Bonded strain gauge

Table of Contents

Structure-

In bonded strain gauge, a fine resistance wire of diameter 0.25mm or less is bonded or pasted on a thin flexible sheet of a paper, tissue, Bakelite or Teflon etc. and this substrate or sheet is directly attached using adhesive to the surface of the structure whose stress or strain to be measured. The grid of fine resistance wire is cemented to carrier. It can be a thin sheet of paper, Bakelite or a sheet of Teflon. To prevent gauge wire from any mechanical damage, it is being covered on top with a thin sheet of insulating material. A diagram of bonded strain gauge is shown as follows-

Working Principle

When bonded wire strain gauges are attached to the structure surface under study and structure is subjected to a force, gauge gets elongated, hence a change in resistance occurs which can be detected and measured by bridge circuits and can be calibrated in unit of measurement of process variable such as force, pressure, stress etc.

Configurations-

There are various configurations of bonded strain gauge available as given below-

Grid type
Rosette type
Torque type
Helical type

Grid type –

This type of strain gauge is the simplest and most common configuration.
The grid can be made of either metal alloys wire or a metal foil.
They are bonded directly to the surface under stress analysis using thin layer or epoxy resin.

Rosette type –

Rosettes type strain gauges are used when either direction or axis of strain in a component is unknown or more than one direction are present So it cannot be measured effectively using single strain gauge.
Rosette is basically an arrangement of combining two or more strain gauge elements in such angles and positioned so closely to measure strains of different directions of the component. 3 element strain gauge rosette is shown in diagram.

Adhesive materials –

Adhesive materials are responsible to transmit the strain from the structure to the gauge wire. The spreading of gauge wire allows uniform distribution of stress over the grid. Various adhesive materials used are given below-

Epoxy adhesive
Ceramic cement
Cyanoacrylate cement
Cellulose nitrate cement
Phenolic adhesive
Polyimide adhesive etc.