EASA.2009.OP.24 LIBCOS – Significance of Load upon Impact Behaviour of Composite Structure

Abstract

A critical safety issue for the design of primary aircraft structures is vulnerability and damage tolerance due to foreign object impact from bird strike, hail, tyre rubber and metal fragments. The damage tolerance strategy for the aircraft industry is based on defining critical damage states for composites which are linked to damage visibility and hence damage detection during service. Currently impact tests on aircraft structures are carried out on test specimens supported in a test fixture in a stress-free condition. However aircraft fuselage and wing structures in flight are subjected to quasi-static loads up to design limit load, hence foreign object impacts usually occur on preloaded structures which may influence damage tolerance. This study commissioned by EASA under the project name LIBCOS focuses on a comprehensive test programme investigating the influence of pre-loads on the residual strength of composite generic panels after impact, representative of fuselage bay panels. To that aim, a total of 78 tests have been conducted: 57 on composite plates and 21 on aluminium plates for comparison. The test programme used representative composite fuselage bay panels situated between frames and stringers. Material selected is an aerospace UD carbon epoxy prepreg Cycom 977-2-35-12KHTS-134-300 from Cytec. Plates with impact area 500 mm x 170 mm are loaded in axial tension or compression in a new hydraulic pre-stressing frame designed to fit in the DLR gas gun target chamber and are then tested under impact. Impact tests are carried out with a 60 mm diameter gas gun at velocities in the range 64 – 136 m/s, and impact energies 36 – 125 J. Two main impact scenarios are considered: notch damage from a 12 mm steel cube projectile, representative of impact from engine fragments or metal objects, and blunt impact damage from glass balls (Ø24-25 mm), which could represent hail or runway stone debris. Impact damage is detected by visual inspection and quantified with lock-in thermography, especially for delamination. Depending on the preload regime, corresponding residual strength tests are carried out on each plate.