Weldments Fracture Properties Study By Digital Image Correlation

Welding is a non-uniform heating and cooling process. It is difficult to avoid complex three-dimensional residual stress and micro-defects in weldments and heat affected zone. While the weldments working in the harsh environment and complex loading conditions under the combined effect of loads, environmental factors, and welding residual stresses, microcracks will propagate and even lead to sudden catastrophic failure. Therefore, it is of significance to study the crack propagation rules under complex residual stress fields.In this paper, fracture properties under the influence of welding residual stress were studied experimentally by digital image correlation technique. Comparative experiments using standard and welding pieces were performed. The weldments which crack tips are located in different residual stress area are obtained by changing the distance between weld center and crack tip. Tensile tests were performed on all specimens after measuring the residual stresses. Speckle images on specimen surface were recorded by digital image correlation measurement system. Displacement field and strain field contours were obtained through image processing using VIC-2D software. Using the displacement and strain data, the fracture parameters such as stress intensity factor and the J integral near the crack tip under small loading conditions were calculated and analyzed. The crack tip opening displacement value and the J integral under large loading conditions were also extracted. Analysis results show that the welding residual stress has significant effects on the fracture behavior of weldments:tensile residual stress will increase the values of fracture parameters, promote the crack opening and expansion; compressive residual stress will reduce the values of fracture parameters, and restrain the crack opening and expansion trend. Under large load conditions, material fracture parameters affected by the welding residual stress was not obvious, and applied loads become the dominant factor in fracture.(1) The paper introduces the background and significance of topic proposed. The domestic and foreign research progress and present situation of welding residual stress influence on fracture toughness are introduced; and the digital image correlation method of domestic and international research status and development trend are also presented.(2) The paper explains the basic theory of this paper. The knowledge of welding residual stress, basic principle and theory of fracture mechanics parameters, and the theory of digital image correlation method are reviewed. (3) The paper describes the experimental method and procedure in this paper, including experiment plan, materials selection and specimen preparing, welding methods and welding parameters, the method and equipment for residual stress measurement, the measuring system for fracture parameters and experimental procedure and so on.(4) The paper describes the experimental results and analysis of the results. The results for residual stress measurement, the image processing, the displacement field and strain field distribution and the fracture parameters are provided and analyzed.(5) The paper summarizes the main research work of this paper, and the results and conclusions are briefly introduced.