New trends in the transportation sector show an increasing demand for energy reduction as a solution for tackling environmental issues and climate change [1
]. Within this context, composite materials have found usage in the transport sector especially as structural components in aerospace [2
] and automotive applications [3
]. Such materials offer better mechanical properties, such as high strength and stiffness to weight ratios, over conventional materials [4
]. However, in the rail sector, carbody shells have been typically manufactured using metallic components such as steel and aluminium, which leads to relative high vehicle weight. The use of composites materials in the rail sector could lead to reduction in carbody mass [4
] and can potentially help the rail transport sector significantly reduce its environmental impact.
The use of carbon fibre or glass fibre components of monolithic and hybrid nature (i.e., sandwich type with foam or honeycomb core) in rail carbodies has been the subject of research by various EU-funded Shift2Rail projects such as PIVOT2 [5
], GEARBODIES [6
], CARBODIN [7
] and Roll2Rail [8
], as well as many research papers on existing prototype carbodies [4
]. When substituting conventional materials with alternatives, transport system manufacturers and operators must ensure safety. Composite components may suffer failures during manufacturing as well as be subjected to harsh operational environmental conditions during their service or even maintenance [11
]. Delaminations, disbonds, impact damage, core crushing, porosity or corrosion are common defects that can occur during the in-service phase of a composite component [12
]. Therefore, such new composite structures would require inspection for surface and subsurface defects, using non-destructive testing (NDT) techniques either during their maintenance phase or even in-service phase.
Infrared thermography (IRT) is an NDT technique that has been well documented in applications of inspecting composite materials. IRT is capable of capturing temperature radiation of a body, which is an electromagnetic radiation. When a heat flux Φ
falls on the surface of a body, part of it is absorbed ΦA
, reflected ΦR
and transmitted ΦΤ
]. The absorbed flux increases the internal thermal energy of the body and thus its temperature.
IRT can be divided into passive and active thermography. In passive thermography the object under inspection is observed using an infrared camera without the use of any external thermal sources, and thermal variations in the specimen, which can be higher or lower than the background, are measured under its environmental temperature [11
]. On the contrary, active thermography studies the object under inspection, using an external source of energy that can induce a temperature difference between defective and non-defective areas of the specimen [14
]. Common types of active IRT are optical methods such as pulsed thermography, lock-in thermography (LT), electromagnetic methods, i.e., eddy current thermography, and mechanical excitation methods, i.e., lock-in vibrothermography and burst vibrothermography [14
]. Ibarra-Castanedo et al. [14
], Wang et al. [15
], Ciampa et al. [16
], Ekanayake et al. [17
], D’Accardi et al. [18
], Vavilov et al. [19
], Liu et al. [20
] and Shrestha et al. [21
] describe the use of LT on composite sandwich and monolithic structures as one of the most popular IRT methods, being capable of defect detection in thicker specimens and being less prone to surface emissivity [11
]. Ramos Silva et al. [22
] compared the application of LT on Poly(methyl methacrylate) samples using two types of excitation. The first is an imperfect sinusoidal excitation coming from a conventional halogen heating system, while in the second case, a proportional–integral–derivative (PID) controller and light sensor have been added to the system to ensure a true sine wave excitation on the samples. This technique led to better stimulus accuracy and improved system frequency response that resulted in cleared images, especially in the case of the amplitude images.
Similarly, applications of step-heating thermography (SHT) on composite structures [23
] are described as an alternative to pulsed thermography (PT) [19
] for overcoming its limitations, as it is more applicable to thinner materials, and as an alternative to LT to overcome the limitations of long inspection time [23
]. Apart from using heat as an excitation method in active thermography, Szymanik et al. [30
] suggest the use of forced cooling as an additional excitation method that can affect heat dissipation within a material (in their case a glass fibre composite) and can cause rapid temperature change in defective areas which can be detected by an infrared camera.
IRT has found application in the inspection of large structures such as wind turbines and aircrafts [16
] that may consist of thick composite components either of monolithic or hybrid nature. An innovative solution of aircraft inspection is presented by Deane et al. [32
] using an unmanned aircraft vehicle (UAV) mounted with tethered halogen lamps that would be used to carry out PT. In addition, the application of IRT has become more popular over the recent years due to characteristics highlighted by [34
]: (1) offering fast inspection time; (2) being a contactless inspection method; (3) offering good spatial resolution and acquisition rate; (4) affordability in terms of cost; (5) advances in thermography image processing. The aforementioned characteristics and competencies of IRT make it a suitable inspection technique for composite rail carbodies that share similar characteristics with such structures where it has already been applied to.
presents a brief overview of IRT research that has been conducted on monolithic and sandwich composite components containing materials that could be suitable for rail carbody application, i.e., carbon fibre reinforced polymers (CFRP), glass fibre reinforced polymers (GFRP) or foam/honeycomb core materials [5
]. Emphasis was given to thicker monolithic materials, i.e., above 5 mm.
