An optical technique for assessing the long-term accuracy of lateral extensometers

An optical interference technique for measuring the change in thickness of a test-piece of a transparent thermoplastic caused by the application of a load is described. Measurements of lateral displacement using this method are compared with data obtained using an electromechanical extensometer in a long term, uniaxial creep test. The optical method is found to be significantly more accurate at the longer times and exhibits less scatter in data. The technique can be applied in assessing the long term accuracy of lateral extensometers and in the design of an optical extensometer which shows potential for enabling a very high level of accuracy to be achieved.