Contactless optical strain gauge for tensile tests - has marked specimen viewed through calibrated adjustable iris

摘要

The contactless, optical strain gauge comprises a telescope-type optical system aligned above a specimen carrying line markings. An object lens focusses the image of the marks through an adjustable aperture. The images are further focussed and can be viewed through an eyepiece. The adjustable iris is calibrated. The images are first made coincident. The specimen is then put under strain and at the end of the tensile test, the optical system is readjusted so that the images of the markings are again coincident. The telescope mounting is fitted so that the images of the markings are again coincident. The telescope mounting is fitted with a tilting me mechanism which is used in conjunction with the iris me mechanism. The ratio between the two readings is then used to calculate the tensile strain from the known original spacing of the markings.