پاسخ کامپوزیت های تقویت شده فیبر به بارهای انفجاری زیر آب Response of fiber reinforced composites to underwater explosive loads

A major threat to ship structures and marine vessels is being exposed to severe shock loads [1,2] which could be due to the underwater explosion of a mine or a torpedo, the structure striking a partially submerged object in water, and/or the slamming pressure that occurs at high sea states when the forefront of the vessel rises above the water surface and then rapidly reenters the water. These shock waves generally generate impulses of very high pressures but short durations, resulting in extremely high strain rates, which may cause severe structural damage. In order to decrease weight of the empty ship and thus increase payload, there is significant interest in developing lightweight structures for replacing conventional plate– beam metallic components in selected areas of a ship. For such structures to provide adequate protection against underwater blast, they must have high resistance to impulsive loads and good residual (post-impact) strength [3]. The estimation of service life requires knowing the progressive degradation of material properties as a consequence of growth of the internal damage. The absorption of energy in ballistic situations depends on the evolution of damage in the target that progressively degrades its material properties. Although several models have been developed to describe the deformation mechanisms of composites, no one model adequately characterizes the entire process due to numerous factors like the difference in behavior between fiber types, fabric and composite constructions, the variation in thermomechanical properties, ductility, anisotropy, rate sensitivity of composite materials, and the fact that composite materials respond differently from monolithic materials (e.g., a metal) upon which fundamentals of the mechanics of high strain rate deformation are based [4]. The initiation and propagation of damage in composites due to impulsive loads has been studied experimentally, analytically and numerically. For underwater shock and  air blast loading, tests are usually performed by subjecting large composite panels (up to 3 m · ۳ m in size) or full scale sections of a ship to increasing levels of shock loads and then examining the laminate for evidence of gross structural damage [1] due to fiber breakage, matrix cracking, fiber/matrix debonding, and delamination. Mouritz [3] used the four-point bend test to measure the residual flexural strength of a glass reinforced polymer (GRP) laminate after it had been impulsively loaded by an underwater shock wave produced by an explosion. The examination by a scanning electron microscope of the laminate tested at a shock pressure of 8 MPa revealed that damage was confined to some cracking of the polymer matrix and a small number of short delaminations; consequently, the flexural strength remained essentially unchanged. However, when the peak pressure of the shock wave exceeded 8 MPa, the laminate was severely damaged by cracking of the polymer, breakage and buckling of fibers, and large delamination zones. High compressive stresses in the area near the impacted surface buckled glass fibers there, and high tensile stresses near the back surface caused cracking of the polymer and glass fibers there. Throughout the laminate, extensive delamination occurred at many interfaces between adjoining plies. The extent of damage, as evidenced by the progressive deterioration of the residual flexural strength and stiffness, increased with an increase in the intensity of the shock pressure from 8 to 28 MPa. Will et al. [5] have pointed out that for high velocity impacts the structure responds in a local mode, a little energy is used to deform fibers and the structure, and a significant amount of energy is dissipated in mechanisms such as delamination, debonding and fiber pull-out. In the remainder of this Section, we summarize literature results regarding effects of different material, geometric and loading parameters on structure’s response to impulsive loads.