When following ASTM D790 for 3-point bend tests, extension taken from the moving crosshead is sufficient for calculating flexural strain. However, some standards, such as ISO 178 require either a direct strain source or corrected extension for determining strain.
When a direct strain source is required, we recommend using a deflectometer, which is a spring-loaded plunger positioned below the specimen. When the specimen starts to flex, the plunger will compress and strain can be measured using an appropriate extensometer. The deflectometer is compatible with certain clip-on extensometers, Advanced Video Extensometer 2 (AVE 2), as well as the AutoX750 automatic contacting extensometer.