhanced MCT results.

The collective documentation for Helius:MCT is divided into several documents. These documentsare listed below along with a brief description of each one.

The Installation Guide explains the installation of the Helius:MCT software on your computer.

Helius:MCT Installation Guide

The Users Guide is a general reference for using Helius:MCT to provide enhanced composite modeling capability for Ansys finite element analyses of composite structures.

Helius:MCT Users Guide


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Nonlinear solutioncontrol parametersare not required for simulations using

Helius:MCT Linear

2 General Requirements for Ansys Inpu t FilesThis section identifies the commands that should be present in an Ansys input file to ensure

complete compatibility with Helius:MCT and to take full advantage of its superior convergence

characteristics for nonlinear, progressive failure problems. More specifically, this section explains theneed for these commands in terms of the unique characteristics of Helius:MCT. This section does notdiscuss the formatting requirements of any individual command, nor

does it discuss any of the specificoptions or data of any individual command; these issues will be covered later in Sections 3 and 4.

2.1 Identific ation and Definition of Helius:MCT Materials

If Helius:MCT is used to provide the constitutive relations for a particular composite material, thenAnsys considers the composite material to be a user-defined material. Consequently, Ansys requiresthe following command for each Helius:MCT user-defined material that is used in the finite elementmodel:

HELIUSMCT , < arguments >

The HELIUSMCT command calls the Helius:MCT macro and the arguments provided as part of the HELIUSMCT command are passed to the Helius:MCT macro. The specific values available for thedifferent arguments of the HELIUSMCT command (along with their formatting requirements) will bediscussed in Sections 3 and 4. For now, it suffices that the reader is aware that the HELIUSMCTcommand will be used to identify each of the composite materials that will be processed by Helius:MCTand to identify the specific form of multiscale constitutive relations that will be used for each of thecomposite materials.

2.2 Nonlinear Solut ion Contro l Parameters for Helius:MCT

It is a widely accepted notion that good convergence (or any convergence at all) is difficult toachieve in a progressive failure simulation of a composite structure. In fact, many progressive failuresimulations terminate early, not due to global structural failure, but rather due to the inability of the finiteelement code to obtain a converged solution at a particular load step.Helius:MCT significantly improves the overall convergence rate and robustness of finite element simulations of progressive failure of compositestructures. Experienced users of Ansys are no doubt familiar with the codestendency to reduce (or cut-back) the time increment size when the code sensesthat convergence is difficult to achieve. However, when Helius:MCT is used in conjunction with Ansys to perform a progressive failure analysis, theincreased robustness of the solution greatly diminishes the need for timeincrementation reductions (or cut-backs), thus the analysis can be completed much faster than withoutHelius:MCT. In order to take full advantage of the superior convergence characteristics of Helius:MCT,the user must change some of the default settings that govern the nonlinear solution process used byAnsys. These changes can be enacted using the NROPT, PRED, NSUBST, NEQIT, and CNVTOLcommands. The specific data and options that are used with the NROPT, PRED, NSUBST, NEQIT, and CNVTOL commands will be discussed later in Sections 3.3 and 4.3.


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2.3 Requesting Output of Solut ion Variables that are Unique to Helius:MCT

Helius:MCT calculates a number of specialized state variables that define the constituent averagestress and strain fields, in addition to the damage state of the composite material. These state variablesare stored by Ansys at each individual integration point within the finite element model. In order that

these state variables can be examined in the Ansys Mechanical APDL, the Ansys input file mustexplicitly specify that SVAR will be written in the Ansys results file. This request is made via theOUTRES command. Specific usage and formatting of the OUTRES command will be discussed inSections 3.4, 4.4, and 5. Appendix B contains a description of each of the MCT state variables.


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3 Using The Ansys Mechanical APDL To Create Ansys Input FilesFor Use With Helius:MCT

Section 3 describes the use of the Ansys Mechanical APDL to create an Ansys input file that iscompatible with Helius:MCT. It is assumed that the reader is familiar with the process of using the AnsysMechanical APDL to create an Ansys input file. Consequently, this section focuses primarily on thoseaspects of model creation that are unique to models that are intended for use with Helius:MCT. In

particular, this section explains the use of the Helius:MCT Graphical User Interface (or GUI) that can beaccessed from within the Ansys Mechanical APDL. The Helius:MCT GUI provides a simple, intuitivemeans for the user to create material definitions that are compatible with Helius:MCT.

3.1 Creating User-Defined Materials with the Helius:MCT GUI

Each composite material that will be processed by Helius:MCT is considered by Ansys to be auser-defined material . The Helius:MCT GUI provides a simple means of creating these compositematerial definitions in the Ansys input file. The Helius:MCT GUI allows the user to select a compositematerial from the Helius:MCT composite material database and then select a number of different optionsfor the multiscale constitutive relations that will be used for the composite material. The Helius:MCT GUI can only be accessed in the model creation preprocessor (/PREP7).

To open the Helius:MCT GUI from within the Ansys Mechanical APDL, go to the Ansys Toolbar and click on the HELIUS button. Figure 2 depicts where this button is located.

Figure 2: Ansys classical GUI at start-up

Once the HELIUS button is clicked, the Helius:MCT GUI will appear as shown in Figure 3.


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Figure 3: The Helius:MCT Graphical User Interface (GUI)

As shown in Figure 3, there are thirteen possible steps involved in using the Helius:MCT GUI to define acomposite material type for Helius:MCT. Each of the thirteen steps is discussed below.

1. Composite Material Selection The user selects a composite material from the Helius:MCTmaterial library. If the material library does not contain a composite material that the user would like to use in an analysis, a material data file must first be created and added to the materiallibrary (refer to Helius Material Manager Users Guide). Once a composite material is selected,the homogenized (or composite average) engineering constants for that material will be displayed in the box labeled Engineering Constants for Your Selected Composite. These constants aredisplayed in Helius:MCTs default system of units (N/m/K). To display these constants in adifferent coordinate system, the user may select a different system of units (see step 2).

2. System of Units The user selects the system of units that should be used by Helius:MCT tocompute constitutive relations and stresses. By default, Helius:MCT expresses constitutive

12

34

5678

910

1112

13


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relations and computes stress in the (N/m/K) system of units. If the finite element model iscreated using a different system of units, then Helius:MCT must convert its constitutivecalculations to the system of units required by the finite element model. For such purposes,Helius:MCT contains conversion factors for four commonly used systems of units: N/m/K,

N/mm/K, lb/in/R, and lb/ft/R. If the finite element model uses one of these four systems of units,the user must select the appropriate system from the drop-down list. In the event that the finiteelement models system of units does not appear in the dr op-down list, the user should select thedefault system of N/m/K and then refer to Appendix A.3 f or details on how to manually modifythe Ansys input file to utilize a custom system of units. The reader can also refer to AppendixA.3 for more detailed information on systems of units in general.

