The figure below shows the S-N curve of three materials subjected to cyclic fatigue about a mean stress of zero. The materials are pushed back and forth with a maximum stress of 350 MPa and a minium of -350 MPa. What value below best estimates how many more cycles the steel sample can last compared to the aluminium sample? Stress amplitude, S (MPa) 500 400 300 200 100 0 10³ O Infinitely more O 2x10² cycles 104 105 1045 steel O 1x10 cycles O None are close to the correct value O 1x10 cycles O 3x105 cycles 2014-T6 aluminum alloy Red brass 106 107 Cycles to failure, N 108 109 11 1 T 80 70 60 50 40 30 20 10 1010 Stress amplitude (10³ psi) 2
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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