Failure in quasi-isotropic (π/4 and π/3) laminates with free edges under on-axis and off-axis loads has been studied experimentally and analytically. Although these laminates are isotropic in stiffness, they are highly amsotropic in strength with respect to loading direction. For these laminates, classical failure criteria are not adequate for laminate strength prediction. A new failure criterion has been developed based on free edge interlaminar stresses. This new criterion was verified experimentally to be quite accurate in predicting failure loads for these quasi-isotropic laminates under off-axis loading.

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Load-frequency effect on fatigue life of a thermoplastic and toughened BMI composite was investigated. Experimental results were obtained using three frequencies at two load levels with three types of laminates, each containing a center hole. Besides fatigue life, the temperature history near the hole and the specimen stiffness were monitored. In addition, the hysteresis loop was recorded at various stages of the fatigue test to observe the variation in viscoelastic property of the specimen. From the fatigue test results, it was found that fatigue life of matrix-dominated thermoplastic composite laminates decreased as load frequency increased. This behavior is opposite to that of epoxy-based fiber composites. For fiber-dominated thermoplastic laminates under tension-compression load, very low load-frequencies (less than 0.4 Hz) could significantly lower their fatigue life. Analytical models were developed to account for the load-frequency effect on fatigue life of thermoplastic composites. Temperature rise near the hole was included in the model to account for temperature effect.