modulus, E 200 700 yield strength, Øy (MPa) 900 TH-SAI-2.55n titanium alloy 600 4340 steel tensile strength, TS (MPa) 1300 500 elongation (%) 20 * 400 fracture toughness, Kie ( MPaſm) 55 1045 steel 300 Duetile cant iron 200 70Cu-30zn brass 2014 T6 Al aley 100 EQ2IA-T6 Mg aloy 10 10 10 10 10 10 Cycles to failure, N Maimum stress, S OMPa
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.
A tie rod made of quenched and tempered 4340 steel is used as a critical linkage in an industrial apparatus. The rod is subjected to an alternating cyclic tensile/compressive stress from 550 MPa to -550 MPa. An inspection of the rod revealed a 3.5 mm deep edge crack on the surface of the rod. Please answer the following question for the rod given the properties in the table and curve below and fracture toughness of 55 MPa√m. (NOTE: Y=1.12 for edge crack and use “a” not “2a” for the length of the crack)
1. Given the S-N curve for 4340 steel, is there an endurance limit? If so, what is the stress value for the endurance limit?
2. If there were no cracks and the stress on the rod still alternated from 550 MPa to -550 MPa with mean stress of zero, what would be the cycles to failure?
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