CH 6 - CH 6: Fatigue Failure Resulting from Variable...

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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 6 Page 1 of 18 CH 6: Fatigue Failure Resulting from Variable Loading Some machine elements are subjected to statics loads and for such elements , statics failure theories are used to predict failure (yielding or fracture). However, many machine elements are subjected to varying or fluctuating stresses ( due to the movement ) such as shafts, gears, bearings, cams & followers,etc. Fluctuating stresses (repeated over long period of time) will cause a part to fail ( fracture ) at a stess level much smaller than the ultimate strength (or even the yield strength in some casses). Unlike static loading where failure usualy can be detected before it happens (due to the large deflections associated with plastic deformation), fatigue failures are usualy sudden and therefore dangerous. Fatigue failure is somehow similar to brittle fracture where the fracture surfaces are prependicular to the load axis. According to LEFM, fatigue failure develops in three stages : - Stage1 : development of one or more micro cracks (the size of two to five grains) due to cyclic plastic deformation. - Stage2 : the cracks progresses from micro cracks to larger cracks (macro cracks) and keep growing making a smooth plateau-like fracture surfaces with beach marks . - Stage3 : occurs during the final stress cycle where the remaining material cannot support the load, thus resulting in a sudden fracture ( can be brittle or ductile fracture ). Fatigue failure is due to crack formation and propagation. Fatigue cracks usually initiate at location with high stresses such as discontinuities ( hole, notch, scratch, sharp corner, crack, inclusions, etc. ).
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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 6 Page 2 of 18 Fatigue cracks can also initiate at surfaces having rough surface finish or due to the presence of tensile residual stresses . Thus all parts subjected to fatigue loading are heat treated and polished in order to increase the fatigue life. Fatigue Life Methods Fatigue failure is a much more complicated phenomenon than static failure where much complicating factors are involved. Also, testing materials for fatigue properties is more complicated and much more time consuming than static testing. Fatigue life methods are aimed to determine the life ( number of loading cycles ) of an element until failure . There are three major fatigue life methods where each is more accurate for some types of loading or for some materials . The three methods are: the stress-life method, the strain-life method, the linear-elastic fracture mechanics method. The fatigue life is usually classified according to the number of loading cycles into: s Low cycle fatigue ( 1≤N≤1000 ) and for this low number of cycles, designers sometimes ignore fatigue effects and just use static failure analysis. s
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This note was uploaded on 10/25/2010 for the course MECHINCAL 2010 taught by Professor علاءحجازي during the Spring '10 term at Hashemite University.

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CH 6 - CH 6: Fatigue Failure Resulting from Variable...

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