Graphical Abstract Figure

Photo and sectional view of the proposed probe

Graphical Abstract Figure

Photo and sectional view of the proposed probe

Close modal

Abstract

This research is primarily motivated by the demand for a convenient and economical nondestructive testing solution in industrial scenarios. We present a novel nondestructive testing probe, which is ingeniously designed with a photoresistor sensor and an illuminated light board, possessing remarkable features such as outstanding portability, minimal power consumption, and low cost. The experimental evaluation of the probe involves a line-scanning process above the crack position of an aluminum plate. As a result, an output signal in the form of variable resistance values of the photoresistor sensor is obtained. Notably, distinct peaks emerge in the output signal precisely when the probe traverses defects. These peaks serve as a crucial indicator for detecting surface defects of aluminum plates. Moreover, the varying amplitudes of the peaks in the output signal offer a reliable means to discriminate between defects of different depths. This probe holds significant potential for application in the quality assurance of aluminum manufacturing, as well as in the routine inspection of metallic structures, facilitating enhanced safety and reliability in engineering applications.

1 Introduction

Nondestructive testing (NDT) is an effective technique for detecting defects, evaluating materials, and predicting remaining useful life without destroying the test specimen being inspected [13]. This technique is not only an important means to discover potential safety hazards and ensure the reliability and effectiveness of equipment but also an effective way to improve product quality and economic benefits [46]. Conventional NDT methods, e.g., ultrasonic testing, radiographic testing, magnetic particle testing, penetrant testing, and eddy current testing, have been extensively applied in modern industrial fields [79].

In recent years, new progress has been made in the field of NDT. Tian et al. put forward a novel microwave NDT utilization to detect and characterize internal defects in coated glass fiber reinforcement plastic pipes. Their research shows that the proposed method can be able to plainly determine the range of flat-bottom holes. Furthermore, depths of flat-bottom holes are successfully characterized [10]. Rodideal et al. evaluated the mechanical properties and fatigue life of thin-walled high-strength low-alloy steel parts manufactured by wire and arc additive manufacturing via NDT and destructive testing. Their research showed that the influence of manufacturing conditions on mechanical properties is more significant than that of part height [11]. Delenkovskii and Gnusin studied the vacuum intensification of the process of filling cracks in stainless steel and glass specimens with penetrants under liquid-penetrant testing experimentally. The study showed a high efficiency of vacuum impregnation with small amounts of penetrants on detected surfaces [12]. Chen et al. proposed a new ultrasonic testing method employing a hemispherical-omnidirectional ultrasonic probe. This probe has an omnidirectional directivity mode and can be able to coat curved surfaces with normal incident ultrasounds at different scanning locations without the necessity of adjusting the probe attitude and location to conform the curved surface contours [13]. Miles et al. proposed a multi-modal hybrid probe for rail rolling contact fatigue inspection, combining electromagnetic and ultrasonic testing. With a novel design of motion-induced eddy current effect and electromagnetic acoustic transducer, it was tested at low/high speeds, showing great potential for high-speed damage detection [14]. Wu et al. proposed combining a two-dimensional phased array with coupling blocks to conduct a complete inspection of fastener holes. The experimental results show that the proposed inspection method can eliminate blind spots and capture all target information from a single inspection location [15]. Chi et al. proposed an ultrasound C-scan image stitching method based on scale invariant feature transformation, which can effectively stitch multiple C-scan images of test blocks containing artificial defects into a panoramic image [16]. In order to improve the degree of automatic detection, Chen et al. presented a thermography-based dry magnetic particle testing method. In their method, dry magnetic particles are heated before use, and then defects are distinguished using thermal data. This method can not only save cost but also distinguish defects without manual assistance [17]. Tout et al. developed a fully automatic crankshaft full surface inspection device based on magnetic particle testing technology, which utilizes convolutional neural networks in deep learning algorithms to improve detection efficiency and accuracy [18]. Eckel et al. proposed a new method for digitizing radiographic film to extract optical density values and grain size, which enables the film system to classify correctly when applying the proposed correction algorithm [19]. The researches mentioned above promote the development of NDT technology; there are still the following imperfections. Ultrasonic testing [1316] requires the use of coupling agents, while magnetic particle testing [17,18] requires magnetic particle cleaning and demagnetization after the testing is completed, making these two testing methods less convenient. Radiographic testing [19] is more expensive and complex for testing large parts. The existence of the above drawbacks has given rise to expectations for simple and inexpensive detection solutions.

