Detection of strain during phase transformation of welds by the laser speckle method: Application of laser speckle method for strain measurement in welding (3rd Report)

Abstract

This paper describes application of the laser speckle method for in situ dynamic strain measurements during welding. The principal objective pursued in the present investigation is to detect the strain during the phase transformation of welds by this method.

The specimens used were 4 mm thick 9%Ni steel and SUS304 stainless steel plate. The former undergoes martensitic phase transformation at around T = 325 °C during cooling, whereas the latter does not. The specimen is heated by a gas tungsten arc (TIG arc) moving along its centreline. During the measurements, the laser beam is arranged to irradiate the specimen back surface at the required location at right angles to the plate surface. Then the change in waveform of the light intensity distribution of the speckle pattern formed by interference of the scattered light on a pair of linear image sensors symmetrically arranged about the irradiated spot is continuously recorded. Several types of specimen of different shape were used to compare the effect of restraint on the strain during phase transformation.

The laser speckle method can be used for successful detection of the strain during phase transformation. Differences in the strain curves due to the direction, i.e. along or perpendicular to the heating line, are clearly detected. The amount of expansion during transformation varies considerably depending on the heat input, measurement direction and specimen size. In some cases where expansion in one direction is heavily restrained, expansion in the other direction greatly exceeds the value of a corresponding stress-free specimen. Such anisotropic strains due to phase transformation correspond well qualitatively with elastic-plastic FEM predictions and confirm the validity of the laser speckle method.