Traditionally, materials testing has been conducted with a device attached to the specimen’s surface to measure the strain. These devices, called extensometers, use very accurate strain gages inside or optical scales. However, despite their extreme accuracy, these devices have three key issues. First, the weight of the device may be enough to bend some specimens, causing errors in your results. Second, there must be sufficient drag force on the specimen to prevent slippage. This is typically applied through very sharp metal contacts called knife edges, which are often sharp enough to cause premature failure in plastic or rubber specimens, giving you low ultimate strength results. Third, the accuracy of contacting extensometers is dependent on sensitive electronics, and the energy released by the break of many materials (especially composite materials or brittle metals) is enough to damage the electronics.
One alternative to contacting extensometers are optical extensometers. Users won’t experience the same grouping of issues as they do with contacting extensometers because optical extensometers do not need to touch the specimen. Therefore, they can’t influence the results of your materials testing and aren’t damaged by the energy released by the failure.