a b s t r a c tThe quasi-static and dynamic crushing response and the energy absorption characteristics of combined geometry shells composed of a hemispherical cap and a cylindrical segment were investigated both experimentally and numerically. The inelastic deformation of the shells initiated with the inversion of the hemisphere cap and followed by the axisymmetric or diamond folding of the cylindrical segment depending on the loading rate and dimensions. The fracture of the thinner specimens in dynamic tests was ascribed to the rise of the flow stress to the fracture stress with increasing strain rate. The hemisphere cap absorbed more energy at dynamic rates than at quasi-static rates, while it exhibited lower strain rate and inertia sensitivities than the cylinder segment. For both the hemisphere cap and the cylinder segment, the inertial effect was shown to be more pronounced than strain rate effect at increasing impact velocities.

An epoxy matrix filled with nonwoven mats of porous polystyrene (PS) fibers processed by an electrospinning was compression tested at quasi-static (1 × 10 −3 s −1 ) and high strain (315 s −1 ) rates. The electrospun PS fibers with a diameter between 6 and 9 μm, accommodated spherical pores on the surface with the sizes ranging from 0.1 to 0.2 μm. The filling epoxy matrix with 0.2 wt % PS fibers increased the compressive elastic modulus and compressive strength over those of neat epoxy resin. The microscopic observations indicated that the surface pores facilitated the resin intrusions into the fiber, enhancing the interlocking between resin and fibers, and increased the deformation energy expenditure of the polymer matrix. KEYWORDS: electrospinning, epoxy matrix, composite, compression, high strain rate T he fields of application for fiber-reinforced polymer composites (FRPCs) over the last few decades have widened in aerospace, defense, transportation, and automotive industries. 1 The strength of FRPCs is derived to a large extent from the inclusion of 10−100 μm size strong fibers or fiber networks into complaint polymer matrices. The effectiveness of fibers in enhancing the mechanical properties of FRPCs is also affected by the interface between fibers and the matrix. The recent rapid progress and pace taken in the field of nanotechnology has resulted in novel techniques of processing submicron and nanometer size fibers of a relatively high surface area. 2 The high fiber surface area associated with small fiber size causes nanometer-scale interlocking between the fiber and the matrix, enhancing the load transfer from the fiber to the matrix, which requires additional energy expenditure to deform the polymer matrix. 3−7 Ultrathin fibers may conveniently be processed by electrospinning. 8 In electrospinning, a charged jet is ejected from a droplet of a solution through grounded collector under a high electrical field when the electrical force overcomes the surface tension of the droplet. By altering the electrospinning parameters including the spinning atmosphere, 9 solvent composition, 10 and calcination, 11 randomly oriented mechanically integrated nonwoven fibrous polymer mats with tunable fiber sizes and surfaces, internal porosities, and pore sizes are processed.There has recently been a rise in interest in exploring electrospun fibrous mats as reinforcement in FRPCs as these mats have the potentials of efficient stress transfer to matrix, high strength to weight ratio, and excellent multifunctional properties. The significant improvement in the mechanical response of FRPCs was also attributed to the nature of the interfacial bonding between nanofibers and polymeric matrices. 12−15 The cross-linked polystyrene-co-glycidyl methacrylate P(St-co-GMA)/ethylenediamine (EDA) nanofiber filled epoxy matrix showed 2.5 times higher storage modulus than P(St-co-GMA) nanofiber filled epoxy matrix, proving the substantial effect of cross-linking between the nanofibers and polymer matrix on the mechanical pro...

a b s t r a c tDynamic crushing and energy absorption characteristics of sandwich structures with combined geometry shell cores were investigated experimentally and numerically. The effect of strain rate on the crushing behavior was presented by the crushing tests at quasi-static, intermediate and high strain rate regimes. It was shown that absorbed energy increased with increasing impact velocity. The effect of confinement on crushing behavior was shown by conducting confined experiments at quasi-static and dynamic rates. Higher buckling loads at lower deformation were observed in confined quasi-static crushing due to additional lateral support and friction provided by confinement wall. By using fictitious numerical models with strain rate insensitive material models, the effect of inertia and strain rate on crushing were shown. It was observed that, increase in impact velocity caused increase in inertial effects and strain rate effects were nearly independent from the impact velocity. The effects of multilayering were also investigated numerically.

a b s t r a c tThe effect of heat treatment on the dynamic crushing and energy absorption behavior of combined geometry shell cores (hemisphere and cylinder) of sandwich structures were investigated both experimentally and numerically. The applied heat treatment on the combined geometry shell cores relieved the stress caused by deep drawing, diminishing the peak transmitted forces. The verified numerical models of the as-received and heat-treated combined geometry shells were used to model blast loading of various sandwich configurations and the additional sandwich configurations of reversing the cylindrical side of the cores to the impacted side. Both the applied heat-treatment and the reversing process decreased the magnitude of the force transmitted to the protected structure. The applied heat treatment increased the arrival time of blast force wave to the protected structure, while the reversing resulted in opposite.