3. Principal Material Coordinate System Helius:MCT expresses constitutive relations and computes stresses in the principal material coordinate system of the composite material. Here theuser selects one of two or three possible orientations for the composites principal materialcoordinate system.

For Unidirectional Microstructures

: Helius:MCTs default principal material coordinatesystem is oriented with the 1 direction aligned with the fiber direction, while the 2 and 3directions lie in the materials plane of transverse isotropy. This default orientation of the

principal material coordinate system corresponds to the selection of "1" from the fiber direction drop down menu. However, in situations where it adds convenience or simplicity tothe model creation process, the user may change the orientation of the principal materialcoordinate system so that the 2 direction is aligned with the fiber direction, while the 1and 3 directions lie in the composite materials plane of transverse isotropy. This particular orientation of the principal material coordinate system corresponds to the selection of "2"from the fiber direction drop down menu. If the user selects the value 2 from the drop-down list, the Helius:MCT GUI updates the contents of the display box labeled EngineeringConstants for Your Selected Composite.

For Woven Microstructures

: Helius:MCTs default principal material coordinate system isoriented with the 1 direction aligned with the fill tow direction, while the 2 directioncorresponds to the warp tow direction, and the 3 direction corresponds with the out-of-planedirection. This default orientation of the principal material coordinate system corresponds tothe selection of "1" from the fiber direction drop down menu. However, in situations where itadds convenience or simplicity to the model creation process, the user may change theorientation of the principal material coordinate system so that the 2 direction is aligned withthe fill tow direction, while the 1 direction corresponds to the warp tow direction. This

particular orientation of the principal material coordinate system corresponds to the selectionof "2" from the fiber direction drop down menu. Additionally, the user may change theorientation of the principal material coordinate system so that the 3 direction is aligned withthe fill tow direction while the 2 direction corresponds to the warp tow direction. This

particular orientation of the principal material coordinate system corresponds to the selectionof "3" from the fiber direction drop down menu.

For more information on the orientation of principal material coordinate systems, please refer toAppendix A.4.

4. Temperature Dependence (unidirectional composites only) If a list of temperatures isdisplayed, then the material data file for the selected material contains material properties atmultiple temperatures. After selecting a temperature, the properties that are stored for that


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Steps 5, 6, 7, 8, 9, 11 and 12 listed below do not apply to Helius:MCT Linear

temperature are displayed in the Engineering Constants for Your Selected Composite. During afinite element analysis, Helius:MCT linearly interpolates the composite and constituent propertiesfor any given temperature that lies within the bounds of the lowest and highest temperature pointsstored in the material file. For temperatures below the lowest stored temperature datum,Helius:MCT will use the material properties stored at the lowest temperature datum (Helius willnot extrapolate properties beyond the bounding stored temperature data points). The same is truefor temperatures above the highest stored temperature datum. For further information on the useof temperature dependent material properties in Helius:MCT, please refer to section 9 of theHelius:MCT Theory Manual. For further information on adding a new temperature dependentmaterial to the Helius:MCT material library, please refer to the Helius:MCT Material Manager Users Guide.

5. Progressive Failure - The user chooses whether or not to perform a Progressive Failure

Analysis. If the user checks this box, then Helius:MCT will routinely evaluate both the matrixfailure criterion and the fiber failure criterion to determine if either constituent has failed. Eachconstituent failure criterion is based on the corresponding constituent average stress state. In theevent that one or both of the constituents fail, the stiffness of the failed constituent(s) and thestiffness of the composite are appropriately reduced instantaneously. It should be emphasized that an instantaneous reduction of the stiffness of a failed constituent effectively results in adiscontinuous, piecewise linear stress/strain response for the constituent and the composite.However, when this type of discrete material response is applied independently at each of theintegration points in a large finite element model, the net result is a gradual (or progressive)degradation of the overall stiffness of the composite structure (hence the name Progressive

Failure Analysis).

The progressive failure analysis feature is the foundation component of Helius:MCTsnonlinear multiscale constitutive relations. Other aspects of material nonlinearity can be invoked,however, these additional forms of nonlinearity cannot be activated unless the progressive failureanalysis feature is also activated. Consequently, if the user chooses not to check the progressivefailure analysis box, then Helius:MCT will use linear elastic constitutive relations. For further information on progressive failure analyses and constituent failure criteria, refer to Appendix A.5of this User's Guide and Section 4 of the Helius:MCT Theory Manual.

6. Calculate Failed Plain Weave Properties - (plain weave composites only) Selecting this optionwill force Helius:MCT to calculate the failed plain weave properties using the matrix and fiber degradation levels specified in steps 11 and 12. If this option is not selected, the failed material

properties that were calculated when the material data file was created using Helius:Material

Manager are used. For example, if the matrix degradation value was 0.7 and the fiber degradation value was 0.015 when the material was created (using Helius:Material Manager) and this option is unselected, the failed material properties corresponding to a matrix degradation of 0.7 and a fiber degradation of 0.015 are used. If, on the other hand, this option is selected and, for example, the user specifies a matrix degradation of 0.8 and a fiber degradation of 0.001 in steps11 and 12, then the failed material properties corresponding to a matrix degradation of 0.8 and afiber degradation of 0.001 are used. Note: The matrix degradation for woven laminae isrecommended to be not less than 0.7.


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7. Hydrostatic Strengthening of the Composite (unidirectional composite only) The user chooses whether or not to account for the experimentally observed strengthening of the compositein the presence of a hydrostatic compressive stress. If the user checks this box, then Helius:MCTwill monitor the hydrostatic compressive stress level in the matrix constituent. If the hydrostaticcompressive stress level in the matrix constituent exceeds a threshold value, then the strength of

both the matrix constituent and the fiber constituent are scaled upwards commensurate with thelevel of hydrostatic compressive stress level in the matrix constituent. For further information onHydrostatic Strengthening of the Composite, refer to Appendix A.8 o f this User's Guide and Section 7 of the Helius:MCT Theory Manual.

8. Pre-Failure Nonlinearity (unidirectional composites only) The user chooses whether or not toaccount for the nonlinear longitudinal shear stress/strain response that is commonly observed infiber-reinforced composite materials. If the user checks this box, then Helius:MCT will employ afour-segment, piecewise linear representation of the longitudinal shear stress/strain response (i.e.,

c

12 vs. c

12, and c

13 vs. c

13

), while the responses of the other four stress and strain componentsremain unaffected by this feature. The entire series of three discrete reductions in the longitudinalshear moduli of the composite is conducted in such a way that the piecewise linear longitudinalshear response closely matches experimentally measured longitudinal shear data for thecomposite.

It should be emphasized that this feature is only available for those unidirectionalcomposite materials where a longitudinal shear stress/strain curve was supplied during the MCTmaterial characterization process. If this feature is requested for a composite material that wascharacterized without a longitudinal shear stress/strain curve, then Helius:MCT will issue an error message at runtime and execution will halt. For further information on the Pre-Failure

Nonlinearity feature, refer to Appendix A.6 o f this User's Guide, Section 5 of the Helius:MCTTheory Manual, and Example Problem 2. For further information on characterizing newcomposite materials with Pre-Failure Nonlinearity capability, please refer to the Helius:MCTMaterial Manager Users Guide.

