Video extensometer
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A video extensometer is a device that is capable of performing stress/strain measurements of certain materials, by capturing continuous images of the specimen during test, using a frame grabber or a digital video camera attached to a PC. The specimen of the material under test is usually cut in a specific shape and is marked with special markers (usually special stickers or with pens that distinguishes the marker from the specimen color and texture in the captured image).The pixel distance between these markers in the captured image are constantly tracked in the captured video, while the specimen under test is stretched / compressed. This pixel distance can be measured in real time and mapped against a calibration value to give a direct strain measurement, and to control the testing machine in strain control, if required.
With a proper calibration value and good image processing algorithms, resolution of much less than one micrometer (1μm) can be achieved. Proper calibration value also depends on the calibration specimen which is usually a specially etched material with great precision. To calibrate, pictures are first captured with the calibration specimen under the same testing conditions to be used for the new specimen.
Video extensometers are used primarily for materials which may damage a traditional contact or digital "feeler arm" extensometer. In some applications the video extensometer is replacing mechanical measurement units - but this is mainly clip-on devices. Note: video extensometers are not the optimum solution for all testing applications due to the limited accuracy and time consuming setup and operation.
When measuring the modulus of elasticity on 50 mm gauge length plastics to ISO 527 an accuracy of 1 µm is required. Some video extensometers cannot achieve this, whilst for production testing it is better to use automated motorized digital extensometry to avoid operators manually attaching marks to the specimen, and spending time setting and adjusting the system. Note that some video extensometers have difficulty in achieving acceptable results when used to measure strain within temperature chambers.
For applications demanding high accuracy, non-contact strain measurement, laser speckle extensometers are now emerging. In certain test applications they are superior due to much higher resolution and accuracy. The major advantage is that there is no need to manually attach marks to the specimen. They are also able to measure strain at high temperatures up to 1,600 oC (2,900 oC) under ideal conditions.
Changing of ambient light conditions during the test can affect the test results. Therefore, some video extensometer suppliers provide a bright background light and recommend shading windows / doors to prevent sudden change in brightness of the room. Most high end systems are designed to eliminate such negative effects.
