Application of the SEM to Study of Mechanisms of Metal Fatigue.
GEORGE WASHINGTON UNIV WASHINGTON D C INST FOR THE STUDY OF FATIGUE FRACTURE AND STRUCTURAL RELIABILITY
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The scanning electron microscopy SEM, already much used in fractography of crack surfaces exposed after full fracture by fatigue, seems in addition eminently suited for studying the microstructural changes preceding fracture. Such changes are illustrated here by work on titanium fatigued by alternating torsion. The observations show, for example, that the extrusion of material from slip zones where deformation concentrates changes from a ribbon mode to a point mode as the strain amplitude decreases from the elastic to the plastic range. They show, too, that, contrary to some current theories, the next stage of fatigue, the formation and growth of fine shear cracks, is not necessarily an extension of the preceding slip-zone activities more often it occurs independently. Finally, they show that the final stage, formation and growth of a tensile crack responsible for final failure, can hardly be treated, as often it is, as a crack in a quasi-continuous medium it behaves rather as one growing in a medium already suffused by fine cracks. Author
- Properties of Metals and Alloys