Probes that detect signals play an important role in NDT. Research on the NDT probe has been performed. Xie et al. designed a kind of magnetic force transmission eddy current array split-type probe for detecting the cracks in the pipeline structure at a long distance. The probe they proposed was composed of an excitation coil with permanent magnets and a plurality of receiving coils with permanent magnets. According to this new probe, they successfully identified the crack defects in aluminum tubes, and the detection efficiency is high [20]. Gong and Yang designed a probe with metamaterial bulks as the cores of the pancake surface probe. Their research found that the electromagnetic properties of metamaterial bulks increased with increasing frequency. In addition, using metamaterial blocks as pancake surface probe cores can greatly change the normalized impedance, so that the probe can better identify cracks [21]. Zeng et al. developed a bridge-type probe including coplanar dual rectangular coils for detecting delamination in multidirectional carbon fiber reinforced polymer (CFRP). The artificial delamination of 20 mm × 20 mm × 50 µm was detected in their experiment [22]. Thereafter, they designed a probe composed of a rectangular excitation coil and a rectangular reception coil that are perpendicular to each other. They detected the fiber waviness in CFRP, and the resolution of the testing fiber direction is as small as 0.5 deg [23]. Abdollahi-Mamoudan et al. designed a rectangular coplanar capacitive probe. This coplanar capacitive probe could detect the rebars in the concrete slab successfully. Their study demonstrates that the capacitive technique has promise as a method for assessing such specimens [24]. Trung et al. developed an eddy current convergence probe which consists of a copper core and single detection coil made of copper enameled wire. The results show that artificial flaws on plate samples can be clearly detected by utilizing the proposed probe [25]. On the whole, sensing coils with different structures and shapes, Giant Magnetoresistance, Tunnel Magnetoresistance, and Hall sensors have been utilized to fabricate probes for NDT, but not involved photoresistor sensors.

A photoresistor sensor that is based on the photoconductive effect is an element whose resistance value changes rapidly when illuminated by light [26]. It is characterized by high sensitivity, stable electrical performance, small size, and low price. Applications of photoresistor sensors include ionizing radiation measurement [27], chemical analysis [28], photoelectric control [29], etc. Surkova et al. utilized photoresistor sensors and light-emitting diodes (LEDs) to make a low-cost optical sensor that could successfully quantify the amount of blood lost during surgery [30]. Conte et al. used a photoresistor sensor to make a 3D printing photometer controlled by Arduino. The 3D printing photometer they developed can be connected online to dynamically measure the absorbance of cell cultures or other technological process flows [31]. Based on the idea of detecting defects through changes in probe impedance [32,33], this article explores the possibility of using photoresistor sensors for nondestructive testing.

The rest of this article is organized as follows. A novel probe that is based on the photoconductive effect for crack detection is designed, and the working principle is introduced in Sec. 2; then the experiment is setup in Sec. 3; after that, experiment results and discussion are shown in Sec. 4; finally, some conclusions are provided in Sec. 4.

2 Proposed Probe and Working Principle

2.1 Proposed Probe.

The proposed probe consisting of a photoresistor sensor 5506 and a light board is shown in Fig. 1. The photoresistor sensor 5506 that is made of semiconducting material cadmium sulfide, whose resistance is inversely proportional to the incident light intensity, can sense the intensity of the light illuminated on it. Its resistance varies from nearly 0.2 MΩ in the dark to a few thousand Ohms in bright conditions, which is approximately two orders of magnitude.

