Analysis of Progressive Matrix Cracking In Composite Laminates II. First Ply Failure

Abstract

The mechanics of transverse cracking in an elastic fibrous composite ply is explored for the case of low crack density. Cracks are assumed to initiate from a nucleus created by localized fiber debonding and matrix cracking. Conditions for onset of unstable cracking from such nuclei are evaluated with regard to interaction of cracks with adjacent plies of different elastic properties. It is found that cracks may propagate in two directions on planes which are parallel to the fiber axis and perpendicular to the midplane of the ply. In general, crack propagation in the direction of the fiber axis controls the strength of thin plies, while cracking in the direction perpendicular to the fiber axis determines the strength of thick plies. The theory relates ply thickness, crack geometry, and ply tough ness to ply strength. It predicts a significant increase in strength with decreasing ply thick ness in constrained thin plies. The strength of thick plies is found to be constant, but it may be reduced by preexisting damage. Strength of plies of intermediate thickness, and of unconstrained thick plies is evaluated as well. Results are illustrated by comparison with experimental data.

Prediction of in situ strengths and matrix cracking in composites under transverse tension and in-plane shear

“…Both experimental [9], [16]- [17] and analytical methods [10], [18] have been proposed to determine the in-situ strengths. Although the models proposed can satisfactory predict the in-situ transverse tensile strength, there is still the need to accurately predict the in-situ shear strength.…”

“…The failure criterion for predicting matrix cracking in a ply subjected to inplane shear and transverse tension and the expression to determine the in-situ strengths proposed are based on the Fracture Mechanics analysis of a slit crack in an unidirectional ply, as proposed by Dvorak and Laws [18]. The slit crack is lying on the 1-3 plane, as represented in Figure 4.…”

“…Following Dvorak and Laws [18], the energy release rate for the case of crack propagation in the transverse direction is obtained as:…”

“…The in-situ tensile strength can be obtained by solving equation (31) for the stress at failure [18]:…”

“…Dvorak and Laws [18] observed that a unidirectional laminate can be considered to be a special case of a thick laminate in which the outer surfaces are unconstrained so that surface cracks can develop. This important observation allows the prediction of the in-situ strengths of thick plies using experimental data obtained for unidirectional laminates.…”