A PARAMETRIC STUDY OF VARIABLES THAT AFFECT FIBER MICROBUCKLING INITIATION IN COMPOSITE LAMINATES - EXPERIMENTS

The objectives of this research are to investigate the effects of stacking sequence (orientation of plies adjacent to the 0-degrees plies), free surfaces, fiber/matrix interfacial bond strength, initial fiber waviness, resin-rich regions, and nonlinear resin shear constitutive 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-degrees-C, 77-degrees-C, and 132-degrees-C. Observations indicate that the notch radius controlled fiber microbuckling initiation, and thus compression strength, by dictating the unsupported fiber length at the notch. The numerical results from a companion paper [1] are compared qualitatively with these experimental results. The results show that increasing the test temperature, locating 0-degrees 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 microbuckling 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.