Crack Growth Monitoring by Strain Measurements

Abstract

Cracks detected during in-service inspections are not always removed when they are judged as hazardous. It is important to monitor the crack growth in order to secure the integrity of the cracked components. The author and a co-worker proposed a crack growth monitoring method, in which the elastic strain caused by internal pressure is continuously measured. The elastic strain acting at the outside surface of a pressurized pipe changes due to growth of a crack in the inside surface, and the magnitude of its change depends on the growth size. In this study, the author uses multiple strain gages to monitor the elastic strain acting on the cracked part of a pipe. An axial crack was introduced at the butt welding portion inside a carbon steel pipe. The strains were then measured under static internal pressure. The crack size was estimated based on the change in strains measured by strain gages attached onto the outside surface of the pipe. This study reveals that such a monitoring procedure could successfully identify not only the crack depth but also the surface length. The maximum estimation errors were 2.2 mm and 0.97 mm for the surface length and depth, respectively. The accuracy of the estimation improved as the number of strain gages increased. It was also apparent that the residual stress had subtle effect on the size estimation, albeit it may have significant influence when the crack propagates.