In Situ Studies Of Full Field Strain And Residual Stress Of Welding Heat Affect Zone Using Digital Image Correlation

The residual plastic deformation and residual stress induced by the non-uniform heating and cooling during welding process affect engineering properties of materials and structural components,notably bearing capacity,fatigue life,brittle fracture and stability.Residual stress analysis is critical in the design of parts and structural components and in the estimation of their structure integrity.Residual stress measurement plays an important role in understanding the residual stress distribution and improving the quality of welds.Digital image correlation(DIC)is a mature full-field non-contact strain measurement technology which relies on cameras and computers.It has a simple set-up and requires less space compared to other measuring methods.In this paper,a method based on DIC and computational solid mechanics is proposed to convert high-temperature welding strain to residual stress.Experiments using this new method for investigating the mechanical behaviors of the heat affect zone(HAZ)in arc welding process were conducted.Factors affecting the full-filed residual stress measurement results of the method were analyzed and validated.First,an alternative algorithm base on 2D-DIC,computational solid mechanics and elastic-plastic mechanics was proposed.The algorithm is able to transform high-temperature welding strain which includes the elastic and plastic part into welding stress.The strain-stress model and strain step width which affect the accuracy of the strain-stress conversion algorithm were also discussed.A reliable and transplantable computer program according to the algorithm was written independently.Second,a high-temperature welding strain and temperature field measurement system based on active version was set up.A novel speckle preparation method was developed.The random speckle pattern can survive at temperatures up to the melt point of the base metal.The pulsed laser based auxiliary illumination and near infrared filtering system were able to suppress the intense welding arc and obtained the clear speckle pattern images in the HAZ during arc welding process.A serial of 304 steel plate welding processes were conducted on the strain and temperature measurement system.The full-field strain and temperature field were calculated by the strain-stress conversion program to acquire the welding stress field and the full-field residual stress of the HAZ.Hole-drilling residual stress measurements for verification were also implemented.factors that influence the accuracy of the welding residual stress measurement method based on 2D-DIC were discussed.Effects of the out-of-plane translation and out-of-plane rotation were analyzed and validated by experiments.To avoid the effects of the out-of-plane translation and out-of-plane rotation,a stereo strain measurement system was set up.Software for synchronous acquisition,calibration and three-dimension reconstruction based on dual cameras was developed independently.Experiments for validation were also conducted on the 3D-strain measurement system.