Thermography is a semiautomatic and non-destructive imaging technique that is used to characterize material defects through visualization of heat patterns of that material (Chen et al., 2016). As such, thermal imaging utilizes a camera consisting of infrared sensors which facilitate the detection and measurement of small temperature differences. The output image is displayed on the PC screen as a grey or color-scale map. There are two types of thermography including (1) passive thermography in which structures and materials are naturally are at different temperatures either higher or lower compared to the background and (2) Active thermography which involves pointing the camera at the test material, and a temperature map is created from the thermal image. However, this process demands external stimulus to thermal product contract in the test material surface since the material flaws are detected by the differences in the temperature decay rate.
Chen et al., (2016) claim that active thermography is widely applied in non-destructive testing (NDT) considering any form of energy can be utilized to stimulate the test material as long as the thermo-physical properties of the final defects are sufficiently different from the non-defective zones to yield quantifiable thermal differences . Besides, the use of an external stimulus can be harmonized with the acquisition, presenting an opportunity for developing a quantitative data analysis model where the subsurface defect depth is proportional to the time its appearance.
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Composite materials are currently widely used in construction aircraft structures, a potential substitute for metals, since they have high tensile strength, fracture toughness as well as resistance to corrosion and light. In addition to the development of structures and materials, specific NDT techniques are required to detect and identify defects and damages specific to these composite materials; and this is why thermography comes in. However, full composite structures misalign with specific hybrid structures and specific requirements hence the incorporation of both metals and composites is still crucial. In such cases adapted NDT must be researched and assessed (Wu, Zweschper, Salerno, & Busse, 1998). Therefore, this research paper is a detailed, accurate and thorough discussion of thermography as a non-destructive inspection method used in testing composite parts and planes with focus on composite parts typically of aircraft.
Historical Background of Thermography
The roots of thermography can be traced back to ancient pyramids era where it was used for medical purposes. Their medical knowledge associated temperatures with diseases and as such, they employed primitive in the form of thermography for prognosis of ailments. Physicians normally applied a thin coat of mud to patients’ body and then made observations based on the patterns formed by the different rates of drying mud which was caused by varying patterns of cold and hot temperatures on the surface of the patient’s body (Meola, 2012).
According to Meola ( 2012), the discovery of ‘thermoscope’ in 1500 AD invigorated the science of measuring temperatures with other following centuries later from simple crude thermometer to sophisticated mercury thermometer. However, the discovery of the thermo-electrical device in 1835 to measure temperatures of inflamed parts of the body with varying temperatures resulted in scientists extending the application of the thermometry. The new application involved photography in recording infrared spectrum . After extensive research on thermal imaging, thermography was in 1972 as an NDT method beyond experimental with Food and Drug Authority (FDA) approved medical thermography for measuring varying skin temperatures on patients
Application of Thermography
Thermography has a wide range of applications including:-
Medical industry for measuring varying skin temperatures in ailments prognosis
Aerospace industry for various applications such as in solid laminates, delamination/impact damage, sandwich panels, and epoxy/carbon composites bonds
Automotive industry for various applications such as in adhesive bonds, spot welds, and composite structures
Power industry for various applications such as in coating uniformity, turbine blades, and delamination in composites (Ibarra-Castanedo, Tarpani, & Maldague, 2013).
Application of Thermography if Defects Detection
The figure below is an illustration some of the elements to consider in a thermography inspection scenario.
Fig 1: An illustration some of the elements to consider in a thermography inspection scenario
Fig. 2: Inspection scenario for thermography techniques
When the above conditions are satisfied the selected waveform dictates the thermography method to be used such as pulsed thermography, lock-in thermography, step heating thermography or square pulse thermography.
Fig. 3: Typical setup of optical pulsed thermography.
According to Ibarra-Castanedo, Tarpani, and Maldague (2013), although no specified standard material size should be tested , 5-15 mm thick specimens are normally used in thermography for NDT. First, the test specimen is first tightly clamped along its edges and subjected to impacts with various energy levels by using a falling mass consisting of a 16 mm diameter steel ball tip. Application of external heat a short period, normally 10 ms, causes high initial temperatures. However, this energy is distributed over a prolonged time through modulation of heat deposition to minimize the thermal load of the tested material to a lower power density. Such conditions are acceptable for aerospace structures.
Fig. 4: Typical setup for optical step heating thermography
Assuming the method of testing is optical heating pulsed thermography, the inspection scenario resembles Fig. 2a: active approach, static configuration, reflection mode, surface scanning technique, optical energy source, and pulse waveform .
Fig. 5: Test specimen (a) standard size and a map of impacts (b) front side and (b) back side.
The working principle of thermography is that the induction of temperature modulation from the impacted surface of the specimen propagates as a thermal wave. Thermal waves are coming back from the inside of the component modify the temperature modulation at the surface as this wave undergoes reflections at boundaries like all other waves (Chen et al., 2016). As such, a sensitive indicator such influences is the phase angle between the local thermal response and energy deposition. Ibarra-Castanedo, Tarpani, and Maldague (2013) assert that performance of Fourier analysis at the individual pixel gives the phase and magnitude of the local response particularly after monitoring temperature field during modulated illuminated phase using thermography camera.
However, optical illumination distribution, optical surface absorption inhomogeneity, and infrared emissions affect the images’ magnitude although these effects can be eliminated by evaluating the individual pixel. In a nutshell, this scenario is significant in testing aerospace structures with standard sizes that pose challenges for uniform optical illumination (Ibarra-Castanedo, Tarpani, & Maldague, 2013). The output data is stored as a 3-dimensional matrix as illustrated in Fig. 7 below where ‘t’ is the time while x and y are spatial coordinates. The obtained data is analyzed using various approaches such as thermal contrast-based, statistical, matrix factorization, and signal transforms techniques (Wu, Zweschper, Salerno, & Busse, 1998).
