Evaluation and Comparison of Creep Parameters in alpha -Phase Zircaloy-2 from Creep Tests Conducted at Constant Stress, Constant Load and Constant Pressure

摘要

A general creep equation relating secondary strain rate epsilon with stress sigma and temperature T has previously been incorporated in a model which predicts the strain-time behaviour of Zircaloy cladding during a postulated LOCA. The equation has the form: depsilon/dT = Asigmasup(n) exp -Q/RT where n is the stress component, Q is the creep activation energy and A is a constant dependent on structure. Conventionally minimum creep rates derived from constant load or constant pressure creep tests are used to establish the general creep equation. It is shown that when minimum creep rates are employed, different creep test methods yield different creep equations, the general equation depending essentially on test method and intrinsic material parameters. An unique secondary creep equation may be defined only from creep tests at constant stress of by correcting minimum creep rates obtained in constant load and pressure tests to secondary rates at zero strain. Creep strain-time behaviour of alpha -Zircaloy for the condition of constant load or pressure can be predicted from experimental strain-time data determined at constant stress. Thus constant stress data reflect intrinsic material behaviour while the modified strain-time behaviour at constant load or pressure results in part from the nature of the test technique. Finally, the primary creep strain epsilonsub(p), measured from constant stress data or calculated from constant load or pressure tests, is small epsilonsub(p)