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Sandwich composites are being aggressively pursued as structural materials by various defense and commercial industries. These include navy, air force, army, automotive and sporting industries to name a few. In the context of structural load bearing members and absorbing dynamic loads, foam core sandwich composites offer unique advantages over traditional composites. The cellular construction of the foam materials not only provides lightweight capability but also a deformation mechanism that allows efficient absorption of energy. Of particular interest in this study is to investigate the behavior of the foam materials and their sandwiches under high strain rate (HSR) loading which are very much prevalent in their actual applications. A systematic approach has been taken to study the response of PVC foam materials at strain rates ranging from quasi-static to around 2000 S −1 . First, room temperature response has been extracted for various density foams, and determined their strain rate sensitivities and failure characteristics. Next, two microstructures in the construction of the core materials, namely linear and cross-linked foams, were considered in the investigation. This study revealed that both the categories of foam had direct dependence on temperatures well below the T g of the base polymer. Consequently, the next phase of the investigation included HSR tests at elevated and sub-ambient temperatures. The study has established that at room temperature (RT), the delamination at the sub-interface zone underneath the actual core–skin interface is the critical mode of failure. This sub-interface is characterized by the separation of the dry and resin-soaked cells near the core–skin interface. Failure modes however changed as the test temperature changed from RT to sub-ambient.

The compressive response of cross-linked and linear PVC foam core materials, namely, H130 and HD130 having identical densities were evaluated. Both cores were subjected to compressive loading at various strain rates ranging from quasi-static to 1560 s1. At quasi-static loading, the compressive strength of the H grade was approximately 30% higher than the HD grade. On the other hand, at HSR loading the HD grade was 20% higher. HSR tests were also conducted at various temperatures ranging from 25 to 12°C. It was observed that degradation of strength was more severe for the HD grade as temperatures increased. A recovery of the cores original volume was also noticed during high strain rate and elevated temperature tests. Accordingly, residual properties were determined. Results show that only the residual properties of the HD grade were affected by temperature.

When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest component of the structure and is the first to fail in shear. In this study the shear fatigue behavior of two closed-cell cellular PVC foams, Divinycell HD130 (linear) and H130 (cross linked), with the same nominal density of 130 kg/m3, were investigated. Static shear tests reveal that HD130 foams are more ductile, have almost twice the energy absorption capability, and an extraordinary crack propagation resistance when compared to the H130 foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, R = 0.1 on the HD130 and H130 foams. S–N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the linear foams was substantially higher than that of the cross-linked PVC foams. HD foams have smaller cells with thicker faces and edges. This microstructure supports absorption of larger amounts of liquid resin forming a resin rich sub interface zone just below the actual core skin interface. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the HD foams. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks form across the width of the specimen and are equidistantly spaced along the length of the specimen. The occurrence of these through the thickness shear cracks signals the final failure event during fatigue. Details of the experimental investigation and the evaluation of the fatigue performance are presented.

When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest component of the structure and is the first one to fail in shear. In this study the shear fatigue behavior of two closed-cell cellular PVC foams, Divinycell HD130 (linear) and H130 (cross-linked), with the same nominal density of 130 kg/m3, were investigated. Static shear tests reveal that HD130 foams are more ductile, have almost twice the energy absorption capability, and an extraordinary crack propagation resistance when compared to the H130 foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, R=0.1 on the HD130 and H130 foams. S–N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the linear foams was substantially higher than that of the cross-linked PVC foams. HD foams have smaller cells with thicker faces and edges. This microstructure supports absorption of larger amounts of liquid resin forming a resin rich subinterface zone in the core. The high-intrinsic toughness of the subinterface delays the initiation of fatigue cracks and thereby increases the fatigue life. For both foams, shear deformation occurs without volume change and the materials fail by shearing in the vicinity of the centerline along the longitudinal axis. In both cases numerous 45° shear cracks form along the length and across the width of the specimen immediately prior to the final failure event. Details of the experimental investigation and the evaluation of the fatigue performance are presented.

