A Method for Calculating Strain Energy Release Rates in Preliminary Design of Composite Skin/Stringer Debonding Under Multi-Axial Loading

Three procedures were developed to determine strain energy release rates in composite skin/stringer specimens for various loading conditions. These procedures may be used for parametric studies in such a way that only a few finite element computations will be necessary for a study of many load combinations. The results were compared with mixed mode energy release rates calculated directly from nonlinear two-dimensional plane-strain finite element analyses using the virtual crack closure technique. The first procedure involved solving three unknown parameters needed to determine the energy release rates. This technique was only applicable if the structure exhibits a linear behavior. A second technique was derived which was applicable in the case of nonlinear behavior. The technique involved calculating six unknown parameters from a set of six simultaneous linear equations and was not time efficient. A third procedure was developed to calculate mixed mode energy release rates as a function of delamination lengths. Only one nonlinear finite element analysis of the specimen is required to obtain a reference solution. The delamination was extended in separate sub-models to obtain the distribution of G with delamination lengths. Although additional modeling effort is required to create the sub-models, this local technique is efficient for parametric studies.