Numerical investigation on the applicability of fiber metal laminates for the crashworthiness of a tiltrotor leading edge under bird strike conditions

The present work, performed in collaboration with the Italian Aerospace Research Centre (CIRA), aims to investigate the applicability of composite materials to the skin of the wing leading edge of the Leonardo Next-Generation Civil Tiltrotor (NGCTR-TD). In particular, the component is designed to satisfy the bird strike requirements while also providing the minimum structural weight. Two materials have been investigated: ARALL and GLARE fiber metal laminates. This study was driven by the industrial demand to find a configuration able to provide a significant weight saving with respect to the current aluminum alloy leading edge structure. The study was performed by means of numerical investigation and all numerical simulations were performed using the FE commercial explicit solver Abaqus. The first step of the study was the calibration of the models and parameters of the materials to be used in the dynamic simulations. Several low velocity impact tests have been carried out by numerical procedures and compared with respect to experimental results available in literature to calibrate the materials strength and stiffnesses, the cohesive properties as well as to validate the numerical models. The quantities used in the comparison are the time history of the reaction force and displacement at the center of the specimen, the impact energy threshold, which corresponds to the barely visible impact damage, the minimum perforation energy, the permanent deflection and the damaged area. Then a series of numerical analyses of bird impacts on a (semi-) rigid target have been conducted. The bird is modeled with the Smooth Particle Hydrodynamics (SPH) method and its material properties have been calibrated to fit experimental results found in the literature. The bird’s material properties selected are similar to those of water and the internal pressure of the bird model is linked to the change in volume by an Equation of State. The quantities examined are: the deformation and velocity of the projectile during the impact event, the evolution of the contact surface envelope, the impact pressure and its radial distribution. Finally, by numerical simulations, the leading edge structure of the NGC Tiltrotor was investigated from the point of view of the crashworthiness, aiming to reduce its total structural mass. Such component must be compliant with bird-strike airworthiness requirements according to the EASA Regulations (tailored requirements drawn from CS-25 for large airplanes and CS-29 for large rotorcrafts for the particular nature of the vehicle). The investigated materials are the aluminum alloy 2024-T3, the GLARE and ARALL fiber metal laminates. For each material, the minimum leading edge thickness able to satisfy the design requirements is determined. The developed numerical models are able to simulate the damage onset and its propagation both in metallic components and in composite ones. The SPH approach was adopted to model the bird with the previously determined material properties. The study shows that fiber metal laminates are potentially well suited for birdstrike applications, since they allow for a significant weight reduction with respect to the conventional aluminum alloy and improved structural performance under impact load conditions.