An equiangular rosette-type extensometer

Abstract

For complete determination of the state of stress at a surface point of an elastic body, it is in general necessary to measure the strain in three directions. This is usually made by means of strain rosettes of the bonded resistance-wire type. In this paper, a new type of extensometer is described which is capable of measuring the strains in three directions simultaneously.

The extensometer consists of three parallel and equidistant legs which are flexibly connected to a central part and supplied with sharp indentors. The change in distances between equivalent points of these legs is proportional to the surface strains. Unbonded resistance wires or reluctance gages have been used for converting the displacements into electrical signals. Attachment is achieved by means of permament magnets or rubber cups.

Restoring forces from flexible connections and strained wires are analyzed in detail. For an extensometer with 12 mm gage length and supplied with strained wires, these forces were found to be 21 g for a principal strain sum of 0.001 and 19 g for a principal strain difference of 0.001. When using reluctance gages, these figures are reduced to 5.8 and 12 g, respectively.