The thermal ratchetting of hydrogen in zirconium-niobium — An illustration using finite element modelling

Abstract

It was recognized early in DHC (Delayed Hydride Cracking) studies in zirconium alloys that crack initiation could often be induced by heating the sample above the test temperature and cooling it back down. Additionally in many cantilever beam test studies of DHC initiation, the samples were routinely thermally cycled once a week from test temperature to room temperature, then to 50[degree] above the test temperature and then back to the test temperature as a means of simulating' the start-up shut-down cycles of real pressure tubes in reactors. Crack initiation often occurred during these cycles. In spite of this little has been done to explain why thermal cycling should be effective in aiding hydride formation at the crack tip. In this paper the way in which thermal cycling can cause large increases in the hydride formation at the peak stress locations will be discussed and the mechanisms illustrated with the aid of a finite element model. Since the thermal cycle can affect the hydrogen concentration near a stressed crack in the same way that a ratchet gear affects mechanical rotation it has been called a thermal ratchet.