Design of Extensometer for Creep Studies

Abstract

The extensometer permits remote operation over a wide range of extensions. The creep specimen is mechanically coupled to a metal core within a mutual inductance bridge. The amount of deformation of the creep specimen determines the position of the core and thus the amount of bridge unbalance. The tolerances of mutual inductance bridges being imperfect, a residual voltage remains which is 90° out of phase with that produced by displacements of the core. A core which produces an output voltage 90° out of phase with the voltage produced by a steel core has been made with laminations of copper and steel. By connecting two bridges in series, one containing a steel core and the other the composite core, the null has been reduced to the noise level of the amplifier to which the bridge system is connected. Requirements for making the phase angle of the bridge signal approximately independent of core position, over the extension range, are discussed.