Influence of pre-load on the impact behaviour of panels: damage tolerance aspects

Kurzfassung

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 materials which are linked to damage visibility and hence damage detection during service. The paper describes an experimental damage tolerance study carried out for EASA on the ‘Significance of Load upon Impact Behaviour of Composite Structure’ (LIBCOS). 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. LIBCOS studied experimentally the effect of preloads on impact damage in carbon/epoxy and aluminium structures, and on subsequent residual strengths of damaged structures. The test programme used representative aluminium and composite fuselage bay panels situated between frames and stringers. The materials selected were alumiunum 2024 and an aerospace UD carbon fibre toughened epoxy prepreg. Plates with impact area 500 mm x 170 mm were loaded in axial tension or compression in a new hydraulic pre-stressing frame designed to fit in the DLR gas gun target chamber. Pre-load levels in tension were representative of design limit loads for fuselage panels of this size, and maximum compression preloads were in the post-buckle region. Impact tests were 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 were considered: notch damage from a 12 mm steel cube projectile, representative of impact from engine fragments or metal objects; blunt impact damage from glass balls, which could represent hail or runway stone debris. Impact damage was detected by visual inspection and lock-in thermography. The influence of preload and impact damage on residual strengths were studied and test results analysed in the context of damage tolerance requirements and aircraft safety. The test programme showed no significant interaction between tension/compression preload and impact damage for the aluminium panels. Composite panels in tension and compression with visible impact damage (VID) from hard body notch impact tests with complete penetration showed very good structural integrity, with no significant reduction in strength due to preloading. However, blunt impact tests led to large delamination damage regions which had a strong influence on plate bending properties and caused lower compression strengths in buckling. Test results showed that compression pre-load intensified this trend. Aircraft damage tolerance for barely visible impact damage (BVID) in composite panels with and without preloads therefore requires more careful assessment, since the damaged structure has to sustain design limit load in flight up to the next inspection.