Temperature measurement and control methods in TMF testing – a comparison and evaluation

Temperature measurement by ribbon-type and spot-welded as well as by coaxial (sheath-type) thermocouples (TCs) placed inside and outside the gauge length of a TMF specimen with different contacting lengths to the test piece as well as measurement by pyrometry was performed. It was found that all investigated types of TCs can be used for temperature measurement and control in TMF testing with temperature rates up to 50 °C/s if a sufficient contacting length (in the case of spot-welded and coaxial) and a sufficient thermal contact to the surface (in the case of ribbon-type) is assured. Temperature measurement and control by TCs placed outside the gauge length is possible with sufficient accuracy up to temperature rates of 5 °C/s if appropriate calibration measurements are performed and an exactly reproducible position of the specimen with respect to the fixtures and the induction coil is assured. Pyrometry should be avoided wherever possible because oxidation induced alterations of the specimen surface may affect the accuracy of measurement.