The effect of specimen constraints on the shape changes and the nucleation of cracks in cyclically deformed Nb crystals

Abstract

Niobium single crystals, with an orientation conducive to deformation by single glide, have been cyclically deformed at 400 K over a wide range of plastic strain amplitudes and cumulative strains. All the Nb crystals exhibited shape changes which were produced by asymmetric glide. The effects of the end constraints on the axial component of the shape changes are investigated. It is shown that these constraints deform the specimen into an S shape and create regions of tension and compression on the crystal ‘side faces’. Fatigue cracks nucleate at the specimen surface at the regions which are in tension, and these cracks cause the specimen to fail prematurely, whereas fatigue cracks which have initiated at the intrusions and extrusions in the unconstrained middle regions of the gauge length remain relatively insignificant. The implications of this effect on the orientation dependence of the fatigue life of b.c.c crystals is discussed.