lec21_notes - MASSACHUSETTS INSTITUTE OF TECHNOLOGY...

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Unformatted text preview: MASSACHUSETTS INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING CAMBRIDGE, MASSACHUSETTS 02139 2.002 MECHANICS and MATERIALS II SPRING 2004 SUPPLEMENTARY NOTES c L. Anand and D. M. Parks DEFECT-FREE FATIGUE 1 1. INTRODUCTION Fatigue Failure is the failure of components under the action of repeated uctu- ating stresses or strains. The word fatigue was introduced in the 1840s and 1850s in connection with such failures which occurred in the then rapidly developing railway industry. It was found that railroad axles failed regularly at shoulders, and that these failures appeared to be quite different from failures associated with monotonic testing. Fatigue failure may be defined as a process in which there is progressive, localized, permanent microstructural change occurring in a structure when it is subjected to bound- ary conditions which produce uctuating stresses and strains at some material point or points. These microstructural changes may culminate in the formation of cracks and their subsequent growth to a size which causes final fracture after a sucient number of stress or strain uctuations. The adjective progressive implies that the fatigue process occurs over a period of time or usage. The occurrence of a fatigue failure is often very sudden, with no external warning; however, the mechanisms involved may have been operating since the beginning of the time when the component or structure was put to use. The adjective localized implies that the fatigue process operates preferentially at specific local areas, rather than homogeneously throughout the body. These vulnerable areas can have high local strains and stresses due to stress and strain concentrations caused abrupt changes in geometry and/or material imperfections. The phrase permanent microstructural changes emphasizes the central role of cyclic plastic deformations in causing irreversible changes in the substructure. Countless investigations have established that fatigue results from cyclic plastic deformation in every instance , even though the structure as a whole is practically elastic. A small plastic strain excursion applied only once does not cause any substantial changes in the substructure of materials, but multiple repetitions of very small plastic strains lead to cumulative damage ending in fatigue failure. We note that although fatigue is popularly associated with metallic materials, it can occur in all engineering materials capable of undergoing plastic deformation. This includes polymers, and composite materials with plastically deformable phases. Plastically non-deformable materials such as glasses and ceramics, in which deformations at ambient temperatures are truly elastic everywhere, do not fail by fatigue due to repeated stresses. However, recent data has shown that polycrystalline ceramics can exhibit fatigue crack growth under certain circumstances....
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This note was uploaded on 02/23/2012 for the course MECHANICAL 2.002 taught by Professor Davidparks during the Spring '04 term at MIT.

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lec21_notes - MASSACHUSETTS INSTITUTE OF TECHNOLOGY...

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