Investigation Of Deformation Field Near Crack Tip By Digital Image Correlation

This paper proposes an experimental method for measuring deformation field near the crack tip,by integrating three-dimensional digital image correlation(3D-DIC)with several techniques,including crack tip determination,incremental matching,correlation coefficient thresholding and local fitting.Specially studying the initial extension and subcritical extension of the crack under the quasi static loading condition,collecting the complete images of the two processes while digital image correlation analysis was carried out.According to 3D-DIC results,making the analysis of the evolution rule of crack failure before the extension.The experimental results show that the 3D-DIC not only can obtain the in-plane deformation resulted by crack opening,extraction of crack tip opening displacement,but also can measure the out-plane displacement near the crack tip and local sag.Through finite element method simulation,comparing the 2D fracture model and the 3D fracture model.It is found that the 3D fracture model is closer to the plane strain state,while the three dimensional simulation stability is poor but the main fracture mechanics parameters of the simulation are closer to the experimental measurement.The proposed method is applied to study the effect of welding on the fracture behavior of metallic materials.In the experiment,Q235 steel is adopted as the base material and compact tension(CT)specimens with fatigue pre-crack are made.By altering the position of the weld,four different CT specimens are prepared: base material only,the weld in the center,the weld deviating from center by 5mm,and the weld deviating from center by 8mm.The experimental results show that 3D-DIC obtains the in-plane deformation induced by the crack opening,extracts the crack tip opening displacement,and also measures the out-of-plane deformation caused by the yield of the material.Among the four specimens,the one with the crack in the weld undergoes the largest deformation before the crack propagation.The two specimens with the crack in the heat-affected zone also produce larger deformation that the base material,and therefore exhibit higher fracture toughness.Large deformation and local damage also occur at the boundary of the weld near the crack tip,which further complicates the fracture behavior of the material.