fea for composite to steel

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Composite structures are used extensively in aircraft, space vehicles, marine, and automotive structures due to their light weight, high stiffness, and high ultimate strength. Thesstructures often are joined to metal structures and other composite structures using me

chanical fasteners, which are expensive and use labor-intensive installation procedures, and thjoints require long-term maintenance. Therefore, new cost-effective processes must be developeto meet the functional requirements of structures and joint longevity.

Edison Welding Institute (EWI) and several partners (Boeing, Applied Research Lab at PenState University, Northrop-Grumman Ship Systems, Bath Iron Works, and the Composites Materials Technology Center) demonstrated an adhesive bonding method to join composite-to-stefor large ship structures. Adhesive bonding can provide load continuity and can be used on bot

internal and external bulkhead type structures[1].Logistics International, Abu Dhabi, Dubai, UAE, obtained two large frigate hulls (141M an

135M), originally belonging to the Dutch government, to convert the hulls into giga-yachts at th

Abu Dhabi MAR shipyard by (among other modifications) removing the steel deckhouse entireand replace it with a composite structure. EWI was engaged to help design and verify the composite-to-metal joint and to provide supporting data pertinent to obtaining the manufacturing procescertification of the ships from Det Norske Veritas (DNV), Baerum, Norway.

Experimental testing and finite element analyses (FEA) were conducted to qualify the adhesive bonding process for joining composite to steel. Experimental and FEA data developed at EWwere submitted to and subsequently approved by DNV. Experimental results for the adhesive werpublished in Ref. 2 together with FEA results. This paper reports the finite element model deveopment for assisting the design of a bonded joint system for a composite-steel interface. The modeling method including the approach, input, validation, and application are discussed.

Composite-to-steel joint

An effective composite-to-steel adhesive joint normally incorporates a double lap shear joindesign, adhesive layers, composite skins, and cores. Many types of steel section were evaluatefor carrying the structural load. The clevis double-leg design (Fig. 1) is capable of transmittin

structural loads and maintaining the integrity of th

composite-steel joint[1].Figure 1 represents a manufacturing approach t

make a composite-to-steel joint[1]. A paste adhesive cabe applied to the steel shoe, into which the compositpart is fitted. After making the composite-to-steel adhesive joint, the bonded steel receiver can be welded to thdeck. Weld points should match the steel legs to existing below-deck stiffeners. As an example, Fig. 2 showan adhesive bonded composite-to-steel structure.

Modeling method

The modeling method was developed based ocommercial finite element software, ABAQUS. A threedimensional (3-D) model was used in which the metaadhesive, and core were meshed with solid brick elements and the composite was meshed with both solibrick elements and cohesive elements. The number olayers and material property orientations in the composite can be considered using this modeling method.

Figure 3 shows the modeling approach used in thfinite element analysis of composite-to-steel adhesiv

joint, which includes modeling input, validation, and application. The model input includes the joint geometr

Finite Element Analyses ofComposite-to-Steel Adhesive Joints

Yu-Ping YangGeorge W. Ritter

David R. SpethEdision Welding

Institute

Columbus, Ohio

Finite element

modeling can

assist in the

design of an

adhesive-

bonded jointsystem for a

composite-

steel interface

to meet the

functional

requirements

of structures

and joint

longevity.

Fig. 1 Adhesive joining of composite to steel[1].

ADVANCED MATERIALS & PROCESSES JUNE 201124

Composite part

Core

Composite

Paste adhesive

Steel H-section


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ADVANC ED MATERIALS & PROCESSES JUNE 2011

and materials properties. Material properties were ob-tained from public literature and supporting tensile testswithin this program. Double-lap shear tests were used to

validate the finite element model, and the model was usedto predict the strength of complex composite-to-steelstructures.

