# Enhancing Prediction Accuracy In Sift Theory

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1 18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS Enhancing Prediction Accuracy In Sift Theory J. Wang 1 *, W. K. Chiu 1 Defence Science and Technology Organisation, Fishermans Bend, Australia, Department of Mechanical & Aerospace Engineering, Monash University, Clayton, Australia * Corresponding author Keywords: SIFT, matrix failure, polymer composite, failure prediction 1 Introduction In the conventional laminate theory widely used to predict the strength of fibre-reinforced polymer composites, the laminae are treated as homogeneous orthotropic materials. Gosse and Christensen [1] adopted a micromechanical approach which predicts separately the failure of the polymer matrix and fibres 1. They used strain invariants as failure criteria and thus this approach was named as Strain Invariant Failure Theory (SIFT). In the application of SIFT, a microstructure analysis is conducted on a unit cell of the composite material that contains a fibre and surrounding polymeric matrix to determine the relationship between the stress-strain states of the whole cell and its matrix and fibre components. This relationship is then used in a structural analysis to predict matrix or fibre failure. In a linear finite element method, the SIFT analysis could be implemented during post processing the results from a computation based on the conventional laminate theory, by correlating the element stressstrain state with matrix and fibre stress-strain states. This theory has been used by a number of researchers and some success has been reported []. For the matrix failure prediction, Gosse and Christensen proposed that two properties that control damage in the matrix, are the first invariant of the strain tensor, J1, and the second invariant deviator, : J = { } 0. 5 = 0.5 [( 1 ) + ( 1 ) + ( ) ] () 1,, are principal strains. where 1 In principle this micromechanical approach may also predict the interfacial failure between the fibre and matrix. These two strain variants can indicate matrix initial failure due to volume increase (dilational strain) and distortional strains respectively. When either of these reaches its critical value, failure will occur. Note that Equation is essentially Von Mises equivalent strain. This paper aims to demonstrate the accuracy of matrix failure prediction could be increased significantly by enhancing these two failure criteria. Discussion about Polymer Failure Criteria In the following discussion four basic load cases will be considered, namely uni-axial compression, pure inplane shear, uniaxial and biaxial, refer to Figure 1. In most part of this paper, only the initial matrix failure is considered, that is the discussion is restricted to the linear elastic condition. (The application to prediction of matrix ultimate failure will be briefly discussed in Section.4.) The stress and strain states under the four load cases are: Uniaxial compression 1 <0, = =0; 1 = 1 /E<0, = = -λ 1 Pure in-plane shear 1 >0, =- 1, =0; 1 =(1+λ) 1 /E>0, =- 1, =0 Uniaxial 1 >0, = =0; 1 = 1 /E>0, = = -λ 1 Biaxial 1 = >0, =0; 1 = =(1-λ) 1 /E, = -λ 1 where 1, and are principal stresses, E is the Young s modulus and λ is the Poison s ratio Fig.1. Four basic load cases considered.1 Uni-axial Compression and Pure Shear Cases In terms of the SIFT theory, the failure criterion based on Equation applies to these two cases.

2 It is well known that the strength of polymer materials could be affected by compressive hydrostatic stress. For those polymer materials, to cover both uniaxial compression and pure shear load cases, a yield criterion such as the Drucker-Prager criterion [] would be more suitable than Von Mises criterion. Table 1 provides yield strength data for two typical resin materials used in laminate composites. Table 1. Typical resin material strength data [4] Resin Tensile () Compression () Shear () Type 1 Yield strength E=.6GPa Ultimate λ=0.5 strength Type Yield Strength E=.9GPa λ=0.5 Ultimate strength For the prediction of yield strength, if we generate the critical value of from pure shear data, the measured yield strength in the uni-axial compression case will be higher than the predicted using Equation by 11% and 60% respectively for these two materials. This discrepancy can be removed by applying the -parameter Drucker-Prager criterion (based on Equation ). + BJ1 = A () where and J1 are calculated using Equations 1 and with the measured failure data from compression and shear tests (Table 1); A and B are two parameters determined from this equation. Failure is predicted when: ' = + BJ1 = A (4) where ' is the revised equivalent strain. Equations (5) and (6) below describe the first invariant of the stress tensor, J1, and the second stress invariant deviator, : J = + (5) { } 0. 5 = 0.5 [( 1 ) + ( 1 ) + ( ) ] (6) Only basic failure criteria that are relatively easy to implement are considered in this study. Note the following relationship: 1+ λ = (7) E 1 λ J1 = J1 (8) E The failure prediction equations can alternatively be expressed using stress variables with full equivalence. For isotropic materials, a stress based failure criterion is more commonly used since it is often easier to use and have clearer physical meaning (e.g. hydrostatic pressure effect shown in Drucker-Prager criterion). Alternative equations of Equations () and (4) expressed in stresses are: BJ1 A (9) + = = + BJ1 = ' A (10) For the materials listed in Table 1, A and B values calculated are listed in Table. Table. Parameter A and B values Resin Strain based equation Stress based equation A B A B Type 1.5x10 µ Type 1.6x10 µ Uni-axial and Biaxial Tension Cases Before discussing the uni-axial and biaxial cases, we may consider an extreme case where the polymer is loaded with uniformly distributed tensile stress in all the three axial directions. It is well known that the Von Mises yield criterion is not valid in this situation. A Drucker-Prager criterion established using parameters determined from compression and shear load cases would also significantly over-predict the strength in this situation. One commonly adopted approach is to use a maximum tensile stress or strain criterion to handle such tensile dominated load conditions, in conjunction with other criteria such as Von Mises or Drucker-Prager to handle other load conditions. In contrast, Equation 1 adopted by Gosse and Christensen contains all the principal strain components in linear combination that indicates the volume increase of the material. Bardenheier [5-6] provided experimental data from uni-axial and biaxial tests of three types of polymer materials. Analysing these data, it is indicated that under a biaxial tensile load condition, prediction based on Equation 1 would significantly

