Measurement of high‐temperature strains in superalloy and carbon/carbon composites using chemical composition gratings

Abstract This paper describes an experimental and signal processing technique to perform high temperature tests on superalloy (INCONEL) and carbon/carbon structures using silica‐based chemical composition gratings (CCGs). The results obtained from applying this technique at 940°C in superalloys and 950°C for carbon/carbon (C/C) composites are benchmarked against data obtained from four different methods. The results show that the wavelength responses of the CCGs bonded on the superalloy and on the C/C plate increase nonlinearly with increasing temperatures. The temperature‐dependent strain transfer coefficients recorded during the superalloy tests show quite stable results below 600 °C and tend to slightly decrease thereafter. The values of the strain transfer coefficients below 1000 °C are significantly affected by the thermal expansion coefficient of the substrate material and the interface. We demonstrate that the strain transfer coefficient calculation method used in this paper is not suitable for low and/or negative expansion material. The results of the relative errors show that the CCGs‐F method based on the quadratic dependence of the wavelength shift versus the temperature appears to be the best to estimate the mechanical strains within the interval of temperatures considered and the measurement accuracy. The relative errors measured between 200 °C and 1000 °C are less than 5%.