Failure criterion for notched fiber-dominated composite laminates

Rajesh Suresh Vaidya, Purdue University

Abstract

Predicting the residual strength of composite laminates in the presence of flaws such as cracks has been an important research problem for the last two decades. Most existing models for predicting notched residual strength are of a 'curve fit' nature, wherein the model parameters, e.g., characteristic distance or damage zone size are chosen so as to fit the experimental data. Such parameters have been shown to depend on notch size and laminate orientation, and as such, cannot be considered material constants for the composite system. In the present research, a new model is proposed for predicting residual strength or fracture toughness of notched fiber dominated composite laminates under pure Mode I or mixed-mode loading conditions. The fracture toughness of such laminates is related to the fracture toughness of the principal load bearing plies within them. A new material parameter is introduced to predict laminate fracture toughness, and it is shown to be independent of laminate orientation. Experimental results indicate that the model can successfully predict the failure stress for notched laminates with arbitrary crack orientations (mixed-mode loading). The effect of ply thickness on notched strength of cross-ply and quasi-isotropic laminates is also investigated in this study. Results indicate that the ply thickness effect is more significant in cross-ply laminate configurations. A 2D finite element analysis is conducted to study the effect of crack tip damage in the form of splitting and delamination on the stress distribution near the crack tip. The experimentally observed damage zones in cross-ply laminates are modeled, and a criterion is established to predict the growth of matrix cracks in the 0$\sp\circ$ layer. It is shown that the matrix crack growth is controlled by the Mode II strain energy release rate. The effect of matrix damage on stress relaxation and subsequent failure is investigated and conclusions are drawn to establish the regime where the failure model is valid.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Aerospace materials|Mechanics|Materials science

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