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This paper presents design data de®ning optimal freeform notch shapes for use in tensile loading conditions for a range of geometries, notably the notch aspect ratio and plate widths. Here, an iterative two-dimensional ®nite element gradientless shape optimization procedure was used to obtain the optimal shapes, which o er the lowest possible stress concentration, subject to the geometric constraints. Precise non-dimensional coordinates are given, for a range of notch aspect ratios and plate widths, and are presented in a tabular form that allows them to be easily used by designers. All cases have been solved with a minimum radius of curvature constraint, which results in a`robust' optimal shape that has no sharp corners. The optimal shapes were found to provide up to a 26 per cent reduction in local peak stress over circular shapes of the same aspect ratio. Optimal geometries are also given for limiting notches that provide a net section stress equal to unity, at a ®nite notch aspect ratio. The results obtained are applicable to both the initial design of notched components and the shape reworking of existing components to remove damage.

Abstract. The fatigue or durability life of a few critical structural metallic components often sets the safe and/or economic useful life of a military airframe. In the case of aluminium airframe components, growth rates, at or soon after fatigue crack nucleation are being driven by near threshold local cyclic stress intensities and thus are very low. Standard crack growth rate data is usually generated from large cracks, and therefore do not represent the growth of small cracks (typically <1mm). Discussed here is an innovative test and analysis technique to measure the growth rates of small cracks growing as the result of stress intensities just above the cyclic growth threshold. Using post-test quantitative fractographic examination of fatigue crack surfaces from a series of 7XXX test coupons, crack growth rates and observations of related growth phenomenon in the threshold region have been made. To better predict small crack growth rates under a range of aircraft loading spectra a method by which standard material data models could be adapted is proposed. Early results suggest that for small cracks this method could be useful in informing engineers on the relative severity of various spectra and leading to more accurate predictions of small crack growth rates which can dominate the fatigue life of airframe components..

The corrections incorporated in fatigue crack growth prediction programs for crack closure are usually tested by their ability to predict retardation following an overload and for the accuracy of their prediction lives for long cracks greater than about 1mm. They should, however, be examined on their ability to predict the life of cracks that grow from small sizes, such as small inherent material discontinuities, to failure, which is more typical of service situations and the growth produced by small cycles as well as the larger cycles. To examine the extent of crack closure in aluminium alloy 7050-T7451 and the prediction of that growth, quantitative fractography measurements of short periods of fatigue crack growth produced with a specially engineered spectrum were conducted and are reported here. The spectrum contained bands of constant amplitude loads with diminishing mean stress designed to examine the extent of closure. The quantitative fractography results are compared to predictions by the common analytical programs FASTRAN and AFGROW and further with a basic effective stress intensity calculation method at a crack depth of about 1mm. The results showed that the analytical programs were able to predict the presence of closure; however, the extent of the closure was not accurately predicted.