Characterization of matrix cracking and delamination in laminated composites

Failure in laminated composite materials often initiates in the form of matrix fractures, namely, transverse matrix cracks and delamination. Transverse matrix cracks refer to intralaminar failures whereas delamination refers to interlaminar failures. The occurrence of matrix crack or delamination will result in degradation of laminate properties or loss of loading carrying capacity. Characterization of matrix cracking and delamination in laminated composites is investigated in this study. Effect of matrix cracking on stiffness and strength is investigated using a finite element model and an experimental program. An experimental method to evaluate shear modulus of a crack layer is presented. A simplified method to predict the onset of open-mode free edge delamination is proposed for a class of laminates. The method takes advantage of a linear relation between average interlaminar normal stress and interface moment. The obtained linear relationship is used to predict the onset of open-mode edge delamination for three laminates. Interlaminar fracture behavior and interlaminar toughness of a multi-directional composite laminate are characterized. Finite element analysis is performed to investigate the strain energy release rate distribution and fracture behavior of the laminate with matrix cracking. An experimental method for characterizing mode II dominated interlaminar fracture toughness of a 0/$\theta$ interface is developed based on experimental observation and finite element analysis, and the method is used to determine the interlaminar toughness of 0/$\theta$ interfaces of AS4/3501-6 composite material with various $\theta$\u27s. Aging effect on interlaminar shear strength and interlaminar fracture toughness is also discussed. Aged as well as virgin composites are tested using a five-point bending device and the ENF loading fixture to determine interlaminar shear strength and interlaminar toughness, respectively