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This paper aims at showing experimental proof of the existence of a shock front in cellular structures under impact loading, especially at low critical impact velocities around 50 m/s. First, an original testing procedure using a large diameter Nylon Hopkinson bar is introduced. With this large diameter soft Hopkinson bar, tests under two different configurations (pressure bar behind/ahead of the supposed shock front) at the same impact speed are used to obtain the force/time histories behind and ahead of the assumed shock front within the cellular material specimen.Stress jumps (up to 60% of initial stress level) as well as shock front speed are measured for tests at 55 m/s on Alporas foams and nickel hollow sphere agglomerates, whereas no significant shock enhancement is observed for Cymat foams and 5056 aluminium honeycombs. The corresponding rate sensitivity of the studied cellular structures is also measured and it is proven that it is not responsible for the sharp strength enhancement.A photomechanical measurement of the shock front speed is also proposed to obtain a direct experimental proof. The displacement and strain fields during the test are obtained by correlating images shot with a high speed camera. The strain field measurements at different times show that the shock front discontinuity propagates and allows for the measurement of the propagation velocity. ARTICLE IN PRESSwww.elsevier.com/locate/jmps 0022-5096/$ -see front matter r All the experimental evidences enable us to confirm the existence of a shock front enhancement even at quite low impact velocities for a number of studied materials. r

This paper reports an original inverse perforation tests on foam core sandwich panels under impact loading. The key point is the use of an instrumented Hopkinson pressure bar as a perforator and at the same time a measuring device. It aims at a high quality piercing force record during the whole perforation process, which is a weak point of common freeflying projectile-target testing schemes.This new testing arrangement allows for the measurement of piercing force-displacement curves under quasi-static and impact loadings of sandwich samples, which is made of 40 mm AlSi 7 Mg 0.5 Cymat foam cores and 0.8 mm thick 2024 T3 aluminium sheets as top and bottom skins (incident and distal side facesheets). Compared with quasi-static top skin peak loads (the maximal load before the perforation of top skins) obtained under same geometric and clamping conditions, a significant enhancement under impact loading (25%) of the top skin peak load is found. However, the used foam core and skin sheet are known and have been confirmed to be hardly rate sensitive by separate tests on foam cores as well as on the skin sheets.A possible explanation of these puzzling results is following: the foam core under the perforator was locally more compressed under impact loading because of the inertia effect. As the used foam cores has a quite important strain hardening behaviour, the strength of foam cores before the failure of the top skin is higher than that under quasi-static loading, which leads to an increase of the top skin peak load under impact loading. Tests on a uniformly pre-compressed sandwich sample exhibit indeed higher top skin peak loads, which supports also this aforementioned concept. r

Numerical simulations of two distinct testing configurations using a Hopkinson bar (pressure bar behind/ahead of the shock front) are performed with an explicit finite element code. It allows us to confirm the observed test data such as velocity and force time histories at the measurement surface. A comparison of the simulated local strain fields during shock front propagation with those measured by image correlation provides an additional proof of the validity of such simulations.Very simple rate insensitive phenomenological constitutive model are used in such simulations. It shows that the shock effect is captured numerically with a basic densification feature. It means that strength enhancement due to shock should not be integrated in the constitutive model of foam-like materials used in industrial FE codes.In order to separate shock enhancement from entire strength enhancement, an improvement of an existing model with easily identifiable parameters for shock enhancement prediction is proposed. For a quick estimate of the shock enhancement level, a simple power law densification model is proposed instead of the classical RPPL model proposed by Reid and co-workers [Tan et al., 2005. Dynamic compressive strength properties of aluminium foams. Part I-experimental data and observations. J. Mech. Phys. Solids 53, 2174-2205]. It is aimed at eliminating the parameter identification uncertainty of the RPPL model. Such an improved model is easily identifiable and gives a good prediction of the shock enhancement level. r

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