Fatigue Essay Example | Topics and Well Written Essays - 2250 words

Fatigue - Essay ExampleBecause of this subtlety, it is important to suffer methods of predicting and controlling cloy during the design process. The progress of fatigue is categorised into three stages (Budynas and Nisbett, 2006). Stage I is the bearing of microcracks, which are the tiny regions of plastic deformation from which the failure process begins. During stage II, these cracks steadily grow and amplification in length. It is during this stage that cracks can be detected and measured, and are apparent in post-failure analysis as visible ridges How quickly a part reaches failure due to fatigue is an important question for robotlike design. It is a complicated process, influenced by many factors. A cornerstone of fatigue behaviour prediction is bulky materials testing. Whatever the mathematical framework used to predict fatigue behaviour, it invariably relies on empirical measurements of object lesson material experiments. Because crack formation and propagation are stoc hastic processes, these tests must be repeated a large number of times and a statistical average used as a guideline. This is peculiarly true for low cycle fatigue. Characterization of Cyclical Loads In order to discuss fatigue and the types of loading that cause it, some standard terminology needs to first be established. Any rotary stress can be characterized by the avocation parameters ?a assay amplitude ?r Stress range ?m Mid-range stress ?max Maximum stress ?min Minimum stress Additionally, at that place is the stress ratio R, and the amplitude ratio A defined by In contrast to stable failure, which occurs when a single threshold value is exceeded, fatigue failure can result from a near-endless combine of the above components. A number of different criteria have been developed to determine which combinations will lead to failure. The following diagram shows some of these criteria, with points on or above each line indicating failure. Some criteria are clearly more conse rvative than others, from the Soderberg line to the Gerber ellipse. These criteria are somewhat crude approaches that do little to describe the personal phenomenon they are deterministic, whilst the phenomenon itself is stochastic. They were developed early in the study of fatigue, and are primarily useful for quick estimation. The following three sections define more rigorous approaches. Fatigue-Life Methods Stress-Life Method The stress-life method is actually convenient and intuitive, and consists of applying repetitive stresses to a sample, then measuring the number of cycles to failure. A sample of material is loaded into a high-speed rotating-beam machine, which places the sample into a state of pure bending, then rotates it to create fully-reversed cyclical axial stresses within it. These samples are very carefully machined and polished to control surface defects that could initiate a crack. Testing begins at near-yield potence, and then bit by bit reduced in subsequent t ests. The result is known as the S-N curve which shows the expected fatigue strength vs. the number of cycles to expected failure. This method of determine material fatigue properties is straightforward to implement, but lacks accuracy, particularly for determining low-cycle endurance. Fatigue failure is stochastic in nature, and even the most carefully-controlled experiments