Abstract
The stop-hole method is a simple and economic repair technique widely used to retard or even to stop the propagation of a fatigue crack in structural components that cannot be replaced immediately after the detection of the crack. Its principle is to drill a circular hole at or close to the crack tip to transform the crack into a notch, reducing in this way its stress concentration effect. In the present study, the stop-hole method was investigated with creating a non circular hole. The aim of the present work is to obtain an optimum stop hole shape that gives maximum fatigue crack initiation life. Fatigue crack initiation life depends at least on the crack size and on the hole diameter. An optimization technique had been used and a finite element program had been built to find this optimum shape of stop hole. It was found that the fatigue life obtained by using the optimum hole shape ranges from 2 to 9 times the fatigue life obtained by using the circular holes depending on the initial hole size and initial crack length. The effect of this optimum hole shape on crack reinitiating life for different specimen geometries has been studied. It was found that the optimum hole shape increased the initial fatigue life for all specimens used whatever its geometry was. A global optimum hole shape (which is practically suitable for all geometries ) is defined. Global optimum hole shape helps engineers to use this global hole shape directly in practice without carrying out any calculation much like the use of the stop hole size found in the literature. Opposite to the traditional stress concentration factor minimization problem where the nominal area remains constant during optimization, here it is allowed to vary nominal area using design variables resulting in decreasing of the nominal stress in addition to decreasing of the stress concentration factor. This leads to higher fatigue life compared to previous studies.
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