Non-Destructive Testing of Joints Used in Refrigerated Vehicle Bodies

Abstract

This paper focuses on the non-destructive evaluation of adhesive joints used in vehicles designed for transporting food products. The research and analysis were limited to the joints used in connecting elements of the cargo space. Two non-destructive methods were employed in the study: ultrasonic and thermographic techniques. Both methods confirmed the feasibility of evaluating adhesive joints in the construction of food transport vehicles, with the thermographic method proving to be much faster in identifying large areas of deadhesion in the plating. The ultrasonic method, on the other hand, allows for the inspection of sheathing and aluminum profiles. The predefined decibel drop in the height of the first two pulses on the ultrasonic defectoscope screen for areas with high-quality joints was less than 3.5 dB. In contrast, for areas with adhesion-related damage, the decibel drop in the first two pulses exceeded 4.5 dB.

1. Introduction

Food transportation is a significant area of the economy, analyzed from various perspectives in research. The first of these is the optimization of fuel consumption. In ongoing work [1], a minimum fuel consumption multi-period optimization model has been developed and solved using constraint programming, and then applied to a local supermarket network. Similar studies have been conducted by other researchers. For example, in [2], a mixed-integer linear programming model for planning the production, storage, and distribution of perishable products, which incorporates interactions with weather conditions, has been formulated. This addresses an open research field that remains largely unexplored. The authors noted that cold chains prevent the spoilage of perishable products but are highly energy-intensive. As much as 15% of the world’s total energy is already used to fuel cold chain infrastructures, and since 40% of food deliveries require refrigeration, the growing global food demand and the expansion of global supply chains will significantly increase energy consumption and associated carbon emissions. Additionally, the authors of [3] observed that emissions from food transport vehicles are responsible for 40% of the global greenhouse gas effect. The capacity, intensity, and environmental impact of these transport systems can be reduced through the use of phase change materials (eutectics). Advanced work in food cooling modeling is being applied to food transportation by motor vehicles. These systems are designed to maintain food temperatures within strict limits, ensuring optimal safety and high-quality shelf life [4].

The aforementioned studies are interesting and valuable, but they overlook the technical aspect related to food transport vehicles. The authors of [4] introduced a noteworthy classification of food transport vehicles.

Several restrictions and regulations apply to the design, construction, and operation of temperature-controlled transport vehicles. The most important piece of legislation in this area is the ATP Convention. This convention introduces a number of regulations that also cover the construction and use of transportation means. The classification of means of transport for transporting perishable foodstuffs is described in the Agreement Concerning the International Carriage of Perishable Foodstuffs and Special Means of Transport for this Purpose (ATP) [5]. There are four categories of such vehicles: isothermal, ice cream, refrigerated, and means of transport with heating capabilities.

A refrigerated body must guarantee that the cargo maintains the right temperature. A key element in the design of a refrigerated vehicle is a tight and well-insulated body. The construction of an isothermal vehicle is influenced by such factors as the thermal conductivity coefficient, the type of insulation used, the thickness of the insulation and the presence of thermal bridges (e.g., door seals, aggregate mountings, hinges, etc.).

The production of isothermal bodies involves the manufacture of insulated walls from sandwich panels, which are formed by foam injection or gluing and then joined together to form a closed, self-supporting box. The walls are joined using an adhesive that is applied to the entire interface. Reinforcing elements may be used (such as screws) to further increase the pressure when gluing the strips [6].

An issue worth exploring is the inspection of the condition of adhesive joints used in ATP vehicle construction. Adhesive joints have numerous advantages: they perfectly dampen vibrations, cause uniform stress distribution and seal the structure. Bonding has two great potential advantages with respect to welding: the speed of application and the absence of thermal distortions [7]. Furthermore, since it presents fewer risks for safety at work, it allows the simultaneous execution of other assembly operations. It is the adhesive joints, which use the phenomena of adhesion and cohesion, that make it possible to produce temperature-controlled food transport bodies. Testing of adhesive joints can be divided into two main groups. These are destructive testing and non-destructive testing. Destructive testing covers many aspects. In [8], the authors showed that the effectiveness of adhesive bonds is notably impacted by the thickness of the cured adhesive layer, underscoring the need for meticulous monitoring within adhesive bonding techniques. A precise adherence to technological protocols and the use of specialized instruments are imperative to ensure the production of high-quality samples meeting the tolerances outlined in relevant standards.