3 under-estimate the material strength, refer to Table. The dilational failure could be more accurately predicted using the critical value of, for example: 0.5 J 1 ' = ( ) (11) This fits more closely to the experimental results. Note that as principal stresses are used, Equation 11 is unaffected by any transformation of coordinates. Table. Data from neat resin biaxial tests [5-6] and errors with different prediction methods Resin Stress applied* Error with prediction method** (%) 1 Eqn Eqn (11) Max stress criterion Type A Type B Type C * Expressed using the ratio of the yield stress in the biaxial test against that in the uni-axial tensile test ** In each method, the yield stress from the uni-axial tensile test was used to determine the critical value for prediction of yield strength in biaxial tests. A + sign means over-prediction and a - sign means underprediction.. Combined Failure Envelope Figure illustrates a failure envelope with the planestress condition ( 0). The portion of the failure envelope formed with the Drucker-Prager failure criterion has two parts. The solid line part ( CS ) is established with the uni-axial compression and pure shear test data as discussed in Section.1, whilst the fine dot line part ( ST ) is established with the pure shear and uni-axial tensile data in a similar way. This figure shows that the portion of the failure envelope formed with the Drucker-Prager failure criterion can intersect with uni-axial compression, pure shear and uni-axial tensile test data points and join with the J 1 curve smoothly, that indicates significantly improvement. (Regarding the curve smoothness, note that a significant improvement of the Von-Mises criterion over the maximum shear (Tresca) criterion is that it smooths the curve and removes the singular points in Tresca criterion.) In the -dimensional space with 1,, coordinate axes, the failure envelope is represented by two cone surfaces (formed with the Drucker-Prager failure criterion, with the axis along the line 1 = = and joined at = 0) truncated by a hemisphere surface (formed with J 1 = the uni-axial tensile yield stress). UC CS J 1 UT ST J 1UT UT = uni-axial tensile; BT = biaxial tensile; PS = pure shear; UC = uni-axial compression; UC = with critical value determined from uni-axial compression test; CS = with parameters determined from uni-axial compression and shear tests; ST = with parameters determined from shear and uni-axial tensile tests; J 1UT = J 1 with critical value determined from uni-axial tensile test; and J 1 UT = J 1 with critical value determined from uni-axial tensile test. Fig.. Illustration of failure envelope (plane-stress).4 Matrix Ultimate Failure Prediction By using the ultimate strength and secant stiffness to replace yield strength and initial linear stiffness, the above analysis may be extended to cover prediction of matrix ultimate failure with linear analyses. Note that: The accuracy of Drucker-Prager criterion over Von Mises failure criterion has been clearly observed particularly for the ultimate strength of many polymer materials; and Use of a linear analysis with secant stiffness to approximate a nonlinear analysis is a widely used analysis method and thus may be significant for practical applications. Lamina Unit Cell Analysis A unit cell analysis using a finite element method was further conducted with the four typical load cases discussed above. The failure prediction based on Equation and Equation () was compared with that based on Equations (10) and (11) when both fibre and matrix are present. 1

