Impact test campaign

Executive Summary:

IMPTEST had involved gas gun impact testing of shields made of composite materials for protection of aircraft against small metal fragments generated by failure of rotating engine components. Three different sizes of steel cylinders were used to represent engine fragments in different applications. The test campaign included three phases, where Phase 1 involved impact perpendicular to the panels to find the ballistic limit velocity for penetration of metallic shields and three different composite shield concepts, using three different impactors at velocities below 600 m/s. Phase 2 involved studies of the influence of impact angle for the two selected design concepts, while Phase 3 studied the influence of material aging for the selected shield concepts. Furthermore, fibre bundles were tested at -40°C, +23°C and +80°C or +120°C, and the results were used in impact simulations to study the influence of temperature. High speed photography was used to record the impact response history of all specimens. The damage in all specimens was characterised quantitatively and qualitatively using various fractographic methods and by 3D scanning of the deformed shape after impact.

The work in IMPTEST is closely linked to the project IMPSHIELD, where the impact shields were designed and manufactured. Both projects involved participation from Imperial College and Swerea SICOMP. Evaluation of the tests in Phase 1 revealed that the two design concepts with polymer fibres were clearly superior, and these were selected for oblique impact testing in Phase 2. The penetration velocity per unit weight of the selected composite shields was 2-3 times higher than for corresponding metal shields. The oblique impacts in Phase 2 demonstrated that the penetration velocity is fairly proportional to the impact velocity component perpendicular to the impact shield. Hence, small impact angles require extremely high velocities for penetration. Furthermore, non-penetrating oblique impacts on laminated composite shields result in a peculiar response, where the projectile is trapped inside the laminate and slides between the plies until it is fully arrested. Panels which had been aged in hot/wet conditions were tested in Phase 3, but the difference in impact performance was insignificant. Tensile tests demonstrated that temperature changes between -40°C and +23°C only had a moderate influence on fibre strength, but that the strength dropped significantly at temperatures around +100°C. The stiffness measurements can be used for future more detailed simulations of the influence of temperature.

The project has demonstrated that ballistic shields made from polymer fibre composites provide efficient lightweight protection of aircraft against engine debris and that their performance per unit weight is 2-3 times better than for corresponding metal shields. Such shields will facilitate the use of open rotor engines mounted on the rear aircraft fuselage, and the reduced weight will contribute to reductions in fuel consumption and harmful emissions. Aging in hot/wet conditions appears to be a minor problem, but use at higher temperatures must be carefully considered, as the mechanical properties of polymer fibres change at higher temperatures. The most efficient composite shields experience significant deflections during impact, which requires consideration during design. Furthermore, the tests highlight the need to consider the attachment of the shields to the substructure to avoid local failure, e.g. at bolt holes.