A solution is presented for the elastic stress intensity factors at the tip of a slightly curved or kinked two-dimensional crack. The solution is accurate to first order in the deviation of the crack surface from a straight line and is carried out by perturbation procedures analogous to those of Banichuk [1] and Goldstein and Salganik [2,3]. Comparison with exact solutions for circular arc cracks and straight cracks with kinks indicates that the first order solution is numerically accurate for considerable deviations from straightness. The solution is applied to fromulate an equation for the path of crack growth, on the assumption that the path is characterized by pure Mode I conditions (i.e., KH = 0) at the advancing tip. This method confirms the dependence of the stability, under Mode I loading, of a straight crack path on the sign of the non-singular stress term, representing tensile stress T acting parallel to the crack, in the Irwin-Williams expansion of the crack tip field. The straight path is shown to be stable under Mode I loading for T < 0 and unstable for T > 0.

The essential work of fracture concept has been extended to cover ductile tearing of polymeric materials that neck before fracture. It is shown that the plane stress specific essential fracture work (w e) can be obtained from deeply edge-notched tension specimens, containing either single or double notches, by extrapolating the straight line relationship between the total specific fracture work (Wy) and ligament length (1) to zero ligament. In this way, specific essential fracture works have been obtained for nylon 66 and two polyethylenes. It seems that w e is a material property for a given sheet thickness being independent of specimen geometry. The straight line relationship between w / a n d / breaks down when the ligament length to sheet thickness ratio is less than about three, since the fracture data fall in the plane stress-plane strain transition region. However, a plane strain specific essential fracture work can still be obtained by extrapolating the least squares curve of the data to zero ligament provided the thickness satisfies plane strain condition. If this condition is not satisfied a near plane strain value is obtained which is dependent upon thickness. This method is also appropriate for ductile polymers like the rubber modified polystyrenes that craze rather than neck.JR curves have also been obtained for nylon 66 and the polyethylenes. Under strictly J-controlled crack growth conditions, it is shown that the intercept and slope of the JR curve, i.e. Jc and dJ/da, are related to the intercept and slope of the w/versus l plot. The limited amounts of J-controlled data available have precluded a more definite and general conclusion to be made. But based on what little is available, the comparisons of these two quantities in the J R -Aa and w / -1 plots are. not unreasonable.

An understanding of the mechanics involved in flake formation provides an opportunity for deriving more behavioral information from flake and flake scar morphology. The mechanics of flake formation are directly relevant to the identification of prehistoric flaking techniques and stone tool use. In this paper we provide a model of flake formation that accounts for much of the variation in flake morphology. Flakes can form in a number of ways and despite popular belief they are not all of the conchoidal variety. The bending flake is common in use wear though it is often misidentified as a conchoidal flake. A third major type of flake, the compression flake, is a common product of bipolar impact. To account for the wide variation in flake morphology we follow a tripartite scheme of flake formation comprising initiation, propagation, and termination phases, within which different mechanisms can operate.

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