# Enhancing the Accuracy of Advanced High Temperature Mechanical Testing through Thermography
## 3 Results
### Infra-Red Considerations
#### 3.1.2 Emissivity  
When using thermography or a pyrometer, for accurate temperature measurement it is essential to know the emissivity of the surface being measured. To assess the stability of thermography upon diverse surfaces, pre-oxidised, metallic, HE23 coated and ceramic surfaces were investigated to determine the time required to form a stable surface emissivity. Knowledge of the conditions at which a stable emissivity can be reached will allow test pieces to be heat treated/oxidised before testing, enabling a stable surface emissivity and accurate non-invasive temperature measurement to be achieved.  
A thermocouple was attached to the centre of the test piece gauge sections and a pyrometer or infra-red camera was aimed aligned alongside, parallel to the TC. The ICS and RLF heat sources were controlled using a feedback loop from the non-invasive techniques to apply a constant temperature to the test piece whilst the temperature was monitored by the TC. The emissivity input of the pyrometer and infra-red camera was adjusted until the measured temperature returned was equivalent to that shown by the TC. These results can be seen in Figure 9.  
Figure 8: View through thermography of a dog bone rectangular specimen at 800 \({}^{\circ}\)C inside a radiant lamp furnaces (RLF). (**a**) Metallic specimen with a pre oxidised polished surface, spot welded thermocouples are attached; (**b**) Metallic specimen with a pre-oxidised metallic surface; (**c**) Metallic specimen coated with HE23 thermal paint, spot welded and tie on thermocouples are attached; (**d**) Metallic specimen coated with HE23, six spot weld thermocouples are attached; (**e**) Ceramic specimen with tie on thermocouples attached; (**f**) Metallic specimen coated with HE23 thermal paint (**g**) Side view of a ceramic specimen; (**h**) Face view of ceramic specimen; (**i**) Temperature reference bar relating to images (**a**) to (**h**).  
In Figure 9, it shows that surface emissivity of the Ni alloy at a constant temperature of 700 \({}^{\circ}\)C does not stabilise until after 75 h, with a resultant stabilised emissivity value of 0.64. At 750 \({}^{\circ}\)C the rate of oxidation is higher and hence the rate of emissivity evolution is rapid with time at temperature. The surface emissivity stabilises after just 7 h at 0.8, an increase of 0.16 compared to the stabilised emissivity value at 700 \({}^{\circ}\)C. Consistent with previous work, the emissivity of the HE23 TP is stable; the emissivity was stable at, \(\varepsilon=0.96\), for the duration of testing. Stability of the CMC surface was also stable for the duration of testing with an emission value of 0.84.  
However, once a stable surface emission has been achieved distortions and inaccuracies may still result from unevenly oxidised surfaces. Small differences in the oxidised polished metallic surface shown in Figure 10a, affect the accuracy of temperature readings through thermography with surface temperature variations along the centred 25 mm vertical \(y\)-axis seen to have a variance of greater than 50 \({}^{\circ}\)C from the target temperature, 800 \({}^{\circ}\)C. Comparing this surface instability to that of a HE23 coated metallic specimen in Figure 10b, it is clear that HE23 thermal paint delivers a stable and uniform surface emission with temperature variations across the centred 25 mm vertical \(y\)-axis line profile are less than 15 \({}^{\circ}\)C. The HE23 delivers a smooth temperature distribution allowing comprehensive thermal analysis. When comparing these results with the un-coated ceramic surface in Figure 10c, a similar overall temperature variation across the 25 mm vertical \(y\)-axis to that of the HE23 TP coated specimen. However a distinct temperature undulation is present, as the \(y\)-axis profile crosses the differing weave orientations of the complex CMC structure.  
Figure 9: Isothermal surface emissivity calibration of a Ni superalloy at 700 and 750 \({}^{\circ}\)C, HE23 TP coating and a ceramic specimen at 800 \({}^{\circ}\)C.