Visualization and Depth Estimation of Inner Crack in High‐Pressure Equipment Based on Organic Mechanochromic Luminescent Materials

Invisible hydrogen‐induced cracking or stress corrosion cracking easily appears in high‐pressure equipment in service. Mechanoresponsive luminogens (MRLs) can convert mechanical force into visible luminescence emission. Thus, MRL‐based detection methods to damage of structures have been paid extensive attention in recent years. However, the structural damage‐induced luminescence response and mechanism are still not fully comprehended. In this study, organic mechanochromic luminescent materials (1,1,2,2‐tetrakis (4‐nitrophenyl) ethene (TPE‐4N)) are proposed to detect the invisible inner crack in high‐pressure equipment. Because the high‐pressure equipment in service is subjected to tensile loading, the inner crack‐induced fluorescence response and mechanism under tension are investigated. The inner crack‐induced local strain concentration can be transformed into a visible green fluorescence, which can be easily observed from the outside. According to the appearance of fluorescence, the position of the inner crack can be detected. Moreover, the depth level of the inner crack can be quantitatively estimated using applied tensile loading and the ratio of fluorescent area. The investigations may provide a new idea of non‐destructive evaluation material and method for invisible damage in high‐pressure equipment.


Introduction
Owing to good hydrogen embrittlement resistance and corrosion resistance, austenitic stainless steel is one of the widely used structural materials in high-pressure equipment, such as highpressure hydrogen storage tank and high-pressure autoclave. [1] Hydrogeninduced cracking or stress corrosion cracking often appears in the inner surface of high-pressure equipment in service. [2] Unfortunately, the inner crack sites are invisible from the outside, thus it is imperative to develop a non-destructive testing (NDT) method for large-scale, and on-site detection of inner crack in the operating high-pressure equipment. Nowadays, many methods have been used for detecting the invisible defects, such as ultrasonic testing, [3] magnetic particle testing, [4] radiographic testing [5] , and eddy current testing. [6] However, due to the limit detection area, the signal acquisition and signal processing are complicated, and the cost of detecting systems are expensive for large-scale detection. [7] Due to non-contract, visualization, and simple acquisition and transmission, luminescence-based detection methods have gained extensive applications in many fields, such as photoelectric sensor, information storage, and biological imaging. Mechanoresponsive luminogens (MRLs), i.e., mechanoluminescent (ML) materials and mechanochromic luminescent (MCL) materials, can convert mechanical force into visible luminescence emission. Thus, MRL-based detection methods for damage of structures (i.e., metals and composites) have been paid extensive attention in recent years. [8][9][10][11][12][13][14][15] Compared with the conventional NDT methods, the luminescence emission is visible, and the signal acquisition and signal processing of MRL-based methods are relatively simple. Zhao et al. [16] produced stress sensor with periodic cellular structures based on ZnS:Cu quantum dots, and explored the quantitative relationship between ML intensity and stress. The ML intensity can be used to represent the local stress of the cellular structures. Kanamaru et al. [17] used SrAl 2 O 4 :Eu 2+ (SAOE) /epoxy resin film to observe the tensile deformation of the aluminum alloy sheet, and analyzed the www.advancedsciencenews.com www.advmatinterfaces.de evolution of the Lüders bands. Besides, ML-based NDT methods have been tried to detect the inner crack of pressure vessel. Fujio et al. [18,19] used SAOE/epoxy resin film to detect the inner crack in high-pressure hydrogen storage cylinder and aluminum alloy structures. Moreover, the depth of inner crack can be evaluated from ML intensity response. In existing research, much attention has been paid to research the ML-based detection methods for stress sensing because the ML intensity is proportional to the applied stress. However, the reported ML materials for stress sensing have poor film-forming capability. It is difficult to prepare the ML sensing film on a large-scale area. Moreover, the duration of ML signal is relatively short (only several microseconds). Therefore, the ML-based on-site detection system needs high-speed camera to receive ML signal, which brings inconvenience to ML signal acquisition and transmission. [20,21] Compared with ML response, the duration of MCL response can last longer, which is convenient to receive and transmit the luminance. Currently, most research are focused on synthesis and luminescence mechanisms of MCL materials (i.e., polymers, photonic crystals, and molecules), less attention has been paid to detect the structural damage. [22][23][24][25][26][27][28][29][30][31] In recent years, preliminary investigations on MCL-based detection methods for stress sensing have been carried out. Toivola et al. [32] used MCL to detect the compression and impact of the carbon fiber reinforced polymer (CFRP) composites. Indentation stress was estimated using the visible fluorescent activation. Tang et al. [33] reported that 1,1,2,2-tetrakis (4-nitrophenyl) ethene (TPE-4N) molecule has good film-forming capability and force ultrasensitivity. Therefore, TPE-4N provides new opportunities for developing large-scale and visual detection methods for structural damage. Qiu et al. [34,35] detected the stree-strain distribution and fatigue crack propagation using the fluorescence response of TPE-4N films. The real-time fatigue crack length and fatigue crack growth rate can be obtained by fluorescence distribution. Zhang et al. [36] investigated the local strain evolution in welding joint under monotonic tension and fatigue loading. TPE-4N film was also used for visual detection of the strain distribution and surface pressure distribution in CFRP composites. [37,38] However, it is noted that the luminescence response of MCL sensing film is related to the substate materials. Moreover, the different structural damages, such as cracking and impact damage, also generate the different MCL response. The structural damage induced luminescence response and mechanism of MCL sensing film are still not fully comprehended.
Compared with MCL polymers and photonic crystals, the TPE-4N is an organic MCL molecule, showing good film-forming capability on the surface of metals and CFRP composites. Therefore, the TPE-4N film is used to detect the inner crack induced damage of the high-pressure equipment in the present work. Because the operating high-pressure equipment is subjected to tensile loading, so the inner crack induced fluorescence response and mechanism under tension are investigated. The depth of the inner crack is estimated based on the applied tensile loading and the fluorescence response. Our investigations may provide a new idea of non-destructive evaluation material and method for invisible damage in high-pressure equipment. Figure 1 shows the fluorescence response of TPE-4N film, which is coated on the outer surface of the specimens with different depth levels of inner crack. It is seen that the fluorescence response of the specimen without inner crack is relatively homogeneous in the gauge area with increasing tensile loading. It indicates that the specimen is deformed homogeneously under tensile loading. By contrast, it is noteworthy that a visible green fluorescence appears in the gauge area of the specimens with invisible inner cracks, indicating that the position of the invisible inner cracks can be seen directly from the outside. When the force is lower than 1400 N, fluorescent area is not significant. When the force is over than 2000 N, X-shaped fluorescent feature pattern also appears. Moreover, it is noted that the higher depth level of inner crack also induces the higher fluorescence intensity and larger fluorescent area. Figure 2 shows the representative comparison between fluorescence response and strain distribution obtained by conventional digital image correlation (DIC) technique and finite element method (FEM) in the specimen with 90% inner crack depth level. The results of the specimens with 60% and 75% inner crack depth levels are also shown in Figures S1 and S2 (Supporting Information), respectively. Both experiment results (DIC) and simulation results (FEM) confirm that the inner crack can generate the inhomogeneous strain distribution near the outer surface under tensile loading. It is notable that the fluorescence distribution agrees well with the local strain distribution. According to the position of the fluorescence, the position of the invisible inner cracks can be easily observed directly from the outside, which provides a great convenience for detecting the inner crack of high-pressure equipment in service. Meanwhile, the fluorescence response, i.e., fluorescence intensity and fluorescent area, also changes with increasing tensile loading and depth levels of inner crack.