9. Post-Failure Nonlinearity (unidirectional composites only) The user chooses whether or not toaccount for the support that is provided to a failed lamina by the surrounding un-failed lamina.When individual matrix cracks appear in a lamina, the surrounding undamaged lamina are able(via interlaminar shear stresses) to divert the load path around the individual matrix cracks and

back into the failed lamina. The net result of this process is that matrix failure in a lamina is not adiscrete catastrophic event, rather it is a gradual process marked by a gradual increase in thedensity of matrix cracks. In this case, the matrix failure criterion is assumed to simply identifythe onset of matrix crack development. As the deformation of the lamina continues to increase,the stiffness of the matrix constituent is subject to a series of discrete reductions until the stiffnessof the matrix constituent finally reaches its minimal level indicating complete matrix failure (i.e.,matrix crack saturation). It is of importance to note a consistent set of material properties isenforced between the microscopic and macroscopic scales to allow for the composite material

properties to degrade along with the matrix.

It should be emphasized that this feature is only available for those unidirectionalcomposite materials where the transverse normal failure strain was supplied during the MCTmaterial characterization process. If this feature is requested for a composite material that wascharacterized without a transverse normal failure strain, then Helius:MCT will issue an error message at runtime and execution will halt. For further information on the Post-Failure


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Nonlinearity feature, refer to Appendix A.7 of this User's Guide and Section 6 of the Helius:MCTTheory Manual. For further information on characterizing new composite materials with Post-Failure Nonlinearity capability, please refer to the Helius:MCT Material Manager Users Guide.

10. Residual Stresses (applicable to unidirectional composites only) This option is used to specifywhether or not to explicitly account for thermal residual stresses in the response of the compositematerial. If this option is checked, then Helius:MCT computes the ply-level and constituent-levelthermal residual stresses that are caused by the post-cure cool down from the stress-freetemperature displayed under Engineering Constants for Your Selected Composite to ambienttemperature (defaults to 72.5 F = 22.5 C = 295.65 K). In this case, these ply-level and constituent-level thermal residual stresses will be present prior to the application of any externalmechanical and/or thermal loads that are imposed during the simulation. If the user chooses toexplicitly account for thermal residual stresses in the analysis, then the user should verify that thestress-free temperature (synonymous with cure temperature) displayed under EngineeringConstants for Your Selected Composite is indeed a reasonable value; otherwise, the predicted thermal residual stresses could be quite erroneous.

If this option is not checked for a particular composite material, then thermal residualstresses are not

included in the response of that particular composite material during thesimulation. In this case, the stress free temperature of the composite material defaults to T sf =0 (regardless of the system of units employed), and the temperature change that is used in theconstitutive relations [ = C ( T)] is simply computed as T = T Tsf = T. Several pointsshould be emphasized here. First, the stress free temperature T sf defaults to 0 even if thecomposite material data file (Mdata file) explicitly defines a non-zero stress free temperature.Second, regardless of the system of units that are employed by the finite element model, thecurrent temperature T completely defines the temperature change T that is used in theconstitutive relations. Third, for composite materials that are characterized at multipletemperatures, the current temperature T will be used to interpolate the various material propertiesthat contribute to the constitutive relations; consequently, it is recommended that a single-temperature characterization (i.e., a single-temperature Mdata file) should be used for thecomposite material in question. In summary, if the user does not request this option, then thecurrent temperature T influences Eqs. 10.1 of the Theory Manual in two different ways: 1) thetemperature change used in the constitutive relations simply becomes T=T, and 2) T is used tointerpolate the temperature-dependent material properties that contribute to the constitutiverelations. Refer to Section 10 of the Helius:MCT Theory Manual for further information on thethermal residual stresses formulation used by Helius:MCT.

It should be emphasized that the default temperature in Ansys is 0 . This defaulttemperature is completely compatible with the default stress free temperature of 0 that isassumed when the ninth user material constant is specified as 0. In this case, the model can still

be subjected to temperature changes by simply imposing a temperature other than 0 ; however,these thermal stresses develop over the course of the analysis, as opposed to being present at thestart of the analysis.

11. Matrix Post-Failure Stiffness This value is a fraction that is used to define the damaged elasticmoduli of the matrix constituent after matrix constituent failure occurs. Specifically, the value isthe ratio of the failed matrix constituent moduli to the unfailed matrix constituent moduli. Avalue of 0.1 would mean that after a matrix failure occurs at an integration point, all six of thematrix constituent moduli (E

m

11, Em

22, Em

33, Gm

12, Gm

13, Gm

23) are reduced to 10% of the original


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Figure 4: Reference Temperature dialog box accessed from within the Ansys GUI.

Figure 5: Uniform Temperature dialog box accessed from within the Ansys GUI.


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Nonlinear Solution Control Parameters are onlyapplicable to the full version of Helius:MCT as they

pertain to progressive failure simulations.

3.3 Nonlinear Solution Contro l Parameters

Helius:MCT significantly improves the overall convergence rate and robustness of finite elementsimulations of progressive failure of composite structures. However, in order to take full advantage of the

superior convergence characteristics of Helius:MCT, the user must change some of the default settings that govern the nonlinear solution process used by Ansys. This sectiondiscusses the use of the Ansys MechanicalAPDL to make the recommended changes to the parameters that govern the nonlinear solution processused by Ansys. These changes can be enacted using the NROPT, PRED, NSUBST, CNVTOL, and

NEQIT commands.

Regardless whether the SOLCONTROL command is ON (default) or OFF, the user will need tooverride these nonlinear solution control parameters.

NROPT

The NROPT command is used to do two things: a) instruct Ansys to use the Full NewtonRaphson algorithm, and b) prevent Ansys from using their Adaptive Descent algorithm to help thesolution process. The NROPT command can only be specified via the Ansys command prompt.Figure 6 shows how to type this command into the command prompt.

Figure 6: Example of NROPT command entered into the Ansys command prompt.

In Ansys, the nonlinear solution process is based on the fundamental assumption of the Newton-Raphson algorithm that the nonlinear response of the composite structure is sufficiently smooth at

both the global and local levels. However, in a progressive failure simulation of a compositestructure, the nonlinear response of the composite structure is not

smooth , especially at the locallevel where material failure results in an instantaneous reduction of material moduli. This non-smooth material response is one of the primary factors responsible for the difficulty in obtainingconvergence in progressive failure simulations. Helius:MCTs method of managing materialnonlinearity is specifically designed to handle this localized non-smooth material response;however, the default settings of Ansyss nonlinear solution control parameters must be changed inorder to allow Helius:MCT to improve the convergence characteristics of the finite elementsimulation.

