1 Introduction

Composites are a large group of materials with exceptional properties developed by combining at least two components that differ greatly. They exhibit high mechanical strength which, in combination with low mass, makes them great construction materials. Therefore, they found applications in many fields such as e.g., aerospace, automotive, medicine, and military. [1]. The combination of two materials with various mechanical and chemical properties may lead to the formation of some defects which may arise during composites manufacturing or their use [2]. The latter relates to large stresses to which composites may be exposed during their work which include among others high humidity, aggressive chemicals, and physical attacks. Therefore, it is highly important to resolve methods allowing for inspection of the composites-based constructions objects during their lifetime to protect them from hazardous failure.

Nowadays, a great deal of attention has been paid to non-destructive testing (NDT) methods which allow for noninvasive evaluation of the properties of materials, components, or systems. The most frequently used methods are based on X-rays, ultrasound, thermography, eddy current or optics however, they have some limitations, especially when it comes to the structural analysis of selected materials such as fiber-reinforced plastics, and hollow or sandwich structures [3]. This forces new experimental methods to be developed. One of such method being a promising candidate for contactless inspection of composites materials is based on the use of terahertz radiation.

Terahertz (THz) radiation band includes electromagnetic waves in the range from 0.1 to 10 THz which corresponds to wavelengths from 3 mm to 30 µm. They may deeply penetrate nonconductive materials such as ceramics, semiconductors, and polymer composites [4]. It allows for higher spatial resolution detection of defects such as delaminations, voids or uneven reinforcement. In comparison to X-ray radiation, THz radiation is unharmful for people and may penetrate clothes. This special property can be potentially used for safe inspection of people e.g., at airports [5, 6], or in biomedical applications e.g., skin diseases detection and treatment [7]. Over the past years, the NDT studies with the use of THz radiation have been focused on composites materials, especially fiber-reinforced composites [8]. The studies allowed detection of both surface and underlying defects [9, 10] as well as the orientation of glass fibers in reinforced plastics [11]. It was also proved that THz radiation may be effective in detecting defects in pyrotechnic materials [6].

Despite its many benefits, the terahertz imaging is not so commonly used in NDT testing. It is connected with the relatively long inspection time of the tested objects which may reach even a few hours for one-meter objects [3]. Therefore, the studies focused on THz radiation are leading to improve efficiency and reduce the time of the investigations. Thus, this work demonstrates the potential of the THz transmission method where the time of the experiment is short (reaching a few seconds only), and the amount of signal reaching the detector is higher when compared to the reflection mode. The latter is connected with the smaller distance between sample and detector which leads to reducing the absorption of Terahertz radiation by the water in the air and hence the thinner defects might be monitored. The studies were performed on military-designated composites based on aramid fibers using two THz sources operated at frequencies 100 GHz and 300 GHz, and X-rays were used as the reference.

2 Materials and Methods

2.1 Materials

As the studies materials, the multilayer composites based on aramid were chosen. The common use of aramid fibers is guaranteed by their high mechanical strength, especially tensile strength, flexural strength, and compressive strength [12] which, in combination with relatively low mass, makes them efficient protection against external forces. The aramid fiber-reinforced composites are commonly used as material for ballistic protection such as e.g., helmets. The harmful and difficult environment in which they work may result in many hidden defects which may weaken their construction and consequently pose a threat. Thus, in the presented work, the most commonly observed in such fiber composites defects were analyzed.

The samples had a form of a rectangular plate with dimensions of 200 × 200 × 10 mm. Tested objects had hidden, intentionally introduced defects in various forms such as delamination, crack, lack of material, and shrinkage of the material. The schematic location of the defects was presented in Fig. 1b. The series of samples contained one sample without defects which plays a role of a reference and four ones with various failures which detailed description in listed in Table 1. They cannot be distinguished by the naked eye.