Fig. 1
Photo and sectional view of the proposed probe
Fig. 1
Photo and sectional view of the proposed probe
Close modal

The photograph of the photoresistor sensor 5506 marked with dimensions is shown in Figs. 2(a) and 2(b). Figure 2(c) shows the structure of the photoresistor sensor 5506. The exterior of the whole structure is a layer of transparent resin moisture-proof membrane, which plays the role of light transmission, moisture-proof, and reinforcement. The two ends of the top of the photoresistor sensor are two cross-comb-shaped metal electrodes. The material exposed between the two electrodes is the cadmium sulfide photosensitive layer. The ceramic substrate is below the cadmium sulfide photosensitive layer. Two metal pins are led out from the end of the comb-shaped metal electrode downward. When the light shines through the resin film onto the cadmium sulfide semiconductor photosensitive layer, the electrons in the semiconductor valence band absorb the energy irradiated by the light. When the absorbed energy reaches the value that can make it transition, the electrons will transition from the semiconductor valence band to the semiconductor conduction band, thus adding a pair of electron-hole pairs. With the increase of the external light intensity, the free charge between the valence band and the conduction band will be more and more, and the conductivity of the photosensitive material will be significantly increased.

Fig. 2
Photoresistor sensor 5506 in the proposed probe: (a) front view, (b) vertical view, and (c) structural illustration
Fig. 2
Photoresistor sensor 5506 in the proposed probe: (a) front view, (b) vertical view, and (c) structural illustration
Close modal

The light board with a dimension of 30 mm in diameter, which is composed of four identical LEDs as shown in Fig. 3, is served as the light source for illuminating the sample under test. Each LED of the light board has dimensions of 5 mm × 3 mm × 0.5 mm and has an output power of 50 W.

Fig. 3
Photograph of the light board in the proposed probe
Fig. 3
Photograph of the light board in the proposed probe
Close modal

The photoresistor sensor 5506 and the light board are covered with a paper shell. The advantage of this configuration is that only the light emitted by the four LEDs reflected from the surface of the metallic plate can shine on the photoresistor sensor 5506 in the probe through the hole in the light board, whereas the light in the surrounding environment outside the probe cannot shine on the photoresistor sensor 5506.

2.2 Working Principle.

Figure 4 shows the principle of NDT of metallic plates with the proposed probe. The proposed probe is placed above the defect-free metal plate and the defective metal plate, respectively. When the circuit of the light board in the probe is powered on, the four LEDs on the light board will give out light. The light that illuminates on the surface of the metallic plate will be reflected, allowing a portion of the light to pass through the hole in the light board to the photoresistor sensor in the probe. If the metallic plate under the probe is free of defects, as shown in Fig. 4(a), the photoresistor sensor in the probe has an inherent resistance. When there is a defect in the metallic plate under the probe, as shown in Fig. 4(b), the light intensity of the light reflected on the photoresistor sensor is different from that without the defect so that the resistance value of the photoresistor sensor changes. Since the photoresistor sensor and the light board are covered by a shell, the resistance value of the photoresistor sensor is only affected by the light reflected from the surface of the metallic plate and not by the light in the external environment of the probe. That is, the resistance value at this time contains information about the defect. By analyzing this resistance value, the defect information in the inspected test sample can be obtained.

Fig. 4
The principle of NDT of metallic plates with the proposed probe: (a) detection of the flawless plate and (b) detection of the defective plate
Fig. 4
The principle of NDT of metallic plates with the proposed probe: (a) detection of the flawless plate and (b) detection of the defective plate
Close modal

3 Experimental Setup

3.1 System Configurations.

The experimental system, as shown in Fig. 5, is designed to generate light and measure the response signal affected by a defect in the specimen. The whole NDT experimental system consists of a power supply, a digital multimeter, and the proposed probe introduced in Sec. 2. The power supply that powers the light board of the probe to make it shine is composed of a power strip and a Universal Serial Bus (USB) power adapter. This USB power adapter converts 220 V of AC from the power strip to 5 V of DC which is utilized as the working voltage of the light board. The digital multimeter DT9205A with enhanced performance is employed to measure the resistance value of the photoresistor sensor illuminated by light reflected from a metal surface. To ensure the stability of the signal, we use wiring terminals to connect the signal line on the probe and the detection line of the digital multimeter DT9205A that collects the signal together. In the experiment, the probe was placed on the specimen to maintain a tight fit with the specimen, and scan the specimen along the path containing defects with a step size of 2 mm. The resistance value is manually recorded during the process for analysis.

