Extracting strain data from high speed video

The problem

When carrying out tests at high speed it is very difficult to extract reliable data on strains and displacements. Conventional solutions such as strain gauges tend to become debonded before the test has proceeded very far and a large number of expensive tests may be needed before a valid result can be obtained, making it extremely difficult to obtain statistically reliable data.

The Solution

Improvements in digital camera technology have made obtaining a video file a straight- forward matter of acquiring a suitable camera system. However, processing that data to obtain accurate displacement or strain data can be much more difficult. In the sample shown here a speckle pattern has been applied before test to give the maximum opportunity for post processing. Whilst the Video Gauge software has been designed around real-time measurements a capability has been demonstrated to post-process standard AVI files to permit strain recording from high speed tests. The data shown here came from a test of a carbon fibre cloth sample at about 400% strain/sec, and the strain is measured between the circles marked.

It should be noted that the circles are not marked on the sample but are included here as an indication of where the measurements have been taken from. The video data can be re-measured as often as is required to map the strains and displacements across the sample under the full control of the user.

Advantages of the Video Gauge

The use of the Video Gauge to process digital images off-line offers the same advantages as operating in real time in terms of minimal specimen preparation, simplicity, non-invasive measurements, scale insensitivity and immunity to the effects of test environment. In addition operating from digital images allows the examination of archived data from tests carried out in the past but previously unused for measurement purposes.