PRED

The PRED command prevents Ansys from using the converged solution at the last substep toestimate the solution for the current substep. This interferes with Helius:MCTs method of managing material linearity. Figure 7 shows how to access this command via the Ansys


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Mechanical APDL. As seen in Figure 7, the DOF solution predictor drop-down menu needs tohave Off selected.

Figure 7: Access of PRED command via Ansys Mechanical APDL.

NSUBST

The NSUBST command specifies the minimum and maximum allowable number of substeps for the current load step. Firehole Composites does not recommend any minimum or maximum valuesand engineering judgment should be exercised. However, provided that enough equilibriumiterations are allowed per substep (discussed under NEQIT heading), Helius:MCT will always find a converged solution. This is a deviation from typical nonlinear solution processes where multiplesubstep size cutbacks may be required. Helius:MCT will converge at each substep, regardless of the size (again provided that enough equilibrium iterations are allowed per substep), so care must

be taken when deciding on substep size. Figure 8 displays where to specify the number of substepsin the Ansys Mechanical APDL.


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Figure 8: Where to specify the number of substeps in the Ansys Mechanical APDL.

CNVTOL

The CNVTOL command is used to define the convergence tolerance for residual node forces.There are two arguments set in this command that allow Helius:MCT to better handle the nonlinear solution process: a) set forces (F) as the convergence label, and b) set the norm selection to infinitenorm (check each DOF separately). Figure 9 displays where to set these arguments in the Ansys

Mechanical APDL.

Figure 9: Access of CNVTOL command via the Ansys Mechanical APDL.


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SVAR1 tracksdamage which is notused in Helius:MCT Linear, all of the

other output

variables are utilized in Helius:MCT Linear.

NEQIT

The NEQIT command is used to specify the minimum number of equilibrium iterations that must be performed before Ansys evaluates the need for cutting-back the size of the current substep.Helius:MCT has the unique ability to converge regardless of the substep size or extent of nonlinearity occurring during the substep. In the experience of Firehole Composites, convergencealways occurs when the number of equilibrium iterations allowed per substep before a substepcutback occurs is set to 1000. Figure 10 displays how to set this argument via the AnsysMechanical APDL.

Figure 10: Access of the NEQIT command via the Ansys Mechanical APDL.

3.4 Requesting MCT state variable output

The solution-dependant MCT state variables are used to track constitutive quantities of interest ateach integration point in the finite element model. If the user checked the boxlabeled Output Constitutive Stress/Strain in the Helius:MCT GUI, then 34MCT state variables are tracked for unidirectional composite materials or 90MCT state variables are tracked for woven composite materials; otherwise, 6MCT state variables are tracked. The default naming convention for thesolution-dependant MCT state variables is SVARi, where i=1,2,3,,6 or 34 (or 90 for woven materials). The most useful of the MCT state variables is SVAR1which is used to track the discrete failure state of the composite material ateach integration point in the finite element model. The exact interpretation of the discrete values of SVAR1 will depend upon the exact set of Helius:MCTmaterial nonlinearity features that are used in the analysis. Appendix B provides a complete descriptionof each of the MCT state variables, including tables that define the interpretation of SVAR1 for variouscombinations of material nonlinearity features.


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Output of the MCT state variables to the results file is accomplished with the OUTRES command.The command, OUTRES, SVAR, ALL, may be issued to write the state variables to the results file atevery substep. For more information on the OUTRES command, refer to the Ansys documentation.


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4 Using A Text Editor To Convert Pre-Existing Ansys Inpu t Files For Use With Helius:MCT

For those users who choose to employ a text editor to manually create an Ansys input file or for users who use a general pre-processor to create an Ansys input file, this section describes the process of manually converting an existing Ansys input file to achieve complete compatibility with Helius:MCT.

4.1 Defining a Heliu s:MCT Material

In an Ansys input file, there is one different command that collectively defines a Helius:MCT user-defined composite material. This command is HELIUSMCT . Consider the following line from anAnsys input file that completely specifies a Helius:MCT user-defined composite material.

HELIUSMCT , MATID , NSTATV , UNITS , PFIB_DIR , PFA , PREFAIL , POSTFAIL , PRESS ,TEMPDEPEND, unused, unused, unused, unused, MDEG , FDEG , FCONV (optional) , LCONV (optional) , TCONV (optional)

An example of an HELIUSMCT command looks like:

HELIUSMCT , 9007, 6, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0.01, 0.01

The HELIUSMCT command calls the Helius:MCT macro and the arguments provided as part of the HELIUSMCT command are passed to the Helius:MCT macro. For any given Helius:MCT material,the number of arguments must be between 5 and 18. The first five arguments are required for allHelius:MCT materials; the last three arguments (i.e., the 16 th, 17 th, and 18 th arguments) are only required if the finite element model is defined using a custom system of units (discussed furth er in Appendix A.3) .Arguments 10-13 are unused and a value of 0 should be entered for these arguments. Appendix A

provides a detailed description of each of the arguments, including the range of allowable values for eachargument and the impact that each argument has on the multiscale constitutive relations used to representthe material. Each of the arguments are shown in Table 1 and listed below along with a brief description.For a more detailed description of any particular argument, refer to the appropriate section of AppendixA.


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Table 1: Helius:MCT Command Arguments

Argument Constitutive Issue Controlled by theUser Material Constant Allowable Values Notes

1 Material Reference Number Integer greater than zero

2 Number of State Variables (SVARs) Unidirectional ~ 6 or 34Woven ~ 6 or 90

3 System of Units

1 N/m/K 2 N/mm/K 3 Lb/in/R 4 Lb/in/R 5 Custom

1 is default

4 Principal Material Coordinate System

Unidirectional:

1 (1 = fiber, 2/3 = planeof transverse isotropy)

2 (2 = fiber, 1/3 = plane

of transverse isotropy

1 is defaultWoven:

1 (1 = fill tow, 2 = warptow, 3 = out of plane)

2 (2=fill tow, 1=warptow, 3=out of plane)

3 (3 = fill tow, 2 = warptow, 1 = out of plane)

5 Progressive Failure Analysis Uni: 0 (off), 1 (on)Woven: 0 (off), 1 or 2 (on)

6 Pre-Failure Nonlinearity 0 (off), 1 (on)

0 is defaultFor unidirectionalcomposites only.

Must have ProgressiveFailure Analysis activated

7 Post-Failure Nonlinearity 0 (off), 1 (on)

0 is default

For unidirectionalcomposites only.