Inner Crack Induced Fluorescence Response and Mechanism Under Tensile Loading
In order to further reveal the inner crack induced fluorescence mechanism, quantitative estimation of relationship between fluorescence intensity and local strain value is investigated. Figure 3 shows the calibration between the relative average grayscale value and local strain value in the specimen with 90% inner crack depth level. As shown in Figure 3a, a 2 mm × 2 mm area with the maximum fluorescence intensity is selected. The average relative grayscale and the average strain in the same region are calculated, thus the relationship between fluorescence intensity and local strain is calibrated. After calibration, the local strain in the specimens with 60% and 75% depth levels of inner crack can be quantitatively analyzed using the fluorescence intensity. Then, comparisons between the local strain obtained using fluorescence intensity and that measured by conventional DIC method are summarized in Figure 3b and Tables S1 and S2 (Supporting Information). It is noted that the errors between the MRL-based method and DIC method are almost less than 15%, indicating that the fluorescence distribution shows a good agreement with the inner crack induced local strain concentration. However, when the local strain is low (<3%), the errors between the two methods become large. It is known that the elastic module of organic TPE-4N film is lower than that of metallic substrate. [34,36] Thus, the relatively small deformation of metallic substrate cannot generate obvious fluorescence emission, which causes the measurement error at small deformation.
The preliminary investigations on mechanoresponsive fluorescence mechanism of TPE-4N film were carried out in our previous works. [33,34,39] TPE-4N exhibits a strong green fluorescence peak at 520 nm in the amorphous state under ultraviolet (UV) excitation. While, the nitrophenyl groups effectively enable the nonradiative intersystem crossing channel, so the emission is quenched as TPE-4N in the crystalline state. The local deformation of the surface causes cracking of the crystalline TPE-4N film into tiny fragments. The molecular packing in the interface of these tiny fragments is destroyed and the intermolecular interactions are changed, leading to the turn on of green fluorescence emission. The larger deformation causes the higher fluorescence intensity. As illustrated in Figures 2 and 3, both fluorescence distribution and fluorescence intensity are in good agreement with the invisible inner cracks induce local strain. Therefore, it is concluded that the invisible inner crack induces local strain concentration near the outer surface under tensile loading, which causes  the destruction of surface crystalline TPE-4N film, generating mechanoresponsive fluorescence emission.