Fig 6: (a) 3-Dimensional thermograph matrix and (b) 1-Dimensional temperature profiles for sound area (continuous line) and impacted zone (dotted line)
Below are typical observations in thermography .
Fig. 7: typical observations in thermograph for active and passive approaches
Advantages and Disadvantages of Thermal Imaging
On the one hand , thermography has several benefits including:-
Contactless – Thermography does not require coupling compared to conventional ultrasound techniques. However, it should be noted that ultrasound thermography demands coupling media amid the test material and the transducer while in induction thermography the coils are required to be relatively close to the test surface of the specimen.
Safe – The user is not at risk of any harmful radiations such as in the case of x-ray radiography. However, external stimulation of high power including powerful flashes necessitates eye protection whereas heat-induced ultrasound necessitates wearing of protective ear plugs.
Imaging capabilities – The output data for thermography is obtained in the form of video formats or images which are relatively easy to interpret.
Flexibility in applications – Thermography is one of the NDT with varies and numerous applications ranging from medical to aerospace, power, and automotive industries. (Ibarra-Castanedo, Tarpani, & Maldague, 2013)
On the other hand, thermography has its limitations including:-
Inhomogeneity in heating – Thermography poses challenges of obtaining fast, homogeneous and highly exited stimulations over large surfaces particularly in pulsed thermography. As such, several processing techniques are required.
Thermal losses – Thermography is affected by thermal losses resulting from heat radiation and convection processes which have the potential of inducing spurious contrasts thereby upsetting the reliability of data for interpretation.
Cost-intensive – Thermographic equipment used in active thermal imaging such as thermal stimulation units and infrared cameras are expensive compared to some NDI techniques that only require visual inspection although such costs cannot be compared to equipment such as x-ray systems and phased array.
Limited defect detection –Thermography is limited to detection of defects emanating from measurable variations in thermal properties of the tested specimen.
Limited test thickness – Thermal imaging can only be used to test specimens of limited thickness under the surface although defect detection is manageable several centimeters under the surface particularly in lock-in thermography if very low modulation frequencies are used .
Emissivity variations – Specimens with low emissivity values reflect thermal radiations from the environment powerfully. However, surface painting can be applied when possible to equalize as well as increase emissions. (Ibarra-Castanedo, Tarpani, & Maldague, 2013)
Detected Defects using Thermography
Thermography excels in detecting various types of defects typically found in adhesive joints including void sand, dis-bonds , delamination, inclusions and kissing defects. These defects are easily detectable since the interfaces of composites have varying densities, heat capacities and heat conductivities which can be characterized and localized compared to the bulk material. The obtained data from thermographs is analyzed using various approaches such as thermal contrast-based, statistical, matrix factorization, and signal transforms techniques (Grys, 2018).
Fig 8: Materials and detectable Defects
A variation on the Thermography Technique
Georges et al. (2018) suggest shearography as an alternative NDT testing solution for thermography. The technique is tailored specifically to address processes related to integrated quality control used in determining the structural integrity of the material . Besides, the technique applies to different materials, and it can detect most of the defects that occur in composite structures depending on the depth of the defects within a specimen as well as the strength of the specimen material. The advantage of Shearography is that it can detect defects in the most challenging places to reach with conventional techniques. Shearography is categorized into two; (1) laser shearography which is an optical measurement approach for quality control and NDT applications in an expedited manner particularly in metals and composite materials and (2) Vacuum shearography which uses a vacuum NDT technique but rarely used (Georges et al., 2018).
In conclusion, thermography is a dynamic and versatile method of NDT. The technique is applicable in various industries including medical, power, aerospace and automotive. Although this technique has benefits such as safety, contactless, imaging capabilities, and flexibility in applications, it has its limitations such as inhomogeneity in heating, cost-intensive , limited defect detection, thermal losses, limited test thickness, and emissivity variations. Thermography excels in detecting various types of defects typically found in adhesive joints including void sand, dis-bonds , delamination, inclusions and kissing defects.
References
Chen, D., Zhang, X., Zhang, G., Zhang, Y., & Li, X. (2016, October). Infrared Thermography and Its Applications in Aircraft Non-destructive Testing. In Identification, Information and Knowledge in the Internet of Things (IIKI), 2016 International Conference on (pp. 374-379). IEEE.
Georges, M., Srajbr, C., Menner, P., Koch, J., & Dillenz, A. (2018, June). Thermography and Shearography Inspection of Composite Hybrid Sandwich Structure Made of CFRP and GFRP Core and Titanium Skins. In Multidisciplinary Digital Publishing Institute Proceedings (Vol. 2, No. 8, p. 484).
Grys, S. (2018). Determining the dimension of subsurface defects by active infrared thermography–experimental research. Journal of Sensors and Sensor Systems , 7 (1), 153-160.
Ibarra-Castanedo, C., Tarpani, J. R., & Maldague, X. P. (2013). Nondestructive testing with thermography. European Journal of Physics , 34 (6), S91.
Meola, C. (2012). Origin and theory of infrared thermography. Infrared Thermography Recent Advances and Future Trends, Bentham eBooks , 3-28.
Tighe, R. C., Dulieu-Barton, J. M., & Quinn, S. (2017). Infrared techniques for practical defect identification in bonded joints in liquefied natural gas carriers. Experimental Techniques , 1-8.
Wu, D., Zweschper, T., Salerno, A., & Busse, G. (1998). Lock-in thermography for nondestructive evaluation of aerospace structures. NDT. Net , 3 (9).
