Critical void content for polymer composite laminates

An experimental program to characterize the effect of voids on the strength of composite laminates is presented, and the adequacy of a fracture criterion to represent the experimental data for the effect of voids on the compressive and interlaminar shear strength of composite laminates is assessed. The experimental program investigates the effect of the material system (epoxy matrix vs bismaleimide matrix), type of reinforcement (unidirectional tape vs woven fabric), and the type of loading (compression vs interiaminar shear) on the critical void content. Laminates produced with carbon fabric/epoxy, carbon tape/epoxy, and carbon fabric/bismaleimide were produced with an intentionally high void content. The ultrasonic absorption coefficient was measured for all specimens and shown to vary approximately linearly with the void content, with the exception of the carbon fabric/epoxy laminates that presented a bilinear relationship with void content, corroborating previous experimental results. The effects of these factors on the strength of composite laminates are discussed in terms of the fracture parameters involved in the fracture criterion. The critical void content is estimated for each case both in terms of void content and ultrasonic attenuation.