Metallurgical investigations aided by electron microscopy and associated techniques have been carried out on failed test specimens and bearings from service, to obtain information of use in the elucidation of the mechanisms of failure. Investigations have revealed that cracks initiating rolling contact fatigue can start at the surface and spread into the material, or start below the surface and spread outwards, the more dominant mechanism depending upon prevailing circumstances. Metallographic changes in surface material owing to rolling and sliding action, and subsurface microstructural changes at depths associated with the region of maximum Hertzian shearing stress can influence the mechanism of failure. Environment can affect the rate of crack propagation and mode of fracture; hydrogen embrittlement can be a contributory factor in catastrophic fracture, and non-metallic inclusions can have a dominant effect on the incidence of failure. SURFACE CHANGESIt is known that fatigue is a surface phenomenon and can be greatly influenced by the physical nature of the surface and the surface finish. Surface finish, the method of surface finishing, and the stresses induced can greatly influence the Vol I 8 1 rt 3 0

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Fractography, the study of fracture surfaces, is useful in failure investigations as the topography and characteristic markings of such surfaces are indicative of the mechanism of fracture which operated during the initiation of failure and crack propagation. Owing to the low depth of focus of the optical microscope, interpretation of some fracture surfaces may be difficult. The microscopic topography, and its relation to the causes and basic mechanisms of fracture, may be conveniently studied by electron microfractography using non-destructive replica methods. Replicas may be taken from selected areas of the fracture surface of large, unwieldy engineering components. Complementary electron optical techniques such as electron diffraction, scanning electron microscopy, and extraction replicas are used where possible to obtain additional fine-scale information of use in the elucidation of failure mechanisms. An explanation of the various techniques and examples of their use in the work of the National Engineering Laboratory in failure investigations is given. The investigations involve fatigue, brittle fracture, corrosion fatigue, stress corrosion, welding problems, and surface phenomena.