Ultrasonic and Mechanical Characterizations of Fatigue States of Graphite Epoxy Composite Laminates

: Fiber reinforced composites are inherently nonhomogeneous materials in which fabrication procedures can affect their service reliability without effect on their visual appearance. Williams and Doll observed in a unidirectional graphite fiber epoxy composite that a small change (14 deg c) in the precure temperature resulted in significant changes in trans fiber compressive fracture strength and the transfiber compression-compression fatigue life. Because (for some modes of) fatigue the failure of graphite fiber composites tends to be sudden and occurs without any visible evidence of damage any means of nondestructively monitoring fatigue damage or predicting fatigue behavior of graphite fiber composites is likely to enhance their effective use. In the investigation conducted by Williams and Doll they observed that the ultrasonic attenuation of the composite in the as fabricated state can be an indicator of the composite fatigue life. The purpose of the present study is to further explore the relationship between ultrasonic attenuation and fatigue survivability of graphite fiber epoxy composites fabricated under various processing temperatures and pressures.