Impact on Laminated Composite Materials
Laminated composite materials are used extensively in aerospace and other applications. With their high specific modulus, high specific strength, and the capability of being tailored for a specific application, these materials offer definite advantages compared to more traditional materials. However, their behavior under impact is a concern, since impacts do occur during manufacture, normal operations, or maintenance. The situation is critical for impacts which induce significant internal damage, undetectable by visual inspection, that cause large drops in the strength and stability of the structure. Impact dynamics, including the motion of both the impactor and the target and the force developed at the interface, can be predicted accurately using a number of models. The state of stress in the vicinity of the impact is very complex and requires detailed analyses. Accurate criteria for predicting initial failure are generally not available, and analyses after initial failure are questionable. For these reasons, it can be said that a general method for estimating the type and size of impact damage is not available at this time. However, a large amount of experimental data has been published, and several important features of impact damage have been identified. In particular, interply delaminations are known to occur at the interface between plies with different fiber orientation. Their shape is generally elongated in the direction of the fibers in the lower ply at that interface. The delaminated area is known to increase linearly with the kinetic energy of the impactor after a certain threshold value has been reached. The effect of impact damage on the properties of the laminate has obvious implications for design and inspection of actual structures. Experimental results concerning the residual strength of impact damaged specimens subjected to tension, compression, shear, bending, and both static and fatigue loading are available. Analyses concentrate primarily on predicting residual tensile and compressive strength. In order to fully understand the effect of foreign object impact damage, one should understand impact dynamics and be able to predict the location, type, and size of the damage induced and the residual properties of the laminate. This article is organized along these lines and presents a comprehensive review of the literature on impact of laminated composites, considering both experimental and analytical approaches.