Investigation and validation of vibratory methods for stress relieving and weld conditioning
Manufacturing processes inevitably induce a state of residual stress into materials and products. These residual stresses pose a large potential problem, in terms of dimensional stability and reduced fatigue life. Ideally, residual stresses should be reducible to low levels. There are three methods in general usage for the relaxation of these stresses, - Annealing, Shakedown and Vibratory Stress Relief (VSR). A previous study had suggested that vibration during and/or after welding may usefully modify residual stresses due to welding. This has been termed Vibratory Weld Conditioning (VWC). A comparative study of the methods is presented in section 1.4. The use of VSR, though widespread, has been adopted on a case-by-case basis, due to the lack of understanding of the processes at work. The purpose of this work was to investigate and validate the VSR/VWC method as a proposed alternative to the processes mentioned above. In order to do this a series of tests were devised in which the specimens were welded as a mechanism of stress induction. The residual stresses were measured before and after welding and vibration by means of a scanning X-ray diffractometer. In Chapter 1, the study of Residual Stress (source, formation etc), Welding Metallurgy and a comparison of VSR/VWC with other treatment methods are presented. In Chapter 2, a detailed review of literature is presented, where the accessible literature on VSR/VWC to date are included. In Chapter 3, the theoretical background of Modal Analysis, the Measurement of Dynamic Induced Stress and Measurement of Residual Stressesis discussed. In Chapter 4, the FE analysis of different structures is presented. In the FE analysis, different properties of the structures were determined using the FE model to aid the VSR/VWC study. The experimental investigations are presented in Chapter 5, which is divided into the following parts: Modal Analysis (experimental), Calibration of the X-ray measurements, VSR/VWC treatments, Cryogenic treatment, Fatigue Test and Metallurgical Investigation of VSR/VWC treated specimens. At the outset of the experimental work, the calibration of the X-ray diffractometer was carried out. After calibration of the X-ray and the X-ray Elastic Constant the error band of the diffractometer was significantly reduced. The practical modal analysis of the "8" frame was carried out to determine the modal characteristics of the frame to aid the VSR investigation of the frame. The VSR/VWC treatments are divided into "during welding" and "post weld" treatments and are presented in 10 different experiments. First, the "during welding" treatments were carried out. Investigation was started with application of tensile and compressive static stress to the specimens during welding and cooling. It was observed that the tensile induced stress decreased, and compressive induced stress increased the residual stresses. Rigid body motion (RBM) vibration showed no effect on the residual stresses. The cantilever beam test of the flexural vibration test showed some important characteristics, where the longitudinal residual stresses were found to decrease with application of small-induced stress. An increase in applied stress or time of vibration did not cause any more reduction. The transverse residual stresses increased with application of small-induced stresses. With increase in the applied stress the residual stresses decreased. High frequency vibration in both RBM and flexural vibration was found to be ineffective in reducing the residual stresses. The flexural vibration of the Four-Roller Supported beam showed a very confusing result, where no particular trend of the residual stresses was found. The combined mode of vibration (longitudinal and flexural) treatment showed no effect on residual stresses. The "post weld" treatment of the specimens showed a significant reduction in the residual stresses, where the reduction in the residual stresses were found to be a function of applied stress, while the vibration time effect was found to be negligible. A complex shape of reduction in the residual stresses were found along the width of the specimens, which made it impossible to develop any plastic flow model for the reduction in the residual stresses. Torsional test revealed a very important property of the residual stresses, where the residual stresses were found to decrease by a significant amount with application of very small induced stress. High induced stress only redistributed the residual stresses. Cryogenic treatment caused no reduction in the residual stresses. Contrarily the same specimen showed a significant reduction after VSR treatment. The fatigue test showed an increased fatigue life of the VSR treated specimens, while the thermally treated specimens showed a decreased fatigue life. The vibrated specimens showed highly oriented ferrite crystals in directions with Miller  to the stress axis. The hardness of the VSR treated specimens was found to increase significantly in comparison to the unvibrated specimens.