JOURNAL OF MATERIALS SCIENCE 26 (1991) 6631-6638 Determination of fibre/matrix interfacial shear strength by an acoustic emission technique A. N. NETRAVALI, Z.-F. LI, W. SACHSE* Departments of Textiles and Apparel and * Theoretical and Applied Mechanics, Cornell University, Ithaca, NY 14853 USA H. F. WU Alcoa Laboratories, Alcoa Center, PA 15069, USA The application of acoustic emission (AE) measurements to locate the sources of fracture of a single high-strength fibre embedded in an epoxy matrix which is loaded in tension is described. From the micromechanical model and the fragment length distribution, interfacial shear strength values were calculated. The technique is demonstrated for small-diameter glass and graphite fibres as well as for fibres which exhibit fibrillar fracture, such as Kevlar and PBZT. Good agreement is found between the mean fragment length values obtained by optical and AE measurements for glass and graphite fibres. Values obtained for interfacial shear strength by the AE technique are comparable with those obtained using other techniques. 1. Introduction Structural composite materials fabricated from epoxy resins and reinforced with high-performance fibres have become increasingly important because of their excellent performance characteristics in terms of their high strength, high modulus and light weight. The properties of such composites, however, depend not only on the strength of the reinforcement or the matrix constituents, but for many of them, also on the charac- teristics of the fibre-matrix interface. It is well-known that the fibre-matrix interface plays an important role in achieving superior tensile properties of a composite [1, 2]. The tensile strength of the composite is dependent on the ability of the composite to transfer the tensile load from the broken fibres to the surviving ones through shear in the matrix and the interface. Improvement in the inter- facial bond strength may increase a composite's tensile strength, but there may be a decrease in the impact strength and toughness [1, 2]. Alternatively, weak interracial bonding may encourage energy-absorbing modes of crack propagation and thereby increase fracture toughness of the composite [1]. Among the many possible tests applicable to inter- faces, two methods which are most often proposed for measuring the fibre/matrix interfacial shear strength (IFSS) include the fibre pull-out test and the single- fibre composite (SFC) test. The fibre pull-out is popu- lar because of its conceptual simplicity. It is, however, not without difficulty. There are clamping and align- ment problems, especially for stiff fibres such as graph- ite and glass, and there are questions related to the required embedded length of the fibre which makes this test not as straightforward as it first ap- pears [3, 4]. In fact, the meniscus at the point of a 0022-2461/91 $03.00 + .12 9 1991 Chapman & Hall fibre's entry into the matrix, makes it sometimes diffi- cult to determine the exact embedded length of the fibre. Nevertheless, the pull-out test has been demon- strated to be an important test procedure for deter- mining fibre/matrix interracial shear strengths as well as permitting an investigation of fibre/matrix frictional effects [3, 4]. Recently, the microbond test which is a modified version of the pull-out test, has been developed by Miller et al. [5]. In this test, a small drop of matrix in the form of a bead is deposited on to the fibre at some point. The fibre with its micro-bead is then mounted in a micro-vice and the fibre is pulled out. Although this test is easy to perform, especially for the purpose of screening materials, there are still several concerns of which a user must be aware. These concerns include the stress concentrations during specimen loading, non-uniform shear stress distribution along the fibre/ matrix interface, the geometry of the resin droplet, the position of the droplet in the micro-vice and the effect of strain rate. All of these factors will significantly affect the test results and the scatter in the test data. The details of the theoretical study of the micro-bond method are contained in a recent study by Wu et al. [6]. The SFC test is a straightforward experimental procedure for determining the fibre/matrix interracial shear strengths. The procedure involves fabricating a specimen in which a single fibre is completely embed- ded along the centreline of a relatively larger dog-bone shaped specimen of matrix material. This single-fibre composite is then strained uniaxially along the fibre axis [8-13]. A requirement of the test is that the strain of matrix fracture is at least three times higher than that of the fibre [9]. When compared to the pull-out 6631