Use of proximity transducers for local radial strain measurements in a hollow cylinder apparatus

This paper describes the use of proximity transducers to accurately measure the radial and circumferential normal strains of the test specimen (covering the pseudo elastic range to shear failure) in a hollow cylinder apparatus. The non-contact proximity transducers, which were located in the outer cell chamber and the inner bore cavity of the specimen, measure the radial displacements of the inner and outer specimen wall surfaces over a central, uniformly stressed zone. The waterproofing and set up of the transducers, including the precision gearing that is used to relocate the instruments from outside the pressure cell during the course of a test, are described. The transducers, which have a measurement range of 6.0 mm and a resolution of 1.1 x 10 -6 mm, were calibrated in water to an accuracy of 1.0 x 10 -5 mm (radial and circumferential normal strains of 7 x 10 -5 %) using an optical table and a laser distance measurement system. The calibration data was non linear and strongly influenced by the target curvature (convex and concave for the outer and inner wall surfaces, respectively). Generalized stress path tests were carried out on fully saturated sand specimens and the deformation response was measured using the local (proximity transducers) and external instrumentation. The external measurements included significant errors, particularly in the case of the outer wall displacement, due to: (a) apparatus compliance; (b) specimen end restraint and bedding effects; (c) inaccuracies in measuring the volume change of the specimen and its inner bore cavity and (d) the fact that the wall displacements were calculated as mean values over the full specimen volume.