Intro to Strain Gauges

An introduction to how strain gauges measure strain, including an introduction to the Wheatstone bridge.

One of the coolest instruments used to record data during test, has got to be the strain gauge. The strain gauge is a zig-zag pattern of ultra fine wires that we mount on structures undergoing strain that is being deformed by loads. We mount the zig zag pattern like this, in line with the strain, and as the structure deforms the wires will stretch or contract. Which shows up as a resistance change which we can measure as a difference in voltage across the strain gauge; but that doesn't really tell us exactly what is happening the the structure. It could be axial load, or it could be bending moment, or it could be torsion that is causing the strain gauge to deform. To know for sure we use this a Wheatstone bridge. This is an arrangement of strain gauges or strain gauges and fixed resistors in as specific pattern. That allows us to see a differential voltage between these two points which is far more accurate that a single strain gauge might be. Now this is a just a wiring diagram. How we actually apply the strain gauges to the structure can allow us to filter out torsion or filter out axial loads or measure the bending moment on that structure. The Wheatstone bridge is extremely versatile and very accurate which is why we love them so much when we're doing testing.