20001549402000200660 Essays (1961 words) - Academic Publishing

20001549402000200660 MAterial FAtigue [Type the abstract of the document here. The abstract is typically a short summary of the contents of the document. Type the abstract of the document here. The abstract is typically a short summary of the contents of the document.]6900096000 MAterial FAtigue [Type the abstract of the document here. The abstract is typically a short summary of the contents of the document. Type the abstract of the document here. The abstract is typically a short summary of the contents of the document.]730005673725center Material Engineering And Metallurgy Research2420096000 Material Engineering And Metallurgy Research 19662653829685 2k15/AE/014400000 2k15/AE/01419853153518535 Ashik Mohd. Saifudeen 400000 Ashik Mohd. Saifudeen Material fatigue is a phenomenon where structures fail when subjected to a cyclic load. This type of structural damage occurs even when the experienced stress range is far below the static material strength. Fatigue is the most common source behind failures of mechanical structures. The process until a component finally fails under repeated loading can be divided into three stages: During a large number of cycles, the damage develops on the microscopic level and grows until a macroscopic crack is formed. The macroscopic crack grows for each cycle until it reaches a critical length. The cracked component breaks because it can no longer sustain the peak load. For certain applications, the second stage cannot be observed. A microscopic crack instead grows rapidly, causing sudden failure of the component. The details of the last two stages are usually considered within the topic of fracture mechanics. The term fatigue applies mainly to the first stage. There is, however, some overlap between the disciplines and the measured number of cycles to fatigue often includes the last two stages as well. Because the largest part of the component's life is spent before it is possible to observe a macroscopic crack, most designs aim to avoid ever encountering such damage. Consider a metal wire. When bent downwards, bending stress induced is in the wire cross section. There will be tension at top area and compression at bottom area. When wire is at equilibrium there will not be any stress on wire cross section. When wire is bending upwards there will be compression at top and tension at bottom. The same phenomenon can happen for axle of this motor where it is undergoing fluctuating stress due to gravity effect of this mass. A rail wheel when it is in contact with the track produces a high contact stress, but when the wheel rotates stress gets relieved. When it comes back to original position again contact stress arises. So this also is a case of fluctuating stress case. Again this will lead to fatigue failure if we do not design it carefully. Same is the case with a gear pair. Here the contact stress arising at contact point fluctuates with time. If stress induced at a point with respect to time is traced, it will vary as a fluctuating stress with time. Initially the point will have positive stress, after that zero, then negative stress. The same cycle repeats again and again. Such fluctuating stress is root cause of fatigue failure. When such fluctuating load act on a material it will initiate something called micro crack. This crack will begin to grow with fluctuating load and over time it will cause an abrupt failure u n like failure due to static load. The graph shows s tress variation at a point is plotted on stress vs. time graph. The stress varies between a maximum stress , and a minimum stress , during a load cycle. In the field of fatigue, the variation in stress is often defined using the stress amplitude , and the mean stress . Further, variables defining the stress range , and the R - value are frequently used to describe a stress cycle. The relation between the different fatigue stress variables is : Fatigue analysis is not always based on a stress response. This branch, however, has historically received much attention since the majority of research has been performed in regimes where stress-based models are useful. Based on the number of load cycles needed to produce a crack, it is customary to make a distinction between low-cycle fatigue (LCF) and high-cycle fatigue (HCF). The limit between the two is not distinct, but it