Functionally graded material hin esti last e cr th a s sh a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing r fabric d per age as l point impact resistance. The basic applications pertaining to these characteristics are packaging of fragile components (e.g. electronic devices) and various protective products like helmets and shield-ing. Another emerging application is usage of cellular structures as the core material for metal sandwich panels, which are shown to have superior performance over the counterpart solid plates of equal mass under shock loading (Dharmasena et al., 2009; Liang et al., 2007; Mori et al., 2007, 2009; Rathbun et al., 2006; phenomena that include buckling and micro-inertial resistance. The fundamental study of Vaughn et al. (2005) and Vaughn and Hutchinson (2006) has provided much insight into these effects, including the interaction between plastic waves and localized buckling under dynamic loading. Xue and Hutchinson (2006) showed that the micro-inertial resistance of core webs of a square honeycomb metal core increases its resisting force remarkably at early stages of dynamic crushing. At later stages of deformation, the dynamics effects results in suppression of the buckling of the metal webs, leading to emergence of buckling shapes with a wavelength much smaller than the core height. These effects lead ⇑ Corresponding author.

Sandwich panel structures with thin front faces and low relative density cores offer significant impulse mitigation possibilities provided panel fracture is avoided. Here steel square honeycomb and pyramidal truss core sandwich panels with core relative densities of 4 % were made from a ductile stainless steel and tested under impulsive loads simulating underwater blasts. Fluid-structure interaction experiments were performed to (i) demonstrate the benefits of sandwich structures with respect to solid plates of equal weight per unit area, (ii) identify failure modes of such structures, and (iii) assess the accuracy of finite element models for simulating the dynamic structural response. Both sandwich structures showed a 30% reduction in the maximum panel deflection compared with a monolithic plate of identical mass per unit area. The failure modes consisted of core crushing, core node imprinting/punch through/tearing and stretching of the front face sheet for the pyramidal truss core panels. Finite element analyses, based on an orthotropic homogenized constitutive model, predict the overall structural response and in particular the maximum panel displacement. 1.

Abstract: A constitutive model for the elastic-plastic behavior of plastically compressible orthotropic materi-als is proposed based on an ellipsoidal yield surface with evolving ellipticity to accommodate non-uniform hard-ening or softening associated with stressing in different directions. The model incorporates rate-dependence aris-ing from material rate-dependence and micro-inertial ef-fects. The basic inputs are the stress-strain responses under the six fundamental stress histories in the or-thotropic axes. Special limits of the model include classi-cal isotropic hardening theory, the Hill model for incom-pressible orthotropic solids, and the Deshpande-Fleck model for highly porous isotropic foam metals. A pri-mary motivation is application to metal core structure in sandwich plates wherein the core is modeled by a con-tinuum constitutive model. The constitutive model is im-plemented within a finite element framework to represent the behavior of square honeycomb metal cores of sand-wich plates subject to quasi-static and dynamic loads. Input identification is illustrated for numerical formula-tions that employ one element through the core thickness. Representations of the core with one element through the thickness are shown to be able to capture most of the im-portant influences of nonlinear core behavior on overall response of sandwich plates under both quasi-static and dynamic loadings.

When sandwich panels with prismatic cores are impulsively loaded, the stresses imposed by the core on the front face, as well as those transmitted through the core govern the response metrics, especially the center displacement, resistance to tearing, and loads transmitted to the supports. This article presents a basic study of the dynamic response with emphasis on the I-core. A prior assessment revealed buck-lewaves induced because of inertial phenomena accompanying the rapid compression of the members. The development of these waves is an integral aspect of the dynamic response. One objective of this investigation is to ascertain the characteristics governing such waves in I-core configurations through a combined experimental and numerical study. A particular emphasis is on the influence of manufacturing imperfections in the core members on the formation and propagation of the buckles. A second goal is to examine the stresses associated with the dynamic compression of the core, again through a combined experimental and numerical investigation. The investigation is conducted for stainless steel I-core panels supported at the back face and sub-jected to a constant velocity at the front. Imperfections to be included in the numerical study have been ascertained by comparing buckle patterns with those found experimentally over the relevant velocity range. The simulations reveal that the stresses induced differ on the front and back faces. On the front they are higher and velocity dependent. On the rear they are velocity invariant and scale with the relative density and material yield strength. The duration of the stress pulses, which is essentially the same on both faces, scales linearly with the core height. It correlates with the time needed for bucklewaves to propagate through the core to the back face. After the pulse terminates, the core continues to compress at a stress level about an order of magnitude smaller. 1.