Fig. 1
figure 1

a The front surface of the selected sample, and b the schematic representation of defects of each sample from 5 to 8

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Table 1 The list of tested samples with description of defects in their interior
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3 X-Ray Studies

The X-ray diagnostic System MU17F (Fig. 2), supplied by YXLON International GmbH, was used as a preliminary non-destructive testing method. X-ray tests were performed subsequently on each sample which was placed vertically in such a way that the larger surface of the sample was oriented perpendicular to the X- ray axis. The operating voltage was 200 kV, and the lamp cathode diameter was 0.4 mm.

Fig. 2
figure 2

X-ray diagnostic system MU17F: 1—radiation booth, 2—control cabinet, 3—control panel, 4—monitor

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4 Terahertz Testing Method

The investigations were performed in two variants. The schematic representation of both experimental methods was presented in Fig. 3 where part (a) corresponds to the experiment operated at frequency 100 GHz, while (b) corresponds to the experiment operated at frequency 300 GHz. One may distinguish the radiation source (1) and the scanner (2) which are placed in such a way to allow for movement along the tested sample (4) on the displacement device (3). The speed of the movement of both elements was optimized to reduce the appearing disturbances. The tests in both cases were performed with the use of instruments from the Terasense Development Labs company. The first experimental setup (Fig. 3a) consisted of a line scanner measuring the reaching radiation—Linear 1024 (256 × 4 pixels image resolution, pixel size 1.5 × 1.5 mm, and frequency of ~ 100 GHz), and terahertz source—Generator Sub-THz—IMPATT (frequency 100 ± 5 GHz and power of ~ 30 mW). The terahertz source has a horn antenna with a specially designed PTFE (Teflon) refractive optical system. The configuration of the PTFE lenses ensures proper focusing of the THz beam onto the linear window of the scanner. The second experimental setup (Fig. 3b) consisted of a line scanner—Linear (512 × 1 pixels image resolution with a pixel pitch of 0.5 mm and frequency of ~ 300 GHz), and terahertz source—Generator THz—IMPATT (frequency 292 GHz ± 5 GHz and power of ~ 10 mW). This source includes novel reflective THz optics based on a specially configured high-gain horn antenna in combination with a metallic mirror. This generator considerably improves the THz imaging capabilities of our linear scanner by increasing the amount of power reaching the sensor array. In both variants the optical system and software—SDK for Terasense Viewer®Software, C/C++, was used.

Fig. 3
figure 3

Schematic representation of experimental setups in transmission mode operated at frequency of 100 GHz (a) and 300 GHz (b), where (1)—source of terahertz radiation, (2) scanner, (3) displacement device, (4) tested sample, and (5) mirror

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The studies were carried out in transmission mode which means that the source of radiation and tested object are placed on the opposite sides, because it provides more information about delaminations and cracks in the fiber composites in comparison to reflection mode (both, source of radiation and tested object, are placed on this same side). The samples were placed in the holder in such a way that the front surface was oriented perpendicular to the terahertz source. Each tested samples were treated this same to ensure similar conditions.

The postprocessing of the obtained images was done with the use of the ImageJ tool. The procedure containing setting high contrast and brightness was performed in order to minimalize the noises’ influence and to expose very thin defects.

5 Results

5.1 X-Ray Studies

The results of RTG imaging were presented in Fig. 4. The studies allowed for defects detection only in two samples: 2 and 3 (Fig. 4b, c, respectively). In the remaining samples, no defects were observed. Figure 4a presents a selected image of sample 1 where no density change was noticed as in sample 4. Such lack of visible disturbances in objects’ interior may be connected with the very small thickness of the introduced defects (reaching even less than 0.5 mm). In such a case there is no visible change in the material’s density, and despite the increasing energy, the defects are not distinguished.

Fig. 4
figure 4

RTG images presenting a selected image of sample 1 where no defects were detected, and b, c sample 2 and 3, respectively. White arrows indicate defects

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5.2 Terahertz Testing Method

Figure 5 presents the THz images obtained at the frequency of 100 GHz. All of them contain a black area in the bottom part which corresponds to the holder and black lines on the left side which are connected with the clips within which the samples were attached to the holder. It is worth to notice here that all images contain noises caused by vibrating to which the THz system is particularly sensitive.

