Creep phenomenon is one of the well-known failure modes that usually happened in the high temperature components. It generally occurs in high temperature condition and material shows visco-plastic behaviour as time passes. Recently, many power-plants are required to operate in higher temperature not only to increase efficiency but also to decrease carbon emission rate. However, this temperature increase is accompanied by more severe creep behaviour of the components. Thus, in order to secure the plant integrity, precise assessment of the creep behaviour arises as the most important research subject. Traditionally, the uniaxial creep test suggested by ASTM E139-11 has been mainly used to evaluate the creep life of metallic materials. However, enormous amount of consumption in testing time and substantial specimen volume requirement have impeded its process. Alternatively, the small punch creep test was introduced as an alternative test method to overcome the shortcomings of the uniaxial creep test. It uses a fingernail-sized specimen and a spherical punch ball to apply load into the specimen. Notwithstanding these positive aspects, small punch creep test has rarely received much attention in industry due to its complex interpretation process requiring further uniaxial creep test and finite element analysis.

 

 In this study, to overcome the aforementioned drawbacks that small punch creep test has, a direct creep life assessment method is proposed analytically based on the Chakrabarty’s membrane stretching theory. Stress converting formulation is derived by relating the applied load and punch displacement with equivalent stress. Thus, equivalent stress values could be directly calculated from small punch creep test results without any additional uniaxial tests. Furthermore, a non-dimensional correction factor is defined to consider the friction effect between the punch ball and specimen. To validate the proposed method, STS316L stainless steel is selected as a test specimen for both the uniaxial creep test and small punch creep test at 650 °C. And, small punch tests were performed using two punch balls made of different materials so that one can formulate the proposed friction correction factor. Both Larson-Miller models and Monkman-Grant models are derived from small punch test results as well as uniaxial creep test results. Those models are compared to each other in order to verify the reliability of the proposed direct creep life assessment method for small punch creep test. As a result, two creep models for STS316L drawn directly from small punch creep tests showed good agreement with those of uniaxial creep tests.


A Direct Creep Life Assessment Method for Small Punch Creep Test

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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