Analysis of local delaminations in composite laminates with angle-ply matrix cracks

In this paper, local delaminations growing uniformly from the tips of angle-ply matrix cracks in composite laminates loaded in tension are modelled theoretically. A 2-D shear lag method is used to determine stresses in a laminate representative segment containing one crack and two crack tip delaminations. For the calculation of the strain-energy release rate (SERR) associated with delaminations, the damaged layer is replaced with an equivalent homogeneous one with effective elastic properties. Closed-form expressions for the total SERR and its mode I and mode II components as a linear function of the first partial derivatives of the effective elastic properties of the damaged layer with respect to delamination area are derived. Dependence of SERRs and the laminate stiffness properties on delamination area, crack density and ply orientation angle is examined for balanced ½02=h2= h2 s and unbalanced ½02=h2 s carbon/ epoxy laminates. The total SERR obtained in this study is compared to a simple closed-form expression for a uniform local delamination derived in earlier work by O’Brien (Local delamination in laminates with angle-ply matrix cracks: Part II Delamination Fracture Analysis and Fatigue Characterization. NASA Technical Memorandum 104076/AVSCOM Technical Report 91-B-011). It appears that matrix crack density and delamination size influence the SERR value significantly