For example, the paper [9] modeled adhesive joints and found that the numerical values of maximum load were close to the experimental values, validating the numerical methodology used to predict the lap shear strength and providing the necessary data to explain the obtained behavior. It also noted that the increase of adherend thickness for CFRP (carbon fiber reinforced plastics) (i.e., from 1.2 mm to 2.1 mm) did not promote a significant increase in joints’ strength; thus, a reduction in mass can be achieved without losing the joint efficiency. In the paper [9], the authors provide an overview of the current developments in the use of “smart” adhesive technology and introduced the reader to early findings on the use of self-healing materials, thermally expandable particles, and nanoparticles, among others, in adhesives and their potential to increase the reliability of adhesive joints. In this work, the possibility of using adhesives in the production of commercial vehicles was confirmed, and the work carried out was also aimed at reducing costs and improving the safety of structures—recycling, healing, or self-healing of bonded structures. The work carried out also included modeling of adhesive joints when joining aluminum [10]. The model developed in work [11] is based on basic mechanical analysis and the application of Adams Peppiatt’s stress equation for single joints, the kind often found in adhesive joints.

Non-destructive evaluation of adhesive joints, especially those used in automotive applications, is an important issue. Already at the end of the last century, R.D. Adams [12] proposed various non-destructive methods for evaluating adhesive joints. According to Adams, the method that provides a fairly good understanding of the properties of an adhesive joint is the ultrasonic method. The quality of adhesive joints using the non-destructive method was also studied in [13]. In this work, three different artificial debondings within composite-adhesive single-lap joints were investigated using ultrasonic immersion pulse-echo technique and induction thermography. Data fusion has been used to increase the performance of different defect detection. Induction thermography NDT performs well with electrically conductive inclusion detection; however, it is not sensitive to nonconductive inclusions. While ultrasonic NDT performs better in detecting release film inclusion, it is an obvious fact that brass inclusion (or any inclusion with high electrical conductivity) is detected better with induction thermography.

In the construction of commercial vehicles and buses, adhesives are widely used, mainly polyurethane and polymeric adhesives. In ultrasonic testing, the acoustic properties of adhesives are important, especially acoustic resistivity. Acoustic resistivity is proportional to the density and speed of ultrasonic waves. Polyurethane and polymer adhesives are high-density adhesives. In the case of temperature-controlled vehicle sheathing, there is an adhesive bond between the insulation layer and the steel sheet. The insulation layer is many times less dense than the adhesives, so it has a different acoustic resistance. The acoustic resistance has a significant effect on the reflection and transmission coefficients of ultrasonic waves. Ultrasonic testing of adhesive joints has been conducted for many years. These are mainly tests of steel-adhesive bonds [14], aluminum-adhesive bonds [15], and adhesive joints of composite materials [16,17]. Ultrasonic evaluation of fatigue damage in adhesive joints is also one of the directions of research conducted by scientists [18]. The quality of adhesive joints was also investigated using non-destructive methods in [19]. Three different types of composite-adhesive joints were examined in this study, utilizing both immersion ultrasonic testing and induction thermography. Similarly, ultrasonic testing was also performed in [20]. In this study, single-layer aluminum adhesive joints were tested in pulse-echo mode. The joints contained defects. The purpose of the test was to increase the probability of detecting defects and to estimate their size. All of these tests involved an adhesive layer that had markedly different acoustic properties from the insulating layer used in temperature-controlled food vehicles. Ultrasonic adhesion bonding has been studied, for example, in the paper [21], B\but it too had completely different adhesive properties. In this work [21], an automotive putty used in vehicle repair, for example, was studied. As a result of vehicle body vibrations, adhesive joints can be damaged. Vibrations occurring during temperature-controlled food transportation have been studied in works [22,23,24,25]. The impact of truck speed and pavement type on vibration levels during transport was analyzed by extracting time-zone data corresponding to predetermined truck speeds and pavement types from vibration records collected along a test route specifically configured for this study. In [26], it was found that vertical total effective G-values were approximately 2.6 and 3.0 times higher than those in the transverse and longitudinal directions, respectively. Such vibrations have a significant impact on the durability of adhesive joints. The vibration frequencies were around 3 Hz for leaf springs and 1.5 Hz for air springs. Another important aspect studied in various environments was the phenomenon of joint detachment under different conditions. Tests were conducted for a variety of materials, including soft materials, as well as joints for hard materials [27]. Vibration and stress undoubtedly cause the deterioration of adhesive joints, which can significantly affect the operational characteristics of refrigeration vehicles [28].

