| Small Punch Creep(SPC) testing technique is a new technique which is applied to evaluate the high temperature creep properties of materials by using miniature specimen. It has advantage of nearly non-destructive sampling and is very suitable for the situations in which taking conventional sample is impossible or uneconomic. It will be an important method to forecast whether the material is invalid and to evaluate the residual life of serviced components in the future. It will be applied widely in many fields including nucleus industry, avigation and spaceflight, electric power and petrochemistry industry and so on.Aimed at presently existed issues of SPC testing, the research has focused on three aspects: basic theory, numerical simulation and experiment research in this paper. Major research works and conclusions are summarized as follows:In the theory research, firstly, by the sheet bend creep model, the relationship of specimen center deflection rate and load was deduced, and the theory formula of creep stress exponent and evaluation method were established. Then, by the membrane stretch model, the relationship of specimen center deflection and equivalent strain, load and equivalent stress were deduced according to geometry connection. What's more, the relationship between the results of small punch creep testing and those of conventional creep testing was obtained.In the numerical simulation research, firstly, the finite element model of SPC testing was established and was validated by comparing to the experiment results. Then, the creep stress exponents of the tungsten-alloyed 9% Cr ferritic steel and 12Cr1MoV steel were calculated by loads and the specimen center deflection rate based on numerical simulation, and the validity of the creep stress exponent theory formula was validated by comparing to the input parameters. Further, the mechanism of SPC testing was discussed by analyzing the specimen deformation, strain and stress state. Finally, the influenced factors of SPC testing were investigated. The results indicate that the influence of friction can be neglected, but the influence of specimen thickness, punch ball and lower model hole diameter is great. Therefore, the machining precision of specimen and mould geometry size should be ensured when SPC testing are carried out.In the experiment research, series of SPC testing for SUS304 stainless steel material were carried out at 600℃. The obtained curves for the central deflection versus time are quantitatively similar to the conventional experimental results. The creep stress exponent(8.058) of SUS304 stainless steel material was obtained by the theory formula and experimental results in this paper. It is good agreement with the conventional experimental results(8.297), and the relative error is two dot nine percent. The experimental results showed that the creep stress exponent which is well consistent with conventional experimental results can he obtained by SPC testing and the theory formula. Further more, the SPC testing for the Si3N4 ceramic and the Si3N4/Ni composite material were carried out firstly. Both the Si3N4 ceramic and the Si3N4/Ni composite material display obvious creep deformation at certain temperature. The dividing line of the first creep phase and the second creep phase is inapparent for the Si3N4 ceramic and the Si3N4/Ni composite material. The values of creep stress exponent of two materials are respectively 5.19 and 2.1 by the theory formula.In this paper, the theory formula of calculating creep stress exponent and the relationship between the results of small punch creep testing and those of conventional creep testing have been established. The validity of theory formula has been validated by the numerical simulation research, and the mechanism and the effect factors of SPC testing were discussed. The creep stress exponents of SUS304 stainless steel, Si3N4 ceramic and Si3N4/Ni composite material have been obtained by using the theory formula in the experiment research. The results indicate that the SPC testing technique is an effective, convenient and credible method for the evaluation of the high temperature creep properties of materials. |
