Dislocation distribution and prediction of fatigue damage

The dislocation density and distribution induced by tensile deformation in single crystals of silicon, aluminum and gold and by tension-compression cycling in aluminum single crystals and Al 2024-T3 alloys were studied by X-ray double-crystal diffractometry. The measurements of dislocation density were made at various depths from the surface by removing surface layers incrementally. In this way, a propensity for work hardening in the surface layers compared to the bulk material was demonstrated for both tensiledeformed and fatigue-cycled metals. Analysis of the cycled Al 2024 alloy as a function of the fraction of fatigue life showed that the dislocation density in the surface layer increased rapidly early in the fatigue life and maintained virtually a plateau value from 20 to 90 pct of the life. Beyond 90 pct the dislocation density increased rapidly again to a critical value at failure. Evaluation of the dislocation distribution in depth showed that the excess dislocation density in the bulk material increased more gradually during the life. Using deeply penetrating molybdenumK _α radiation, capable of analyzing grains representative of the bulk region, the accrued damage and the onset of fatigue failure could be predicted nondestructively for 2024 Al, cycled with constant stress as well as with variable stress amplitude. The dislocation structure produced in the bulk by prior cycling was unstable when the work-hardened surface layer was removed. It is proposed that the deformation response of the bulk material is controlled by the accumulation of dislocations and associated stresses in the surface layer. Formerly Research Associate, Rutgers College of Engineering.