Infrared thermography, IRT, is a science that engages in the acquisition and processing of thermal information captured by contactless measuring devices. This technology can be applied to a great number of different fields, from the industrial sector, in process and product control, as well as medicine, environmental sciences, etc.
Thermography can be grouped into two types: active thermography and passive thermography. In passive thermography, no excitation source is required to inspect or analyze an object or a biological organism, as the heat generated or discharged by the object studied is used. In contrast, in active thermography, an external excitation source is applied, which heats the object studied, thus generating temperature gradients. Figure 1 shows different possible forms of excitation.
Figure 1. Different possible excitation sources in active thermography. Source: https://workswell-thermal-camera.com/wp-content/uploads/2020/01/ati_scheme.jpg
At Lortek, we apply active thermography as a non-destructive testing technique.Moreover, passive thermography permits process control and the early detection of faults in production processes.
Nowadays, traditional NDT inspection methods are generally based on manual techniques, such as Magnetic Particle or Dye Penetrant Testing, which are both time-consuming and user-dependent. The purpose of Active Thermography is to replace these traditional methods, not just to reduce inspection times, but also to permit clean and contactless inspections, something that, for the majority of manufacturing industries, is invaluable. Another important advantage of thermography is that this technique is sufficiently robust so as to guarantee the repeatability both in the measurements and in the subsequent automatic detection of defects; a characteristic that traditional inspection techniques will never fulfil.
The work methodology, once the inspection viability has been proven, is summed up in the figure below:
Figure 2. Normal procedure in active thermography applied to NDT, after having proven the technology viability.
On the other hand, passive thermography permits controlling different processes, and even predictive maintenance. A clear example of the latter is the use of passive thermography in the world of steel to predict the degradation of the refractory material on ladles, and thus send them to be repaired before they fail in production, with the risk that this could represent otherwise.
Figure 3. Example of passive thermography application in casting. In this case, the objective was not to inspect the cast part, but to develop a model to estimate the life of the dies, in such a way as to achieve two crucial results for the manufacturer: (i) increase the working life of the tools, and (ii) ensure good part manufacturing quality (less rejected parts per deteriorated die).
Guided Waves non-destructive testing is an inspection method based on low frequency ultrasound waves, which permit inspecting very long and thin surfaces by propagating ultrasound signals through these surfaces, using the actual material as a wave guide. This technique has applications in different sectors, such as aeronautical, wind, maritime, etc. Depending on the different propagation modes, by means of low frequency probes and advanced signal processing, the behaviour of the Guided Waves within the actual material can be analyzed, determining possible anomalies located both on the inside and on the outside.
To optimize quality control processes of parts from different sectors, Lortek is conducting an in-depth study of different advanced ultrasound techniques by means of Guided Waves.
The implementation of these techniques affords multiple benefits for industrial fabric companies that require an optimized quality control process, as they reduce inspection times, they permit inspecting areas that are difficult to access, and do not entail any risk for the operator.
Figure 4. Inspection of offshore chains via inductive thermography. On the left of the figure, a comparison is made of the results obtained by means of magnetic particles and inductive thermography in the inspection of offshore chains (viability). On the right, the proposed inspection system is shown, indicating the critical steps to be taken to reach a totally automated and robust inspection system.
Figure 5. Inspection of crankshafts via inductive thermography. On the left, the results obtained in the viability study. The centre image shows a possible setup for inspection automation. Finally, on the right, the Lortek setup is shown and the result of the automatic inspection.
Nowadays, to inspect the quality of samples prepared by means of additive manufacturing, a Guided Wave-based methodology is being developed. This consists in using a laser vibrometer to detect anomalies in a contactless manner.
Figure 6. Diagram of the setup to use the modular Polytec laser vibrometer OFV-5000 to detect anomalies.
The Lortek inspection cell is a space used for the industrial research and development of active thermography. It is divided into two zones: a manual inspection zone and a robotized inspection zone. The manual zone is used for preliminary tests and viability studies of the technology. It has two thermographic cameras: a microbolometer camera, Flir A655sc, and a high end cooled camera, Flir X6541s. Lortek also now possesses three excitation sources: Laser, Induction, and Flash and Halogen Lamps.