Must have Progressive

Failure Analysisactivated


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8 Hydrostatic Strengthening 0 (off), 1 (on)

0 is default


Must have ProgressiveFailure Analysis

activated

9 Stress Free Temperature (ThermalResidual Stress)

0 (0)1 (temperature read from material file)

0 is default


10 Not used 0 or blank 11 Not used 0 or blank 12 Not used 0 or blank 13 Not used 0 or blank

14 Matrix Post Failure Stiffness 0 < value 1

0.1 (10%) is default


15 Fiber Post Failure Stiffness 0 < value 1

0.01 (1%) is default


16 Force Conversion for CustomUnitsMust be greater than

zero

17 Length Conversion for CustomUnitsMust be greater than

zero

18Temperature Difference

Conversion for Custom UnitsMust be greater than

zero

1. MATID The first argument allows the user to specify a material reference number to beassociated with a Helius:MCT material. When Helius:MCT is installed for Ansys, aHeliusMCT_MatDB.xml file is created in the < Helius Root Directory > \ Materials folder (bydefault this is defined as C:\Firehole\Materials). The purpose of this file is to link the materialreference number (MATID) with the name of a material stored in the < Helius Root Directory > \Materials folder. The material reference numbers for the composite materials that come with theHelius:MCT install are already included in this file. If a new material file is created using theHelius Material Manager, the HeliusMCT_MatDB.xml file is automatically updated to includethe new material and is assigned a material ID number. However, if the material files aremanually copied and edited, the HeliusMCT_MatDB.xml file must be updated to include the newlink between the material reference number (MATID) and the name of the newly created composite material. If the HeliusMCT_MatDM.xml file is opened using a text editor or internet

browser, the contents will look similar to the following (if opened with a text editor):


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To add a link between a newly created material file and an Ansys material reference number ( MATID ), copy the format of the existing file to add an additional line that links the two items. For example, if the newly created material file was saved as example_composite_material , the filewould be modified as:

The value 9018 would be used as the first argument in the HELIUSMCT command and tellsHelius:MCT to use the material example_composite_material .


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2. NSTATV The second argument is used to identify the number of solution-dependent MCT statevariables (SVARS) that must be tracked at each integration point in the finite element model. Thenumber of solution-dependent MCT state variables is dependent upon whether or not the user desires access to constituent average stresses and strains and the microstructure of the composite(unidirectional or woven). Allowable values for this argument are 6 or 34 for unidirectionalmaterials and 6 or 90 for woven materials. 6 state variables should be requested unless the user desires access to the constituent average stresses and strains. In this case, 34 state variablesshould be requested for unidirectional materials and 90 state variables should be requested for woven materials.

3. UNITS The third argument specifies the system of units that should be used by Helius:MCT incomputing the constitutive relations and stresses. In the example provided above, the third argument has a value of 1, indicating that Helius:MCT should compute the constitutive relationsand stresses in its default system of units (N/m/K). There are three other systems of units(N/mm/K - 2, lb/in/R - 3, and lb/ft/R - 4) that can be requested via specific values of the firstuser material constant, in addition to a custom (or user-define d) system of un its which would bespecified using the value of 5. For more information, refer to Appendix A.3 which provides adetailed discussion of the third argument.

4. PFIB_DIR Helius:MCT expresses constitutive relations and computes stress in the principalmaterial coordinate system of the composite material. The fourth user material constant specifiesthe specific orientation of the principal material coordinate system that will be used byHelius:MCT.

For Unidirectional Microstructures: Helius:MCTs default principal material coordinatesystem is oriented with the 1 direction aligned with the fiber direction, while the 2 and 3 directions lie in the materials plane of transverse isotropy. However, in situationswhere it adds convenience or simplicity to the model creation process, the user maychange the orientation of the principal material coordinate system so that the 2 directionis aligned with the fiber direction, while the 1 and 3 directions lie in the compositematerials plane of transverse isotropy. The numerical value (1 or 2) of the fourth user material constant specifies which of the principal mate rial coordinate a xes will be aligned with the fiber direction. For more information, refer to Appendix A.4 w hich provides adetailed discussion of the fourth argument.

For Woven Microstructures: Helius:MCTs default principal material coordinate systemis oriented with the 1 direction aligned with the fill tow direction, while the 2direction corresponds to the warp tow direction and the 3 direction corresponds with theout-of-plane direction. However, in situations where it adds convenience or simplicity tothe model creation process, the user may change the orientation of the principal materialcoordinate system so that the 2 direction is aligned with the fill tow direction, while the1 direction corresponds to the warp tow direction. Additionally, the user may changethe orientation of the principal material coordinate system so that the 3 direction isaligned with the fill tow direction while the 2 direction corresponds to the warp towdirection. For more information, refer to Appendix A.4 wh ich provides a detailed discussion of the fourth argument.

5. PFA The fifth user material constant activates or deactivates Helius:MCTs progressive failureanalysis feature. If the progressive failure feature is activated, then Helius:MCT will routinelyevaluate both the matrix and fiber failure criterion to determine if either constituent material has


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Progressive Failure Analysis is notavailable in Helius:MCT Linear,

its default value is zero and anonzero entry will be adjusted back

to zero by Helius:MCT Linear atruntime. All other nonlinear material constants will also default

to zero.

Pre-Failure Nonlinearity is notavailable in Helius:MCT Linear,

its default value is zero.

failed. Each constituent failure criterion is based on thecorresponding constituent average stress state. In theevent that one or both of the constituents fail, thestiffnesses of the failed constituent(s) and the stiffnessesof the composite are appropriately reduced to therespective post-failure stiffnesses. It should beemphasized that the progressive failure analysis feature isthe foundation component of Helius:MCTs nonlinear multiscale constitutive relations. Other aspects of materialnonlinearity can be invoked via the 6 th, 7 th, and 8 th user material constants; however, theseadditional forms of nonlinearity cannot be activated unless the progressive failure analysis featureis also activated. For more information on the progressive failure analysis feature, refer toAppendix A.5 of this document and Section 4 of the Helius:MCT Theory Manual.

Unidirectional Microstructures: A value of 1 activates the progressive failure analysisfeature, while a value of 0 deactivates the progressive failure analysis feature.

Woven Microstructures: A value of 0 deactivates the progressive failure feature, avalue of 1 activates the progressive failure feature and uses the matrix and fiber degradation levels from the material data file to calculate the failed material properties,and a value of 2 activates the progressive failure feature and uses the matrix and fiber degradations levels specified by the fourteenth and fifteenth user material constants tocalculate the failed material properties. Selecting a value of 2 for plain weaves will add approximately 45-60 seconds to the pre-processing time per woven material. A value of 1 will not add run-time during pre-processing because the failed material properties (atthe matrix and fiber degradation levels specified during material creation inHelius:Material Manager) are already stored in the material file.

