The strain at break A or At is a measurement of the ductility, as well as the flow properties of a material.
The strain at break At can only be determined with extensometers, which remain on the specimen up to and including the point of break, and measure the extension of the specimen.
Strain at break A was normally measured manually, while today it is also measured with extensometers. For automatic measurements, correct determination of the point at which the specimen breaks (point of break) is therefore of significant importance.
Modern algorithms, which automatically analyze the stress-strain curve, ensure reliable specification of the break point and accurate determination of the strain at break. The break location along the specimen, more specifically along the parallel length of the specimen, is also important for reliable and accurate determination of the strain at break. If the break or point of failure is not within the gauge length of contact-type extensometers, the plastic deformation that occurs during necking and the point of failure cannot be correctly determined. Modern evaluation algorithms estimate the point of failure, or break point, relative to the measuring points of the extensometer and denote an unreliable strain at break value.
With optical, non-contact extensometers, which record the entire parallel length of the specimen, the point of break or failure can be determined. If the point of break is outside of the initial gauge length, according to ISO 6892-1:2017, Appendix I, the strain at break can nevertheless be determined, if an appropriate number of gauge marks were considered and measured during the test. The laserXtens Array as well as the videoXtens Array provide an optional solution for this task. With their use, the strain at break is automatically determined both reliably and accurately on 100% of the specimens.
The JIS Z2241 provides a classification of the break point. This is normally done manually through visual testing or by separate non-contact measurement. Both methods are personnel and time consuming. With modern optical, non-contact extensometers this task is automatically handled for tensile tests: indication of the class (depending on the break point A, B or C) is then part of the determined and recordable results.