Monitoring of inside surface crack growth by strain measurements of the outside surface: Application of multiple strain measurements technique to fatigue crack growth

Cracks detected by in-service inspections are not always removed when they are judged to be not hazardous. In order to secure the integrity of the cracked components, it is important to monitor the crack growth. The author and a co-worker proposed a crack growth monitoring method, in which the elastic strain caused by internal pressure is measured continuously. The elastic strain at the outside surface of a pressurized pipe is changed due to growth of a crack existing in the inside surface and the magnitude of its change depends on the growth size. By using multiple strain gages, it was shown to be possible to estimate not only the crack depth but also the surface length. In this study, the monitoring method using multiple strain gages was applied to fatigue crack growth. An axial crack was introduced at the butt welding portion inside a carbon steel pipe. Then the crack was grown by applying a cyclic internal pressure and the change in crack size was estimated from change in strains measured by strain gages attached onto the outside surface of the pipe. It was revealed that the monitoring procedure could successfully identify the changes in crack depth, surface length and inhomogeneous growth in the surface direction. The maximum estimation errors were 1.2 mm and 2.0 mm for the depth and surface length, respectively. The accuracy of the estimation was better for the larger crack size and it was improved by introducing weights in the estimation procedure. Since the welding residual stress affected the strain measurements, it is important to exclude the influence of residual stress for accurate monitoring.