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COMPOSITE DRAPING SIMULATION TO ENHANCE STRUCTURAL ANALYSIS

Paul Van Huffel Altair Engineering, Inc

Abstract

With composite analysis and optimization on the rise, the accuracy of our assumptions is becoming more and more important to product development. In the case of woven fiber composites with organized fiber orientations, the need for accuracy in the orientation definition in Finite Element Models early on in development is critical. For this there are two ways this can be established. One can simulate the forming process of the fiber composite using explicit simulation and use the results to condition a model for product performance simulation, or one can use a drape estimating program to implicitly calculate and set the fiber orientations. This paper will cover these two methods and compare the net resulting predictions between these two methods using a B-Pillar model with Impact and Normal Modes simulations.

Background and Requirements

Composite optimization has become an important design process as structured composites gain popularity in mass reduction and performance improvement initiatives. There are a growing number of tools for optimizing composites in the design and development process.

The first principle in this process is understanding that manufacture can affect the resulting composite in spite of all the design efforts invested in advance. The result may still be within the specifications for that design or, for more sensitive designs, they may not.

Forming simulation of composite materials can be done with explicit solution finite element analysis tools. This captures anomalies and subtle variations that arise from the forming process, whereas as drape estimating algorithms tends to give an idealized answer. The central question here is how significant this difference is to the performance in the final product. If manufacturing variation is low, and quality is high in forming, then the results should be close.

There is one particular problem in performing a comprehensive forming simulation of a composite material. The data provided by material suppliers gives information for the cured material, but testing is not generally done on the uncured material or raw fabric because the demand for full forming simulation of this type is not yet high enough to warrant characterization of the uncured material. As such, this leaves the need for some means of approximating those properties reasonably enough to perform this part of the simulation in order to get the get fiber orientation of the resulting formed product and thus compare to a drape estimating algorithm.

To the point of this paper, a drape estimating solver can help capture the effects of forming without having to get the properties of the uncured composite material, thus saving significant time and effort. The question remains as to how comparable the solution is to the more comprehensive forming process as this captures the effects and sensitivities inherent throughout the forming process. The effect of wrinkles on the resulting orientation angles is one specific example for which drape estimating solvers don’t account. As such, this study has value in the form of correlation of results and comparison of time involved, both modeling & computation.

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In evaluating the results, there will be a comparison of impact and normal modes analyses on a B-Pillar of these two methods using a 4 layer stack.

Theory and Process for Study

Theory

Composite layer properties are orthotropic, and thus the material characteristics used to describe them must capture this behavior. For the forming simulation, a bi-directional orthogonal weave of glass fiber is assumed, and directions “warp” & “weft” are used to describe directions along the fibers in each orthogonal direction in the plane of the fabric construction respectively. Essentially, one is aligned with the direction of the continuous manufacture of the fabric (warp), and the other is the cross-stitch (weft). The weft direction is typically straight, or nearly so, in its alignment where the warp, as the name suggests, bends alternately about the weft fibers as shown below.

A “stack” denotes a collection of weave layers that eventually make up the composite structure when that point in the process is reached. It’s intuitive that fabrics regardless of composition, are stiffest in their warp and weft directions, but not so in shear or bending to the point where there is little interrelation between them. The only correlation or interrelation worth noting is between the behavior in the fiber directions (warp and weft) and bending. This is easier to describe in words than in math models because the bending behavior, while we need to describe this as a function contained in the material definition, it is actually a property of the physical construction of the fibers and thus would correlate to a function of the element behavior.

Intuitively, the tensile behavior in the warp and weft directions are expected be, by some measure, lower in stiffness and will gradually approach the stiffness of a similar series of straight fibers. Some data found in a book on fabric construction and testing shows a slightly higher stiffness for the weave versus raw glass fiber (REF #1) in the warp direction and slightly lower in the weft direction. The difference for glass is not great enough to affect the results here.

The purpose behind having two inherently different tests is to capture potential sensitivities to the results. Composite properties can significantly alter the behavior of a complex structure just as they can in simple structures. Normal modes, both mode shapes and frequencies, may highlight sensitivities from the process that may seem less significant in an impact test.

The use of impact analysis evaluates the nonlinear behavior of the structure in an impact event using an explicit-dynamic solution. The structure’s stiffness is evaluated with a specific load under sophisticated conditions. Normal modes are very good at comparing the overall stiffness matrix and behavior of each structure as not only will the modes generated vary, but the shapes of the modes may vary as well, thus providing greater depth of understanding when trying to overcome mode-based engineering challenges such as Noise, Vibration, & Harshness (NVH).Don’t forget that you need to have more than one sub-head in a section to use sub-heads. If you only have one sub-head, leave it out and just make an additional section.

