Issue 11

D. Taylor, Frattura ed Integrità Strutturale, 11 (2009) 3-9; DOI: 10.3221/IGF-ESIS.11.01 5 this paper established that the critical stress  o was equal to the ultimate tensile strength of the material as measured in tests on plain (i.e. unnotched) specimens. In other classes of materials this is not always the case but it appears to be true universally for composite laminates, and to my knowledge no one has proposed otherwise. A significant limitation of the paper was that only two types of notches were considered: circular holes located centrally in flat plates, and sharp edge notches in which the notch root radius was very small. Figure 2 : Data and predictions (using the point method) fro m Whitney & Nuismer [3], showing the effect of hole radius on fracture strength (nominal stress, normalised by the plain-specimen strength) in a quasi-isotropic glass/epoxy laminate. The three prediction lines were drawn using slightly different values of the critical distance, d o which is equivalent to L/2. Figure 3 : Data fro m Whitney and Nuismer [3] s howing the effect of notch length (c) on fracture toughness, for sharp notches in a graph-epoxy laminate material. Prediction lines using the point method, drawn at different values of the critical distance d o (equivalent to L/2). Within a decade of the publication of this original paper, a lot of experimental data had been generated to demonstrate the truth of Whitney and Nuismer’s proposal. Awerbuch and Madhukar [9] p resented an enormous study covering over 2800 test results, and Wetherhold and Mahmoud [10] also considered a large set of data, both reports showing that the TCD could be applied successfully to a wide range of materials, mostly polymer-matrix long-fibre materials but also including some metal-matrix composites and some materials with discontinuous fibres. Interestingly, despite the wide range of strengths and toughnesses in these materials, it was found that L fell within a narrow range of values, being almost always between 1 and 5mm, sometimes as high as 15mm. More recent literature has extended the subject in various ways, which I have reviewed elsewhere [7]. However, the overwhelming impression is that most workers have followed closely the lead taken by Whitney and Nuismer. As a result,