Tài liệu tham khảo giáo trình nhôm trong thiết kế xây dựng bằng tiếng anh - Chương 12 Fatigue GENERAL DESCRIPTION It is well known that seemingly ductile metal components can fail in a brittle manner at a low load, far below their static strength, when this load is applied many times. Aluminium is more prone to this problem than steel. The phenomenon, known as fatigue, results from the presence of localized details or irregularities in zones carrying tensile stress, especially at welds | CHAPTER 12 Fatigue GENERAL DESCRIPTION It is well known that seemingly ductile metal components can fail in a brittle manner at a low load far below their static strength when this load is applied many times. Aluminium is more prone to this problem than steel. The phenomenon known as fatigue results from the presence of localized details or irregularities in zones carrying tensile stress especially at welds. These act as stress-raisers and although they have no effect on static resistance they become critical under repeated load. Elastic analysis predicts a peak stress at such positions that greatly exceeds the basic stress found using conventional stress formulae. The ratio of peak to basic stress the stress-concentration factor can reach a value of 3 or more. The peak stress which is highly localized causes a microscopic crack to form initiate at a relatively low level of basic stress which then grows propagates each time the load is applied. At first the rate of propagation per load cycle is minute but after many cycles it speeds up eventually leading to catastrophic failure. In non-welded construction a fatigue crack may form at a bolt or rivet hole at a sudden change of cross-section or at any other geometric irregularity. Just the very slight surface roughness of the aluminium itself well away from any joint or change of section may be sufficient to cause fatigue. Welded components fare worse. Even when the welding is to the highest standard there are still inevitable stress-raisers at the toe or root of a weld and also in the ripples on the weld surface. These all lead to an inferior performance in fatigue. With lower standards of fabrication the welds are likely to contain additional unintended defects micro-cracks undercut lack of penetration which will reduce the fatigue strength still further. The level of inspection specified to the fabricator can be crucial. The number of cycles N to failure the endurance at a given detail is found to relate .
