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ISO 75

A general test method for measuring the temperature of deflection under load for plastics, high-strength thermosetting laminates, and ebonite, a hard, inelastic rubber that is commonly used as an electrical insulator. This type of test is commonly (but unofficially) referred to as HDT, which stands for "heat distortion test" or "heat deflection test." Three test methods are used within the scope of ISO 75, and are as follows:

Method A: flexural stress of 1.80 MPa is applied

Method B: flexural stress of 0.45 MPa is applied

Method C: flexural stress of 8.00 MPa is applied