ISO 75

An International Organization for Standardization (ISO) standard test method for determination of heat deflection temperature (HDT) and deflection temperature under load (DTUL). HDT is a relative measure of a material's ability to perform for a short time at elevated temperatures while supporting a load. The test measures the effect of temperature on stiffness; a standard test specimen is given a defined surface stress and the temperature is raised at a uniform rate. Alternate test methods for HDT and DTUL are DIN 53461 and ASTM D648.

In both ISO and ASTM standards, a loaded test bar is placed in a silicone oil filled heating bath. The surface stress on the specimen is either: low - for ASTM and ISO both 0.45 MPa; high = for ASTM 1.82 MPa and for ISO