Material properties

Typically, steel, composite, adhesive, and core are in-volved in a composite-to-steel joint. Isotropic elastic andplastic material behavior was assumed for the steel, the ad-hesive, and the core. Orthotropic elastic material behaviorwas assumed for the composite. Steel material propertieswere obtained from public literature. Adhesive materialproperties were obtained from testing, because they aredetermined by the bonding process. The composite andthe core material properties were provided by materialsuppliers.

Type 316 stainless steel was selected for hull structures

based on the corrosion requirements. Material propertiescan be obtained from material handbooks such as ASM

specialty handbook[3]. In the analysis, elastic-plastic mate-rial properties were input into the finite element model.The adhesive selected for the exterior joint is 3M 2216translucent epoxy adhesive (3M Co., St. Paul, Minn.) towhich an accelerant was added to boost its cure rate andtemperature resistance. Tensile tests of adhesive were con-ducted from cast specimens configured as ASTM D638Type I dogbones. Tensile properties were measured atroom temperature (23C) and at 60C. The higher temper-ature was selected based on the possible service conditionsof composite to steel structures at sea. The strain rate waskept constant at 12.5 mm/min.

Poissons ratio, tensile elastic modulus, and plastic fail-ure strain at room temperature and elevated temperaturefor the epoxy adhesive are shown in the table below.

Figures 4a and 4b show the stress-strain curves for theadhesive at room temperature and (60C), respectively.Tensile tests show that the failures at the higher tempera-ture were caused by stretching the adhesive to the materiallimit (most deformation is plastic). Three duplicate testswere conducted. The average material properties wereinput to the models.

The core material was Diab Divinycell H200 (Diab Inc.,DeSoto, Tex.). Elastic and plastic material properties takenfrom the product literature were used for the analyses:Poissons ratio is 0.32, tensile modulus is 0.23 GPa, yieldstress is 1.6 MPa, and tensile strength is 6.4 MPa.

Composite material properties were provided by thematerial supplier. Table 1 shows tensile and shear elastic

Temperature, C

23 60

Poissons ratio 0.38 0.38

Density, kg/mm3 1.13E-06 1.13E-06

Tensile modulus, GPa 1.3 0.177

Failure plastic strain, % 18.0 56.5

Fig. 2 Adhesive-bonded composite-to-steel structure.

Fig. 3 Modeling approach.

Fig. 4 Tensile material properties of 3M 2216 translucentepoxy adhesive at room temperature (a) and 60C (b).

Composite

Steel

Geometry

Model input Tensile test

Material

properties

Model

validationDouble-lap shear test

Design

Modeling

application

35

30

25

20

15

10

5

0

Stress.

MPa

Stress.

MP

a

Average

Rt-5

Rt-4

Rt-6

0 0.04 0.08 0.12 0.16 0.2

Strain

0 0.01 0.2 0.3 0.4 0.5 0.6

Strain

Average

T4 T2 T1

12

10

8

6

4

2

0

(a)

(b)


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moduli and Poisson ratio. The composite is strong in plane(x-y plane) and weak in the thickness direction (z-direc-tion). The composite failure was modeled using the pro-gressive damage and failure method. Failure parameterswere calibrated with experimental testing results and pub-lished in Ref. 2.

Modeling validation

FE model was validated by analyzing a double lap shear(DLS) test sample, as shown in Fig. 5. The DLS specimenconfiguration is in accordance with ASTM 3528, Type A.A single large bonded plate about 300 mm wide was pro-duced and individual 25-mm test specimens were cut from

the plate. The adhesive cured for at least one week at roomtemperature prior to testing.

Tensile tests were conducted by fixing the steel end anapplying the load at the composite end (Fig. 5). Load-diplacement curves were used to validate the finite elemenmodel.

A finite element model was built based on the DLconfiguration (Fig. 6). The steel and adhesive were meshewith an 8-node brick element. There were 2040 nodes an1200 elements for the steel and 640 nodes and 360 elements for the adhesive. The composite was first modeleas skin only with cohesive interlayers (no core). The composite skin was meshed with 8-node brick elements, anthe composite cohesive interlayers were meshed with cohesive elements. There were 2550 nodes and 1000 elemenfor the skin and 2520 nodes and 976 elements for the cohsive strength.

