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Numerical calculations become a more and more efficient way to estimate the lifetime of structures under thermomechanical loading. But these life estimations cannot be reliable if the needed parameters have not been correctly identified and measured and if not all the damage causes have been considered. The disk testing under gas pressure is an experimental technique used for the mechanical characterization of thin sheets with various conditions: monotonous loading (biaxial rupture tests at strain rates from 10-6 to 10° s-1), constant loading under high stresses (sustained load) at elevated temperature (creep tests), cyclic loading (mechanical fatigue tests); the temperature may be chosen between 20 and 900 °C. The disk testing quickly reveals the damage when crossing cracks through the membrane create leakages detected by a mass spectrometer. Besides, we have developed a calculation method in order to measure the true mechanical properties of the pressurized disk. We evidence the quality of the results obtained with this method in the thermomechanical characterization of thin sheets and its sensitivity to secondary parameters. The analytical method of calculation will be optimized in order to obtain the best modelling of thermomechanical behaviour of materials loaded at temperature and strain rates within very large ranges and subject to different environments as gaseous hydrogen. [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]

A mechanical modelling is proposed in order to describe the viscoplastic behaviour without hardening at high temperature (900 °C) of a nickelbase super alloy with strain rates varying within a large range (from 10-1 to 10-4 S-1). A mathematical law is associated to the viscoplastic model; the law parameters 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 for the whole strain range; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in bibliography. The parameters identified from biaxial tensile tests of thin sheets have been used to calculate accurate stresses during stabilized tension-compression loops of cylindrical specimens. The proposed experimental method is interesting because the disk biaxial testing is much easier to be performed at high temperature than the tension-compression testing of cylindrical specimens. [ABSTRACT FROM AUTHOR]