In turn, the robotized zone is comprised, in summary, of three components.The robot, the positioner and the respective associated virtualization. The robotized zone is prepared to be able to perform inspections with all the equipment mentioned in the manual zone.
Figure 7. Inspection cell (a) shows the inspection cell from the outside. (b) View of the automated zone from the manual zone. (c) View of the manual zone from the robotized zone. Finally (d) shows the laser station inside the manual zone.
Figure 8. Robot and positioner in the Lortek inspection cell.
Figure 9. On the left, a thermographic head still in the virtual phase, and on the right, the manufactured part, in reality.
Figure 11. (a) and (b) show a laser thermography setup in LMD test pieces. (c) corresponds to an inductive thermography setup for welded Inconel 718 test pieces.
This section includes a brief overview of all the thermographic equipment available in the Inspection Cell.
Thermographic cameras
Lortek has two thermographic cameras: the Flir A655sc and the Flir X6541sc.The main difference lies in the type of detector and the cooling system.
Excitation Sources
LASER
INDUCTION GENERATOR
FLASH AND HALOGEN LAMPS
Figure 11. (a) and (b) show a laser thermography setup in LMD test pieces. (c) corresponds to an inductive thermography setup for welded Inconel 718 test pieces.
Polytec laser vibrometer (OFV-525/-500): Equipment to detect anomalies by measuring vibrations.
Figure 12. Polytec OFV-5000 modular laser vibrometer (Controller and laser head OFV-505), used in the setup to detect anomalies.
Phased System Array (PSA): Portable system based on SITAU technology, by means of which it is possible to obtain real time ultrasound images.
Figure 13. Portable Phase Array Sitau system and configuration/evaluation software.
Multi-channel Difrascope ultrasound instrument: Equipment for ultrasound signal generation and acquisition. As well as the respective software to configure ultrasound signals and monitor them.
Figure 14. From left to right: setup, multi-channel Difrascope equipment for signal generation and acquisition, and configuration/monitoring equipment.
Amplus 32 Preamplifier: Equipment to amplify ultrasonic signals.
Figure 15. From left to right, amplifier and setup.
Transducers: Transducers operating at different frequencies, used for the different techniques. Lortek also has positioners for the piezoelectric transducers, specifically designed for certain applications.
Figure 16. This shows the tooling developed to position the transducers for the different positions.
One of the first projects in the area of active thermography at Lortek was none other than to inspect the weldings of the international project, ITER, whose objective is to manufacture the first nuclear fusion prototype in the world. In this case, the excitation source chosen was laser.
Figure 17. Welding inspection of the Vacuum Vessel (VV) Assembly for the ITER fusion reactor. (a) shows the VV structure and how the sectors are welded; (b) is a TIG narrow gap coupon with a root pass on which liquid testing has been carried out. Finally, (c) shows the respective result by means of laser active thermography, where the greater resolution, compared to the traditional technique, can clearly be seen.
To improve fault detection by means of non-destructive testing on compound materials of the aeronautical industry (samples manufactured by the company, SAAB Aeronautics). Lortek participated in the European project, CRO-INSPECT, as leader of activities related to research on Guided Waves, to inspect difficult-to-access areas. Contributing with an inspection procedure by means of Guided Waves geared towards the detection of defects associated with the production process.
The figure below shows the set-up implemented to inspect the stringers that make up a flap. Showing the comparison between the acquired ultrasonic signals for the case of a zero-defect stringer, and for the case of a stringer with delamination (blue signal).
Figure 18. This shows the diagram and set-up for the inspection of flaps manufactured by SAAB for the detection of both surface and internal defects, by means of Guided Waves. The figure on the right shows the acquired ultrasonic signal for a zero-defect sample (grey) and a sample with defect (blue).
Challenges to be faced in the coming years:
Despite this, thermographic non-destructive testing has so many advantages that in recent years has it has become of special interest for sectors such as oil & gas, Automotive and Aeronautics. All these sectors are interested in replacing their current inspection techniques (magnetic particles and dye penetrant testing) with clean and automated technologies, such as thermography.
Ultrasonic inspection is of great interest for different industrial sectors such as aeronautics, wind or maritime, as it permits the semi-automatic inspection of large surfaces that are difficult to access.
