The vast majority of the short-term properties that appear in a material data sheet are measured at room temperature. The heat deflection temperature (HDT) represents the only systematic attempt to characterize elevated temperature performance. The HDT test describes a particular response to temperature under a very specific set of conditions, however it is often used in the material selection process as a maximum continuous use temperature. As the trend toward the computerization of property data has progressed, the tendency to rank order properties for a large number of materials from different families has increased the separation between the property value and its significance to the design engineer.

This paper will briefly review the HDT test as defined in the ASTM Test Method for Deflection Temperature of Plastics Under Flexural Load (D 648) and the International Standards counterpart, ISO 75. It will then discuss an alternative method for capturing a more complete picture of the effect that temperature has on modulus. This technique, known as dynamic mechanical analysis (DMA), provides an excellent tool for evaluating materials and comparing their mechanical performance over a wide range of temperatures.