The purpose of the research conducted in the present study was to verify the evaluation of the condition of the refrigerated superstructure in terms of the quality of adhesive joints. The results of the research may be important not only at the stage of vehicle production but also at the stage of operation.

2. Research

2.1. Research Procedure

The entire study was carried out according to the scheme shown in Figure 1. Due to the actual thickness of the walls and the attenuation of ultrasonic waves, the study used the ultrasonic echo technique. Classical ultrasonic testing uses frequencies ranging from 2 to 6 MHz. Since the thickness of the sheet metal used in the construction of refrigerated vehicle bodies is about 1mm, sometimes 0.8 mm, it was necessary to use high-frequency probes, above 6 MHz; thus, the stage of selecting ultrasonic probes for the tested joints was also planned. Tests using a thermal imaging camera were also planned. For this purpose, it was necessary to determine the temperature to which the samples should be frozen and to check what emissivity factor should be adopted.

2.2. Materials and Methods

2.2.1. Materials

The subject of the study was selected nodes of a temperature-controlled food transport vehicle. For ecological and economic reasons, vehicle bodies are becoming lighter, which involves a reduction in material usage. Polyurethane or extruded polystyrene, often 40 to 60 mm thick, is commonly used as an insulating layer. Since the insulation layer has low mechanical strength and poor resistance to environmental conditions, steel sheets (about 1 mm thick) or laminates (about 2 mm thick) are used on both sides. When producing such walls, insulation core sheets are glued together with laminate on a vacuum table. In terms of fixing hinges and joining walls, different technologies are used, but the most common is a combination of adhesives and screw connections. When selecting areas for research, the results of studies obtained by other research centers were considered. The phenomenon of joint detachment under different conditions, for various materials, including soft materials [29], and connections for hard materials were also studied [30]. Vibration and stress undoubtedly cause the deterioration of adhesive joints.

After analyzing the structure, properties [31], and trends in the development of temperature-controlled food transportation vehicles [32], specific areas were selected for testing. The first area is a section of the side wall of the body, and the second is the junction of an angle-shaped profile with a thin steel sheet (Figure 2).

On the prepared samples, damage in the nature of adhesive failure was simulated. A sample was also prepared to evaluate the effect of the thickness of the insulation layer from the signal obtained on the ultrasonic defectoscope.

A view of the main sample is shown in Figure 3. Figure 4a shows the dimensions of the sample along with the plotting of the measurement points, while Figure 4b shows a photograph of the sample. Figure 5a shows a view of the specimen after removing some of the plating and insulation, while Figure 5b shows a view of the specimen after the insulation layer was pasted in place, allowing the lack of adhesive damage to be mirrored.

The second element tested was the combination of an aluminum profile with a section of sheathing (Figure 6). Two profiles were used in the study; they had the same dimensions, but one was anodized. Hybrid glue was used to attach the aluminum profiles to the sheathing of the cold storage unit. Figure 7a shows the dimensions and measurement points, while Figure 7b shows the specimen model. In the main tests, the connection was evaluated for two profiles. The third profile, shown in the figure, was used as a reference profile for calibrating the apparatus, establishing measurement ranges and apparatus settings.

