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Heat Deflection Temperature

Above-room temperature at which a polymeric material deflects by a specific amount under a specific flexural load. Useful for setting approximate boundaries on elevated service temperature, for applications where stiffness is important. Not measured for metals or ceramics.

The terms heat deflection (or distortion) temperature (HDT) and deflection temperature under load (DTUL) are synonymous. Testing is standardized under ASTM D648 and ISO 75. The two standards are generally considered equivalent, but they are not identical.

Both standards allow testing at two loading levels: 1.82 MPa or 264 psi, and 455 kPa or 66 psi. Under ISO, the two levels are called Method A and Method B, respectively. The ASTM test limits itself to just the two levels, while the ISO also has a third level (Method C, at 8 MPa). This third level is rarely used.

Material Properties Explained

Material Properties Explained is a handbook published by It is a concise encyclopedia of engineering material properties. It is not tied to the material properties database, and covers a far larger number of properties than those used on the website. Subject matter includes:

  • Differences in definition and testing between types of materials.
  • Common testing variations and their significance.
  • What additional information is required to correctly interpret a value.
  • Any equations to calculate or estimate property values.
  • Synonyms and related terms.
  • Relevant international testing standards, including both ASTM and ISO.

To purchase a digital (PDF) copy: Material Properties Explained.

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