Fracture mechanism of cross-ply carbon/carbon composites

Abstract

Mode I fracture toughness tests were carried out on (0/90) cross-ply carbon/carbon composite laminates in order to investigate the applicability of the concept of the fracture toughness based on linear elastic fracture mechanics. Tests were carried out by using both compact tension (CT) and single edge notched beam (SENB) specimens. Effect of the root radius of the starter notch was also investigated in order to find out its sensitivity on the fracture behavior. Although the load-displacement relation showed nonlinear behavior, the relation between the specimen compliance calculated by the straight line from the origin of the load-displacement axis and the measured crack length followed the polynomial equations based on the linear elastic fracture mechanics. Fracture toughness values were calculated by using the stress intensity factor. The relation between the fracture toughness and the increment of crack length (R-curve) was independent of the specimen shapes, the starter notch lengths, and the root radius of the starter notch. The fracture toughness values first increased rapidly until the increment of the crack length equaled 1 mm, and then leveled off. For the case of SENB specimens, tests without starter cracks were also carried out. These tests indicated that both the crack propagation behavior and the resultant R-curve were similar to those of normal SENB specimens. Comparison between the fracture toughness value and the bending strength yielded that the length of the latent crack is about 1 mm. X-ray observation showed that plenty of microcracks had already existed before the fracture load was applied. Microscopic observation showed that the crack growth process was characterized as pull-out of fibers. The length of fiber pull-out with the increment of crack growth was discussed in conjunction with the R-curve behavior.