Delamination Growth in Composites under Cyclic Loads

A semi-empirical model was developed for estimating the growth of delaminations inside fiber reinforced organic matrix composites subjected to cyclic loads. At any point on the circumference of the delamination mode I, mode II or mixed mode conditions may exist. The form of the proposed delamination growth model follows Elber's expression for the growth of a mode I crack in an aluminum sheet. By employing dimensional analysis, the parameters were established which are to be included in the model. The expression obtained depends on the material properties (transverse tensile ply strength, transverse tensile ply modulus, longitudinal ply shear strength, critical energy release rates) and on the energy release rate at the location under consideration. Tests were conducted with Fiberite T300/976 and IM7/977-2 graphite epoxy composites under mode I, mode II and mixed mode conditions using double cantilever beam, end notched cantilever beam, and mixed mode bending test coupons subjected to both static and cyclic loads. From these tests, first, the constants needed in the model were determined. Second, data were generated and compared to predictions of the model, and good agreement was found between the measured and predicted delamination growths. Results of the present model were also compared to data reported previously, and the model predictions agreed well with these previous data.