6. PREFAIL (optional, for unidirectional composites only) The sixth user material constantactivates or deactivates Helius:MCTs Pre-Failure Nonlinearity feature. A value of 1 activates the

pre-failure nonlinearity feature, while the default value of 0 deactivates the pre-failurenonlinearity feature. If the pre-failure nonlinearity feature isactivated, then Helius:MCT will explicitly account for thenonlinear longitudinal shear stress/strain response that istypically observed in unidirectional fiber-reinforced composite materials. The Pre-Failure Nonlinearity featureimposes a series of discrete reductions in the longitudinal shear stiffness of the matrix constituentmaterial, causing the composite materials nonlinear longitudinal shear response to closely matchexperimentally measured data. It should be emphasized that the Pre-Failure Nonlinearity featureonly affects the longitudinal shear moduli of the composite (i.e., c12 vs.

c12, and

c13 vs.

c13),

while the responses of the other four composite stress and strain components remain unaffected by this feature. Also, the Pre-Failure Nonlinearity feature will not alter the shear stress level atwhich the composite fails; however, it will result in an overall increase in longitudinal shear deformation of the composite prior to failure. For further information on the Pre-Failure

Nonlinearity feature, refer to Appendix A.6 of this document and Section 5 of the Helius:MCTTheory Manual.

Note: The Pre-Failure Nonlinearity feature is only available for unidirectional compositematerials. This feature is ignored by woven composites.


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Post-Failure Nonlinearity is not availablein Helius:MCT Linear, its default value

is zero.

Note: The Pre-Failure Nonlinearity feature is only available for those unidirectional compositematerials where a longitudinal shear stress/strain curve was supplied during the MCT materialcharacterization process. If this feature is requested for a composite material that wascharacterized without a longitudinal shear stress/strain curve, then Helius:MCT will issue an error message at runtime and execution will halt. For more information on using measured longitudinalshear data during the material characterization process, please refer to the Helius MaterialManager Users Guide.

7. POSTFAIL (optional, for unidirectional composites only) The seventh user material constantactivates or deactivates Helius:MCTs Post-Failure Nonlinearity feature. A value of 1 activatesthe Post-Failure Nonlinearity feature, while the default value of 0 deactivates the Post-Failure

Nonlinearity feature. If the Post-Failure Nonlinearityfeature is activated, then Helius:MCT will graduallyreduce the stiffness of the matrix constituent modulito their minimum values. When the Post-Failure

Nonlinearity feature is activated, the matrix failurecriterion simply identifies the initiation of the matrix failure process (or the initiation of matrixcracking). After the matrix failure criterion is triggered, the matrix constituent stiffness isgradually reduced via a series of discrete stiffness reductions that are applied as the matrixaverage strain state continues to increase beyond the level presen t at failure initiation. For further information on Post-Failure Nonlinearity, refer to Appendix A.7 of this document and Section 6of the Helius:MCT Theory Manual.

Note: The Post-Failure Nonlinearity feature is only available for unidirectional compositematerials. This constant is ignored by woven composites.

Note: The Post-Failure Nonlinearity feature is only available for those unidirectional compositematerials where the transverse tensile failure strain ( 22

ult) was supplied during the MCT materialcharacterization process. If this feature is requested for a composite material that was

characterized without a transverse tensile failure strain, then Helius:MCT will issue an error message at runtime and execution will halt. For more information on the MCT materialcharacterization process, please refer to the Helius Material Manager Users Guide.

Note: If the Post-Failure Nonlinearity feature is turned on , then the matrix post-failure stiffnessvalue is ignored.

8. PRESS (optional, for unidirectional composites only) The eighth argument activates or deactivates Helius:MCTs hydrostatic strengthening feature. A value of 1 activates thehydrostatic strengthening feature, while a value of 0 deactivates the hydrostatic strengtheningfeature. If the hydrostatic strengthening feature is activated, then Helius:MCT explicitly accountsfor the experimentally observed strengthening of the composite in the presence of a hydrostatic

compressive stress. If the hydrostatic compressive stress in the matrix constituent exceeds athreshold value, then the strength of both the matrix constituent and the fiber constituent arescaled upwards commensurate with the level of hydrostatic compressive stress level in the matrixconstituent. For further information on the hydrostatic strengthening feature, refer to AppendixA.8 of this document and Section 7 of the Helius:MCT Technical Manual.

Note: The Hydrostatic Strengthening feature is only available for unidirectional compositematerials. This feature is ignored by woven composites.


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9. TEMPDEPEND (optional, for unidirectional composites only) - The ninth argument (0 or 1) isused to specify whether or not to explicitly account for thermal residual stresses in the responseof the unidirectional composite material. If the ninth argument is specified as 1, then Helius:MCTcomputes the ply-level and constituent-level thermal residual stresses that are caused by the post-cure cool down from the stress-free temperature (i.e. cure temperature) to ambient temperature. Inthis case, the stress free temperature is read from the material data file (mdata file) and ambienttemperature corresponds to 72.5F, 22.5C or 295.65K. If the ninth argument is specified as 1,

ply-level and constituent-level thermal residual stresses will be present in the composite material prior to the application of any external mechanical and/or thermal loads that are imposed duringthe actual simulation. If the user chooses to explicitly account for thermal residual stresses in theanalysis, then the user should verify the material data file (mdata file) actually contains a defined stress free temperature; otherwise, the stress free temperature will default to 0 and the predicted thermal residual stresses will be quite erroneous.

If the ninth argument is specified as the default value of 0, then thermal residual stresses are notincluded in the response of that particular composite material during the simulation. In this case,the stress free temperature of the composite material defaults to T sf =0 (regardless of the systemof units employed), and the temperature change that is used in the constitutive relations

[ = C ( T)]

is simply computed as

T = T T sf = T.

Several points should be emphasized here. First, the stress free temperature T sf defaults to 0 evenif the composite material data file (mdata file) explicitly defines a non-zero stress freetemperature. Second, regardless of the system of units that are employed by the finite elementmodel, the current temperature T completely defines the temperature change T that is used in

the constitutive relations. Third, for composite materials that are characterized at multipletemperatures, the current temperature T will be used to interpolate the various material propertiesthat contribute to the constitutive relations; consequently, it is recommended that a single-temperature characterization (i.e., a single-temperature mdata file) should be used for thecomposite material in question. In summary, if the user does not request this option, then thecurrent temperature T influences the constitutive relations in two different ways: 1) thetemperature change used in the constitutive relations simply becomes T=T, and 2) T is used tointerpolate the temperature-dependent material properties that contribute to the constitutiverelations.

It should be emphasized that the default reference temperature in Ansys is 0. This defaulttemperature is completely compatible with the default stress free temperature of 0 that is

assumed when the ninth argument is specified as 0. In this case, the model can still be subjected to temperature changes by simply imposing a temperature other than 0; however, these thermalstresses develop over the course of the analysis, as opposed to being present at the start of theanalysis.

ARGUMENTS 10-13 ARE NOT USED AND SHOULD BE SET TO 0.


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Matrix Post-Failure Stiffness is part of progressive failure analysis and is not available

in Helius:MCT Linear, its default value is zero.

Fiber Post-Failure Stiffness is part of progressive failureanalysis and is not available in Helius:MCT Linear, its

default value is zero.

