stressed case (equivalent to 6000 µε) and the shear loading produced the greatest impact damage area on the specimen. Whittingham et al.[10] performed similar experiments on carbon-fibre/epoxy laminated plates under tensile and shear pre-stress. They also found that low pre-stressed levels (< 1500 µε) had no significant effects on the peak force of impact, absorbed energies and penetration behavior. Heimbs et al.[11] tested carbonfibre/epoxy laminated plates under an in-plane compressive pre-load. An increase in deflection and energy absorption was observed under a pre-load of 80% of the buckling load. Sun et al. [12] and Choi [13] analytically investigated the effects of pre-stress on the dynamic response of composite laminates. They found that the initial in-plane tensile load increased the peak contact force while reducing the total contact duration and deflection. The compressive load reacted oppositely. However, contact loading, such as impact loading, will induce localized damage, which is different from air or underwater blast loading. Thus, these results cannot be extended to the blast response of composite structures. There are very few theoretical and numerical studies [14-15] related to the blast response of structures with inplane compressive loading. In the author’s previous work, the dynamic behaviors and failure mechanisms of sandwich composites with uni-axial in-plane compressive loading have been studied under intensive transverse shock wave loading [19]. However, the bi-axial in-plane compressive loading are more common than uni-axial loading in reality. The present study focuses on the dynamic behavior and failure mechanisms of sandwich composites with bi-axial in-plane compressive loading while experimentally subjected to a transverse blast loading (as shown in Figure 1). A specially designed fixture was utilized to implement the uniformly passive biaxial in-plane compressive loading on the sandwich composites. Three compressive loading levels were chosen. A high-speed 3-D Digital Image Correlation (DIC) technique was used to capture the real-time full-field deformation data during blast loading. Post mortem visual observation of the test samples was also carried out to indentify the failure mechanisms. 2. MATERIAL AND SPECIMEN The sandwich specimen used in this study has two composite face sheets and a foam core. The VARTM procedure was carried out to fabricate sandwich composite panels. Figure 2 shows a sample image and its overall dimensions. The foam core itself was 25.4 mm thick, while the skin thickness was 3.8 mm. The average areal density of the samples was 17.15 kg/m2. Materials Properties E-glass Vinyl Ester composite Nominal Density: 1800 kg/m3 Compressive Modulus: 13.6 GPa Compression Strength: 220 MPa CoreCellTM P600 Nominal Density: 122 kg/m3 Compressive Modulus: 125 MPa Compression Strength: 1.81 MPa Shear Modulus: 56 MPa Table1. Material properties of the components in sandwich composites [16, 19] Figure 1 A sketch of a blast loading upon a structure with in-plane compressive loading Figure 2 Real specimen and its dimensions 384
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