2.2.2. Ultrasonic Method

In ultrasonic testing, ultrasonic probes with frequencies above 4 MHz were pre-selected for assessing the condition of the joint, due to the attenuation and length of the ultrasonic wave. It was verified which probes were able to obtain pulses on the screen of the ultrasonic defectoscope that are useful for the purpose of testing the systems. The choice of ultrasonic probes with relatively high frequencies is due to the relationship between wavelength, frequency, and speed. For a steel sheet, a wavelength comparable to the thickness of the sheet to be tested (1 mm) is obtained at a frequency of 6 MHz. Increasing frequency increases measurement accuracy by shortening the ultrasonic wavelength (for a frequency of 20 MHz, the wavelength will be about 0.3 mm), while increasing attenuation, which is unfavorable. The research was carried out both for the side plating and for the profile, which can be used for joining the walls and doors of the cargo space. It was decided to use ultrasonic probes such as P10-10L (frequency 6.37 MHz), DS6MB 4–12 (frequency 12.21 MHz), SBS3PB 6–16 (frequency 13.70 MHz), S6WB10WM (frequency 7.68 MHz), and G20MNX (frequency 20 MHz). The choice of ultrasonic probes with a relatively high frequency is due to the relationship between wavelength, frequency, and speed. For a steel sheet, a wavelength comparable to the thickness of the sheet to be tested (1mm) is obtained at a frequency of 6 MHz. Increasing frequency increases measurement accuracy by shortening the ultrasonic wavelength (for a frequency of 20 MHz, the wavelength will be about 0.3 mm), while increasing attenuation, which is unfavorable. Two digital ultrasonic defectoscopes—USM35XS and CUD—were used in the first stage of the study. Due to the relatively high frequencies and bandwidth range of the defectoscope, the USM35XS was selected for alkaline testing, which allowed the desired signals to be obtained for both the SBS3PB 6–16 and G20MNX ultrasonic probes. A view of the test stand for the first stage of testing is shown in Figure 8. Figure 8a shows the test attempt for the CUD defectoscope and the S6WB10WM ultrasonic probe, while Figure 8b shows the attempt to obtain the desired signal for the same ultrasonic probe (S6WB10WM) and the USM35XS defectoscope. In practice, the authors have used various measurement systems [33], while temperature-controlled transfer vehicles are not feasible for immersion testing due to their dimensions. Attempts have been made to model adhesive joints in terms of nondestructive testing [34], but the work did not include the types of joints studied in this paper.

Figure 8c,d shows that at a frequency of 10MHz, it is possible to obtain a string of pulses on the screen of the defectoscope, and that it is possible to make measurements of the thickness of 1mm thick sheet metal; however, the decreases and increases in the height of the pulses on the screen of the defectoscope are indicative of testing conducted in the near field of the ultrasonic probe. Working in the near field makes it impossible to use known ultrasonic measures to evaluate adhesive joints, and the values obtained may be random. The ultrasonic transducer G20MNX was selected to test the adhesive joint of the sheet metal. In the case of testing the adhesive joint of an aluminum profile (with a thickness of about 2.5 mm) with a hybrid adhesive, the SBS3PB 6–16 transducer was selected for use. It allowed us to obtain useful signals for testing. In the aluminum profile, the attenuation of the 20 MHz ultrasonic wave from the G20MNX ultrasonic probe was too high.

The decibel drop in the height of the first two pulses was used as a measure of the quality of the adhesive bond. The choice of such a measure is dictated by limiting the influence of the force and stability of pressing the probe against the surface to be tested, which is particularly important when testing objects in road or field conditions. The principle of determining the ultrasonic measure is shown in Figure 9.

2.2.3. Thermographic Method

In order to perform thermal imaging tests, the test construction sample was frozen to −18 °C. Thermograms obtained with a FLIR thermal imaging camera were captured under consistent ambient conditions to ensure accuracy. The air temperature was maintained at 22 °C with a relative humidity of 60%, providing a controlled environment for the measurements. The distance from the object was precisely set at 1 m to standardize the imaging conditions.

For the analysis of the thermographic images, a value of 0.90 was introduced as the emissivity coefficient. This choice was crucial, as it allowed for the alignment of temperature readings obtained from both the multimeter and the thermal camera. By ensuring that both devices operated under the same emissivity assumption, the results became comparable, enhancing the reliability of the thermal imaging tests. This methodology not only validates the temperature measurements but also contributes to a deeper understanding of the thermal properties of the construction materials evaluated.

3. Results

3.1. Results Obtained for the Ultrasonic Method

As part of the work, the measurement error was initially determined. For this purpose, the measurement was repeated 150 times in two areas (with and without thermal insulation). A summary of these results is shown in

Table 1. Measurement errors for the ultrasonic method (the complete set of results is available in the Appendix A, Table A1).

