# Enhancing the Accuracy of Advanced High Temperature Mechanical Testing through Thermography
###### Abstract  
This paper describes the advantages and enhanced accuracy thermography provides to high temperature mechanical testing. This technique is not only used to monitor, but also to control test specimen temperatures where the infra-red technique enables accurate non-invasive control of rapid thermal cycling for non-metallic materials. Isothermal and dynamic waveforms are employed over a 200-800 \({}^{\circ}\)C temperature range to pre-oxidised and coated specimens to assess the capability of the technique. This application shows thermography to be accurate to within \(\pm\)2 \({}^{\circ}\)C of thermocouples, a standardised measurement technique. This work demonstrates the superior visibility of test temperatures previously unobtainable by conventional thermocouples or even more modern pyrometers that thermography can deliver. As a result, the speed and accuracy of thermal profiling, thermal gradient measurements and cold/hot spot identification using the technique has increased significantly to the point where temperature can now be controlled by averaging over a specified area. The increased visibility of specimen temperatures has revealed additional unknown effects such as thermocouple shadowing, preferential crack tip heating within an induction coil, and, fundamental response time of individual measurement techniques which are investigated further. thermography; high temperature; dynamic temperature; thermocouple; pyrometer 201  
control. Previous work has highlighted its significance to deliver the required phasing between mechanical and thermal loading [4]. Deviations in phase angle of \(\pm 10^{\circ}\) have shown to have clear effects on the cyclic deformation behaviour of metallic materials. As such it has been recommended to keep phase shifts to within \(\pm 2^{\circ}\) of the target phase angle [2; 3; 4]. However, despite the standardisation of test methods such as TMF, uncertainty still remains with regards to how accurately temperature can be measured and controlled, especially in dynamic circumstances such as TMF. Previous studies have shown that temperature accuracy has a significant impact when considering data scatter [5; 6; 7; 8; 9]. Traditional isothermal heating methods comprise of a conventional resistance furnace, with temperature monitored by tie on thermocouples (TCs). The technique is not suitable for the rapid dynamic thermal cycling required by TMF as the heating and cooling rates generated by the furnace are too slow. Furthermore, the accuracy of tie on TCs under such rapid thermal cycling is not acceptable when used for dynamic temperature control and must be spot welded. To overcome the established impact of spot welding on the specimen gauge section upon crack initiation [10], spot welding TCs on the specimen shoulder for dynamic high temperature control has been recommended [2; 3]. However, despite these recommendations, the accuracy and practicality of such a technique under rapid dynamic thermal cycles is questionable, particularly in specimens with a long parallel gauge section, where large distances exist between the material under investigation and the TC location on the test piece shoulder [11]. Understanding the dynamic temperature effects associated with TMF is particularly important in the aerospace sector. In order to achieve desirable temperature ramp rates to simulate engine flight cycles which may exceed \(\pm 10\)\({}^{\circ}\)C/sec, radiant lamp furnaces (RLF) or induction coil systems (ICS) are typically employed [11; 12]. A variety of temperature control options now exist for these heating systems with the predominant control methods being spot welded TCs, pyrometers and most recently a technique using an infra-red camera known as thermography [11; 13]. However, the use of pyrometers or thermography for dynamic temperature control is not yet recommended by the relevant standards involved with cyclic high-temperature testing such as TMF [2; 3].