25 year perspective Aspects of strain and strength measurement in miniaturised testing for engineering metals and ceramics

Abstract

Testing in the microregime is relevant to many technological issues, including, for example, evaluating the performance of established plant or new components through removal of small scale samples; assessing point to point variation in welded structures; characterising expensive advanced alloys that typically are only available in small quantities; and testing of subsized components. The use of miniature testpieces requires attention to the scale of the underlying microstructure relative to the overall testpiece dimensions, and presents challenges to the measurement of stress and strain in small volumes. The subject is extensive and the emphasis of the current paper is on a discussion of issues associated with the measurement of strength and strain, particularly with the latter assessed using non-contact optical techniques based on digital image correlation. The current article thus concentrates on the testing situation at the intermediate scale (in the range 0·5–5 mm) where testpieces are small compared with the size of engineering components and in the range where representative mechanical behaviour of the underlying microstructure must be considered. Metrology issues are addressed through specific case studies concerning; microuniaxial tests; small punch tests; subsized ceramic strength tests and the microindentation technique. Particular attention is paid to the uncertainties in measurement arising from the use of microscale tests to provide quantifiable levels of confidence in the ability of such tests to discriminate material behaviour.