	Decibel Drop for High Quality Area	Decibel Drop for Area with Adhesion Damage
Average value dB	3.009	7.147
Standard deviation	0.121	0.143
The t-student coefficient	1.976	1.976
Measurement error dB	0.240	0.282

, while the aggregate results are presented in Appendix A—

Table A1. Measurement results to determine measurement errors (No—measurement number, GA—area in which adhesive joints were of high quality, BA—area in which adhesion damage occurred.

No	GA	BA	No	GA	BA	No	GA dB	BA dB
1	3.19	7.33	51	2.95	7.00	101	3.20	7.38
2	2.86	7.08	52	2.97	7.06	102	2.84	6.92
3	2.83	7.02	53	3.07	6.95	103	3.09	7.04
4	2.92	7.18	54	2.97	7.01	104	3.14	7.32
5	2.81	6.95	55	2.88	7.35	105	3.02	6.94
6	3.18	7.23	56	3.06	7.09	106	3.19	7.17
7	2.84	7.03	57	3.06	7.39	107	3.19	6.94
8	2.92	7.16	58	3.05	7.27	108	3.05	7.34
9	3.08	7.37	59	2.95	7.06	109	2.87	7.19
10	2.94	7.02	60	2.91	7.24	110	3.17	7.07
11	2.95	7.36	61	3.15	7.36	111	2.86	7.18
12	3.14	7.35	62	3.06	7.33	112	3.13	7.38
13	2.83	7.13	63	2.82	6.96	113	2.84	7.01
14	3.00	7.05	64	3.05	7.08	114	2.91	6.98
15	3.03	7.23	65	3.22	7.33	115	3.19	7.06
16	3.05	7.16	66	3.08	7.00	116	3.13	7.10
17	3.16	7.09	67	3.09	7.04	117	2.92	7.11
18	2.85	7.31	68	2.81	7.09	118	3.21	7.39
19	2.88	7.23	69	2.89	7.23	119	2.91	7.35
20	3.03	7.23	70	3.04	6.99	120	2.89	7.17
21	2.98	7.20	71	2.88	6.99	121	2.94	7.22
22	2.87	7.33	72	3.19	7.27	122	2.92	7.06
23	3.18	7.21	73	3.17	7.29	123	2.98	6.95
24	3.01	7.02	74	3.03	6.94	124	2.91	6.93
25	2.80	7.26	75	2.85	7.00	125	2.99	7.13
26	2.94	7.38	76	2.84	6.98	126	3.22	7.05
27	2.94	7.38	77	2.97	7.11	127	3.17	7.38
28	3.21	7.03	78	2.97	7.00	128	3.20	6.97
29	2.87	7.27	79	2.89	7.18	129	3.01	6.96
30	2.88	7.16	80	2.88	7.08	130	3.17	6.92
31	3.01	6.96	81	3.16	7.35	131	3.12	6.97
32	3.09	7.36	82	3.00	7.05	132	2.86	7.09
33	3.21	6.93	83	3.20	7.34	133	2.81	7.05
34	3.14	6.92	84	2.97	7.26	134	2.95	7.23
35	2.97	7.00	85	3.10	7.34	135	2.99	7.13
36	2.97	7.21	86	3.10	7.17	136	2.82	7.37
37	3.11	7.27	87	3.11	7.19	137	2.83	7.08
38	3.08	7.34	88	3.08	6.93	138	3.06	7.27
39	2.80	7.32	89	2.90	7.31	139	3.01	7.20
40	3.18	6.92	90	2.92	7.12	140	3.00	7.11
41	3.07	7.15	91	3.11	7.34	141	2.98	7.34
42	2.95	6.93	92	3.02	6.95	142	3.21	7.01
43	2.95	7.16	93	3.12	7.21	143	3.00	7.25
44	3.21	7.13	94	3.02	7.31	144	3.20	7.23
45	3.08	7.25	95	3.11	7.01	145	3.08	7.16
46	3.10	7.12	96	3.22	7.10	146	2.85	7.32
47	2.96	7.25	97	2.89	7.18	147	2.86	7.24
48	2.88	7.13	98	2.99	7.19	148	3.04	7.19
49	2.94	7.08	99	2.97	6.92	149	3.05	7.06
50	3.06	6.92	100	2.81	7.06	150	2.98	6.96

.

