Stress intensity factors for high aspect ratio semi-elliptical internal surface cracks in pipes

Surface cracks with aspect ratio a/c>1.0a/c>1.0 can occur in pipes subjected to corrosion attacks. There are few studies on stress intensity factors for semi-elliptical surface cracks with such high aspect ratio. With the failure rate for cast iron pipes unacceptably high there is a need to examine the accuracy of the prediction of stress intensity factors for pipes with high aspect ratio. However, little research has been undertaken on stress intensity factors with high aspect ratio. This paper attempts to develop a method to determine the stress intensity factors for pipes with high aspect ratio semi-elliptical internal surface cracks and subjected to any service load. Two analytical models are presented to determine the stress intensity factors in terms of influence coefficients. A three-dimensional finite element model is employed to derive a solution to analytical models. Based on numerical results it is found in the paper that the maximum stress intensity factor for surface cracks with high aspect ratio occurs at different point to that with low aspect ratio and hence may lead to different modes of failure. The paper concludes that the method presented in the paper can be used to determine the stress intensity factors for pipes with surface cracks of a large range of aspect ratios. This will enable more accurate prediction of the strength of pipes with various longitudinal semi-elliptical surface cracks.

► A three-dimensional finite element model for stress analysis of pipes with high aspect ratio semi-elliptical surface crack has been developed.
►  Stress intensity factors for pipes with a variety of geometry of high aspect ratio surface cracks have been obtained.
► The maximum stress intensity factor occurs at a different position along the crack with high aspect ratio to that with low aspect ratio.
► The type of loading is the most significant single factor that affects the stress intensity factor.