Influence of Low-Velocity Impact on Composite Structures

Abstract

Composite materials are sensitive to out-of-plane loads because they are weaker through the thickness than in the plane of lamination. Consequently, composite structure subjected to impact may suffer significant damage, which results in a loss in strength. The effect of impact energy on different thicknesses of laminate with respect to ultimate compressive strength was evaluated using the NASA ST-1 specimen test. Impact damage was measured using ultrasonic C-scans and quality was assessed by visual observations. A semiempirical model for impact damage was proposed to determine residual strength in terms of laminate properties, static influence coefficients, and impact energy. To determine residual compression strength, single- and multiple-impact damage was inflicted on bonded stiffened structures. The results were verified by the residual strength method, which uses laminate properties and measured damage size to predict the residual strength of composites. The correlation between predicted and actual strength was very encouraging. Although the damage was inflicted with different impactor masses, velocities, and impact energy, the data indicated that residual strength is a function only of damage present and does not reflect the manner in which the damage is inflicted. Finally, a semiempirical relationship was developed for damaged areas in stitched laminates.

Damage Tolerance of Thick-Section Composites Subjected to Ballistic Impact

“…This behavior exists until a plateau is reached, after which no further strength reduction is seen. Madan (1989) has also shown that thinner laminates required less energy to delaminate, but the plateau strength was approximately the same that of the thicker two laminates. The thermoplastic systems showed a 10% plateau strength improvement over the thermoset resins.…”

“…investigations have been performed on low-velocity impact as another source of delaminations (Wanthal et al 1993;Chai 1982;Naval Air Development Center 1986;Madan 1989;Horton and Whitehead 1988;Ramkumar 1982;Williams 1984;Guynn and O'Brien 1985). Impact damage can occur at almost any time during the service lifetime of an aircraft.…”

“…Although a considerable amount of research has been conducted towards understanding impact damage on the strength of composite structures, the effects of geometry, layup, thickness, etc., are still not well understood. Madan (1989) presented an investigation of the effect of impact energy and laminate thickness on the in-plane compression strength of a toughened carbon/epoxy system. Impact tests were conducted on test specimens using a Dynatup impact machine, whereby a hemispherical projectile is dropped IYom a certain height with weights attached.…”