In the next step, it was checked whether the thickness of the insulation layer affects the value of the ultrasonic measure obtained. The tests were carried out on a sample, the model of which, along with the ultrasonic probe used, is shown in Figure 10. The results of the tests are shown in Figure 11. It is clear that the ultrasonic method makes it possible to determine whether there is adhesion between the plating and the iso-layer in the test area, but there is no way to estimate the thickness of the insulation layer.

In the next step, a survey was conducted to identify possible separations between the insulation layer and the shell of the refrigerated vehicle body. Measurements were taken at 900 measurement points, and the results are shown in Figure 12.

In the final step of ultrasonic testing, an attempt was made to locate areas of glue detachment from aluminum profiles. The tests were conducted on two profiles (aluminum and anodized aluminum). On each of the tested profiles, 120 measurement points were determined. In these tests, due to the damping, the SBS3PB 6–16 probe was used. It is a probe equipped with a rigid delay line, which allows fast and efficient testing of aluminum profiles. The test results are shown in

Table 2. Ultrasonic measurement results for aluminum profile adhesive joint

	Ultrasonic Measure R, dB
Location of the Measuring Point	1	2	3	4
1	5.00	3.14	2.88	5.06
2	4.93	2.64	5.19	4.94
3	4.92	2.97	2.78	2.80
4	5.05	2.77	2.72	2.87
5	5.15	2.67	2.99	2.99
6	5.32	3.16	3.06	3.29
7	4.93	3.23	3.05	2.83
8	5.47	2.66	2.91	3.06
9	5.44	3.05	3.27	2.65
10	4.84	3.26	2.78	2.96
11	5.29	3.00	3.14	2.76
12	5.21	2.63	2.98	3.29
13	5.13	2.71	2.92	2.89
14	5.57	5.18	3.25	3.18
15	4.97	5.05	2.95	2.96
16	5.62	5.14	5.42	3.31
17	2.77	2.62	3.30	3.26
18	3.22	3.00	2.76	3.20
19	2.86	3.16	2.90	3.07
20	3.23	3.01	2.71	3.13
21	2.96	2.62	3.28	3.01
22	2.80	3.18	2.61	3.01
23	2.96	2.89	2.68	2.79
24	3.01	2.93	2.66	3.09
25	3.28	2.69	3.31	2.78
26	5.41	2.99	3.21	3.21
27	5.65	3.00	3.03	4.84
28	4.76	2.72	3.09	5.43
29	5.18	5.50	2.66	4.95
30	5.35	5.46	5.14	4.99

and

Table 3. Ultrasonic measurement results for the joint of aluminum profile anodized adhesive

	Ultrasonic Measure R, dB
Location of the Measuring Point	1	2	3	4
1	5.54	5.00	5.49	5.47
2	4.96	4.74	5.21	4.88
3	4.90	3.01	2.91	4.76
4	2.64	3.20	2.69	5.42
5	3.23	3.14	2.94	4.83
6	2.93	2.61	2.93	5.59
7	2.86	3.21	3.22	5.51
8	2.61	2.84	2.71	2.98
9	2.83	3.22	2.77	2.74
10	3.14	2.89	2.61	3.17
11	2.85	2.75	3.02	2.77
12	3.21	2.73	3.20	3.16
13	4.91	3.28	2.79	3.15
14	5.26	5.31	3.02	2.64
15	4.84	5.19	2.75	2.75
16	5.59	5.62	3.23	3.14
17	5.55	5.08	3.09	3.28
18	5.01	3.01	2.93	2.96
19	5.60	2.88	2.61	3.06
20	4.89	3.24	2.98	2.68
21	5.60	2.75	2.68	5.15
22	5.53	2.95	2.81	5.21
23	5.04	2.67	3.29	5.50
24	5.34	3.08	2.70	5.36
25	5.41	2.90	2.63	5.33
26	5.11	2.80	2.86	5.16
27	5.21	3.14	3.01	4.97
28	5.27	3.26	2.86	5.12
29	5.34	2.81	3.24	5.19
30	5.66	5.09	4.98	5.31

.