Finite element analysis (FEA) was performed to predithe strength and failure mode for DLS testing at room temperature and 60C. The load capacity was predicted ancompared with the experimental results as shown in Tab3(2). Three replicates (E3, E4, and E10) were tested at room


Fig. 5 Dimensions and loading methods of double lap shear test.

Fig. 6 Finite element model of a double-lap shear test sample.

Table 1 COMPOSITE MATERIAL PROPERTIES

Tensile modulus, Shear modulus, PoissonsGPa GPa ratio

17.50 (Ex) 6.90 (Gxy) 0.30 (Vxy)

17.50 (Ey) 6.90 (Gxz) 0.30 (Vxz)

3.0 (Ez) 6.90 (Gyz) 0.30 (Vyz) Fig. 7 Failure initiation during loading at room temperature(a) and 60C (b).

Fix this end

Adhesive

Steel (2.5 mm)

Composite

(4.25 mm)

Apply load at this end

25.4mm

152.4mm

152.4mm

Skin

Cohesive

BrokenFailure

initiation

Failure Initiation

(a)

(b)


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temperature and three replicates (E13, E14, and E15) athigh temperature, all at strain rate of 1.25 mm/min. Theaverage load capacity is 23.2 kN with a displacement 3.4mm at room temperature and 9.8 kN with a displacement2.5 mm at high temperature. The predicted load capacity is23.9 kN with a displacement 3.3 mm at room temperatureand 9.6 kN with a displacement 2.6 mm at high tempera-ture. The model predicted load capacity that correlatedwell with the testing results.

In addition to the load capacity, the model can pre-dict the joint failure modes and failure locations. Asshown in the Table 2, interlaminar-shear fracture was ob-

served at room temperature[2]. The fracture location is theexterior composite skin. Finite element analysis results(Fig. 7a) shows that the crack started in the cohesive area

between plies and then induced skin failure. This corrob-orates the observed failure mechanism. The failure modeat high temperature is cohesive tearing in the adhesive,rather than failure in the composite skin. Finite elementanalysis (Fig. 7b) shows that the crack started and propa-gated in the adhesive.

The comparison of the peak load, the displacement at

the peak load, the failure mode, and the failure location be-tween the experiment and prediction indicates that themodel accurately predicts the load response. Therefore, themodel is ready to be used to predict the joint strength oflarge composite-to-steel joint.

Modeling application

in optimizing adhesive thickness

The validated FE model was used to assist the compos-ite-to-steel joint design and optimize the adhesive thick-ness. To understand the effect of adhesive thickness on the

joint strength, the adhesive thickness shown in Fig. 5 waschanged. The following cases were analyzed at high tem-perature:

2 mm thick at one side and 2 mm thick at another side

(2 mm-2 mm) 2 mm thick at one side and 4 mm thick at another side(2 mm-4 mm)

4 mm thick at one side and 4 mm thick at another side(4 mm-4 mm)

Figure 8 shows the plastic strain distributions and fail-ure locations for the three analyzed cases after applying 3.5

mm displacement at the com-posite end. For the 2 mm-2mm case, a crack started inboth sides of adhesive. For the2 mm-4 mm case, a crackstarted on the 2 mm side and

there was no crack in the 4 mmside. For the 4 mm-4 mm case,no crack was observed in theanalysis. This shows that thickadhesive bondline allows moredisplacement.

