ME 3263 Introduction to Sensors and Data Analysis (Fall 2018)

Lab #2 - Static beam deflections with strain gage

What is a Strain Gage?

A strain gage consists of a looped wire that is embedded in a thin backing. Two copper coated tabs serve as solder points for the leads. See Figure 1a. The strain gage is mounted to the structure, whose deformation is to be measured. As the structure deforms, the wire stretches (increasing its net length ) and its electrical resistance changes: $R=\rho L/A$, where $\rho$ is the material resistivity, $L$ is the total length of the wire, and $A$ is the cross sectional area of the wire. Note that as $L$ increases, the cross sectional area changes as well due to the Poisson contraction; the resistivity also changes.

Figure 1: a) A typical strain gage. b) One common setup: the gage is mounted to measure the x-direction strain on the top surface. It's engaged in a quarter bridge configuration of the Wheatstone bridge circuit.