In-situ Measurement of Dynamic Strain in Welding by the Laser Speckle Method

Application of the Laser Speckle Method to Strain Measurement in the Welding Process (Report 2)

Abstract

The laser speckle strain measurement is a non-contacting and highly accurate method. It has been developed mainly for the measurement of static strains.
In the previous report, the authors proved the applicability of the laser speckle method to the dynamic strain measurement in welding. However, we indicated in the report that there was a range of distance between the heat source and the measuring point in which the strain could not be measured as the heat source approached the measuring point and passed through the point. It was caused by a rapid change of the speckle patterns and the slow responsibility of the measuring instruments.
In this report, the authors improved the sampling rate of the measuring system up to 8 ms/frame and could follow the change of the speckle patterns through almost all the period during and after welding. The obtained strain curves generally agreed not only with qualitative prediction but also with quantitative testing results obtained by sequential photographs. Only the strain perpendicular to the weld line deviated from them when the heat source was in the vicinity of the measuring point.
High refreshing rate of the reference frame in the strain calculation and the local deformation of the sample surface were considered as possible causes for the discrepancy, whereas the oxidation of the surface seemed have little effects.