A laser extensometer for measuring strain at incandescent temperatures

A laser-extensometer system has been developed and installed in an electron-beam tensile vacuum furnace to measure the strain of tensile-test specimens at incandescent temperatures. The laser extensometer operates by measuring the length of time during which the laser light can be “seen” by a photosensitive device while the light beam is rotated at a constant angular velocity. The light can be “seen” by the photosensitive device only when the light passes through a gap between two opaque flags which are affixed to the specimen so that the change in the gap width is proportional to the strain in the specimen. Tests indicate that the laser extensometer is capable of measuring strain with a maximum error of less than 100 μin./in. based on a 1-in. specimen gage length. The sensitivity and accuracy of the first model of the laser extensometer increase with a decreasing strain rate and are well within the range of requirements for metallurgical evaluation of modern metals and alloys for use at very high temperatures. Operation of the extensometer has indicated that greatly improved sensitivity and reduction of electronic noise and drift can be achieved by simple modifications yet to be made.