# Enhancing the Accuracy of Advanced High Temperature Mechanical Testing through Thermography
## 4 Discussion  
The bespoke thermography based mechanical test setup for non-metallic materials testing at high temperature has proven to be an excellent means to assess these materials under arduous conditions. It has been shown in this work that in circumstances where external thermal radiation can be kept to a minimum, and a stable and known surface emissivity exists, accurate temperature control can be achieved through thermography. Careful selection of the control mode and point, area or line-based control is crucial and is ultimately determined by the combination of heating technique and specimen geometry. The addition of the non-contact monitoring thermocouple within the bespoke thermography based mechanical test setup provides an additional means of measurement to ensure accuracy. Figure 17: Thermographic image of a Ni-base superalloy 7 \(\times\) 7 mm corner cracked specimen under induction coil systems (ICS) heating during In-Phase thermo-mechanical fatigue (TMF) cycling. Thermographs are taken at peak load and temperature, 750 \({}^{\circ}\)C. (**a**) View of the entire specimen gauge section within an ICS; (**b**) Magnified view of starter notch and initial crack growth; (**c**) Magnified view of starter notch and secondary crack growth (**d**) Magnified view of the starter notch and further crack growth. Trials to achieve a stable surface emissivity were successful upon the three surfaces investigated, metallic, HE23 TP coated, and a non-metallic ceramic. The HE23 TP provided a stable surface emissivity of 0.96 at 800 \({}^{\circ}\)C consistent with previous work, [30]. In the author's experience, meticulous care must be taken with metallic surface preparation prior to exposure to ensure a uniform surface, enabling accurate and repeatable temperature control. Despite achieving uniformity, a negative effect on fatigue life has been previously reported as a result of such pre-test thermal exposure [27]. Concerns to this effect could be justified when considering the timescale to surface stabilisation of the Ni superalloy at 700 \({}^{\circ}\)C, Figure 9. Although the surface emissivity did not fully stabilise until after 75 h of exposure, it should be noted that the rate of emissivity evolution with time reduced significantly after 24 h. Indeed, the surface emissivity increased by only 0.05 (0.59-0.64) in 51 h of exposure at 700 \({}^{\circ}\)C after the initial 24 h of exposure. Considering this stabilisation timescale is at the peak isothermal temperature of a dynamic test, 24 h of pre-exposure could be justified to limit any negative effect on fatigue life whilst ensuring a stable surface condition. Despite the efforts to achieve a stable surface emissivity from a metallic surface within a timescale suitable not to impact on fatigue life, uncertainties still exist with regard to the accuracy of non-invasive temperature measurement. Despite efforts to meticulously prepare metallic test specimen surfaces prior to heat treatments, inhomogeneous surfaces can still prevail. These surface inconsistencies cause severe inaccuracy across measured surfaces, Figure 10a. However, both the HE23 TP coated and ceramic surfaces delivered a stable surface emission and thus accurate temperature control, allowing detailed visualisation of thermal gradients, Figure 10b. It must be noted that slight differences were seen in the surface emission of the complex woven ceramic structure between the fibres and surrounding matrix, Figure 10c. However, controlling the temperature through thermography by averaging over an area dissipated this effect and provided excellent accuracy in comparison to tie on thermocouples at isothermal temperatures.