Effects of voids on the mechanical strength and ultrasonic attenuation of laminated composites

The effects of voids on the mechanical strength and ultrasonic attenuation have been investigated on a number of laminated composite systems containing a wide range of void content. Graphite fiber reinforced specimens were fabricated both from unidirectional tape and woven fabric prepreg and their interlaminar shear strength and frequency dependent ultrasonic attenuation were measured together with a careful study of microstructure. The measured strength and attenuation slope were found to depend not only on the void content but on the void geometry associated with the prepreg structure of composites. A shear fracture criterion similar to the Mar-Lin equation for notched tensile fracture stresses was used successfully to correlate the fracture strength of porous laminates. The unidirectional laminates showed much higher void sensitivity than the fabric laminates, i.e., more reduction in shear strength and higher attenuation slope for a given void content. It was shown that the void content may be estimated from the measured attenuation slope and that a good correlation exists between the attenuation slope and the shear strength of porous composite laminates.