Analytical Process

From the part data, forming tool models were generated. These included a die, punch, and blank holder. The process was developed initially using a previously established unidirectional glass fiber material model, but the results proved unusable due to excessive forces needed to form the part. The blank holder force was set as high as 50,000N and still it ejected upward during forming. 100kN was also attempted. This led to time step instabilities and a rework of the material

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properties was needed.

Based on research into composite properties, fabric testing, and a series of trial and error forming simulations while observing the blank holder force for forming raw glass fiber, properties for the blank in the forming process were established (Ref #2). The variables of particular interest here are:

(E11, E22, E33, v12 G12, G23, G31, Rho).

The stiffness in the warp and weft directions, for the purposes of this study, were assumed to be that of the glass fibers pulled tight. The shear properties were then adjusted until a fairly realistic result was achieved. The properties used for the forming and draping simulations were as follows (with respect to the list above):

(70000, 70000, 50, 0.3, 300, 300, 0.2, 2.5e-9).

The unit system employed was mm, Mg, s, MPa. The blank holder force for this simulation was 20kN.

A: Formed Glass Weave: 4 layers with (0,45,-45,0) orientations. With very low bending stiffness and almost no shear stiffness in the plane of the sheet, wrinkles form easily.

In illustration A above shows the result with several wrinkles in the formed part. Anyone who

has seen stamped aluminum parts knows this is common. Ideally this effect is kept to a minimum, but it is not always possible to completely remove them.

Being as an uncured resin in the mix of the woven fabric will be negligible to the mechanical response in the forming simulation, and curing behavior (reaction kinematics) during forming is assumed to be negligible, it is ignored for the forming simulation and the only result of import

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from this simulation is the resulting fiber directions. The fabric construction tested in this paper is a 4-layer stack composed of the following angles in degrees (0, 45, -45, 0).

The resulting fiber directions are then mapped to a clean mesh aligned to the resulting formed composite, and the angles are mapped to the clean mesh based on their respective positions. There may be some sensitivity to the accuracy of the relative positioning, but this variable exceeds the scope of this paper at this time.

The material properties for the cured composite are then mapped to the new structure along with the fiber directions, and the model is formatted and exported for the respective mechanical tests. The properties for the cured resin are assumed to be as follows (again respective of the list previously given):

(70000, 70000, 5000, 0.2, 3000, 7000, 7000, 2.5e-9).

Once the forming operation was completed the results were mapped to a clean mesh and exported to Bulk Data, and Block formats. Those exports were then imported and setup for Normal Modes or used as an include file to condition the clean mesh for the impact analysis respectively. OptiStruct® was used for Normal Modes analysis and Radioss® for the impact analysis.

A single clean mesh was used to generate the 2 different drape models (Modes and Impact). The drape model for the impact was used to create the mapped result models for both the normal modes and impact simulations.

Normal modes were setup with constraints on 3 well separated nodes to prevent rigid body motion (DOFs 123, 23, 3). This was done to aid the comparison of the two results. As stated previously, the Drape model for the impact was used directly with the addition of an included state file to condition the element orientations to the mapped results.

The drape estimation, accessible from the pre-processing GUI, was setup by creating ply layers referencing the elements in the part, the material data (same properties were used in draping as in forming as compatible models were present in both codes), and a coordinate system with assigned alignments. From this, a laminate is constructed that references the plies. The drape estimator is invoked to create the drape tables. This took about 10 minutes to complete with the simple structure used here. The laminate is then “realized” in order to complete the assignment of element orientations.

The impact analysis used a 250kg rigid pole with an initial velocity of 1000 mm/sec. The part was held in translation only, along each edge, at the ends (DOFs 1-3). The event simulated spanned 10ms.

Future work on this topic will start with testing methods for the materials being formed once the desired material is identified for the performance and the properties are known for the cured state. This took the most time in the development of this paper. Beyond that, a more in-depth study of the number of layers, and the effect of composite optimization over the design process affecting the manufacture and subsequent analytical method could provide a wealth of understanding into identifying best practices.

Results & Conclusions

Result Data

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The normal modes did show some difference in the frequencies identified. The drape result showed frequencies of 2.18, 7.73, 18.2, 32.1, 52.6, and 60.9 Hz (Illustration B).

B: Normal Modes Result for Drape Estimated model via Eigen Solver

The mapped result, on the other hand, showed frequencies of 4.61, 21.1, 36.8, 60.1, 66.8, and 78.2 Hz (Illustration C).

C: Normal Modes Result for Mapped Forming Result model via Eigen Solver

The mass of the models was about 256 grams and the difference in mass between the two was less than 1 gram.

The mode shapes are shown here in illustrations D through I with the respective models set side-by-side. Deflections are scaled at 5x:

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D: Mode 1 Comparison.

E: Mode 2

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F: Mode 3: Mode shapes are beginning to differentiate

The scaling for the next 3 modes was 2.5 as the shapes became more distorted in the view.