Fig. 5
figure 5

a-e THz images obtained at frequency 100 GHz for samples 0, 1, 2, 3, and 4, respectively where the white arrows indicate defects, white square in e) corresponds to the area in which the defect should be located. The grey area on the bottom part of the images is a holder in which the samples were supported

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The THz image presenting the sample without defects (Fig. 5a) contains noises in a form of wide, alternating arranged lighter, and brighter areas. Such areas relate to the propagation of the electromagnetic wave in homogeneous medium. This effect was not observed within other samples where defects were introduced. The result of THz imaging of sample 1 was presented in Fig. 5b. High contrast allowed to observe a white square in the central part of the sample (indicated by a white arrow). This object may be easily distinguished as a defect due to its sharp edges however, one may also perceive another area (indicated by a blue arrow) which may be wrongly recognized as material’s damage. Thus, the objects’ inspection needs to be improved to exclude the impact of noises on the registered data.

Sample 3 (Fig. 5c) contains a long crack running through the central part of the sample. The indication of the crack has irregular width which may be connected with the penetration of THz wave through the crack. The lack of material was also easily detected in the central part of sample 4 (Fig. 5d). Unfortunately, the shrinkage of the material introduced to sample 5 was not detected. Figure 5e presents the THz image where white dashed lines delimit the area in which the defect should be located.

Figure 6 presents the THz images obtained via transmission mode at a frequency of 300 GHz. The black area at the bottom part of the images comes from a holder in which the samples were placed.

Fig. 6
figure 6

a-e THz images for samples 0, 1, 2, 3, 4, respectively where the white arrows indicate defects, and e’ defect area of sample 4 (collected from the area pointed by a white square in e) with a marked area of defect and lines which indicate the selected fibers. The black area on the bottom part of the images is a holder in which the samples were supported

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Figure 6a, obtained from sample 0 confirmed the lack of defects. Sample 1, presented in Fig. 6b, contained delamination of the material in its central part. On the THz image only some shadow in this area was observed. Comparing this sample to the one without defects, in the central part one may distinguish two sharp edges (in the area indicated by a white arrow) which may come from defect. However, one cannot confirm with certainty the presence of a defect in this place. Thus, the THz method, operated at a frequency of 300 GHz may not be useful in detecting such damages.

The best defect detection was observed for sample 2 which contained a long crack in the central part of the sample’s interior (Fig. 6c) what is also connected with relatively high depth of the crack (5 mm). The crack, visible as a black line going through the sample, is surrounded by a thin brighter layer which is a resin in which the aramid fibers are embedded. The lack of aramid material was detected in sample 3 (Fig. 6d). It has a form of a square object located in the center of the sample. Finally, results for sample 4 with shrinkage in its interior was presented in Fig. 6e. This defect was prepared in such a way that every second roving in onerovings’ layer was removed. As it is presented, the square defect is hardly visible in the central part of the sample. In order to expose the fibers, the test was performed on a sample rotated at about 45°. In the Fig. 6eʹ, the singular fibers inside the defect area may be distinguished. The white lines in the picture are an extension of the fibers.

6 Discussion

The registered with the use of THz radiation images confirmed the presence of defects in the samples’ interior. It turns out, that the methods using THz waves are more effective than X-rays with which it was impossible to detect thin defects. However, visible differences in the obtained results were noticed with changing frequency and a high contribution of noises was detected.

In both techniques, the noises result from the high sensitivity of Terahertz radiation to various kinds of resonances [13]. Such noises, visible as alternately arranged lighter and darker areas (for images acquired at frequency 100 GHz), and horizontal rounded lines (for images registered at frequency 300 GHz), are introduced by the vibrations of the environment and setup elements such as movement of the displacement device and investigated sample, or by a scanner. The higher noises content for the first discussed case (100 GHz) is connected with different scanner construction. In this case the scanner contains 256 detectors arranged in four rows, and in comparison to the scanner used in the second experiment (300 GHz), which has 512 detectors arranged in one row, is more sensitive to every single small vibration of the environment. In some cases, the high contribution of noises leads to the formation of shadow defects in the registered images. These unreal defects are highly undesirable, and their presence may result in an incorrect assessment of the condition of the tested element.

