فرسودگی فولادهای با استحکام بالا
Abstract
1-Introduction
2-Weld quality
3-Measurement systems for quality assurance
4-Improved fatigue strength using alternative methods
5-Fatigue design for improved weld quality
6-Fatigue and quality aspects of cut edges
Conclusions
References
Abstract
In this study, several aspects regarding effect of quality on the fatigue strength in welded cut HSS have been investigated and are discussed. A novel numerical algorithm has been developed which assesses the welded surface and calculates and quantifies weld quality parameters and the presence of defects which are critical in fatigue applications. The algorithm is designed for implementation in serial production. It will provide robust and reliable feedback on the quality being produced, which is essential if high strength steels are utilized and high quality welds are necessary for the structural integrity of the welded component. Two welding procedures which can increase the weld quality in as welded conditions have been assessed. It was found that by using these methods, the fatigue strength can be increased with 20% compared to normal weld quality. Furthermore, two fatigue assessment methods ability to account for increased weld quality in low cycle and high cycle fatigue applications has been studied. One of these methods showed sufficient accuracy in predicting the fatigue strength with small scatter and also account for increased weld quality. The influence of surface quality on cut edges was studied and the fatigue strength was estimated using international standards and a fatigue strength model for cut edges. It was found that the fatigue strength in testing was 15-70% higher compared to the estimation, thus proving a weak link between the international standard and fatigue strength.
Introduction
Design and manufacturing of welded structures is an important task which requires accuracy, especially for robotic welding in serial production. For lightweight welded structures however, where thinner and high strength steels are utilized, the increased nominal stress levels require consideration of other design aspects such as buckling, plastic collapse and fatigue strength. High strength steels suffer from an increased sensitivity to notches and defects compared to mild steels. For welded components, the fatigue strength will be the same for high strength steel and mild steel if no improved weld quality is achieved [1]. Thus, improving the design of the welded structure by using high strength steel requires improved weld quality, which in turn demand an improved quality assurance. Today, most of the quality assurance for welded components is carried out by the audit process, separate to the production line, using standard gauges. Hammersberg and Olsson [2] concluded that basic standard gauges and methods for weld quality assurance are out-dated if care is not taken to investigate and improve the used measurement systems relative to the actual variations occurring in production. Thus, to fully achieve lightweight design in welded structures, manufacturing companies which utilize serial production will face challenges in quality assurance when introducing high strength steel in their products.