Figure 9 shows the result-ing force (load) after applyinga 3.5 mm displacement. Thehighest peak load was ob-tained for the 2 mm-2 mmcase. The lowest peak load

ADVANCED M ATER IALS & PROCE SSES JUNE 2011

Table 2 COMPARISON OF PEAK LOAD AND DISPLACEMENT BETWEEN EXPERIMENTAND PREDICTION

Displacement at peakPeak load, kN load, mm

Temp., C Sample Test Avg. Test Avg. Failure mode Failure location

23 E3 24.1 23.2 3.0 3.4 Interlaminar shear Exterior composite skin

E4 21.3 3.5

E10 24.1 3.8

Prediction 23.9 3.3

60 E13 8.7 9.8 2.4 2.5 Tearing Adhesive

E14 8.8 2.5

E15 11.8 2.5

Prediction 9.6 2.6

Fig. 8 Effect of adhesive thickness on joint failures at 60C.

2 mm-2 mm 2 mm-4 mm 4 mm-4 mm

Two-side One-side

failure failure No failure


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was obtained for the 4 mm-4 mm case, for which no faiure was predicted. This study shows that the thicker thadhesive layer, the more flexible it becomes. As a resulfor the same load, the elongation goes up as the bondlinthickness increases.

Modeling application in predicting joint strength

The validated model was expanded to predict the joinstrength of a composite-to-steel joint shown in Fig. 10. Thdesign includes a steel section, adhesive, composite, annow the core. The adhesive has the same thickness on botsides. The end of the steel was welded to the hull, whicwas simulated by fixing the end numerically during thanalysis. To save computation time, only one half of the design was analyzed. Symmetric boundary conditions werapplied on the plane of top surface.

Finite element analyses were conducted to predict thload capacity of the structure by applying load at the composite end and fixing the steel end. Figure 11 shows thpredicted load capacity at room temperature and high temperature; the joint can carry a higher load at room tempeature than at high temperature.

Summary

A finite element analysis method was developed to predict the load carrying capacity of adhesively bonded composite-to-steel joints. In the procedure, the materiproperties of steel, adhesive, and core were assumed to bisotropic and the material properties for composite werassumed to be orthotropic. Progressive damage and faiure were modeled by defining failure criteria (damage intiation and evolution) of the adhesive and composite. Thfailure parameters were obtained from experimental tesing results. The analysis procedure was validated by analyzing a DLS sample and comparing the calculations witexperimental testing results. The validated model was applied to assist the composite-to steel-joint design and optimizing adhesive thickness.

Reference1. J. Simler and L. Brown, 21st Century Surface CombatanRequire Improved Composite-to-Steel Adhesive Bonds,AMPTIAC Quarterly, Vol 7, No. 3, p 21-25.2. G.W. Ritter, D.R. Speth, and Y.P. Yang, Qualifications of Adhesive for Marine Composite-to-Steel Bonded Application

J. of Ship Production, Vol 25, No. 4, p 198-205, Nov. 2009.3. J.R. Davis, Stainless Steels, ASM Specialty Handbook, 199

Acknowledgement: Johan Valentijn, CEO, Logistics Interntional; Brian Climenhaga, BJC Design; Mark Bishop, MB Design; Frank Crane, J. Frank Crane Inc; and Jim GardneCompmillenia. Visual imagery is available at www.abudhabmar.com.

For more information: Dr. Yu-Ping Yang is a senior enginein modeling group (614/688-5253; email: [email protected]), DGeorge W. Ritter is technology leader (614/688-5253; [email protected]), and Dr. David R. Speth is senior engineer adhesive bonding (614/688-5253; email: [email protected] Welding Institute, 1250 Arthur E. Adams Dr., Columbus, OH 43221.


Fig. 9 Predicted load-displacement curves at 60C.

Fig. 10 Complex design of composite-to-steel joint.

Fig. 11 Load responses during tension.

Load,

kN

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Displacement, mm

2 mm-2 mm

2 mm-4 mm

4 mm-4 mm

12

10

8

6

4

2

0

Load,

kN

Adhesive (3.5 mm)

Fix

156 mm

Symmetric plane

76.2

Metal Core

Composite (5 mm)

Load

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Displacement, mm

RT

60C

300

250

200

150

100

50

0

fea for composite to steel

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