A testing laboratory for an engineering consortium has been asked to characterise the fatigue life properties of two materials (steel and aluminium bars of unknown grades) having identical geometry which will be used in the analysis for a failure accident report. Both bars are of square shape of 200 mm and the failed structure is known to operate under the following service conditions; 1. Both bars undertake a service temperature of T = 260 °C (500 °F) 2. The reliability target for both materials when they first designed was 99.9% Additionally, the steel is observed to have been hot-rolled finish and the ultimate tensile properties for both materials are SUT, steel = 650 MPa and SUT, aluminium = 340 MPa, respectively; a) Firstly, for the steel, devise an estimation of the S-N diagram based on the available
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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