It is customary to study the creep deformation of materials at high temperatures and the incubation and growth of creep cracks using constant load test machines. However, this highly idealised loading condition does not accurately reflect the practical circumstances that occur when operating high temperature plants. Real loading conditions often lie between load and displacement control and correspond to situations where there is elastic follow-up, with low values relating to near displacement control and high values near to load control.

This paper explains a series of experiments where pre-cracked martensitic P92 steel compact tension specimens are loaded and tested for different values of structural elastic follow-up, ranging from constant load to near fixed displacement. It is found that the degree of elastic follow-up significantly changes the time taken for creep crack incubation. This is a consequence of the relaxation of the load applied to the specimens. Elastic-plastic creep finite element simulations are used to reveal the underlying mechanical behaviour of the specimens. The simulations were confined to 2D analyses for plane stress and plane strain conditions. It is observed, that, irrespective of the initial loading and boundary conditions, the predicted mechanical response for plane stress and plane strain lies either side of the experimental results.

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