14. MDEG (optional, for unidirectional composites only) The fourteenth user material constant isa fraction that is used to define the damaged elastic moduli of the matrix constituent after matrixconstituent failure occurs. Specifically, thevalue is the ratio of the failed matrix constituentmoduli to the unfailed matrix constituentmoduli. A value of 0.1 would specify that after amatrix failure occurs at an integration point, all sixof the matrix constituent moduli (E m11 , E

m22, E

m33 , G

m12, G

m13, G

m23) are reduced to 10% of the

original undamaged matrix constituent moduli. The matrix post-failure stiffness value must begreater than 0, and less than or equal to 1. In the event that the fourteenth user material constant isnot specified, the default value of 0.10 is assu med. For more information on the matrix post-failure stiffness, please refer to Appendix A.9 of this documentation.

Note: If the Post-Failure Nonlinearity feature is turned on, then the fourteenth user materialconstant will be ignored since the matrix post-failure stiffness is determined by the Post-Failure

Nonlinearity feature.

Note: The matrix post-failure stiffness (fourteenth user material constant) can only be specified for unidirectional composites; the fourteenth user material constant is ignored by wovencomposites.

15. FDEG (optional, for unidirectional composites only) The fifteenth user material constant is afraction that is used to define the damaged elastic moduli of the fiber constituent after fiber constituent failure occurs. Specifically,the value is the ratio of the failed fiber constituent moduli to the unfailed fiber constituent moduli. A value of 0.01would specify that after a fiber failureoccurs at an integration point, all six of the fiber constituent moduli (E f 11 , E

f 22, E

f 33, G

f 12, G

f 13,

G f 23) are reduced to 1% of the original undamaged fiber constituent moduli. The fiber post-failurestiffness value must be greater than 0, and less than or equal to 1. In the event that the fifteenthuser material constant is not specified, the default value of 0.01 is assumed. For more informationon the fiber post-failure stiffness, please refer to Appendix A.10 o f this User's Guide.

Note: The fiber post-failure stiffness ( fifteenth user material constant) can only be specified for unidirectional composites; the fifteenth user material constant is ignored by woven composites.

The last three user material constants (i.e., the 16 th, 17 th and 18 th constants) are only required if the finiteelement model is defined using a custom system of units. If using a custom set of units, please refer toAppendix A.3 f or formatting details.

Ansys 11 Note: A longitudinal elastic modulus (EX) must be defined for each Helius:MCT material.Ansys simply uses this parameter as a place holder and its value has no effect on theanalysis. This is only required for Ansys 11 and is not required for Ansys 12 and later.In order to specify EX, add the following line after each HELIUSMCT command :

MP,EX, matID ,value


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Nonlinear Solution ControlParameters are not used in

Helius:MCT Linear.

Where matID is the ANSYS material ID that corresponds to the Helius:MCT materialand value is the value of EX for that material. Again, the particular value of EX doesnot influence the results.

4.2 Modeling Issues for Imposing Temperature ChangesThe composite materials that are stored in the Helius:MCT Composite Material Database are

defaulted to have a stress-free temperature of zero in the system of units that is specified by the value of the 3 rd argument unless it is specified by the 9 th argument that the stress-free temperature stored in thematerial file should be used. Any initial temperature specified via the TREF command (including thedefault value of 0 ) will be used to calculate the residual stresses present in each of the constituents and the composite due to the unmatched coefficients of thermal expansion between the fiber and matrix. For details on how the temperature changes imposed in a model affect how Helius:MCT calculates residualstresses, please refer to Section 10 in the Helius Theory Manual.

4.3 Nonlinear Solution Contro l Parameters for Helius:MCT

It is a widely accepted notion that good convergence (or any convergence at all) is difficult toachieve in a progressive failure simulation of a composite structure. In fact, many progressive failuresimulations terminate early, not due to global structural failure, butsimply due to the inability of the finite element code to obtain aconverged solution at a particular load step. In light of this

problem, one of the major advantages of Helius:MCT is that it has been optimized to significantly improve the overall convergencerate and robustness of finite element simulations of progressive failure of composite structures.However, in order to take full advantage of the superior convergence characteristics of Helius:MCT, theuser must change some of the default settings that govern the nonlinear solution process used by Ansys.These changes can be enacted using the NROPT, PRED, NSUBST, CNVTOL, and NEQIT commands.

Regardless whether the SOLCONTROL command is ON (default) or OFF, the user will need tooverride these nonlinear solution control parameters.

NROPT

The NROPT command is used to do two things: a) instruct Ansys to use the Full NewtonRaphson algorithm, and b) prevent Ansys from using their Adaptive Descent algorithm to help thesolution process. An example of the NROPT command fulfilling these two requirements is shown

below.

NROPT, FULL, , OFF

In Ansys, the nonlinear solution process is based on the fundamental assumption of the Newton-Raphson algorithm that the nonlinear response of the composite structure is sufficiently smooth at both the global and local levels. However, in a progressive failure simulation of a compositestructure, the nonlinear response of the composite structure is not smooth , especially at the locallevel where material failure results in an instantaneous reduction of material moduli. This non-smooth material response is one of the primary factors responsible for the difficulty in obtainingconvergence in progressive failure simulations. Helius:MCTs method of managing materialnonlinearity is specifically designed to handle this localized non-smooth material response;however, the default settings of Ansyss nonlinear solution control parameters must be changed in


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order to allow Helius:MCT to improve the convergence characteristics of the finite elementsimulation.

PRED

The PRED command prevents Ansys from using the converged solution at the last substep toestimate the solution for the current substep. This interferes with Helius:MCTs method of managing material linearity. An example of the PRED command that fulfills this is shown below.

PRED, OFF, , OFF

NSUBST

The NSUBST command specifies the minimum and maximum allowable number of substeps for the current load step. Firehole Composites does not recommend any minimum or maximum valuesand engineering judgment should be exercised. However, provided that enough equilibriumiterations are allowed per substep (discussed under NEQIT heading), Helius:MCT will always find a converged solution. This is a deviation from typical nonlinear solution processes where multiplesubstep size cutbacks may be required. Helius:MCT will converge at each substep, regardless of the size (again provided that enough equilibrium iterations are allowed per substep), so care must

be taken when deciding on substep size.

CNVTOL

The CNVTOL command is used to define the convergence tolerance for residual node forces.There are two arguments set in this command that allow Helius:MCT to better handle the nonlinear solution process: a) set forces (F) as the convergence label, and b) set the norm selection to infinitenorm (check each DOF separately). An example of the CNVTOL command that fulfills these twoitems is shown below.

CNVTOL, F, , , 0

NEQIT

The NEQIT command is used to specify the minimum number of equilibrium iterations that must be performed before Ansys evaluates the need for cutting-back the size of the current substep.Helius:MCT has the unique ability to converge regardless of the substep size or extent of nonlinearity occurring during the substep. In the experience of Firehole Composites, convergencealways occurs when the number of equilibrium iterations allowed per substep before a substepcutback occurs is set to 1000. An example of what this command looks like is shown below.

