Accuracy of Video Gauge for strain measurement

The problem

Strain gauges are the standard way to generate high quality and reliable strain measurements in both sample testing and full scale structural tests. However, strain gauge application and use is time consuming and requires high levels of skill and training. The Video Gauge offers an alternative method of measuring strain that can often save time and reduce costs, but the approach must first be demonstrated to be equivalent to the use of strain gauges. The purpose of this experiment was to establish the accuracy of the Video Gauge compared to strain gauges.

The solution

A standard flat aluminium dog-bone specimen was instrumented with strain gauges on front and back and a speckle pattern was applied to act as a target for the Video Gauge. The specimen was then strained in a universal servo-hydraulic test machine to yield and the strain gauge outputs were compared to the Video Gauge outputs.

The first graph below plots the front strain gauge output against the Video Gauge output. The second graph plots the front and back strain gauges against each other.

Ideally the two strain gauges and the Video Gauge would give identical results. In practice there is a 0.5% difference between the Video Gauge and the front strain gauge, but a slightly larger difference between front and rear gauges (0.7%). Thus the Video Gauge results are within the range of gauge to gauge variability for strain gauges.

advantages of the Video Gauge

In this case the preparation needed for the Video Gauge was a few moments to apply a speckle pattern compared to more than an hour to prepare and apply the strain gauges - for results that are essentially indistinguishable between strain gauges and Video Gauge.

Additional advantages of the Video Gauge in this case are that Poisson ratio or additional strain measurements (for example, to check strain uniformity) can be obtained for minimal additional effort.