An experimental investigation of the impact response of composite laminates

Abstract

In this study, the impact response of unidirectional glass/epoxy laminates has been investigated by considering energy profile diagrams and associated load–deflection curves. Damage modes and the damage process of laminates under varied impact energies are discussed. Two different stacking sequences, [0/90/0/90]_s and [0/90/+45/−45]_s, were chosen in tests for comparison. An alternative method, based on variation of the excessive energy (E_e) versus impact energy (E_i), is presented to determine penetration threshold (Pn). The penetration threshold for stacking sequence [0/90/+45/−45]_s is found to be smaller than that of [0/90/0/90]_s. The primary damage mode was found to be fiber fracture for higher impact energies; whereas, it was indentation resulting in delamination and matrix cracks for smaller impact energies. Contour plots of the overall damage areas are also depicted for several impact energies.

Introduction

There has been a growing interest, particularly in the few last decades, in the use of composite materials in structural applications ranging from aircraft and space structures to automotive and marine applications. However, their behavior under impact loading is one of the major concerns [1], since impacts do occur during manufacture, normal operations, maintenance and so on. Especially, unidirectional laminated plates are highly susceptible to the transverse impact loads resulting in significant damages such as matrix cracks, delaminations, and fiber fractures. Therefore, a lot of studies have been carried out to help understand and improve the impact response of composite materials and structures [2], [3], [4], [5], [6].

In the literature, a lot of experimental, numerical, and analytical studies on the impact response of laminated composite structures in many aspects can be found. Among them, Sadasivam and Mallick [7] have studied on the low energy impact characteristics of four different E-glass fibers reinforced thermoplastic and thermosetting matrix composites. Caprino et al. [8] have performed low velocity impact tests on carbon/epoxy laminates of different thicknesses. They have examined the force and absorbed energy at the onset of delamination, the maximum force and related energy, and penetration energy. Some experimental investigations have been carried out by Hosur et al. [9] to determine the response of four different combinations of hybrid laminates subjected to low velocity impact loading. They have indicated that there was considerable improvement in the load carrying capability of hybrid composites as compared to carbon/epoxy laminates with slight reduction in stiffness. Datta et al. [10] have investigated the effects of variable impact energy and laminate thickness on the low velocity impact damage tolerance of GFRP composite laminates. Critical values of impact energy and laminate thickness were also defined. Baucom and Zikry [11] have addressed an experimental study to understand the effects of reinforcement geometry on damage progress in woven composite panels under repeated impact loading. Fuoss et al. [12], [13] have worked on the effects of key stacking sequence parameters on the impact damage resistance in composite laminates.

Wu and Chang [14] have conducted a transient dynamic finite element analysis for studying the response of laminated composite plates subjected to transverse impact loading by a foreign object. They have calculated displacements, the transient stress and the strain distributions through the thickness of laminate during the impact event. A finite element analysis of fiber-reinforced composite plates subjected to low velocity impact has been also done by Tiberkak et al. [15]. Cho and Zhao [16] have investigated the effects of geometric and material parameters such as span to stiffness ratio, out-of-plane stiffness, stacking sequence on mechanical response of graphite epoxy composites under low velocity impact. Aslan et al. [17], [18] have done a numerical and experimental analysis to investigate the effects of the impactor velocity, thickness and in-plane dimensions of target and impactor mass on the response of laminated composite plates under low velocity impact. They have concluded that the peak force in an impact event increases with the thickness of the composite as the contact time decreases.

Mitrevski and co-authors [19], [20] have investigated the effect of impactor shape on the impact response of composite laminates using a drop weight test rig. A very useful work regarding the effect of an initial pre-stress on the response of carbon–fiber/epoxy laminated plates subjected to low velocity impact has been carried out by Whittingham et al. [21]. Prior to being impacted, the samples in their study were loaded either uniaxially or biaxially using a specially designed test rig. An energy profiling method, which has been used by some recently [22], [23], seems to be useful to characterize some impact properties, e.g. penetration and perforation thresholds. Hence, the damage process of individual laminates can be reconstructed from comparing the corresponding load–deflection curves, energy profile and images of damaged specimens.

In this study, impact response of the cross-ply and angle-ply glass/epoxy laminates has been investigated. [0/90/0/90]_s stacking sequence was chosen for the cross-ply lamination while [0/90/+45/−45]_s for the angle-ply one. Damage modes and damage process of laminates under varied impact energies are also discussed. Thanks to the optically transparent nature of glass–epoxy composite, in determining damage mechanism, the impacted samples were visually inspected by using a strong backlighting.