Investigation on the Effect of Thermo-Mechanical Tensioning on Weld Induced Out-of-Plane Distortions

Abstract

Fabrication of ship structures involving welding of thin plates, poses a real challenge in limiting the out-of-plane distortion due to buckling. In this study, the effect of thermo-mechanical tensioning (TMT) on minimizing out-of-plane distortion due to buckling in fabrication of stiffened panels has been investigated through experimental and numerical studies. Experiments were conducted using 3-mm-thick plates. The pretensioning of the plates was done along the stiffener welding lines using the shrinkage forces of the tensioning lugs. The lug width was decided based on the possible width of the tensile stress zone due to the welding of stiffener. The tensioning lugs were heated to a temperature so that the plastic deformation of the lugs are minimized thereby required tensioning of the plate could be achieved. Through the implementation of TMT, a noticeable reduction in the out-of-plane distortions were observed. The influence of the sequence of lug removal on the buckling distortions was studied numerically. Fairly good agreement was obtained between experimental and numerical results. The process of TMT was found to be quite effective as an active in-process distortion mitigation technique for fabrication of thin stiffened panels. 1. Introduction Fusion welding is one of the widely used processes of joining. It is extensively used in heavy fabrication industries such as railways, shipbuilding. Fusion welding being a thermal process, it may lead to structural distortions and residual stresses due to nonuniform thermal expansion and contraction of the weld metal and the adjoining the base material. Welding distortions adversely affect the dimensional accuracy of the welded structures and as a consequence act negatively on productivity because of the postweld corrective work that becomes necessary. On the other hand, residual stresses due to fusion welding are the major contributors in crack generation and impairing the fatigue life of the structure. Therefore instead of postweld treatments, in-process active distortion and residual stress mitigation techniques are more beneficial to enhance product quality and productivity. in ship building industry, thin plates are widely used to reduce the weight and thereby cost of fuel (Huang et al. 2004). Welding of these thin plates are more prone to buckling because of their lesser buckling strength. Masubuchi was the first to report about the weld-induced buckling distortions caused by compressive residual stresses in thin plates (Masubuchi 1953). Further Watanabe and Satoh (1958) made an attempt to find out the minimum half wavelength, below which a plate does not buckle, and maximum radius of curvature after buckling for bead welding of thin steel plates. The effect of thermal tensioning on the mitigation of weld-induced buckling distortions was studied by Michaleris and Sun (1997). They employed both finite element analysis and experimental technique in their study. Thermal tensioning was done by simultaneous cooling the bottom of the weld zone by water spraying and heating of the adjacent areas with resistive heating blankets. it was observed that too high a temperature gradient had a detrimental effect on the weld-induced buckling distortions and too low a value of the same resulted in insufficient tensioning. Also it was concluded that the cooling time should be as low as possible for the process to be effective and economic (Michaleris & Sun 1997). Deo and Michaleris (2003) evaluated experimentally the influence of transient thermal tensioning on the reduction of buckling distortions. They also compared the case of Time Temperature Transformation (TTT) at 200 and 250°C with the case of plates welded with only mechanical restraints. It was observed that the buckling distortions cannot be mitigated only with the use of mechanical restraints. The introduction of tensile stresses in the direction of welding through the side heating reduced the compressive residual stresses at the free edges of the plate to a value below the critical buckling stress.