LAUSR

Abstract In composite laminates the layer-wise material mismatch induces highly localized stress concentrations at the free edges. This is referred to as free-edge effect. As a consequence interlaminar crack initiation may occur in addition to the typically considered intralaminar failure modes. The stresses do not necessarily have to be caused by mechanical loads. Also temperature differences may induce significant interlaminar stresses. In the present study, delamination onset in symmetric angle-ply laminates caused by thermal stresses is investigated by means of the coupled stress and energy criterion within the framework of finite fracture mechanics on the basis of the well-known generalized plane strain state. Using this relatively young criterion, both failure load and the length of the initiated crack can be predicted. Besides the intrinsic material properties in terms of strength and fracture toughness, only the interlaminar stresses as well as the released energy during crack initiation are required for the evaluation of the coupled criterion. Since no analytical closed-form solution of the free-edge effect is known, the required field quantities are obtained by employing a finite element model. Effects as the influence of the thickness and the orientation of the individual layers on the failure temperature are studied. For that purpose, a dimensional analysis is performed allowing to scale the results of a certain boundary value problem to self-similar configurations yielding a significant reduction of the numerical effort. It is revealed that especially for thick angle-ply laminates with large fiber orientation angles and temperature differences as they may occur in aeronautical applications interlaminar crack initiation can be the predominant failure mode. For validation purposes the results obtained by the coupled criterion are compared to the widely-used cohesive zone model.