Specimen size effects on the notch sensitivity of composite laminates loaded in compression

The most important variables for scaling effects on the strength of notched composites from the experimental tests have been identified as notch size, and ply and laminate thickness. These have been scaled both independently and simultaneously over as large a range as possible. The specimens are fabricated from commercially available (Hexcel Composites Ltd.) carbon/epoxy pre-impregnated tapes 0.125mm thick (IM7/8552). The material is laid up by hand in 0.25 m × 0.3 m unidirectional plates [04]ns with n = 2, 3, 4, and 8 (i.e., 2, 3, 4 and 8mm thick) and two quasi-isotropic lay-ups, one fabricated with blocked plies [45n/90 n/-45n/0n]s and the other with distributed plies [45/90/-45/0]ns with n=2, 4 and 8. It is shown that the critical failure mechanism is in the form of fibre microbuckling or kinking and that the unidirectional compressive strength in thicker specimens (>2 mm) is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects in the form of ply waviness, fibre misalignment and voids. An scaling effect existed in the unidirectional specimens ([04]ns - scaling factor 1 to 4) and 2-dimensionally (in-plane size) scaled multidirectional open hole specimen. The notched compressive strength results obtained are compared to a simple cohesive zone fracture model for the prediction of notched strength to assess its applicability.