Effect of thickness and pre-crack length on delamination of unidirectional l-shaped laminated composites

It is widely assumed that delamination initiates at the curved region of the Lshaped laminate leading to loss of loading carrying capacity. However, as shown in this paper, under certain conditions a second failure mode in thick L-shaped laminates is observed in which a secondary crack initiates at the arm region. Delamination in L-shaped laminates is modeled using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. The 2-D model consists of 24 plies of unidirectional CFRP laminate with an initial crack at the center of curved region. Loading is applied parallel to one arm quasi-statically and the observed delamination occurs dynamically. Moreover a sequential analysis with implicit method followed by explicit is used for the sake of efficient computational analysis. For thicker L-shaped laminates with an initial crack at the center of curved region, formation of secondary crack in the arm region is observed. After the length of the initial crack reaches a critical value, initial crack at the curved region starts propagating. Therefore the size of the initial crack at the center of the curved region mainly decides the failure mode. The failure at the initial crack is under opening stresses whereas the failure due to secondary crack is under shear stresses. In addition, the change of the failure mode from the failure at the initial crack to failure due to secondary crack result in a decrease of the maximum load that the L-shaped laminate can resist. Thus the assumption of failure at the initial crack leads to the underestimation of failure load in the load displacement curve