Introduction

Composite materials are used extensively in sandwich constructions, and compared with skin-stiffener panels, sandwich construction presents definite advantages: improved stability, weight savings, ease of manufacture, and easy repairs. Using composite materials instead of aluminum for the facesheets results in higher performance and lower weight, even though composite materials are more susceptible to impact damage. Composite sandwich construction has been used extensively for side skin panels, crown skin panels, frames, and longerons in the Boeing 360 helicopter (Llorente 1989). Compared to skin-stringer aluminum construction, the use of sandwich structures lead to an 85% reduction in the number of parts, a 90% reduction in tooling costs, and a 50% reduction in the number of work-hours needed to fabricate the helicopter. In spite of these advantages, the problem of impact on sandwich structures has received only minimal attention.

Contact laws for sandwich structures are significantly different that those for monolithic laminates. With sandwich structures, the indentation is dominated by the behavior of the core material, which becomes crushed as transverse stresses become large. To predict the contact force history and the overall response of the structure to impact by a foreign object, mathematical models should account for the dynamics of the projectile, the dynamics of the structure, and the contact behavior. Failure modes involved in impact damage of sandwich structures and the influence of the several important parameters will be discussed. We will also examine the effect of impact damage on the residual properties of the structure.