A diametral extensometer for the measurement of small strains in tension-compression testing

Abstract

Diametral extensometers are often used in tension-compression testing. When very small strains are being recorded, indentation of the surface of the specimen by the contacts, which is an effect of the combination of axial cyclic strain and radial contact pressure, is a source of appreciable error. This error can be kept to a minimum by the use of the lightest possible contact pressure. When the specimen has to support the weight of the extensometer or, at least, to keep it in position, it is not possible to use a very low value of contact pressure. With the extensometer here described the weight of the instrument is supported independently of the specimen. Its body incorporates two transducers and therefore does not have to undergo lateral displacements. As contact with the specimen is made through two long probes it is not necessary for the body of the extensometer to rise and fall as changes of length occur in the specimen. Tests on the equipment show that it will record small strains with a high degree of precision. Because of this the testing machine functions well when operated under the strain control system.