A Parametric Study of Variables That Affect Fiber Microbuckling Initiation in Composite Laminates: Part 2 — Experiments

The objectives of this research are to investigate the effects of stacking se quence (orientation of plies adjacent to the 0° plies), free surfaces, fiber/matrix interfacial bond strength, initial fiber waviness, resin-rich regions, and nonlinear resin shear consti tutive behavior on fiber microbuckling initiation. Three thermoplastic composite material systems are used in this investigation. The materials are the commercial APC-2 (AS4/ PEEK), QUADRAX Interlaced Tape, and a poor interface experimental material, AU4U/ PEEK. Notched compression specimens are studied at 21°C, 77 ° C, and 132°C. Observa tions indicate that the notch radius controlled fiber microbuckling initiation, and thus compression strength, by dictating the unsupported fiber length at the notch. The numeri cal results from a companion paper [1] are compared qualitatively with these experimental results. The results show that increasing the test temperature, locating 0° plies at the free surface of the laminate, and degrading the fiber/matrix interfacial bond strength reduce the resistance to fiber microbuckling initiation in these notched laminates. The fiber micro buckling initiation strain is shown to be a constant, regardless of stacking sequence, for these notched laminates. Experimental results show that resin-rich regions also reduced the resistance to fiber microbuckling initiation.