# Effect of local stress on the heat-checking morphology in high temperature tool steels under thermal fatigue: Transition from multi-axiality to uniaxiality

Abstract : Thermal fatigue experiments are performed on a high temperature tool steel X38CrMoV5 (AISI H11), under various maximum temperatures and heating rates. A microscopic interconnected crack pattern (named heat-checking'') forms on the oxidised surface of the laboratory tubular specimen. A gradual transition is observed, from a cell-type'' cracking at the centre of the specimen to a parallel cracking'' at its extremities. This variation of the morphology is well demonstrated by geometrical and topological characteristics of the crack network (micro-crack orientations, cell shape and node density), which change along the longitudinal axis of the specimen. The thermo-elasto-plastic loading of the specimen is estimated by Finite Element Calculations using ABAQUS (TM). Whatever the thermal fatigue conditions, a linear correlation can be established between the longitudinal and hoop stress amplitude ratio Delta sigma(zz)/Delta sigma(00) and the hoop and longitudinal inter-crack spacing ratio d(00)/d(zz) It is shown that a stress amplitude ratio close to 1 results in a multi-axial heat-checking, while a uni-axial cracking is generated when Delta sigma(zz)/Delta sigma(00) is close to or below 0.6. This means that the morphology of the heat-checking pattern (cell or parallel type) can be used as an indicator for the local stress ratio of the thermal fatigue specimen or industrial real tools.
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https://hal.archives-ouvertes.fr/hal-01611608
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Farid Medjedoub, Sabine Le Roux, Gilles Dour, Farhad Rezai-Aria. Effect of local stress on the heat-checking morphology in high temperature tool steels under thermal fatigue: Transition from multi-axiality to uniaxiality. Mechanics of Materials, Elsevier, 2014, 69 (1), p. 159-172. ⟨http://www.sciencedirect.com/science/article/pii/S0167663613002056⟩. ⟨10.1016/j.mechmat.2013.09.014⟩. ⟨hal-01611608⟩

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