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.

Grain size and particle dispersion effects on the tensile behavior of friction stir welded MA956 oxide dispersion strengthened steel from low to elevated temperatures

“…Previous research showed that large gradients in microstructure and hardness exist at the retreating side (RS) and advancing side (AS) of the weld nugget, and the gage section of the tensile specimen was intentionally designed to avoid any possible effects from these gradients. Extensive literature review and finite element modeling was conducted to verify the suitability of the specimen geometry prior to testing, and although the specimen geometry does not conform to an ASTM standard, it is similar to those used or proposed for use in miniature mechanical testing of irradiated materials [42][43][44][45][46]. The specific aspects considered were the aspect ratio of the specimen (defined as the thickness to width ratio of the gage section) and the radius of curvature from the grip to the gauge section to ensure that yield stress and ultimate strain values were not adversely affected by design and that failure would occur in the gage section.…”

Use of Miniature Tensile Specimen for Measurement of Mechanical Properties

“…These days different techniques are being employed for measurement of displacement in miniature testing. LVDT, electrical resistance based transducer, capacitance gauges, line scan cameras, laser interferometry, digital speckle pattern interferometry, digital image correlation (DIC), photoelastic stress analysis and thermoelastic stress analysis are such techniques having their own advantages and disadvantages [10]. c) Validation of test procedures: Even in case of conventional test specimens, the ductility varies due to change in shape and sizes of test specimens.…”

“…that can influence the test results. The various issues with respect to development of Miniature Test techniques can be listed as follows [10]: a) Conformity of geometry of the test specimen with standards: The test specimen may not represent the bulk material due to various methods applied on it for its fabrication, such as EDM, grinding, polishing etc. The damage caused due to specimen preparation methods may be significant in volume-wise so that the result can be affected.…”

Characterization of dynamic damage mechanisms in Palmetto wood as biological inspiration for impact resistant polymer composites

“…Lord et al (2010) reviewed the experimental techniques for miniaturized testing for strength and strain measurement, and detailed the evolution and strengths of the DIC method. DIC has been found to be particularly valuable when capturing spatially inhomogeneous deformation fields.…”