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High chromium ferritic/martensitic steel T91 (9% Cr, 1% Mo), on account of its radiation resistance, is a candidate material for nuclear reactor applications. Its joining by an impact method to create a cold joint is tested in the realm of scoping tests toward the safe operation of nuclear fuels, encapsulated in representative T91 materials. Hitherto, T91 mechanical characterization at high strain rates is relatively unknown, particularly, in relation to impact joining and also to nuclear accidents. In this study, the mechanical characterization of T91 steel was performed in tension by varying the strain-rate (10−3 up to 104 s−1) and temperature (20-800°C) on dog-bone specimens, using standard testing machines or Hopkinson Bar apparati. As expected, the material is both temperature and strain-rate sensitive and different sets of parameters for the Johnson-Cook strength model were extracted via a numerical inverse procedure, in order to obtain the most suitable set to be used in this field of applications.

Strain rate and temperature are variables of fundamental importance for the definition of the mechanical behavior of materials. In some elastic-plastic models, the effects, coming from these two quantities, are considered to act independently. This approach should, in some cases, allow to greatly simplify the experimental phase correlated to the parameter identification of the material model. Nevertheless, in several applications, the material is subjected to dynamic load at very high temperature, as, for example, in case of machining operation or high energy deposition on metals. In these cases, to consider the effect of strain rate and temperature decoupled could not be acceptable. In this perspective, in this work, a methodology for testing materials varying both strain rate and temperature was described and applied for the mechanical characterization of Glidcop Al-15, a copper-based composite reinforced with alumina dispersion, often used in nuclear applications. The tests at high strain rate were performed using the Hopkinson Bar setup for the direct tensile tests. The heating of the specimen was performed using an induction coil system and the temperature was controlled on the basis of signals from thermocouples directly welded on the specimen surface. Varying the strain rate, Glidcop Al-15 shows a moderate strain-rate sensitivity at room temperature, while it considerably increases at high temperature: material thermal softening and strain-rate hardening are strongly coupled. The experimental data were fitted using a modified formulation of the Zerilli-Armstrong model able to reproduce this kind of behavior with a good level of accuracy.