High temperature full-field strain measurement based on digital image correlation during arc welding

Experiments for measuring high temperature full-field strain were conducted during GTAW welding process using digital image correlation (DIC) technology. A special visual sensor system was equipped with image acquisition module. In order to reduce the influence of intense arc interference, a special laser-based illumination and filtering system was utilized. The in situ total strain close to the fusion line was measured during arc welding process. According to the total strain measured by DIC method and thermal strain calculated by the coefficient of thermal expansion, the mechanical strain was obtained, illustrating the sum of elastic and plastic strain distribution and deformation in the heated affected zone of the joint. Results showed that there remained compressive strain in both horizontal and vertical direction near the welding joint after welding. It demonstrated that full-field strain measurement could be achieved by DIC method during in situ arc welding.

I. Introduction

In the welding process, the metal experiences non-uniform rapid heating cooling, resulting in stresses and deformation [1]. At the end of welding, residualstress would remain in the inner part of the welding structure. Therefore, understanding and controlling residual stresses is crucial to design welding procedure and improve welding joint quality. Over the past decades, there has been significant research in accurately measuring the residual stress values to reduce the influence on service performance of components. There are a large number ofresidual stress measurement techniques at present, which can be generally classified into stress release and diffraction methods [2]. Stress release methods are destructive since changes in strain are measured while nearby parts of components are removed [3], including hole drilling, deep hole drilling and contour method, etc. In recent years, researchers in Europe, the United States, Japan and China have carried out some in-situ measurement by using neutron and high-energy synchrotron diffraction [4]–[7]. The internal stress can be calculated by Hooke's Law, according to the elastic strain based on Bragg's Law [8]. Apart from residual stress, many factors such as structure and chemical inhomogeneity may affect the measurement precision. In addition, it is generally recognized that near-surface residual stress cannot be accurately measured by neutron diffraction method due to the “surface effect” [9]. The complicated equipment requiring neutron sources makes it impossible to realize in situ measurement, which limits the practical application. As photodynamic detection technologies continue to develop, the laser technology such as moire interferometry [10] and laser speckle interferometry [11], and the image collecting and processing technology such as DIC method, have been applied for strain measurement. The laser method generally involves complicated measuring system, which could only detect single-point strain, while DIC provides full-field strain.