Small punch test method assessment for the determination of the residual creep life of service exposed components

The small punch test, also known as miniature disk bend test, is a relatively new method for the creep test of specimens of small dimensions. Experiments have shown that the small punch test can be used to describe the time to failure by means of an equation of the Dorn type, in which stress is replaced by load. The activation energies calculated using such a modified equation are somewhat smaller than the activation energies calculated from the results obtained using conventional creep testing methods. However, the constant value of the activation energy and the almost constant value of the load exponent in the modified Dorn equation confirm the hypothesis about the almost iso-stress nature of the small punch creep test. A finite element analysis of the small punch test has been carried out taking into account the constitutive theta -projection concept of the law of creep of the experimental steel 14 MoV 6 3, obtained from conventional, constant load creep test data. The viscosity parameters were evaluated using the theta -projection concept, and numerical simulation was performed by prescribing the experimentally determined history of the displacement of the punch. An incremental timestepping procedure was used, which included updating of a state variable, which monitors the evolution of creep in variable load problems. There was found to be good agreement between the calculated and actual loads for the secondary stage of creep.