The effect of vibratory stress on the welding microstructure and residual stress distribution

Previous studies have suggested that weld microstructure may be modified by the presence of static stresses. In this investigation, vibratory stress was applied to mild steel specimens while they were being welded to observe its effect on the residual stress, microstructure and hardness of the material. Residual stresses were found to decrease in response to vibration whether it was applied during welding or after welding. It was found that the applied stress influenced the grain growth process in the weld. As a result of the treatment the hardness of the material was found to be increased by 25 per cent

The influence of vibratory treatment on the fatigue life of welds : a comparison with thermal stress relief

A comparison has been made between the fatigue lives of welded specimens a) in the as welded condition b) after heat treatment, and c) after post-weld vibration. By comparison with the aswelded specimens, the fatigue lives of the thermally relieved specimens were found to decrease by 43%, while the vibration treated specimens showed an increase of between 17% and 30%. While these findings are interesting in that they offer a way of extending the fatigue lives of welded joints, they confirm the view that the mechanism of residual stress relieving in the vibratory stress relief (VSR) method, and its relationship with fatigue life is poorly understood

Divided communities and contested landscapes: Mobility, development and shifting identities in migrant destination sites in Papua New Guinea

Internal conflicts at the local and national levels in several South Pacific countries have revealed the fragility of national unity and the difficulties nations face in governing and managing their own economic development. In Papua New Guinea, the focus of this paper, an uncertain economic future for many rural and urban communities, and rising inequalities in income opportunities and access to resources, have coincided with greater intolerance of migrants at sites of high in-migration by customary landowners and provincial and local authorities. This paper draws on fieldwork undertaken in the major oil palm growing regions of Papua New Guinea where migrants from densely populated regions of the country have settled on state land alienated from customary landowners. We examine how struggles over land, resource control and development are polarising migrant and landowner identities resulting in increasing tensions and episodic communal violence. A settler identity is emerging based on a narrative of nation building and national development, while an ethno-regional identity amongst customary landowners is undermining the citizen rights of migrants and challenging the role and authority of the state in land matters

A method for determining X-ray elastic constants for the measurement of residual stress

The X-ray diffraction method is arguably the most convenient method of measuring residual stresses in terms of cost, spatial resolution, measurement time and the accuracy of measurement. The normal methods for calibrating X-ray diffractometers are not conveniently applied to automated scanning systems, however, and so a new approach is required. In this study, a scanning X-ray diffractometer was calibrated and the X-ray elastic constant for a steel alloy was determined using a customised four-point bending rig. The bending rig, in turn, was calibrated by dead loading. This study also described a simple alternative method for determining the X-ray elastic constant, without the use of specialised software. After calibration, the error band of the diffractometer was found to be less than ±10 MPa. As this is ±5% of the yield stress for a typical steel, this level of accuracy was deemed to be acceptable for the measurement of residual stress

Use of static stress for modification of welding residual stress

Previous studies have suggested that the application of stress during weld cooling could change the microstructure and residual stress state of a weld. Flat bar specimens were welded in a prestressed condition of tensile or compressive stress. The longitudinal residual stresses were found to decrease with application of tensile pre-induced stresses and to increase with application of compressive pre-induced stresses. Away from the weld toe the transverse residual stresses were found to decrease and near the weld toe the residual stresses were found to increase with application of tensile pre-induced stresses. With application of compressive pre-induced stresses the transverse residual stresses were found to increase. The present study suggests that the prestressing method should not be used as a residual stress relieving mechanism

Modal analysis of a lightweight structure - investigation of the effects of the supports on the structural dynamics

This study grew out of the use of vibration to modify welding stresses, and the corresponding need to understand the modal vibration properties of the test structures. Supporting the structure is a practical problem in experimental modal analysis. In this study the effect of the supports on the dynamic properties of a lightweight structure were investigated. Thick rubber bungee supports and thin elastic band supports were used in the investigation. The effect of the position of the supports was found to be insignificant if a proper type of support is used. It was observed that with the thick bungees the modes become invisible or obscure on the frequency response function (FRF) curves. On the other hand, the thin elastic band supports provide very clear FRF curves and hence clear mode shapes. It has been suggested that the bungees should be chosen considering the weight of the structure and the requirement of the vibration amplitude in the modal analysis and/or in the subsequent treatments. The position of the bungees should be chosen at the node points of a particular mode of the structure to reduce the support effects further

Modification of residual stress by post-weld vibration

Previous studies have suggested that welding residual stresses can be reduced by the application of controlled vibration. In the present study welded specimens were processed after being cooled to room temperature, with varying amplitude of applied stress and time of vibration. An increase in the applied stress led to a significant decrease in the residual stresses. The effect of time of vibration was found to be very small for a lower range of applied stresses (230 MPa), the residual stresses were found to redistribute with increasing time of vibration. It is shown that the energy concept of the vibratory stress relief mechanism was not validated

Modification of welding stresses by flexural vibration during welding

Flexural vibration was applied to specimens during the welding process to observe its effect on welding residual stresses. The study was carried out in three phases, namely, (i) investigation of the effect of amplitude of vibration, (ii) investigation of the effect of time of vibration, and (iii) investigation of the effect of high frequency vibration. The results of the present study provide a basis for relieving the residual stresses in practice. It has been shown that there is an optimum applied stress that will maximise the reduction in both longitudinal and transverse residual stresses. The effect of time of vibration on residual stresses was found to be negligible. After high frequency vibration, the change in longitudinal and transverse residual stresses showed no consistent trend