Investigation of Bulk Damage Progression and Inspection for GFRP Laminate Properties

A study has been accomplished to compare bulk material degradation in two graphite fiber-reinforced plastic (GFRP) laminates under cyclical fatigue. This study is the first in a series of experiments to characterize and quantify acoustic response to large-area bulk properties of field systems with hybrid structural composite-metallic systems. Long before failure of load-bearing composite components, stiffness reduction creates loading path changes, potentially overloading or fatiguing metallic substructure. The composite materials used in this study represent potential components and their respective correlation of stiffness changes to acoustic signal degradation through direct area measurement techniques. Specimens tested were fatigued using a sinusoidal waveform at continuous max loading and stress range at 30, 50, 60, and 70% of each layup, and UTS and cyclic stress-strain data were collected over 100,000 cycles to generate discrete levels of stiffness change. Additional specimens were cycled to one million cycles to generate higher levels of damage and determine failure life in specimens examined. All the specimens in this study were 8-ply, woven, 30.5-cm-long, 2.54-cm-wide, 0.297-cm-thick, rectangular specimens. A damage parameter based on material stiffness, as determined by changes in Young’s modulus (D = 1 − E/E0), was employed to compare the overall stiffness changes in the specimens. This damage parameter was correlated to ultrasonic scan results to provide a basis for field characterization of bulk material damage and, subsequently, life estimates of in-use structural parts. Examination of these effects showed a strong correlation of back-face signal strength to stiffness degradation in both laminates.