No abstract

One of the most important mechanical properties of a fibre-polymer composite is its resistance to delamination. The presence of delaminations may lead not only to complete fracture but even partial delaminations will lead to a loss of stiffness, which can be a very important design consideration. Because delamination may be regarded as crack propa­gation then an obvious scheme for characterizing this phenomenon has been via a fracture mechanics approach. There is, therefore, an extensive literature on the use of fracture mechanics to ascertain the interlaminar fracture energies, G c , for various fibre-polymer composites using different test geometries to yield mode I, mode II and mixed mode I/II values of G c . Nevertheless, problems of consistency and discussions on the accuracy of such results abound. This paper describes a detailed study of the methods of analysing the experimental data obtained from fracture mechanics tests using double-cantilever beam, end loaded split and end notched flexure specimens. It is shown that to get consistent and accurate values of G c it is necessary to consider aspects of the tests such as the end rotation and deflection of the crack tip, the effective shortening of the beam due to large displacements of the arms, and the stiffening of the beam due to the presence of the end blocks bonded to the specimens. Analytical methods for ascertaining the various correction constants and factors are described and are successfully applied to the results obtained from three different fibre-polymer composites. These composites exhibit different types of fracture behaviour and illustrate the wide range of effects that must be considered when values of the interlaminar fracture energies, free from artefacts from the test method and the analysis method, are required.

No abstract

A detailed study on the interlaminar failure of carbon-fibre/poly(ether-ether ketone) unidirectional composite (termed PEEK composite: "APC-2" from ICI (UK) plc) is presented. A fracture mechanics approach has been adopted and Mode I, Mixed-Mode I/II and Mode II tests have been conducted and the effects of specimen geometry, test rate and test temperature have been investigated. It is shown that for the interlaminar fracture of the PEEK composite the value of the interlaminar fracture energy, G,, generally in creases as the crack propagates through the composite, i.e., a rising "R-curve" is observed. Thus, it is not usually possible to assign one unique value to the interlaminar fracture energy, Gc, for any given Mode of loading for the PEEK composite. We have therefore defined both an initiation value, Gc(init), and a steady-state propagation value, Gc(s/s prop). The variation of these parameters with the Mode of loading, method of precracking and the test temperature is described in detail. From optical and electron microscopy studies it is shown that in Mode I the "R-curve" behaviour mainly arises from the degree of fibre-bridging increasing as the interlaminar crack grows, whilst in Mode II it appears to mainly arise from the increasing degree of microcracking and plastic deformation damage which develops around the tip of the advancing crack. The failure loci for the in terlaminar fracture of the PEEK composite have also been established, and various theo retical models to describe these data are considered.

Mode I, Mode II, and Mixed-Mode I/II interlaminar tests on unidirectional carbon-fiber composites have been conducted using beam specimens. Both a thermosetting-based (an epoxy resin) matrix and a thermoplastic-based (poly(ether-ether ketone)) matrix have been employed. The fracture energy, Gc, has been ascertained and the various correction factors that need to be applied if accurate results are to be obtained are described. Where Mixed-Mode I/II loadings may have been present, the measured value of Gc has been partitioned into the separate GI and GII components. The experimental results all suggest that the partitioning of G on a global energy basis, as opposed to using local stress-field solutions, is the most appropriate for the laminates. Thus, the differences in energy absorption are a consequence of opening as opposed to sliding, and the symmetry, or otherwise, of these motions is not important. It is thus apparent that a global energy analysis is entirely satisfactory and appropriate for these types of systems. Finally, it is also shown that when the failure locus is plotted in the form of GI versus GII then it may be theoretically described by a simple interaction parameter model, where the interaction parameter, Ii, has a linear dependence upon the value of GI/G.