4 As the discussion in Section indicated that the failure criteria proposed in this paper could predict neat resin failure more accurately, Equations (10) and (11) are used as a yardstick in this comparison..1 Finite Element Model The following typical material properties are assumed: Fibre Young s modulus (E f ) = 00 GPa Fibre Poisson ratio (λ f ) = 0. Matrix Young s modulus (E m ) =.6 GPa Matrix Poisson ratio (λ m ) = 0.5 Fibre volume fraction (V f ) = 0.5 Referring to Figure, with a relatively low fibre volume fraction of 0.5, a square pattern fibre distribution in the matrix can be assumed (as opposed to a hexagon pattern). A unit cell was extracted with two different orientations. A FEM model is shown in Figure 4. Considering the symmetrical condition, only one eighth of the unit cell was built. remaining unloaded surfaces were given a uniform displacement, that results in the gross load on the surface being zero, to reflect the Poisson effect. The loading and boundary conditions applied ensure all the surfaces are kept straight, as required for simulation of a unit cell in a lamina structure.. Results and Discussion..1 Compression and Shear Load Cases The unit cell shown in Figure (a) was considered. As expected, for the matrix material in the uni-axial compression case, high compressive stress concentration (over 4 times the average strain of the unit cell) occurred at the area indicated in Figure 5(a). Due to the Poisson effect from the stiff fibre, the other two stress components perpendicular to the applied load are also compressive in the high stress concentration area. This results in a relatively high hydrostatic stress in this area and thus the strength predicted with the Drucker-Prager type criterion is much higher than that with Von-Mises criterion. In contrast, in the pure-shear case, high compressive and tensile stress concentration occurred separately at the areas shown in Figure 5(b). This results in strength predicted with Drucker-Prager type criterion to be much less than that with Von-Mises criterion in the tensile stress concentration area. Compression stress concentration area Tensile stress concentration area (a) Orientation 1 (b) Orientation Fig.. Unit cells with two orientations (V f = 0.5) (a) (b) Fig.5. Stress concentration areas in uni-axial compression and pure shear load cases Fig.4. A unit cell FEM mesh (one-eighth model of the unit cell in Figure a) The load condition is applied in the form of uniform surface displacement. Symmetry boundary conditions are applied to three orthogonal surfaces. The With a load applied that resulted in 0.01 gross strain, the calculated maximum effective stress values are listed in Table 4. For practical applications of the SIFT approach, the failure strength of the matrix is generally determined using laminate specimen tests rather than from neat 4

6 4 Summary The SIFT failure theory separably predicts fibre and matrix failure in a fibre reinforced composite material. Matrix yield prediction is based on a Von-Mises strain and first strain invariant criteria. Improvement of the matrix failure criteria for prediction accuracy is discussed and demonstrated in this paper. For two typical resin materials, Von-Mises yield criterion is unable to fit both yield strength values from uni-axial compression and pure-shear tests. When calibrated using the measured shear yield strength, the discrepancy between the predicted and measured uni-axial compression strength is found to be 11% and 60% respectively, whilst using a Drucker-Prager criterion these discrepancies could be removed. Use of the critical value of the first invariant strain, when calibrated using the uni-axial tensile yield strength, under-predicts the biaxial tensile strength significantly by over 0% on average, compared with available measurement data of neat resin materials. A revised criterion proposed in this paper could reduce the discrepancy to less than 10%. The proposed failure envelope contains Drucker- Prager failure criteria, that intersect with all the uni-axial compression, pure shear and uni-axial tensile test data points, and a revised tensile failure criterion. The areas governed by these criteria join each other smoothly. For a unit cell with a fibre and surrounding matrix from a lamina with 50% fibre volume fraction and typical material properties, a FEM analysis conducted in this study indicates that the difference between the yield strength of the matrix material predicted using Von-Mises and Drucker- Prager type criteria is over 60% in the pure shear load case, when the critical values of these yield criteria are determined in the uni-axial compression load case. The FEM analysis showed that the difference between the yield strength of the matrix material in a unit cell predicted using the first strain variant and the revised criterion reaches 6.1% in the biaxial tensile load case, when the critical values of these yield criteria are determined in the uniaxial tensile load case. By using ultimate strengths and secant stiffness to replace yield strengths and initial linear stiffness, the analysis in this paper may be extended to prediction of matrix ultimate failure using linear analyses. Acknowledgement The authors would like to acknowledge that this paper is outcome of a research project of the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS). References [1] Gosse JH, Christensen S., Strain invariant failure criteria for polymers in composite materials, AIA A. Paper AIAA/ASME/ASCE/ASC conference, Seattle, WA, United States, 001. [] Li R, Kelly D, and Ness R. Application of a first invariant strain criterion for matrix failure in composite materials. Journal of composite Materials, 00. 7(): [] Drucker D C and Prager W. Solid mechanics and plastic analysis for limit design. Quarterly of Applied Mathematics, 10 () , 195. [4] Fei Y P. Mechanics of fibre reinforced composite. Tongji University Press. Shanghai [5] Bardenheier. Mechanicaches Versagen von Polymerwerkstooen. Hanser-Verlag [6] Kolling S, Haufe A, Feucht M, and Du Bois P A. A constitutive formulation for polymers subjected to high strain rates. 9 th International LS-Dyna Users Conference. Michigan, June 4-6,

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