Can Hail Impacts Trigger Delaminations?

Abstract

The current generation of commercial aircrafts extensively use composite materials such as Carbon Fibre Reinforced Polymers (CFRP) in both exposed and primary structures. These materials lack through-thickness reinforcement and are hence susceptible to out-of-plane impact damages. Barely visible impact damage caused by low energy impacts poses a unique problem, since delaminations, de-bonding and cracking may be present below the surface layers without any indication of damage on the surface. The focus of this research is on one such scenario, multiple site low energy hail impacts, while the aircraft is on the ground. Taking into account the relevant parameters, a hail impact envelope was established both in terms of the initial kinetic energy and peak impact force. Further, contradictions found in literature over the influence of the compressive strength of hailstones were resolved. These were accomplished with the aid of a state-of-the-art finite element model and experiments in the laboratory. With help from a custom designed and assembled impact force measurement experimental setup a relation was established between impacts carried out with steel impactors and those with simulated hail ice impactors. Based on this relation, predictions are made on which hailstones have the potential to cause damages to CFRP structures.