It needs to be noted that in the identified literature regarding inspection of sandwich components listed in Table 1
, all samples contained defects only in the front surface of the sample, which was the inspection side. Therefore, there was no intention of detecting defects on both sides of the sample in the aforementioned literature.
The aim of the current research is to present the application and qualification of active IRT and more specifically optical step heating and lock-in thermography techniques, where specific inspection requirements have been applied from the rail manufacturing industry for the detection of defects in new composite rail carbodies. The research presented in this paper has been carried out within the framework of the EU-funded project GEARBODIES [6
]. Specifically, the paper presents inspection protocols and results using the aforementioned IRT techniques that have been applied to two thick samples of composite components manufactured with artificial defects. The two samples that were inspected are (1) a monolithic carbon fibre reinforced polymer (CFRP) component of 20 mm thickness and (2) a sandwich block consisting of CFRP skins (5 mm thickness each) and a polyethylene terephthalate (PET) foam core (30 mm thickness). These samples were designed (thickness and choice of materials) and manufactured according to input from the rail manufacturing industry to resemble structural sections of future composite carbodies, as is presented in Section 3.1
The contribution of the current research is based on the application of IRT on the aforementioned samples that exhibit significant thickness, have thermal insulating properties (in the case of the sandwich foam core) and offer very low surface emissivity, thus making their inspection extremely challenging using only one-sided inspection. According to the authors’ knowledge and literature review, no other research has been identified covering the application of IRT on monolithic CFRP composites with 20 mm thickness or 40 mm sandwich and the combination of materials of CFRP and PET in the case of the sandwich sample. In addition, the inspection procedures that were followed were based on requirements that have been set according to input from the rail manufacturing industry. The first requirement was that the inspection of the rail carbody had to be carried out only using one-sided inspection, thus excluding the use of transmission mode thermography that can be used to overcome issues of emissivity and component thickness. Instead, the issue of emissivity was overcome by removing the heating lamps out of the IR camera’s field of view and placing them at an angle, while the camera had to be placed at larger distance from the sample. The second requirement was a time limit of 12 h that was set for inspecting both lateral sides of the carbody. Therefore, this research could be potentially useful for readers looking into identifying NDT methods for the combination of these materials in composite components as well for readers asking to what extent IRT could be used in terms of capability in detecting defects in these type of materials, as well as for new rail applications, in a more practical way.
The current paper presented the application of SHT and LT IRT on monolithic and sandwich composite components that will find structural usage on future composite rail carbodies, with the aim of presenting to the reader the capabilities of IRT as an inspection method for such specific application (although not limited). The inspection procedures had to comply with specific requirements set by the rail manufacturing industry that required only one-sided inspection as well as time limitations. The paper presented the challenges, signal processing results and limitations from the application of these IRT techniques. A gap in literature was identified regarding the use of IRT for inspecting the aforementioned components with low surface emissivity, such thickness or the combination of the sandwich materials.
The key challenges for the IRT inspections were the reflections caused by the low-emissivity shiny surface of the CFRP, the thickness of the samples and the thermal insulating material used for the sandwich core. Being restricted to reflection mode IRT, meant that the only solution to overcome this issue was to place the halogen lamps out of the IR camera’s field of view and at an angle. Hence, the use of longer heating protocols was another solution for the thickness of the samples. Furthermore, the Teflon® inserts also proved to be problematic due to inhibiting similar thermal behaviour with the CFRP material. In addition, the thermal properties of the PET foam meant that the core acted almost as the end of the component, thus making defects visible only in the front skin.
SHT and LT provided similar results, although step heating offered better results in an overall shorter duration. For the monolithic sample, both techniques detected defects up to 10 mm depth and gave indication of the defects at 15 mm depth. Overall, the inspection of the monolithic CFRP sample was easier due to the thickness of the 1 mm Teflon® inserts as well as that they consisted of a single material. For the sandwich component, only defects on the front skin were detected using both IRT techniques, limiting the detection to 5 mm depth. However, the SNR of the defects was low, possibly due to the thin Teflon® inserts. In retrospect, defects of larger size and thickness could potentially make detection easier and be tested in future work.
The results from this research conclude that SHT and LT are viable NDT techniques that can be used to inspect composite rail carbodies. However, further experimentation is required to improve detection using either other materials for creating artificial defects or inducing actual defects (i.e., impact damage or drilled holes) on the samples. Further assessment of the capabilities of these IRT techniques can be conducted using larger defects to establish a relationship between defect size and detection. Furthermore, part of future work is also to explore transmission mode IRT, simply to verify any potential changes in detecting all the defects throughout the thickness of the components.
Scalability of the inspection procedures to accommodate the scale of a commercial rail carbody or other transport vehicles that could use such composite components is needed. Using an IR camera with enough resolution and the same heat sources, larger carbody areas could be inspected in the same amount of time in order to decrease overall inspection time. Automation of the IRT techniques and their integration into an automated inspection platform is part of the ongoing research within the GEARBODIES EU project.