During in-pile incidents in fast breeder reactors, the cladding can experience higher temperatures than normal; it is necessary to know the mechanical properties of the cladding material during such thermal transients to predict the time and location of rupture.

Two types of tests have been developed. First, tension (constant strain rate) tests after a heating at a constant rate and, secondly, constant load tests where heating is performed until rupture occurs.

The tension tests clearly show the role of the heating rate: that is, the higher the heating rate, the lower the cold-work recovery.

Constant load tests were conducted with either uniaxial or biaxial (burst tests) loading. The same stress/failure temperature relationship is found in both types of loading using the Von Mises equivalent stress. To predict failure, the Larson Miller parameter is not adequate, as well as all parameters based on a time/temperature equivalence.

The yield stress measured in the two types of tests are very different probably due to a strain rate effect. Indeed, the tension tests are dynamic ones (ε = 50/s) to avoid thermal recovery during the test duration, while the strain rate measured in constant load tests ranges only from 10⁻⁵/s to 10⁻³/s, being an increasing function of heating rate (ranging from l°C/s to 100°C/s).