Experimental and numerical investigation of small punch creep test

Abstract

The small punch creep testing (SPCT) technique has received much attention because it can provide information on the creep behaviour of materials with a very small specimen being tested. However, the nature of the test is complex and several aspects of the behaviour of the specimen, characterised by various non-linear concurrent processes, still need investigation. This thesis reports the findings of experimental investigations and numerical analyses of SPCT carried out with the aim of improving the understanding of various features which characterise the behaviour of the specimen and to develop a novel technique to correlate the SPCT experimental output with the corresponding uniaxial creep test data, which is also presented. The experimental programme consisted of SPCTs and pre-strained uniaxial creep tests, all performed at 600°C on the same batch of P91 steel. The pre-strained uniaxial creep tests have been used to evaluate the effects of large initial plasticity on the subsequent creep behaviour of P91 steel. For different stress levels, the results of the experiments have shown that creep was resisted for low pre-strain levels and enhanced for high pre-strains. The SPCT specimens have been investigated by use of scanning electron microscopy (SEM) to identify the effects of the punch load on the fracture surface of the failed specimens and the evolution of microstructural features in the material during the test. When the punch load was increased, the failure mechanism changed from creep-governed to plasticity-governed, as the presence of fresh dimples in the fracture surface increased. For the low-load tests, a macro crack was found to nucleate on the bottom surface of the specimen at approximately 20% of the failure life, and it subsequently propagated along the circumferential direction and through the thickness of the specimen. A modified creep constitutive model has been developed based on the results of the pre-strained uniaxial creep tests and it has been implemented in a FE model of a SPCT capable to take into account the effects of the large initial plasticity, generated by the load application, on the creep response of the SPCT specimen. A global creep resistance in the SPCT specimen, due to the combination of localised different effects in various regions of the sample, was observed when these effects were included. FE calculations have also been performed to investigate the effects of the eccentricity and the misalignment of the punch loading conditions on the punch minimum displacement rate (MDR) and on the time to failure. A correlation equation for these effects has also been reported. When the punch load was eccentric and misaligned, the MDR decreased and the time to rupture increased. Further numerical analyses have been carried out to evaluate the effects of the friction coefficient modelling procedure on the behaviour of the specimen. The results obtained using the classical Coulomb friction theory are compared with those obtained by a more modern friction formulation, which takes into account the dependency of the friction coefficient on the contact pressure. Finally, a nobel interpretation technique for SPCT data has been developed using the results of experimental tests and numerical analyses. The interpretation technique takes into account the effects of the initial, large plasticity on the behaviour of the SPCT specimen, in order to correlate the SPCT results with the corresponding uniaxial data. A significant improvement in the accuracy of the correlation for rupture SPCT data with the corresponding uniaxial test results has been obtained