Shock enhancement of cellular structures under impact loading: Part I Experiments

Abstract

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

Volume changes in a filled elastomer studied via digital image correlation

“…The specific difficulty of using DIC for elastomeric materials is the large amplitude of displacements that may prevent a proper determination of the displacement [38][39][40]. To overcome this difficulty, images were acquired at small loading increment (typically for an incremental axial strain of order 2-3 %) and the displacement analysis was partitioned into subseries (the displacement field was computed from time t i to time t j , for t j − t i < 50 images).…”

Propagation of a stress wave through a virtual functionally graded foam

“…They noted that strain rate effects were higher for a higher density and attribute this to the kinetics of internal gas flow. Elnasri et al [9] reported a limited rate sensitivity for ALPORAS foam at strain rates up to 1300 m/s. Zhao and Abdennadher [10] stated that the rate sensitivity of metallic foam is due to inertia effects in dynamic buckling of cell walls, although the foam is made of strain rate insensitive material.…”

Stress Distribution in Graded Cellular Materials Under Dynamic Compression

“…Significant enhancement of crushing stress was observed in the dynamic impact experiments of woods (Reid and Peng, 1997;Harrigan et al, 2005), aluminum honeycombs (Zhao and Gary, 1998;Hou et al, 2012) and foams (Mukai et al, 1999;Deshpand and Fleck, 2000;Tan et al, 2005a;Elnasri et al, 2007). Several shock models and mass-spring models have been proposed to understand the shock wave propagation in cellular materials under dynamic impact, such as the R-PP-L (rate-independent, rigid-perfectly plastic-locking) model (Reid and Peng, 1997), the mass-spring model (Li and Meng, 2002), the E-PP-R (elastic-perfectly plastic-rigid) model (Lopatnikov et al, 2003), the power law densification model and the D-R-PH (dynamic, rigidplastic hardening) shock model .…”