Tensile stress-strain curves can be measured on specimens with an overall length of 3 mm using this new approach. The square test section of the specimens is 0.2 mm on each side.
The load frame is made from stainless steel by electrical discharge machining (EDM) and is 20 mm wide by 12 mm high and 3 mm thick. Load is applied and measured with an external translation stage and the load cell connected to the load frame with a fine wire. The load frame has wedge-shaped grips into which the ends of the specimen fit. The specimens have a “dog-bone” shape and are also manufactured by EDM.
Strain is measured directly on the specimen with laser-based interferometry from two tiny indentations placed in the specimen with a Vickers microhardness tester. Motion of the interference fringes emanating from the indentations is measured with two linear diode arrays and a microcomputer to enable real time strain measurement.
Preliminary tests have been conducted on brass samples to demonstrate the test technique. The relative uncertainty in stress measurement is high in these tests, but that arises from lack of precision in measuring the area. The relative uncertainty of strain is also high because of the crude mounts for the diodes. This paper demonstrates that the approach is feasible; straightforward refinements in the procedures will reduce the uncertainties.