Deformation and Fracture Mode during Small Punch Creep Tests

Abstract

The creep damage condition of components under elevated temperature is a requirement to guarantee safe life extension and continued operation. Destructive and/or non-destructive assessments are regularly applied to assess the remaining life of components during service. Uniaxial creep specimens have been traditionally employed for conventional tests to examine a series of high temperature creep properties. However the ability to remove these relatively large uniaxial specimens is limited due to the required size of the specimens with respect to the component dimensions. To overcome this shortcoming, small element testing techniques such as miniature creep (MC) and small punch creep (SPC) tests have recently been proposed to investigate creep properties. However their applications are limited as there is no established standard for the testing procedures and subsequent data evaluation. In order to aid the standardization of the SPC test method, this paper investigated the deformation and fracture of interrupted SPC tests. Results showed that the creep deformation in the SPC test could be classified into three conventional stages. Firstly, the crack of about 1mm in the diameter developed on the extended surface of the disc specimen at the end of the primary creep stage. Secondly, during secondary creep, circumferential cracking progressed in the through-thickness direction by about 0.1mm. Lastly, the tertiary creep region was extremely short and only appeared just before final fracture and failure. The result showed that the ratio of load in the SPC test to stress in the uniaxial creep proposed past was smaller than the experiment value. This result was due to the early crack formation in the disc specimen and the shear type crack development, and the difference of loading ball's diameter.