Significant reductions in the fluid structure interaction regulated transfer of impulse occur when sand-wich panels with thin (light) front faces are impulsively loaded in water. A combined experimental and computational simulation approach has been used to investigate this phenomenon during the compression of honeycomb core sandwich panels. Square cell honeycomb panels with a core relative density of 5% have been fabricated from 304 stainless steel. Back supported panels have been dynamically loaded in through thickness compression using an explosive sheet to create a plane wave impulse in water. As the impulse was increased, the ratio of transmitted to incident momentum decreased from the Taylor limit of 2, for impulses that only elastically deformed the core, to a value of 1.5, when the peak incident pressure caused inelastic core crushing. This reduction in transmitted impulse was slightly less than that previously observed in similar experiments with a lower strength pyramidal lattice core and, in both cases, was well above the ratio of 0.35 predicted for an unsupported front face. Core collapse was found to occur by plastic buckling under both quasistatic and dynamic conditions. The buckling occurred first at the stationary side of the core, and, in the dynamic case, was initiated by reflection of a plastic wave at the (rigid) back face sheet-web interface. The transmitted stress through the back face sheet during impulse loading depended upon the velocity of the front face, which was determined by the face sheet

a b s t r a c t The dynamic compressive response of a sandwich plate with a metallic corrugated core is predicted. The back face of the sandwich plate is held fixed whereas the front face is subjected to a uniform velocity, thereby compressing the core. Finite element analysis is performed to investigate the role of material inertia, strain hardening and strain rate hardening upon the dynamic collapse of the corrugated core. Three classes of collapse mode are identified as a function of impact velocity: (i) a three-hinge plastic buckling mode of wavelength equal to the strut length, similar to the quasi-static mode, (ii) a 'bucklewave' regime involving inertia-mediated plastic buckling of wavelength less than that of the strut length, and (iii) a 'stubbing' regime, with shortening of the struts by local fattening at the front face. The presence of strain hardening reduces the regime of dominance of the stubbing mode. The influence of material strain rate sensitivity is evaluated by introducing strain rate dependent material properties representative of type 304 stainless steel. For this choice of material, strain rate sensitivity has a more minor influence than strain hardening, and consequently the dynamic collapse strength of a corrugated core is almost independent of structural dimension.

e e ob st ual e o ame and The results show the possibility to tailor the elastic buckling properties at each hierarchical level, and observ t al., nd the

strength of the sandwich. It has also been experimentally shown that the forces transmitted to supports when rigid back supported sandwich panels are impulsively loaded in water are significantly less thanequivalent solidplates [11e14]. These reductions arise from acombinationof beneficial FSI effects (which reduce the transmitted The deformations of solid plates due to impulse loading in air have been investigated [18e22]. Recent analytical [9] and numer-ical [23,24] assessments of sandwich panels indicate the FSI effect is much weaker in air and only becomes significant when (i) the overpressure is high, (ii) the core crush strength is small and (iii) the face sheet exposed to the impulse has a very low mass (inertia) per unit area. Even so, recent experiments and confirmatory numerical simulations have shown that sandwich panels with thick, strong square honeycomb cores and thick face sheets still * Corresponding author. Contents lists availab International Journal o lse

us c esul l co rugated and stacked adjacent blanks over their areas of contact. The computational analyses employed ed com estigat ted ma 1.1. Basics of sandwich structures 1.1.1. Definition In the broadest sense, sandwich structures can be defined as laminated hybrid structures consisting of (top and bottom) rela-tively-thin face sheets (typically made of stiff and strong materials) terial (as well as hole) can p rties; and tructures a face sheets, while the core provides structural support for t sheets and controls the overall in-plane shear stiffness/stre the sandwich structure. 1.1.2. Sandwich-structure core construction As mentioned above, the core of the sandwich panels generally possesses a low relative mass. The low core mass is sometimes at-tained by choosing bulk materials with low densities (e.g. balsa wood). However, in most cases a non-bulk form of the core mate-rial is used, in which a relatively small fraction of the core space is

Numerical simulation of metallic honeycomb sandwich panel structures under dynamic loads