Impact Damage in Graphite-Fiber-Reinforced Composites

Theoretical and experimental studies are presented on the failure modes in graphite-fiber-reinforced composite plates subjected to low-velocity impact. Influence of the following parameters is studied theoretically or experimentally or both: fiber and matrix properties, fiber orientation, stacking sequence, and laminate thickness. A quasi-dynamic approach is used to predict theoretically the influence of the noted parameters on the impact response of composite plates. A previously developed approach is used and extended to predict the time-dependent surface pressure and its distribution in a generally orthotropic target impacted by a body of revolution. The triaxial stress state in the composite target (assumed to be generally orthotropic) is determined using a finite-element computer program. These stresses are used with failure criteria for generally orthotropic solids to determine the extent of impact damage. To verify predictions of the impact-induced failure modes, ball-drop tests are conducted on circular composite plates incorporating different fiber-resin combinations, fiber layups, and stacking sequences. Types of graphite fibers used in the experimental program include Thornel 300, Modmor II, and Celion GY-70. Fiber layups investigated include unidirectional, 2:1 bidirectional, 1:1 bidirectional, and tridirectional (pseudo-isotropic).