Problem 2: A catastrophic failure occurred to a 7079-T6 aluminum alloy shaft. The maximum tensile stress was specified as 30% of the yield stress (oy = 300 MPa) and the stress ratio R was zero. Post-failure examination found that fatigue crack growth striation started at a depth of 2.0 cm from the surface, and the striation spacing was 1.3 x 10-4 cm. The fatigue-crack growth behavior of the aluminum alloy is given in the right figure. The manufacturer claimed that the part was overloaded by the user, but the user denied the allegation and sued the manufacturer. As a mechanical engineer, what is your verdict based on your calculation?
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
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