Residual stresses in condition monitoring and repair of thermal power generation components

Residual stresses have a significant impact on fatigue and fracture of engineering components and structures, with an effect that is largely dependent on the sign of the residual stress relative to that of the applied stress, i.e. on whether they add to, or subtract from, the applied stress. The present paper will emphasise the importance of detailed knowledge of residual stresses to applications in thermal power generation. The context of the examples is condition monitoring and repair procedures where assessment of the influence of residual stress fields is important to both fatigue and fracture performance, and to certification of the repair procedure itself. The main conclusion in the paper is that the innovative use of solid-state friction taper hydro-pillar processes can offer additional capability in condition monitoring of through-thickness creep damage in thermal power plant, as well as provide cost-effective local repair of creep or fatigue damage in, for example, thick-walled steam pipe and blade-disc attachment holes

Mechanical properties and microstructure of Friction Stir and Laser Beam Welded 3mm Ti6Al4V Alloy

Abstract: In this paper the authors report the study done on welded Ti-6Al-4V alloy sheets by friction stir and laser beam welding. Fusion and solid-state techniques were compared to determine the most performance favourable welding process. Welds were accomplished by varying the process traverse speed. High traverse speeds indicated increased hardness in the weld nugget and wider nugget area for laser beam as compared to FSW. Tensile strength for both processes showed similar performance to that of the parent plate. Welds from laser beam welding exhibited superior fatigue strength under tension-tension loading than friction stir welds at intermediate speeds. Microstructure was discussed to explain the effect of the process parameters on joint integrity in relation to the parent plate. Additionally, residual stresses measured in the weld nugget are explained

Crack initiation and propagation paths in small diameter FSW 6082-T6 aluminium tubes under fatigue loading

This paper reports results of fatigue tests of friction stir welded (FSW) aluminium tubes. Relatively small 38 mm diameter tubes were used and hence an automated FSW process using a retracting tool was designed for this project, as the wall thickness of the aluminium tube was similar to the diameter of the FSW tool. This is a more complex joint geometry to weld than the more usual larger diameter tube reported in the literature. S-N fatigue testing was performed using load ratios of R = 0.1 and R = -1. Crack path analysis was performed using both low magnification stereo microscopy and scanning electron microscopy, in order to identify crack initiation sites and to determine the direction of crack propagation. Work is still in progress to follow the crack path through the various microstructural zones associated with the weld. A simple statistical analysis was used to characterize the most typical crack initiation site. This work forms part of a wider project directed at determining multiaxial fatigue design rules for small diameter 6082-T6 aluminium tubes that could be of use in the ground vehicle industry