High-strain-rate Fracture Energy of Unidirectional Composites

摘要

The objectives of this study are to develop a new experimental method to directly characterize the composite interphase properties under variable rates of loading; and evaluate the influence of low and high temperatures on the high-strain-rate interlaminar fracture toughness. The primary concern of composites in any critical structures is its premature failure. Composites failure occurs first mostly at the interphase, which is a small zone between the reinforcing phase (usually strong fibers) and the bulk resin matrix. When cold, common expectation is that they will fail with very small amount of strain, with more violence and with high energy release. This early failure is often attributed to the existence of critically sized processing and/or material defects and interfacial problems in the interphase region between the matrix and the reinforcing phase (Drzal, 1983). In understanding failure at the interphase, one must closely examine the polymer matrix and its interaction with the interfacial surfaces (Wool 1995). Many studies, as reviewed by Cantwell and Morton (1991), have concluded that composites are particularly susceptible to impact damage by delamination, which is particularly dangerous because it is often not visible from surface. Thick section composites typically fail at stresses and strains that are well below the expected failure limits. The property commonly measured in the fracture mechanics study is fracture toughness K_(C). Three modes of crack loading can occur, namely mode I (tensile opening), mode II (in-plane shear) and mode III (out-of-plane shear). In this study we focused only on mode I fracture toughness. Test methods for measuring the interlaminar fracture toughness (K_(C)) at slow rates in mode I, II and mixed I/II are well established and several standards exist for mode I (ASTM D5528, ASTM E399, ISO CD 15024 version 97-02-24, and JIS K 7086 of 1993). Various test methods are currently being pursued for the other modes. However, currently no appropriate high rate-loading test exists, and all previous attempts to extend the slow speed test methods to high rates have met with significant obstacles (Blackman et al 1996). The first obstacle is in experimental test equipment to be capable of rapidly accelerating the test specimen and then accurately recording the forces applied and the non-recoverable deformation (fracture) occurred. Second, the dynamic effects are invariably induced at the high rate tests and it is critical that these effects are carefully considered, and accurately accounted for, if accurate and valid K_(C) values are to be measured. Indeed, this probably accounts for the conflicting nature of some of the test results reported in the literature. For example, Smiley and Pipes (1987) pointed to very large reductions in the values of K_(IC) of K_(IIC) for brittle epoxy as well as for thermoplastic Polyether Ether Ketone (PEEK) composites, as the test rate was increased from a few mm/min to about 1m/s. On the other hand, Beguelin et al (1991) reported mode I results of a PEEK matrix carbon composites only a small reduction in the value of K_(IC) as the test rate was similarly increased. In a third study by Aliyu and Daniel (1985) on similar materials, increasing followed by decreasing values of K_(IC) were reported, as the test rate was increased. The differences in experimental results reported were further highlighted in a recent review by Cantwell and Blyton (1998). Their review indicated that the rate sensitivity of the composites was dominated by the toughness of the matrix, with brittle matrix composites exhibiting much less of a rate effect than tough matrix composites.