# Enhancing the Accuracy of Advanced High Temperature Mechanical Testing through Thermography
###### Abstract
Not only have infra-red techniques proven sensitive and accurate enough to control and monitor temperature, but have been employed to uncover valuable material effects. Infra-red has been used to characterise the dynamic properties of materials to thereby pre-determine their fatigue limit [14], and also used in the assessment and separation of the effects of elastoplasticity and thermoelasticity [15], are two examples. Thermography has also been a vital tool to aid investigations into quantifying the effects of frequency in ultrasonic or very high cycle fatigue (VHCF) testing upon temperature [16]. Such effects are not prevalent in the current work as typical dynamic temperature tests such as TMF are undertaken at low frequencies, characteristically 0.0333 Hz equivalent to a thirty second cycle at a maximum. Infra-red techniques are sensitive to changes in the surface condition of the test specimen predominantly caused by progressive oxidation at high temperatures that in turn affects the surface emissivity and the accuracy of the technique [17]. Depending on the specific temperature and wavelength, the energy radiated by a metallic surface is highly sensitive to any change in the surface condition and therefore is directly proportional to the spectral emissivity of the object [5]. Without a stable emissivity value, it is extremely difficult to achieve accurate temperature measurement with a non-invasive technique such as a pyrometer [17; 18; 19; 20; 21; 5; 22]. Pyrometry does offer non-invasive temperature monitoring and control [23], however the measurement reading is limited to a particular location on the specimen, a limitation shared with TCs [24]. A solution to this issue is to pre-oxidize the specimen to produce a stable oxide layer and hence stable surface emissivity [1]. However, pre-heat treatments and thus consistent oxide formation has previously been found to lack repeatability where previous work has found that the surface emissivity evolution with temperature is dependent on the material, surface preparation, thermal treatment, and the chemical reactions on the surface [25]. Further work suggests that there is no conclusive trend of emissivity observed as a function of oxidation time (\(15\pm 60\) min) at a given oxidation temperature [26]. However, provided a stable oxide layer, and thus a constant surface emissivity can be achieved, the high-temperature pre-exposure and resultant oxide formation can influence the fatigue life of the test piece [27]. A reduction in fatigue life occurs for some alloys subjected to pre-test temperatures \(\geq\)500 \({}^{\circ}\)C with reductions in fatigue life resulting from surface oxide scale cracking in combination with increased crack growth rates from enhanced susceptibility to intergranular cracking [28; 29]. This effect was investigated on two Ni-base superalloys in the temperature range 650-704 \({}^{\circ}\)C for exposures of 100 to \(\geq\)1000 h [28]. The investigation concluded that pre-exposure increased scatter in the results and reduced fatigue life by up to 70% compared to unexposed specimens. An alternative to pre-oxidation is to paint the surface to achieve an adequate emissivity. The use of thermal paints (TP) to measure, correct, and reduce the influence of surface emissivity values have been described in previous work [30]. These paints can be applied to encompass the entire specimen surface as a permanent coating. Alternatively, the paint can be applied in a single smaller location or a small spot to determine the emissivity by comparing the measured radiance temperature of this spot with that of the uncoated surface [30]. To avoid complications and inaccuracies with respect to radiation from nearby heat sources, such TPs should yield as high an emissivity value as possible, close to the emissivity of a blackbody, which is rated with a value of 1 [8].