This section of IEC 6021 6 specifies the experimental and calculation procedures for deriving the thermal endurance characteristics, temperature index (TI), and relative thermal endurance index (RTE) of a material using the “fixed time frame method (FTFM)”. In this protocol, aging takes place over a small number of fixed times, utilizing the appropriate number of aging temperatures throughout each period. The properties of the specimens are measured at the conclusion of the relevant time interval. This differs from the procedure of IEC 6021 6-1, where aging is conducted at a limited number of fixed temperatures, with property measurements occurring after ageing times that depend on the progress of aging. The diagnostic tests employed in the fixed time frame method are restricted to destructive tests. This method has not yet been applied to non-destructive or proof test procedures. Both the TI and the RTE determined according to the FTFM protocol are derived from experimental data obtained in accordance with the instructions of IEC 6021 6-1 and IEC 6021 6-2 as modified in this standard. The calculation procedures and statistical tests are modified from those of IEC 6021 6-3 and IEC 6021 6-5.