Depth Analysis of Inner Crack Using Applied Tensile Loading and Mechanochromic Luminescence
As illustrated in Figure 2, the fluorescent area is dependent on both the applied tensile loading and the depth level of inner crack. Thus, a detection method to evaluate the inner crack depth level is proposed based on the applied tensile loading and fluorescent area. In this paper, the fluorescence image needs to be processed to extract the fluorescent area. First, the background of the images is normalized using MATLAB to reduce the brightness change of the different specimens. Then, the threshold value is adjusted to carry out binary segmentation of the images. The fluorescent area with higher grayscale than the threshold value is white, while the other area is black. Finally, it is necessary to fill the holes and then extract the connected area due to the discontinuity of fluorescence pixels. The representative original fluorescence image, the grayscale distribution, and the extracted fluorescent area are shown in Figure 4. The fluorescent area in the binary image is calculated by the Bwarea function in pixels. For the convenience of analysis, the ratio of the fluorescent area (r[%]) in the whole image is calculated by the following equation: where S FL is fluorescent area (white part in the binary image), and S is total area of the image. The extracted fluorescent areas of the specimens with different inner crack depth levels under tensile loading after image processing are shown in Figure 5 and Table S3 (Supporting Information). It is clearly seen that the fluorescent area ratio increases with increasing inner crack depth level and applied tensile loading. The X-shaped fluorescent feature patterns become more obvious in the specimen with high inner crack depth level. Furthermore, the relationship between the fluorescent area ratio and the inner crack depth level under different tensile loadings is summarized in Figure 6. The ratio of the fluorescent area is dependent on both applied tensile loading and inner crack depth level. It is known that the applied tensile loading can be determined according to thickness and the operating inner pressure of highpressure equipment. If the fluorescent area is detected during regularly on-site safety inspection of high-pressure equipment, the estimation of depth level of the invisible inner crack can be carried out.

Challenges and Prospects
In the present organic MCL-based method, TPE-4N sensing film is coated on the surface of the samples. The inner crack generates the deformative discontinuities when the specimen is elongated, which causes a local deformation of the surface. The local deformation of the surface is transformed into the visible green  fluorescence using TPE-4N film. Compared with conventional NDT methods, the main advantages of the organic MCL-based method for inner crack detection are: 1) The detection is not influenced by the thickness and shape of the equipment; 2) The inner crack induced fluorescence can be seen directly by the nakedeyes, which brings convenience for rapid detection in large-scale equipment; 3) The inner crack induced fluorescence still exists even the applied force is removed. 4) The fluorescence acquisition and image processing are not complicated and expensive. The proposed organic MCL-based method has great potential in rapid and visual detection of inner crack induced structural damage. However, the organic MCL-based method still has some limitations: 1) The applied force is necessary for detection; 2) The shape and depth of the invisible inner crack cannot be seen directly from the fluorescence distribution.
3) The sensitivity becomes poor when the inner crack is far from the surface of equipment.
In the past decades, although preliminary investigations on organic MCL-based detection methods have been carried out, there are still many challenges for applications in real engineering industry. [35,40] The metals and composites are widely used as structural materials of the equipment. The local strain concentration of the equipment in service is limited. However, most of the reported organic MCL materials have poor stress sensitivity. Namely, the MCL response often occurs when the materials are largely stretched or ground. Therefore, improving the stress sensitivity of organic MCL materials is a big challenge for using in NDT evaluation. Moreover, because organic material aging may occur in high temperature and humidity environment, thus the durability of organic MCL sensing film should be considered before engineering applications. Furthermore, fluorescence response is dependent on the materials, applied force, and working conditions (camera exposure time, UV lamp power, and incidence angle, etc.), which brings difficulty for quantitative analysis of fluorescence intensity. Therefore, the calibration between fluorescence intensity and structural damage as well as the standardization of detection equipment are highly needed. The database of structural damage induced fluorescence response should be further investigated. Computer vision combined with machine learning or deep learning may be suitable to recognize and quantitatively evaluate the structural damage. [41] Although many challenges have to be overcome before real engineering applications, it is still expected that the organic MCL molecule may have great potential to be applied in nondestructive evaluation.

