Test specimen are aged in a load-free state in ventilated hot-air cabinets at different high temperatures so that subsequently, at room temperature, the change in specific properties can be calculated in relation to the aging period.
Generally, the changes in properties are investigated by means of a mechanical shock test, a quasi-static and an electrical measurement. The test is complete once the property limit (50 % of the original value) has been undershot. On the basis of the results obtained in this way, the time until the property limit is reached is calculated for each individual property (temperature/time pairs).
These pairs are then used for the thermal resistance diagram for the investigated material.
By extrapolating the results, a temperature index (TI generally after 5,000 and 20,000 hours) can be drawn up according to (IEC) DIN EN 60216-1.
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