Characterization of the strain-rate dependency of the behaviour of shock protection based on aluminum foam

Abstract

Shock protection of structure and equipment has always been a challenge for nuclear installation, both for new build or for retrofit of existing structure. Because of the inertial effects of structures and the viscosity of materials behaviour, the design process of a shock protection is strongly related to the strain rate, which depends on shock characteristics, such as the mass and the velocity of the projectile, but also the stiffnesses of both projectile and target, including the shock protection itself. Under a dynamic load, most of shock absorbers such as polymer foams or woods have a compaction strength which is strongly dependent on the strain rate. To design a shock protection for several and distinct threats, it is interesting to take a shock absorber which behaviour is the less dependent as possible on the strain rate to simplify the design process. NuFoam Impact® is an aluminum foam which can answer to that. Nuvia Structure, Dewesoft and Thiot Ingénierie has performed compaction tests on NuFoam Impact® with three distinct facilities, from static tests (100 mm/min) until gas gun shock tests (30 m/s). These experimental tests show a very low- dependent behaviour of NuFoam Impact® on impact speed. To have a medium compaction speed, we instrumented a drop tower which can generate 1 300 J impact energy by falling a 52 kg mass from a height until 2,50 m. The compaction-strain curves are established by measuring the transmitted load with a force cell, and the velocity with an optical fork. The optical fork is associated to a multi holed plate, which constitutes an original measurement tool for this kind of application. The measurement method is validated by the good accuracy of the results with the two other compaction test facilities.