Growth Monitoring of Internal Surface Crack by Strain Measurement of External Surface (Part I: Development of Multiple Strain Measurement Method)

Abstract

Cracks detected by in-service inspections are not always removed when they are not hazardous according to fitness-for-service assessment. In order to secure the integrity of cracked components, it is important to monitor the growth of the cracks in order to validate the assessment. A monitoring method using a strain gage has been proposed by the present author. In this method, the crack depth is estimated from the change in outside strain due to internal pressure. In this study, this method was extended for estimating not only crack depth but also surface length by using multiple strain gages. From a combination of two strains measured at different position, the crack depth and surface length was estimated using the relationship between the crack size and strains obtained by finite element analyses. By using several gages, the average of the estimated sizes from each combination could improve the accuracy of the estimation. Further improvement could be achieved by introducing weights in the averaging of the estimated crack sizes. A Monte Carlo technique was applied to evaluate the influence of the error in strain measurements. By using this technique, it was shown that the estimation error was less than 4% of the pipe depth even when the standard deviation of the error in strain measurement was 5%.