G: Mode 4: Differences in mode shape are now distinct

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H: Mode 5: While similar at one end, the other is more distinct between the two.

I: Mode 6: Continued distinction between the two methods of model development..

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The first mode shapes of the two parts were nearly identical. The second showed only slight differences, but by the third mode shape, it was clear that there were differences in the structure that meaningfully differentiate the two.

The impact analysis showed only a very slight change in deflection (Illustration J).

J: Impact Deflection/Deformation

The difference was only 0.002mm. This can be retested at higher speeds, and like the modes it may show a point where the two will differentiate. That up-scaling is out of the scope of this paper but would certainly be part of a future work.

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K: VonMises Stress: The peak stress in the mapped-result model is notably higher.

The VonMises stress in the formed part showed a peak at 96.0MPa versus 47.6 in the Drape Estimated model (Illustration K). Also, as seen in the image above, there are some patches of elevated stress near the point of impact on either side of the bar. This means that the peak is not simply a single-element outlier. While the deflections were almost identical, the stresses represent some differentiation between the two approaches. The peak stress points appear to be outliers (high stress gradient versus neighboring elements), but when the scale is brought into focus for relative to the drape model, there is enough difference in the neighboring elements to take note for further study in future work.

Interpretation of the Data

The impact analysis showed very little difference between the mapped and drape-estimated models with respect to deflection for the simulation performed. The stress results showed that the wrinkles from the manufacturing process can create weak spots in a part that are not captured in drape estimation.

While significant time can be saved in using drape estimating software, some critical parts being made with composites should be at least once evaluated using forming if there is any question to the potential for significant variation in manufacturing. If the risk of product variation in manufacturing is low for a given part geometry, then drape estimation will likely perform well.

The differences in the modal analysis were more pronounced and the subsequent effects to frequency response analyses can be expected to show similar differentiation. Most of the differentiation appear to be from the wrinkles in the formed part. These cause orientation aberrations that appear like fault-lines in the part. These sudden and radical changes in orientation will contribute to elevated stresses. Further study with better forming results is needed to evaluate the effectiveness of draping over forming for product performance evaluation.

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Summary and Next Steps

Getting the properties of the pre-formed composite (glass weave or uncured SMC) for forming simulation is critical to getting an accurate answer via that method. It is also the most difficult as this testing is generally not performed by suppliers. For anyone seeking to add composite forming to their company’s development path, they may need to add testing procedures to characterize the pre-formed material. There are many more variables in the forming solution that need to be confirmed in order to effectively use that path.

The wrinkling in the forming simulation likely has the most to do with the differences between these models. A longer process development with greater focus on final part quality after forming would need to be part of the next step in this research so that the effects of those wrinkles can be best minimized.

It can be concluded that drape estimation for standard use impact analyses where the product is expected to survive is reasonably robust and insensitive to the method of obtaining fiber orientation where the risk of manufacturing variation (deviation from the design intent) is low.

As for mode-based performance analyses, this does not yet appear to be the case. The effects of wrinkles and manufacturing variation can materially alter the results. Evaluation of composite structures for noise and vibration are likely quite sensitive to the effects of manufacturing variation.

While more work is needed in this area, this work does highlight the potential for downstream effects of manufacturing variation, and for its inclusion in quality planning.

Acknowledgments

Special Thanks to SPE and the Composites Conference hosts for their patience. Also to Subir Roy and Ravi Kodwani for their assistance with some parts in the simulation.

Nomenclature

Warp – The direction of fiber in a fabric along the direction of continuous manufacture.

Weft – The direction of fiber in a fabric that crosses orthogonally to the warp direction.

Bulk Data Format – A file format for implicit structural analysis originally developed by NASA.

Block Data Format – A file format used for input to RADIOSS®, an explicit dynamic analysis software developed in France by Mecalog Group.

Ply – A single layer of an organized composite structure.

Laminate – A collection of plies also called a “stack.”

Bibliography

1. Forster, B., M. Mollaert R. Blum, H. Bögner, G. Némoz., European Design Guide for Tensile Surface Structures, TensiNet, 2004 pg.221-225. http://www.tensinet.com/files/Design_Guide/09-tensinet.pdf.

2. Kovačević Stana., “Analysis of the Mechanical Properties of Woven Fabrics from Glass and Basalt Yarns”, FIBRES & TEXTILES in Eastern Europe 2015, Vol. 23, 6(114), Pg 83-91. http://www.fibtex.lodz.pl/pliki/Fibtex_(m3tya70oirsxtqw5).pdf.

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3. Divine, V., E. Beauchesne, S. Roy, H. Palaniswamy., “Forming Simulation of Woven Composite Fibers and Its Influence on Structural Performance”, AIP Publishing LLC, 2013/2014.

http://scitation.aip.org/content/aip/proceeding/aipcp/10.1063/1.4850148.