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

Fatigue - Essay Example Because of this subtlety, it is important to develop methods of predicting and controlling fatigue during the design process. The progress of fatigue is categorised into three stages (Budynas and Nisbett, 2006). Stage I is the presence of microcracks, which are the tiny regions of plastic deformation from which the failure process begins. During stage II, these cracks steadily grow and increase 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 mechanical design. It is a complicated process, influenced by many factors. A cornerstone of fatigue behaviour prediction is extensive materials testing. Whatever the mathematical framework used to predict fatigue behaviour, it invariably relies on empirical measurements of representative material samples. Because crack formation and propagation are stochastic processes, these tes ts must be repeated a large number of times and a statistical average used as a guideline. This is particularly 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 cyclical stress can be characterized by the following parameters: ?a Stress amplitude ?r Stress range ?m Mid-range stress ?max Maximum stress ?min Minimum stress Additionally, there is the stress ratio R, and the amplitude ratio A defined by: In contrast to static failure, which occurs when a single threshold value is exceeded, fatigue failure can result from a near-endless combination 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 conservative than others, from the So derberg line to the Gerber ellipse. These criteria are somewhat crude approaches that do little to describe the physical 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 very 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 strength, and then gradually reduced in subsequent tests. 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 determining 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