Test specimens are heated under an applied stress in an oil bath at a programmed heating rate. The temperature at which the specimens deflect is recorded. In the HDT Test, a specimen utilizes a three-point flexural stress state. The Vicat test uses a cylindrical probe to indent the specimen.
Stress: 0.455 MPa, 1.8 MPa, 8.0 MPa
Temperature: ambient to 280°C
heat deflection temperature (deflection temperature under load)
Vicat softening point