A component is subjected to a variable load history that can be split into two load blocks: (i) a cyclic stress range of 500 MPa for 1 million cycles and (ii) a cyclic stress range of 700 MPa until failure. Estimate the number of cycles to failure in the second load block if the material's fatigue resistance is described by the following fatigue equation: S = 3500N-0.15 where S is cyclic stress amplitude (expressed in MPa) and N is number of cycles to failure at a given stress. ANS: n₂ = 4.536x106 cycles
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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