Abstract A series of dynamic tests were conducted on a closed-cell aluminum alloy foams in order to determine experimental failure surface under impact loading conditions. Quasi-static tests have also been performed to investigate failure mechanism under different stress paths. Three typical types of deformation modes can be observed, which corresponds to the different failure mechanism. The failure loci of the foam in principal stress plane are explored from quasi-static to dynamic loading conditions. A significant strength enhancement is identified experimentally. The expansion of the failure locus from the quasi-static to the dynamic test is almost isotropic. A modified failure criterion for the metallic foam is proposed to predict failure locus as a function of strain rate. This rate-dependence failure criterion is capable of giving a good description of the biaxial failure stresses over a wide range of the strain rates.

Abstract The dynamic compressive behavior of ice is investigated using a large-sized (37 mm in diameter) modified aluminum split Hopkinson pressure bar (SHPB) with pulse shaper at the strain rate from 500 s-1 to 1200 s-1. A series of relatively stable experimental results of dynamic compressive strength versus strain rate and a linear fitting curve have been obtained by controlling data scatter within 25%. The composition of incident wave has been discussed. The effects of pulse shaper diameter and velocity of striker bar have been tested. The properties and principles of incident wave in different stage has been elaborated when using pulse shaper. A theoretical analysis of pulse shaper and bar size effects on the rising time of incident wave has been conducted. Results show the thickness of pulse shaper is proportional to the rising time. Enlarging the diameter and reducing the velocity of the striker bar could increase the rising time and suppress the dispersion. The diameter and wave impedance of bars also contribute to the rising time.

Abstract The dynamic response and energy absorption capabilities of clamped shallow sandwich arches with aluminum foam core were numerically investigated by impacting the arches at mid-span with metallic foam projectiles. The typical deformation modes, deflection response, and core compression of sandwich arches obtained from the tests were used to validate the computation model. The resistance to impact loading was quantified by the permanent transverse deflection at mid-span of the arches as a function of projectile momentum. The sandwich arches have a higher shock resistance than the monolithic arches of equal mass, and shock resistance could be significantly enhanced by optimizing geometrical configurations. Meanwhile, decreasing the face-sheet thickness and curvature radius could enhance the energy absorption capability of the sandwich arches. Finite element calculations indicated that the ratio of loading time to structural response time ranged from 0.1 to 0.4. The projectile momentum, which was solely used to quantify the structural response of sandwich arches, was insufficient. These findings could provide guidance in conducting further theoretical studies and producing the optimal design of metallic sandwich structures subjected to impact loading.

Abstract Finite element analysis was used to investigate the impact behavior of the foam-cored sandwich shells under the ballistic impact. The ballistic limit, failure mode of the face-sheet and energy absorption of the uniform/graded core were discussed in this paper. Based on the impact phase diagrams, the effects of several factors (i.e. projectile shape and diameter, face-sheet curvature and thickness) on the normal/oblique impact behavior were analyzed in detail. Results indicate that blunter and larger projectiles, higher impact velocity and thicker face-sheets can significantly raise the impact-resistance performance of the targets. In addition, the behavior of ballistic impact becomes more complex as the result of the increasing oblique angle.

Dynamic compressive and tensile properties of mortar under impact loading were investigated experimentally by using a split Hopkinson pressure bar (SHPB) apparatus with pulse shaping technique. Firstly, the basic principle, experimental limitations and some feasible improvements/modifications of SHPB technique used for dynamic tests on concrete-like materials were summarized briefly. And then the dynamic compressive strength, stress versus strain response, and failure modes of mortar were discussed and analyzed. Finally, a dynamic Brazilian disc test was conducted to obtain the splitting tensile property of mortar, and some typical experimental results were presented. Both compressive and splitting tensile results show that mortar is a strain-rate sensitive material. Either compressive or tensile strength enhances with the increase of strain rate, especially when the strain rate is greater than the transition strain rate, which is around 20 s-1 for the dynamic compression and 2.0 s-1 for the splitting tension, respectively. These findings are helpful to guide the design and application of concrete structures.

AbstractA new circle-arc model was established to present the cellular structure. Dynamic response of models with density gradients under constant velocities is investigated by employing Ls-dyna 971. Compared with the uniform models, the quasi-static plateau stress of different layers seems a significant parameter correlated with the deformation mode except for inertia effect when the density gradient is introduced. The impact velocity becomes much more vital on the deformation of the unit cell than the density gradient. The stress at both the impact and stationary sides is investigated in details. Furthermore, the stress-strain curve is compared with the modified shock wave theory. The density gradient does have some remarkable influence on the energy absorption capability, and a certain density gradient is not always beneficial to the energy absorption. Irrespective of the impact velocity, there seems always a critical strain where the energy absorbed by all these specimens could approximate to nearly the same value. So the critical strain-velocity curve is plotted and gives the beneficial area for energy absorption pertinent to density gradients and impact velocity.

The dynamic response of cylindrical sandwich shells with aluminum foam cores subjected to air blast loading was investigated numerically in this paper. According to KNR theory, the nonlinear compressibility of the air and finite shock conditions were taken into account in the finite element model. Numerical simulation results show that the compression strain, which plays a key role on energy absorption, increases approximately linearly with normalized impulse, and reduces with increasing relative density or the ratio of face-sheet thickness and core thickness. An increase of the impulse will delay the equalization of top and bottom face-sheet velocities of sandwich shell, but there is a maximum value in the studied bound. A limited study of weight optimization was carried out for sandwich shells with respect to the respective geometric parameters, including face-sheet thickness, core thickness and core relative density. These numerical results are of worth to theoretical prediction and engineering application of cellular metal sandwich structures.

This paper presents an experimental investigation on the yield behavior of Nomex honeycombs under combined shearcompression with regard to out-of-plane direction. Four different types of specimens were designed in order to investigate the influence of in-plane orientation angle on the yield behavior of honeycombs under combined loads. Two different failure modes of honeycomb specimens, i.e. the plastic buckling and the extension fracture of cell walls, are observed under combined shear-compression. The experimental results validate that the in-plane orientation angle has a significant influence on the developments of the experimental yield surface. The experimental yield surfaces are compared with a phenomenological yield criterion capable of accounting for anisotropic behavior. The comparative analytical results indicate the experimental yield surfaces are approximately consistent with the theoretical yield surfaces in the normal-shear stress space. These experimental results are useful to develop constitutive models of Nomex honeycombs under combined shear-compression.

The dynamic response of the clamped sandwich beams was investigated by impacting the beams at mid-span with metal foam projectiles. Efects of applied impulse, face-sheet thickness and core thickness on the impact resistance of sandwich beams are discussed. Using strain gauge measurement technique, the deformation mechanism of sandwich beams was analyzed in the paper. The results indicate that the structural response is sensitive to the configuration of sandwich structures. Based on the experiments, corresponding finite element simulations have been undertaken using the LS-DYNA software. A good agreement has been obtained between the numerical and experimental results. The deformation and failure modes of sandwich beams under impact loading are presented, i.e. large inelastic deformation, core compression and core shear with interfacial failure. Numerical results also show that the velocity-time histories of the mid-span of the front and back sheet of sandwich beams are related to not only core strength but also core thickness. The research results are of worth to the theoretical prediction of the dynamic response of metallic sandwich beam.