An Experimental Approach to the Application of Digital Image Correlation to Small Punch Creep Testing

Abstract

Deterioration assessment of materials is essential to the continued effective operation of critical components operating, at high temperatures and stresses, in thermal power plants. Within progressive inspection strategies, small punch creep testing (SPCT) presents an effective method for establishing the degree of creep exhaustion of in-service power engineering alloys on account of the small volume of material required (Ø8 mm × 0.5 mm discs). These tests make the application of complex strain fields, analogous to the strain field experienced by the service components, possible. In addition, this work explores the application of a noncontact digital image correlation (DIC) method to the full-field measurement of strains which is not possible through traditional extensometry. The accelerated SPCT tests were performed on 12 % Cr ex-service steels subjected to 600 °C. Experimental methodologies and challenges associated with ex situ DIC-SPCT are discussed including equipment and parameter selection, speckle pattern application, preforming requirements and loading method. It is shown that the developed DIC technique using a stereo microscope, airbrushed speckle patterns, sombrero-preformed discs and interrupted tests can be used to compare surface strain development in various aged forms of the ex-service steel. Such an approach is valuable to understanding strain development in small punch tests and to assist in the comparative damage assessment of ex-service power plant materials.