High-Temperature Measurements With a Fabry–Perot Extensometer

We have developed a Fabry-Perot extensometer that is able to operate at high temperature (up to 400 °C), for in pile experiments in material testing reactor, under high neutron and gamma flux. For that purpose, we have revised the basic scheme of an extrinsic Fabry-Perot interferometer (EFPI) to limit the effects of radiation and especially of the fast neutron fluence and to adapt the sensor to high temperature. After a description of the sensor and its operation, we present, in this article, two types of tests that were carried out to check the accuracy of the sensor at high temperature. First, four sensors are fixed either on stainless steel plates or on tantalum plates and the temperature is raised from room temperature to 400 °C. Knowing the thermal expansion of both the supports and the sensor parts, it is possible to predict the expected variation of the sensor output. The comparison between the predicted and experimental outputs provides the information about the accuracy of the sensor. Second, we performed high-temperature tensile testing at 300 °C and 350 °C. The Fabry-Perot measurements are compared, in the elastic domain, with the expected strain given by Hooke's law, and on a larger strain domain, with the measurements of a high-temperature axial extensometer. The experiments and the results are presented. The error appears to be limited to about $1~\mu \text{m}$ when the strain is low (0.1%) and can be of few micrometers on a larger range (up to 2%). Strain measurements up to more than 5% are also reported.