Conclusion
The present study investigated the application of the organic mechanochromic luminescence response of TPE-4N film in detecting the invisible inner crack in the operating high-pressure equipment. The inner crack induced fluorescence response and mechanism were analyzed under tensile loading. The main conclusions are drawn as follow: 1) The inner crack can induce the local strain concentration near the outer surface under tensile loading, which generates a visible green fluorescence of TPE-4N film. Thus, the position of the invisible inner crack can be easily detected from the outside.
2) The inner crack induced fluorescent area increases with increasing applied tensile loading and depth level of inner crack. Based on the applied tensile loading and the ratio of fluorescent area, the depth level can be quantitatively estimated.
3) Compared with the conventional NDT methods, the fluorescence is visible, and the signal acquisition and signal processing are relatively simple. It is convenient to prepare the large-scale TPE-4N sensing film on the surface of equipment. The proposed organic MCL-based method may provide a new idea for NDT evaluation material and method for inner crack in high-pressure equipment in service.

Experimental Section
Materials: Commercial 316 L austenitic stainless steel was used in the present work. The shape and dimensions of the specimens with inner crack are shown in Figure S3 (Supporting Information). A non-penetrating pre-crack was machined on the side of specimen (i.e., inner surface) by milling process. The depths of the pre-cracks were ≈0.96 mm (60% thickness), 1.20 mm (75% thickness), and 1.44 mm (90% thickness), respectively. Because the specimens have thin thickness and small size, it is difficult to detect the depth of inner crack by eddy current testing and ultrasonic testing. Thus, the 3D shape of the pre-crack was roughly measured by 3D optical microscope (VH-Z250R, KEYENCE). Figure S3b (Supporting Information) shows the front and back views of the specimen with a crack depth of ≈0.96 mm. Owing to the shape of the cutter, the depth of the crack was not uniform. In the experiments, TPE-4N film was coated on the other side of specimen (i.e., outer surface).

Film-Forming:
The TPE-4N/chloroform solution with a concentration of 0.03 g mL −1 was prepared, and the TPE-4N was coated on the outer surface of the specimen by dip coating. In this situation, TPE-4N film is in the amorphous state, showing green fluorescence emission under UV excitation. The coated specimens were subsequently heated at 150°C for 10 min to crystallize the TPE-4N film. Meanwhile, the fluorescence was quenched.
Experiment Procedures: The setup of the organic MCL-based detection system is shown in Figure S4 (Supporting Information), which consists of a mechanical testing system and an image acquisition system. The loadcontrolled uniaxial tensile tests were conducted on an in situ uniaxial tensile fatigue testing machine (IBTC-5000, CARE) controlled by a computer control system. The loading rate was 100 N s −1 . The tests were carried out in a dark room under UV lamp excitation. During tension, the real-time fluorescence images of TPE-4N film were recorded by a conventional charge coupled device (CCD) camera. Furthermore, fluorescence images were transformed into corresponding grayscale distribution images by MATLAB R2021b. MATLAB has been extensively used in data processing. [42][43][44][45] Strain Measurement: The local strain near the outer surface was measured by DIC technique. As shown in Figure S5a (Supporting Information), the outer surface of the specimen was sprayed to form random speckles, and the pictures were recorded during tension to capture the displacement of each speckle on the surface of the specimen. The von Mises local strain distribution was obtained by GOM Correlate software. The typical local strain distribution on the outer surface of the stretched specimen with inner crack is shown in Figure S5b (Supporting Information).
Finite Element Method Simulation: The local strain distribution near the outer surface was also verified by finite element method simulation (ANSYS 17.1). The 3D finite element model of the specimen with inner crack was developed. Considering the symmetry of the specimens, a quarter model was developed to reduce computation time. The elastic modulus of 181 GPa and Poisson's ratio of 0.3 were adopted as elastic properties of the 316 L stainless steel. Tetrahedral solid element (SOLID185) was used. This element has four nodes, and each node has six degrees of freedom. Figure S6 (Supporting Information) shows the 3D model of the finite element mesh, and the mesh size is refined in the vicinity of the crack to ensure more reliable results. After defining the boundary conditions and the applied loading, a nonlinear finite element analysis was performed, and the equivalent von Mises strain distribution near the outer surface of the specimen were obtained.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.