PRCI Report 195

L51654e

University of Newcastle Upon Tyne

Need: There was a need to define the spatial density of stress-corrosion crack arrays that develop in operating gas-transmission pipelines and in laboratory test specimens of line-pipe steel, to improve understanding of the factors that control the density and provide data to test models of pipeline cracking. Within the broad definition of crack density are included the locations, numbers, lengths, depths, and degree of linkage of cracks.

Benefit: An analysis has been conducted of location, numbers, lengths, depths, and degree of linkage of stress-corrosion crack colonies in samples from the field and from laboratory tests. This has provided data to test any model of the cracking of pipelines. Thus, it is shown that crack densities are of the order of 5 cracks/mm of distance normal to the crack length direction and that the depths and lengths of cracks are distributed according to log normal and Weibull functions. During the growth stage of cracks in a colony, their spatial distribution is not random, because of the interaction of their associated stress fields. That interaction also results in the coalescence of cracks, an extremely important part of the overall growth process, and that interaction is defined quantitatively to provide further information to which modeling must aspire.

Result: Most of the data available relating to crack nucleation is from laboratory tests, which have established in a number of programs that cracks continue to nucleate, but at a diminishing rate with increase in test time. It is now reasonably well established in laboratory tests that stress-corrosion cracks do not grow continuously. There are considerable periods of time, especially as the maximum stress is reduced, during which even relatively long cracks do not grow. In laboratory tests, where the environmental requirements for cracking are maintained, the most probable explanation of cracks ceasing to grow is that the strain rate in the crack tip region falls below that required for growth.

Product Details

Published:

04/01/1992

Number of Pages:

72

File Size:

1 file , 1.3 MB