Besides those noises, which may be softened by image postprocessing, the presented images (for both, 100 GHz, and 300 GHz) are characterized by significantly higher quality in comparison to previously published ones where the 100 GHz source was used to detect guns and polypropylene-polyester samples [5], and pyrotechnic materials [6]. The improved quality is a result of the application of more advanced experimental setup equipped with the metallic mirror. The use of the mirror on which the radiation is impinged and from which it is next reflected to the sample’s surface increased the amount of the signal reaching the detectors. On the other hand, this improved defect detection was ensured by changing the experimental setup configuration from reflection to transmission. In such a case the THz wave transmitted through investigated object is not absorbed or scattered in the air because the distance between sample and detector is significantly smaller than in the reflection mode. However, it is worth to notice here that the better quality 300 GHz scanner was not able to clearly detect the whole delamination while the 100 GHz scanner was not able to detect shrinkage of the material, thus both testing frequencies should be used in order to characterize such materials accurately.

The defect in sample 2 is in the form of a crack. To obtain such a crack, approximately 10–12 layers of aramid prepreg were cut. The edges of this defect obviously have an influence on the flow of terahertz radiation. The dimension of the crack width in relation to the sample dimensions (200 × 200 mm) in terahertz images (Figs. 5c, 6c) is definitely larger than in reality (1 mm). This does not occur with thin defects. The dimensions of the defect edges in terahertz images correspond to their real dimensions.

The successfully non-destructive detection of defects located inside aramid fiber-reinforced composites has been previously reported by Świderski et al. [12] and Pracht et al. [14] with the use of active (ultrasonic) thermography. The studies using THz radiation have an advantage over this method because there is no need to use an external heat source required to expose defects in the sample’s interior and the experiment is carried out contactless. Moreover, the presented experimental method based on the use of THz radiation is faster.

Despite many advantages, Terahertz monitoring method has also some limitations that reduce its applicability. The main problem results from a relatively small penetration depth which is related to a lack of higher power THz sources. Using our method, we are able to penetrate through objects whose thickness do not exceed 20 mm which may be below expectations. Thus, the new materials able to generate higher power THz waves are being searched. Moreover, the obtained results include variable disturbances visible in THz images as e.g., black areas, shadows, and black lines, which may cover hidden defects. This effect is related to the high sensitivity of THz waves. In doing so, each vibration coming from the environment such as e.g., movement device, or the sample itself, can cause the noises. Taking this into consideration, the efforts are now put into optimizing the experimental setup in such a way as to possibly minimalize the influence of different external factors on the final results.

7 Summary

In the frame of the presented study, experimental tests with the use of Terahertz radiation in transmission mode on aramid fiber-reinforced composites were carried out. The tests were performed at the frequency of 100 GHz and 300 GHz. The obtained results allowed to detect the defects such as crack and shrinkage in materials’ interior and lack of material. It is worth to notice here that the presented defects, due to their small thickness (even one layer of aramid fibers which is below 0.5 mm) were not so easily distinguishable. In comparison to X-rays, the transmission mode using THz radiation allowed for exposure defects that were located at various depths below the surface.

To summarize, the presented studies showed that the transmission investigations using Terahertz radiation may be successfully used in non-destructive testing of aramid fiber-reinforced composites. The results are promising for future studies of military-designated elements such as ballistic covers or helmets. The continuous large share of noise related to the sensitivity of this method requires the development of the experimental setup and implementation of such image processing methods to get rid of the environmental impact as much as possible. Next studies will be focused on improving the efficiency of this method.

The obtained results have led to the conclusion that NDT techniques using THz radiation provide a robust and reliable inspection system in the detection of defects within military-designated aramid fiber-reinforced composites. Despite its limitations and regardless the frequency used, it allows detecting discontinuities in materials that may be very useful not only in military applications, and due to that it is worth for further study.

In further work, we also intend to improve the quality of terahertz imaging by reducing vibrations during scanning and using image processing methods. We also intend to investigate how the depth of the crack affects its imaging in the terahertz spectrum.