BEHAVIOR OF TENSILE PRELOADED COMPOSITES SUBJECTED TO LOW-VELOCITY IMPACT LOADS

Composite Materials are being used increasingly in aircraft structures due to their high specific strength and stiffiiess, and the resultant weight savings. Traditionally, the use of graphite fiberreinforced composites have been confined to secondary structures. However, requirements for reduced structural weight, improved aircraft performance, and efficiency are making the composite materials increasingly competitive for expanded usage in the primary, load carrying structures. In comparison to conventional metals, an understanding of the complex behavior of composites is still in its infant stage, and applications are based on knowledge gained through extensive experimental programs. Past experiences and experiments have confirmed that the graphite fiber-reinforced composite laminates have low ultimate strains, no plastic deformation range, and no usable strength in the thickness direction. These limitations become very obvious when laminates are subjected to impact loads. In this paper, the effect of low-velocity impact on composite laminates is addressed. Due to the low-velocity impact loading laminates suffer an extensive internal damage such as delaminations and the damage on the back surface known as spalling. Therefore the study of impact damage susceptibility of these composite laminates is increasingly important. While considerable experimental and analytical studies have been made on the low velocity impact phenomenon, very little work is done in the area of effect of preloads on the damage mechanics in thin and thick composite laminates subjected to low velocity impact loading.