Maximum service temperature assessment

Over the years many attempts have been made to provide some measure of the maximum service temperature which a material will be able to withstand without thermal degradation rendering it unfit for service. Quite clearly any figure will depend on the time the material is likely to be exposed to elevated temperatures. One assessment that is being increasingly quoted is the UL 746B Relative Temperature Index Test of the Underwriters Laboratories (previously known as the Continuous Use Temperature Rating or Index). 

The so-called flow temperature cannot be considered to be either the processing temperature or the maximum service temperature. It is obtained using the highly arbitrary Rossi-Peakes flow test (BS 1524) and is the temperature at which the compound is forced down a capillary of fixed dimensions by a fixed load at a specified rate. It is thus of use only for comparison and for quality control purposes. Since the rates of shear and temperatures used in processing are vastly different from those used in this test, extreme caution should be taken when assessing the result of flow temperature tests. 

It can therefore be seen that the UL rating is not an adequate method of assessing maximum service temperature if a material is to support stress or undergo strain in service. 

In addition, a specific tests for composites ASTM 2733 [35] uses the double notched specimens (c.f single lap joint) to assess the shear strength at elevated temperatures. An extension of this test is in an internal aerospace test [36]. which uses thermal spiking to assess performance. In this test a sample is conditioned at the aerospace standard 70 C 85%RH for two weeks prior to a rapid excursion for a maximum of one minute to a higher service temperature. The specimen is then conditioned under the standard conditions for a further two weeks prior to the evaluation of the weight gain. 

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