Impact and post impact response of laminated beams at low temperatures

Abstract

In this study, glass fiber reinforced unidirectional and crossply laminated composite beams were subjected to low velocity impact and compression after impact (CAI) testing at low temperatures. A total of fifty effective 152.4 mm × 50.8 mm × 3.2 mm laminated beam specimens were prepared. Low velocity impact tests were conducted on the prepared specimens using an instrumented drop-tower impact machine at frozen temperatures 0 °C, −10 °C, and −20 °C. Temperatures at 20 °C and 10 °C were also used for comparisons. CAI tests were conducted using a hydraulic-servo MTS machine to determine the residual load carrying capacity of the impact damaged specimens. Damage observation was conducted to aid in the understanding of the damage mechanism. The effect of environmental temperatures on the impact damages and on the residual compressive buckling strength and elastic modulus was evaluated based on the test results.

Introduction

Over the years, there has been mounting concern over the safety of laminated composites subjected to a low velocity impact. A low velocity impact on laminated composites can cause various types of damages including delamination, fiber breakage, matrix cracking, and fiber–matrix interfacial debonding. These types of damage are very dangerous because they cannot be detected visually and lead to structural failure at loads well below design levels. A number of researchers have investigated the low velocity impact behavior of laminated composites at an ambient environment. Baker et al. [1] has summarized the work on damage of laminated composites including the techniques for nondestructive testing and observation of crack propagation, damage observation, application of failure criteria, effect of impactor mass, target geometry, impact velocity and initial stress and the residual strength. Abrate [2] and Cantwell and Morton [3] have also given reviews on the impact of laminated composites that cover both theoretical and experimental aspects of the problem, such as impact modeling, impact damage, damage prediction, and residual properties.

Because most composite structures are used out-of-doors, it cannot be avoided that composite structures are subjected to various environmental conditioning. The study of impact and post impact response of laminated composites subjected to environmental conditioning other than ambient is more realistic. Karasek et al. [4] have evaluated the influence of temperature and moisture on the impact resistance of an epoxy/graphite composite. They have found that only at elevated temperatures has moisture had a significant effect on damage initiation energy and that the energy required for initiating damage has been found to decrease with temperature. The investigations by Bibo et al. [5] have shown that temperature is capable of altering the nature and extent of impact induced damages. Parvatareddy et al. [6] have investigated the low velocity impact behavior of laminated composites aged at elevated temperature in both air and nitrogen environments. They have indicated that the aging environment has a significant effect on the residual tensile strength. Hale et al. [7] have found that the effect of temperature and moisture is interactive. The loss of strength and stiffness of laminated composites at elevated temperatures is exacerbated by the increased rate of water absorption at high temperatures. Li et al. [8] have investigated the effect of cycling moisture on the low velocity impact behavior of laminated composites at elevated temperature. Their results show that the first moisture cycle has a significant effect on reducing the low velocity impact resistance of laminated composites. Elevated temperature accelerates the damaging effect of cycling moistures. Pang et al. [9] have investigated the effect of ultraviolet radiation on the low velocity impact response of laminated beams. They have found that UV radiation alone has a significant effect on reducing the residual load carrying capacity of impact damaged laminated beams. The presence of water enhanced the damage effect of UV radiation.

From the above literature survey, almost all the previous studies were focused on impact and post-impact response at elevated temperatures. There is currently a lack of understanding of the low velocity impact behavior and the post-impact load carrying capacity of laminated composites at low temperatures. In this present study, low velocity impact response and post-impact compressive buckling strength of laminated composites at low temperatures were investigated experimentally. The effect of temperature variation on the low velocity impact behavior and residual compressive buckling strength were evaluated based on the test results.