New aspects of fatigue and fracture mechanics

Abstract Recent Scanning Electron Microscope observations of the development of microstructural changes under reversed cyclic straining in soft, medium-strong and high strength metals have established the existence of two dominant fatigue mechanisms associated with, respectively, the development of glide fissures by reversed plastic slip in soft metals and of slip-less shear cracking in strong metals, as well as of combinations of various complexity of both in medium-strength metals. The stage of micro-crack multiplication and growth preceding the stage of propagation of the final macro-crack and following a very short stage of formation of point-defects, the significance of which has essentially been lost between the attempts to reduce fatigue either to a lattice defect phenomenon in physics or to a single crack-propagation phenomenon in mechanics represents the principal engineering aspect of fatigue except when, as the result of the pre-existence of a dominant local defect or stress-concentration, the fatigue life is completely determined by the propagating crack. Analytical treatment of this stage for consideration in fatigue life prediction and reliability assessment must await the development of suitable methods of statistical fracture mechanics. Some aspects of this approach are discussed.