THE INITIATION OF DELAYED HYDRIDE CRACKING AT A NOMINALLY SMOOTH SURFACE

As part of a wide-ranging theoretical research programme on the initiation of delayed hydride cracking (DHC), this paper is concerned with modelling the initiation of DHC at a nominally smooth surface. The central assumption is that a critical tensile stress, defined in a continuum sense, has to be attained in the immediate vicinity of the surface. It is shown that this stress is attainable provided that a hydrided region is sufficiently long, in relation to its thickness, such that the applied stress overrides the compressive transformation stress generated within the hydrided region, to the extent that the critical local tensile stress is attained. The theoretical analysis highlights the importance of the hydrided region's geometrical parameters, i.e. thickness and length, together with the applied stress level, in enabling the critical local tensile stress level to be attained; however, with a long hydrided region, length has little influence on the attainment of the critical stress. Once the initiation barrier has been overcome, it is shown that a crack will extend almost the whole length of the hydrided region.