Fig. 5
The constructed experimental system: (a) diagram and (b) photograph
Fig. 5
The constructed experimental system: (a) diagram and (b) photograph
Close modal

3.2 Specimen Preparation.

To verify the performance of the probe based on photoconductivity proposed in Sec. 2, a sample was designed with cross-sectional and top views as shown in Figs. 6(a) and 6(b), respectively. The specimen is an aluminum plate and has dimensions of 200 mm × 150 mm × 5 mm. Artificial slots having a length of 10 mm and a width of 2 mm and at different depths of 1, 2, 3, and 4 mm (cracks 1, 2, 3, and 4, respectively) are machined on the surface of the aluminum plate. The distance between two adjacent slots is 45 mm.

Fig. 6
Cross-sectional view and top view photo of the aluminum test piece used in the experiment: (a) cross-sectional view and (b) top view photo of the aluminum sample
Fig. 6
Cross-sectional view and top view photo of the aluminum test piece used in the experiment: (a) cross-sectional view and (b) top view photo of the aluminum sample
Close modal

4 Results and Discussion

In this section, an aluminum plate sample as described in Sec. 3.2 is tested by employing the experimental system elaborated in Sec. 3.1. The proposed probe scans over the test sample with a step size of 2 mm and the scanning path for inspecting the test sample is illustrated in Fig. 7. The test sample is scanned three times with the probe, and the measured resistance values of the photoresistor sensor in the proposed probe are averaged.

Fig. 7
Schematic diagram of the probe linearly scanning the aluminum plate sample along the path containing defects in the experiment
Fig. 7
Schematic diagram of the probe linearly scanning the aluminum plate sample along the path containing defects in the experiment
Close modal

Figure 8 shows the average resistance value of the photoresistor sensor in the proposed probe when scanning the aluminum plate sample three times along the path shown in Fig. 7. It can be seen in Fig. 8 that when the probe is located above the defect with the depth of 2 mm, 3 mm, or 4 mm in the aluminum plate, the resistance of the photoresistor sensor will appear a peak value, indicating that the defect can be detected. The reason for the peak resistance value is as follows. When the probe is close to the defect, the light intensity of the light reflected to the photoresistor sensor through the hole in the center of the lamp board of the probe is smaller than that when the probe is far away from the defect. According to the photoconductive effect, with the decrease of light intensity, the resistance of the photosensitive resistor increases, and then the peak appears. In addition, the detection signals in Fig. 8 have a trend, that is, with the increase of defect depth, the peak resistance value increases, which means that defects with different depths can be distinguished according to the peak resistance value.

Fig. 8
Experimental results of scanning testing using the proposed probe
Fig. 8
Experimental results of scanning testing using the proposed probe
Close modal

5 Conclusions

NDT plays an important role in ensuring the safe operation of equipment and protecting the safety of life and property. In this article, a novel probe deployed a photoresistor sensor, and an illuminated light board for NDT was proposed. An experiment of detecting a sample of aluminum plate with multiple defects by the proposed probe was carried out, and the experimental results prove that defects on the surface of the aluminum plate can be detected effectively by the proposed probe. Besides, the peaks in the output signal can be used to distinguish defects with different depths. This probe does not have the imperfections of traditional detection methods such as ultrasonic testing and radiographic testing, which are inconvenient to operate and expensive. In future work, we will test the probe in an industrial environment and further improve its sensitivity to enhance its applicability.

Funding Data

  • This research is supported by the Taizhou Science and Technology Plan Project (Grant No. 24nyb09), Collaborative Education Project of the Ministry of Education (Project's No. 230805635245326), and National College Students' Innovation and Entrepreneurship Training Program Project (Project's No. 2024103500028).

Conflict of Interest

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent was obtained for all individuals. This article does not include any research in which animal participants were involved.

Data Availability Statement

The authors attest that all data for this study are included in the paper.

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