Alloy 617 has been selected as a reference material supporting the Very High Temperature Gas Cooled Reactor (VHTR). However, current simplified design methods in Subsection NH have been deemed inapplicable at very high temperatures because, at these conditions, it is not possible to decouple plasticity and creep which is the basis for the current methods. Also, the alternative use of inelastic analysis requires development and verification of material modeling at these very high temperatures. A test procedure has been developed and implemented to support verification of new simplified methods and material modeling of Alloy 617 at very high temperatures. The procedure is based on two bars tested in series using two coupled servo-controlled testing machines to achieve equal displacement and constant applied load, mimicking the behavior of a pressurized cylinder subjected to through wall thermal transients. The tests were conducted with a hold time at 950°C. The bars were heated and cooled out of phase to generated thermal induced loading superimposed on a constant mean stress. The results are presented for different mean stress levels, heating and cooling rates, and thermal histories.

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