NEQI T, 1000

4.4 Requesting Output of the MCT State Variables

The solution-dependant Helius:MCT state variables are used to track constitutive quantities of interest at each integration point in the finite element model. The number of solution-dependentHelius:MCT state variables is defined via the second argument of the HELIUSMCT command in theinput file (see Section 4.1). The default naming convention for the solution-dependant MCT state


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variables is SVARi, where i=1,2,3,,6 or 34 (or 90). The most useful of the MCT state variables isSVAR1, which is used to track the discrete failure state of the composite material at each integration

point in the finite element model. The exact interpretation of the discrete values of SVAR1 will depend upon the exact set of Helius:MCT material nonlinearity features that are used in the analysis . Appendix B

provides a complete description of each of the MCT state variables, including tables that define theinterpretation of SDV1 for various combinations of material nonlinearity features.

Solution-dependant Helius:MCT state variables are used to track the histor y of certain quantitiesthat are computed in the Helius:MCT user programmable feature. Appendix B de scribes each of the MCTstate variables. These MCT state variables are not written to the database unless explicitly requested inthe Ansys input file. Listed below is a method for requesting SVAR output to the results file:

OUTRES, SVAR, ALL .

This command will write only SVAR to the results file and is useful for limiting the size of the resultsfile. For more information on the OUTRES command, refer to the Ansys documentation.

4.5 Modeling Damage Tolerance

Damage tolerance is the ability of a structure to retain required structural strength or stiffnessafter it has sustained damaged. When a composite part is damaged, there are numerous failure modes thatcan exist. These failure modes are constituent-level defects (i.e. fiber and matrix level defects) so it isappropriate to model damage at this level. Helius:MCT is well-suited for modeling damage tolerance

because it allows the user to specify constituent-level damage in elements at the start of the analysis. For example, if a plate was impacted by a mass and there is diffuse matrix damage in impacted region, theuser can designate a region that represents the damaged region and assign matrix failure to that region

prior to the start of the analysis. At the start of the analysis, this region will have an SVAR1 value of 2(matrix failure) and as the simulation progresses, the region can undergo fiber failure which will result inan SVAR1 value of 3. The initial value of SVAR1 that is assigned to the damaged region is not fixed and can change if either the matrix or fiber failure criterion is satisfied.

The initial value of SVAR1 must be an integer value equal to 1, 2, or 3. For unidirectionalmaterials, 1 corresponds to no failure, 2 corresponds to matrix failure, and 3 corresponds to fiber and matrix failure. For woven materials, 1 corresponds to no failure, 2 corresponds to matrix failure in alltows and the matrix pocket, and 3 corresponds to fiber failure in all tows plus matrix failure in all towsand the matrix pocket. The Ansys commands listed below are used to activate damage tolerance:

HELI USMCT, TB, STATE, < MATID >, , < NSTATV > TBDATA, 1, < initial value of SVAR1 (2 or 3) >

Note: The TB and TBDATA commands required to specify that a material is using degraded material properties must immediately follow the HELIUSMCT command that defines the undamaged material.

Note: The MATID and NSTATV arguments in the TB command above must exactly match the MATIDand NSTATV arguments supplied during the HELIUSMCT command (see section 4.1).


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This section mainly deals with progressive failure, which is not available inHelius:MCT Linear. Helius:MCT Linear users may however find the portion

on the selection of section points helpful.

5 Examining Helius:MCT Results with Ansys Mechanical APDL

This section discusses the use of Ansys to examine the finite element results that are generated byHelius:MCT. It is assumed that the reader is familiar with using Ansys to view finite element results and

perform any necessary post-processing of finite element results. Therefore this section focuses on issuesthat are unique to the processes of examining and interpreting the progressive failure results generated byHelius:MCT. This section is divided into two parts. The first part focuses on the generation of contour

plots for the MCT state variables. The second part deals with the issue of correlating damage distributionwith overall changes in structural stiffness, including the detection of global structural failure.

5.1 Using contour plots to view the MCT state variables

The MCT state variables (SVARs) are element output variables stored at each integration pointwithin each element; consequently, the same methods are used to view the MCT state variables as thefamiliar stress and strain variables. However, in order to view any of the MCT state variables in Ansys,the Ansys input file must first request that they be written to the Ansys results file (see Sections 3.4 and 4.5). For a complete description of each of the MCT state variables, refer to Appendix B.

Contour plots are usually the most appropriate means of examining the distribution of the MCTstate variables. To generate a contour plot of the MCT state variables within Ansys, first open the resultsfile by clicking Main Menu General Postproc Data & File Opts and selecting the results file.Data from a set must be read, which is accomplished via the SET command or by clicking Main Menu

General Postproc Read Results . To view the MCT state variables that are computed byHelius:MCT, use the PLESOL or PLNSOL commands. As an example, to create a contour plot of statevariable 1, enter PLESOL,SVAR,1 into the command prompt. The number of SVARs that are availableto plot depends entirely on the number of SVARs requested for output by the Ansys input file. Note, thatin order to view SVARs greater than 11, power graphics must be turned off by issuing the /GRAPHICS,FULL command.

The fundamental MCT state variable is SVAR1 which indicates the discrete damage state of thecomposite material. SVAR1 is an integer variable. The range of integer values that can be assumed bySVAR1 depends entirely upon the specific set of material nonlinearity features that were used byHelius:MCT during the finite element solution (see Appendix B). As a specific example, Appendix Bstates that if Helius:MCT is used with its progressive failure feature activated and its pre-failure and post-failure nonlinearity features de-activated, then SVAR1 can only assume the integer values 1, 2, or 3 asshown in the table below.


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Figure 12 shows two different 3-color contour plots of the integer variable SVAR1 for an axiallyloaded composite plate with a central hole. In Figure 12, the use of three discrete colors makes it easy toidentify the regions where each of the three discrete composite damage states occur. The blue elements(SVAR1=1) are completely undamaged; the green elements (SVAR1=2) have failed matrix constituentsand undamaged fibers; the red elements (SVAR1=3) have failed matrix constituents and failed fiber constituents. Note that the PLNSOL contour plot shows contours that are continuous across element

boundaries, while the PLESOL contour plot shows contours that are discontinuous at element boundaries.In other words, a PLNSOL plot will average values across element boundaries, while a PLESOL plotsimply uses the values of SVAR1 at the individual integration points to establish the color contoursindependent of the element boundaries.

Figure 12: Comparison of a PLNSOL contour plot and a PLESOL contour plot using threediscrete color contours to represent distribution of SVAR1=1,2,3

The remaining MCT state variables (SVAR2, SVAR3, SVAR4,,SVAR34 or 90) are continuousreal variables . Therefore, in generating contour plots of these variables, it is not critical to manage thenumber of color contours. Furthermore, the standard practices used in viewing stress and strain

distributions are a

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