A Clamp-On Strain Transducer for Use on Geologic Materials
Authors: Hardy Reginald H. , Kim Y.S. 
Published: December 2007

The design, calibration and application of a clamp-on type strain transducer for the simultaneous monitoring of axial and transverse strain under both uniaxial and triaxial test conditions is described The transducer attaches directly to the test specimen and utilizes a number of linear variable differential transformers to provide average strain data. Tests have been carried out on specimens of a number of metallic (steel, brass and aluminum) and geologic materials (granite, limestone and sandstone) at confining pressures of up to 5,000 psi. No difficulties are contemplated in using the psi. No difficulties are contemplated in using the transducer at confining pressures of 10,000 psi and higher.

Introduction

In order to determine the deformation characteristics of a solid, the requirements are: a suitably shaped test specimen; a system for applying the necessary loads to the test specimen; and a system for monitoring the resulting strains. This paper is concerned with the third requirement and describes the development of a clamp-on strain transducer for the measurement of both axial and transverse strain in specimens of geologic material under both uniaxial and triaxial loading conditions. For studies under uniaxial test conditions several workers have designed transducers for specific research projects. In many cases sufficient time was not available to develop and qualify these transducers for general use. At least one commercial firm manufactures a clamp-on type axial and transverse-strain transducer that utilizes differential transformers and is designed specifically for use on geologic materials. The development of suitable strain transducers for use on geologic materials under triaxial test conditions** has been extremely limited. Several methods have been utilized in which axial specimen strain was determined by external measurement of loading piston displacement. Although such methods have proved more or less satisfactory for experiments where large strains were involved, their use in experiments where small strains (less than 0.5 percent) and small strain increments ( 0.001 percent) are of interest are unsatisfactory due to a percent) are of interest are unsatisfactory due to a number of factors. For example, even with extremely fine polishing of specimen end-surface will compress surface will contain a large number of asperities. Under a compressive load the surface will compress rapidly upon initial loading due to failure or the development of plastic deformation in the individual asperities. Strain values based on deformation measurements made across such a surface would be seriously in error, particularly at low loads. Furthermore, the mechanical stability and accuracy of an external strain measuring system, although not too difficult to achieve in large strain experiments, becomes a major difficulty when small strains are being considered. Bonded resistance-type strain gauges provide one means of conveniently measuring axial and transverse strain in both uniaxial and triaxial tests. These have been used successfully for the investigation of the mechanical behavior of geologic materials. The inability to calibrate directly this type of gauge and the cost of gauges required for any extensive test program suggests that considerable effort should be directed toward the development of a clamp-on type strain transducer that could be utilized under both uniaxial and triaxial test conditions. Besides the transducer described, we are familiar with two other clamp-on transducers designed for use under triaxial test conditions one a purely experimental unit, incorporating bonded resistance strain gauges, being utilized by Mazanti, and the other made by Structural Behavior Engineering Laboratories in Phoenix that utilizes linear differential transformers as sensing elements.