The results presented in Table 2 and Table 3 confirm that it is possible to control the adhesive bonding for both tested profiles made of aluminum alloy. The areas where adhesion damage occurred are indicated. The decibel value of the height drop of the first two pulses on the screen of the ultrasonic defectoscope for areas with a high-quality connection was less than 3.5 dB. For areas with adhesion-like damage, the decibel value of the height drop of the first two pulses on the ultrasonic defectoscope screen was above 4.5 dB.

3.2. Results Obtained for the Thermographic Method

Non-contact temperature measurement techniques can be used to assess the performance of thermal insulation panels. Any object with a temperature above absolute zero emits thermal radiation. Objects warmer than room temperature emit more radiation at shorter wavelengths, making them easier to detect. Thermography is the process of creating an image in the infrared band with a wavelength of 0.9 to 14 μm, which makes it possible to record the thermal radiation emitted by an object under typical environmental conditions. Thermal imaging can be used to monitor the manufacturing processes of thermal insulation panels, as well as to assess the quality and tightness of thermal insulation. It is a fast diagnostic method that can also check the quality of adhesive joints.

The accuracy of the measurement is affected by a number of factors, primarily the emissivity of the surface of the object under study. Emissivity is a measure of the intensity of the thermal radiation of an object compared to that of a blackbody of the same temperature. Emissivity values range from 0.1 to 1.0. In addition to emissivity, precise measurements should pay attention to the value of the reflected apparent temperature, the distance of the camera from the object, and the ambient conditions, including relative humidity and ambient temperature. Due to the difficulty of accurately determining the emissivity, a qualitative approach is often used, which involves locating areas of increased or decreased temperature rather than making accurate quantitative measurements. For quick comparative studies, qualitative measurement, which can be performed with simple thermal imaging cameras, is sufficient.

The study used a FLIR camera with an infrared detector resolution of 640 × 480. The temperature measurement range was −20 °C to +120 °C, with an accuracy of 0.1 °C, and the minimum distance from the object was 0.15 m. All thermograms were obtained under the same conditions: the air temperature was 22 °C, the humidity was 60%, and the distance from the object was 1 m. An emissivity value of 0.9 was assumed, and two measurements were taken. One element had an identical surface, and the other had stick-on angles of different materials. The entire first surface was made of a single element—a laminate—so a single emissivity value was assumed. The thermogram of this element (Figure 13) shows a spot with different temperatures, suggesting differences in thermal conductivity. The image shows a possible defect in the form of a circle on the tested surface. In the second case (Figure 14), elements made of different materials were placed on the surface of the laminate, resulting in infrared reflection. In order to obtain an accurate measurement, the emissivity value would have to be changed, which, in practice, is inconvenient for a quick survey. With modern thermal imaging cameras, however, emissivity values can be changed in software after the image is taken. The emissivity value can also be accurately determined based on independent temperature measurements; however, in comparative studies to detect damage, this is not necessary.

4. Conclusions

A study was conducted to confirm the feasibility of non-destructive evaluation of adhesive joints. Two different methods were employed. The thermal imaging method is notably faster and enables efficient localization of adhesive damage. In contrast, the ultrasonic method provides more accurate damage localization but requires significantly more time. Using a thermal imaging camera, the entire cargo space plating can be examined in less than 30 min. The same inspection using the ultrasonic method takes approximately 8 h. An additional advantage of the thermal imaging method is that it can be performed from ground level, thereby reducing the risk of accidents, such as falls from working platforms.

For the inspection of profiles, the ultrasonic method proved effective, whereas the thermographic method did not yield satisfactory results. However, thermal imaging can be used for preliminary screening. In the future, it may also be possible to automate inspections using thermographic cameras in maintenance and repair facilities, as well as diagnostic stations.

The use of thermal imaging technology can facilitate the rapid detection of heat losses in structural nodes. It can also be applied to monitor manufacturing processes in the production of thermal insulation, including refrigeration vehicles. Thermal imaging allows for the assessment of the quality and tightness of bonded joints and serves as a quick diagnostic tool to evaluate insulation quality and workmanship, as well as to identify discontinuities in adhesive joints.