Indirect damage in composite materials due to raindrop impact

Indirect damage in composite substrate materials caused by propagation and interaction of elastic compression waves from the impact of water droplets on a coated surface of the composite is described and analyzed. The damage from these waves takes the form of microcracks which are propagated along resin-fiber boundaries and extend to a considerable depth in the composite itself. The coating material may appear to be undamaged. Multiple impact of droplets results in the interaction of reflected waves from microstructural discontinuities and positive wave interferences which generate tensile stresses with an amplitude greater than the dynamic ultimate strength of the material. The protective effect of polyurethane coatings is due to damping of the stress waves and reduction in the stress at the impact surface and hence the stress which is transmitted to the substrate. In the case of nickel coatings, the modulus mismatch between the nickel and the composite substrate will reduce the transmitted stress in this system.