The objective of this study is to investigate the dynamic response of S2-glass–vinylester foam core sandwich specimens at low temperatures. Sub-ambient testing condition was achieved using liquid nitrogen. The effect of the sandwich core density as well as microstructure was explored. Results from the stress–strain curves suggested that the energy absorption of the sandwich specimen was predominantly through progressive failure. Although a nominal increase in compressive strength was observed at sub-ambient temperature in respect to its room temperature, the effect of low temperature on the failure modes was severe. Each category of sandwich specimen portrayed its own unique mode of failure.

Cross-linked polyvinyl chloride closed-cell foams were examined under quasi-static and high strain rate compression loading using a servo-hydraulic testing machine and a modified split Hopkinson pressure bar apparatus consisting of polycarbonate bars for strain rates up to 1900 s −1 . Three foam densities were examined viz. 75, 130, and 300 kg/m 3 . Each core density has been subjected to compressive loading at room and elevated temperatures. A reverse trend in failure modes was observed when moving from room to elevated temperatures at high strain loading, which was not found in quasi-static testing at elevated temperatures. Accordingly, post-impact tests were conducted to evaluate the residual strength of the foam cores subject to elevated temperatures and HSR. Results of the post-impact test revealed that the foam cores are still capable of taking some loading. The residual strength of cores was fairly constant regardless of temperature therefore recovery of volume does not signify an increase in residual strength of cores.

Flexural fatigue tests were performed on cross-linked PVC foams of densities in the range from 75 to 300 kg/m3 at a frequency of 3 Hz and at a stress ratio, R=0.1. S–N diagrams were generated, and the failure mechanisms were examined. The fatigue behavior was found to be similar to structural materials with a fatigue strength that increased with increased foam density. The final failure event was catastrophic due to crack propagation initiating at the tension side of the specimen. SEM analyses performed on the R300 foam specimen revealed cell wall cracking and densification of the foam during the earlier part of the fatigue life. This densification contributed to stiffening of foam specimens. Viscoelastic parameters of the foams were determined using a dynamic mechanical analyzer over a frequency range of 1–10 Hz. For the virgin specimens it was found that the viscoelastic moduli and damping ratio were quite independent of frequency over this range of frequencies. Viscoelastic parameters were also extracted at intermediate stages of fatigue life and the residual properties were studied with respect to the number of fatigue cycles. It was observed that the variation of dynamic mechanical analysis (DMA) parameters with the number of fatigue cycles was consistent with the variation of residual global stiffness of the material. Experimental details and the analyses of fatigue behavior are presented in this paper.

Flexural fatigue tests were performed on cross-linked PVC foams of densities in the range from 75 to 300 kg/m3 at a frequency of 3Hz and at a stress ratio, R = 0.1. S-N diagrams were generated, and the failure mechanisms were examined. The fatigue behavior was found to be similar to structural materials with a fatigue strength that decreased with increased stress and increased with increased foam density. The final failure event was catastrophic due to crack propagation initiating at the tension side of the beam. SEM analyses of unfailed and failed 300kg/m3 density foam specimens revealed cell wall cracking and densification of the foam. The densification contributed to stiffening of foam specimens. Viscoelastic parameters of the foams were determined at room temperature using a Dynamic Mechanical Analyzer (DMA) over a frequency range of 1–10Hz. For the virgin specimens it was found that the viscoelastic moduli and damping ratio were quite independent of frequency over this range of frequencies. Except for the lowest density foam (75kg/m3), the damping ratio was quite independent of foam density.

In this study, cross-linked poly-vinylchloride (PVC) closed-cell foams were examined under high strain rate compression loading using a servohydraulic testing machine and a modified Split Hopkinson Pressure Bar (SHPB) apparatus with a steel incident bar and a polycarbonate transmitter bar for strain rates up to 2000 s−1. Three foam densities were examined, viz. 75, 130, and 260 kg/m3. The stress and strain-time history and stress-strain behavior were evaluated. An increase of stress and strain was observed for all categories of foam as strain rate increased. A post impact study was also performed to evaluate the failure modes of the foam cores. A densification band of collapsed cells was the major mode of failure.