Effects of temperature and deformantion rate on cyclic strength and fracture of Alpha-Iron

Cyclic dcformnrlo nb chuviour and f r ac t u r c mcchnn Lsms o IF r r ov,ic ”I’:” subject vd LI) l ow I’,VC 11’ I”:Il:i ”,11’ <ll v.rr Lous L ’IlIP rr a t.u r csn nd dcf o rum t iOI1 r a t.c-n wv ro ::LIIII l cd , 1’:XpJ:Jl1;ILioll::; arc o I I or ’u fur t h ecrystal g ra Ln s I z e Oil c yc l I c s t r cs s l”l’::ponse, d Ls Lo cu t Iou sub s t ruc t ur ,St.c ady=s t o t o cyclic dc fo rm.u l o n r e apous c \.,08 ac h Lcvcd by acritical amount of prior strain. An equation expressing the amount ofprior monotonic strain required for subsequent stable cyclic materialnesponse in terms of the cyclic strain amplitude illustrates theequivalence of the monotonic and cyclic hardening processes.Supportingevidence \Vas obtained from substructural investigations.The influence of cyclic deformation rate on material behaviouris discussed in terms of dislocation dynamics theory.It is shown thatthe internal stress component in cyclic deformation is higher than thevalues reported for monotonic defonmation at identical stresses, but thedislocation velocity exponent is independent of the deformation mode. Onincreasing the cyclic deformation rate, transient yielding behaviour isobserved, followed by a reduction in stress to a saturation level identicalto that observed when continuously cycling at the new rate. This behaviouris associated with an adjustment in cell size.The stress sensitivily to deformation rate increases sharplyu 8er conditions which penuit changes in both internal and effectivestress components.At 370oC, c ha ng e s in the internal stress componentare due to an increase in the rate of dislocation multiplication under