Experimental Validation of Residual Stress Simulations in Welded Steel Tubes with Digital Image Correlation (Experimentele validatie van eigenspanningssimulaties in gelaste stalen buizen met digitale beeldcorrelatie)
Experimental Validation of Residual Stress Simulations in Welded Steel Tubes with Digital Image Correlation
As each production process introduces residual stresses in the material, cold-rolled steel tubes are not delivered stress-free. The motivation o f this research can be traced back to the laser cutting of tubes. During the cutting process, residual stresses are relieved, which causes defor mation of the tube and a reduction of the precision of cut. Residual str esses also play a role in stress corrosion and sudden collapse of tubula r structures. This work concentrates on the residual stresses in cold-rolled steel tub es due to the welding process. Insight into the origin, influencing para meters, distribution and magnitude is gained with a finite element (FE) simulation of the welding process. An FE model allows for the visualisat ion of these aspects in the entire tube. A welding simulation requires t he quantification of numerous input parameters: thermal and thermomechan ical material properties (and their evolution at elevated temperatures), thermal and mechanical boundary conditions, and a model for the heat-in put of the welding torch. The simulation is split up into a thermal anal ysis which calculates the temperature field in the tube during the weldi ng and subsequent cooling and a thermomechanical simulation which calcul ates the deformation and stress. Since welding simulations require an appropriate value for numerous para meters, validation of the simulation results is of utmost importance. Fo r this purpose, a laboratory setup which allows measuring the temperatur es and deformations during the welding of the tube is built. The tempera tures are measured with 5 thermocouples. The deformation is measured wit h strain gauges and the digital image correlation (DIC) technique.<b r> Although DIC has some advantages over other techniques measuring deforma tions during welding, to the author's knowledge it is the first time tha t DIC is used in a welding application. First, it is investigated whethe r it is possible to measure the thermal expansion of the types of stainl ess steel used in this work (austenitic SS304 and ferritic SS409) on 3D objects. It is shown that this is possible up to 600°C. From these measu rements the thermal expansion coefficient is determined, and this is use d as input in the thermomechanical simulations. Although the strain that occurs in the welding is rather small compared to common DIC applicatio ns, the strain evolution during the welding and subsequent cooling of th e tube can be recognised easily. These strain evolutions are in agreemen t with the strain evolution measured with the strain gauges. Strain gaug es have in this application the disadvantage that the measured strain mu st be compensated for the actual material temperature, which makes their result dependent on this temperature measurement. In the thermal simulation, the predicted temperature field is brought in agreement with the measured temperature field. For this purpose, the va lue of three poorly known input parameters was optimised: (i) the effici ency of the welding process; (ii) the actual room temperature around the setup; and (iii) the convection film coefficient. This procedure result s in physically acceptable values for these parameters. The difference b etween the measured and the simulated temperature curves at the thermoco uple points is minimised, but they are certainly not yet coincident, whi ch is directly reflected in the simulated strain evolutions. In the thermomechanical simulation, the thermal field is combined with t he thermal dilatation of the material to calculate the stress and strain evolution during the welding process. The tube in the experiments was s ubjected to a stress relief treatment before the experiment was started and also the simulations start from a stress-free tube. The simulated st rain evolutions are compared with the measured ones. The residual stress profile in the middle section of the tube is compared to an X-ray diffr action residual stress measurement. A sensitivity analysis shows that pa rameter variations are visible in both the strain evolution and in the r esidual stress profile. Validating the simulations with the strain evolu tion is preferred to a residual stress measurement as in this way the wh ole process is checked and not just the final state. The methodology developed for a SS304 tube with a diameter of 60mm and w all thickness of 1.5mm is applied to a SS409 tube with a diameter of 60m m and wall thickness of 2mm. Especially the larger heat input and the lo wer thermal expansion coefficient define this case study. The results ar e similar to the SS304 tube, but a better material characterisation of t he SS409 steel is necessary to improve the results. Finally, the effect of an initial residual stress field on the strain ev olution during and the residual stress after the welding is investigated . For that reason the stress/strain state inferred from exploratory roll ing simulations was implemented as initial field in the welding simulati ons. From these simulations it is concluded that the final residual stre ss pattern is mainly determined by the welding process.