Fiber-reinforced Laminated Composite Delamination Behavior Research

Continuous fibre-reinforced composite laminates have been used in manyimportant structures due to their excellent properties, such as high ratio strength andmodulus to weight etc. But the strength of laminates in the direction along thicknessis much lower than that along fibers. So delamination occurs easily in the case of highinterlaminar stress or impact on surface. This can lead to premature failure of thecomposite laminates because of the stiffness degradation, and the full potential of thematerials is not utilized. Delamination is a main failure mode and a serious hiddendanger of the composites. Hence, delamination mechanism, its effects and methodsfor characterizing and improving its toughness have been discussed in a great amountof literatures in recent years. Owing to the complex natures of these problems, furtherworks have been still done till now. In this paper several major problems aboutdelamination have been studied thoroughly in theoretical and numerical analysis.Some valuable conclusions are gotten.In this paper, based on nonlinear continuum mechanics theory, virtual workprinciple is applied to deduce quasi-3D finite element formulations. A nonlinear finiteelement program with more than 5000 lines is developed to study geometricnonlinearity, material nonlinearity and damage evolution of composite laminates.Post-buckling of delaminated composite laminates is an important problem ofdelamination analysis. In this paper, considered geometric nonlinearity only, the effectof delamination on critical load of composite structures and effects of delaminationlength and stacking sequences on stresses, displacements and energy release rate arediscussed.Material nonlinearity is an important characteristic of composite materials. Toanalyze the effect of material nonlinearity on delamination, the present researchextends the modified Hahn-Tasi elastic nonlinear constitutive relations for plane strain.A new, convenient and effective modified converse power method is set forth tocompute the critical loads of nonlinear laminates. Compared with linear cases theresults indicate that: (1) the nonlinear critical load of composite laminates decreases,and the decrease depends on the value of critical load and the stacking angle θ ofthe ply. (2) In post-buckling regime, stress singularity at delamination-tip region isreduced but displacements are increased obviously. Energy release rate G and itscomponents GI and GⅡ are all increased evidently. Both intralaminar nonlinearity andinterlaminar nonlinearity occur in post-buckling regime of delamination. Interlaminarnonlinearity occurs at delamination-tip region, which has some effect on crack-tipfields and energy release rate, but the effect on post-buckling displacements is verysmall. The results show that it is necessary to study buckling and post-buckling ofcomposite laminates with delamination based on nonlinear constitutive relations.Failure mechanics method is applied to analyze the effect of micro-cracks of matrixparallel to the fibres on delamination. Considering both interlaminar and intralaminarmatrix micro-cracks damages in composite laminates, a multi-scalar damage model ispresented and applied to the post-buckling analysis of laminates with through-widthdelamination. The numerical results indicate that: (1) The stress singularity atdelamination tip vanishes, and higher stresses occur at the tip region of limit damagezone. (2) Displacements at crack-tip increase while energy release rate decreasesobviously. Because J-integral isn't conversation and there is no direct relationbetween energy release rate and damage zone, it seems that delamination growthcriterion based on damage process is more reasonable. (3) With increase of loading,both intralaminar and interlaminar damage zones expand and interact on each other.The direction of limit damage growth depends on stacking angle θ of the ply aboveinterface of delamination. (4) The analyses show that it is not reasonable to considerdelamination as Griffith crack, but to study it on concept of Failure Mechanics, andthe multi-scalar damage model is convenient and effective in dealing with thedelamination problems.