Due to their low cost, light weight, resistance to corrosion, and general versatility, plastics have established a significant presence in many different industries. It is important to determine the material properties of plastics in order to characterize their expected behavior in real-world applications. One important property is the relationship between stress and strain while a plastic sample is being bent or flexed: in other words, the flexural properties of the material. To regulate an internationally standardized method for determining this property, the International Organization for Standardization (ISO) developed ISO 178.

ISO 178 is a test method for determining the flexural properties of rigid and semi-rigid plastics by performing a three-point bend test on a universal testing system. A three-point bend test applies force at the midpoint of a rectangular specimen, which is freely supported at either end. The applied force is measured by a Load Cell, and the resulting deflection is measured by either the system’s crosshead displacement (with results corrected for system compliance) or by a direct strain measurement device. There are four test types outlined in ISO 178, each specifying a deflection measurement method (i.e. via crosshead or direct strain transducer) and an associated calibration accuracy requirement.