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A mechanical modelling is proposed in order to describe viscoplastic behaviour without hardening of a nickel-base super alloy loaded at high temperature (900 °C) with strain rates varying within a wide range (from 10−1 to 10−4 s−1). A mathematical law is associated to the viscoplastic model; the parameters of the law are identified from monotonic biaxial tests of membranes loaded by pressure of inert gas (disk pressure testing under helium). The viscoplastic law provides calculated stresses with accuracy better than 1% at the highest strain rates and 4% at the lowest strain rates; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in the bibliography. The parameters identified from biaxial tensile tests of disks have been successfully used to calculate the stresses during stabilized tension–compression loops of cylindrical specimens. The proposed experimental method and behaviour model are interesting because the disk biaxial testing is much more easily performed at high temperature than the tension–compression testing of cylindrical specimens. [ABSTRACT FROM AUTHOR]

Since three decades, the French nuclear centre (CEA) has defined and systematically used the disk pressure testing for studying and measuring the metals sensitivity to gaseous hydrogen. In 1990, this experimental method became a French standard to select metallic alloys used to construct high-pressured vessels under gaseous hydrogen. The disk pressure testing compares the pressure resistance of membranes similarly tested under helium and hydrogen. The experimental method is simple and sensitive and its results have always been sure; the disk testing allows original and various studies and reveals parameters that are not significant with less sensitive methods. First, we present an overview of different experimental results: the influence of the mechanical resistance on the hydrogen embrittlement, the influence of the chemical composition and the mechanical or thermal treatments of the materials, the influence of the gas composition (hydrogen content and impurities content)... Then, we present different types of metals damage: the metallurgical embrittlement of different alloys used at medium and high temperatures, the influence of the thermal ageing on gaseous hydrogen embrittlement, the embrittlements by gaseous and internal hydrogen when hydrogen enters the material before or during mechanical loading, the influence of the initial hardening mechanism on thermal ageing and on the hydrogen embrittlement, the shortened fatigue life due to thermal ageing and gaseous hydrogen and the synergy induced by the mutual presence of both these phenomena. The embrittlements are discussed and extrapolated to the loading and the extreme environment found within a space launcher engine: high temperatures with low partial pressure of hydrogen, rapid cooling, hydrogen super-saturation and high thermal cyclic strains. [ABSTRACT FROM AUTHOR]