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Filament-wound composite pressure vessel inspection based on rotational through-transmission laser ultrasonic propagation imaging
Journal Pre-proofs Filament-Wound Composite Pressure Vessel Inspection based on Rotational Through-Transmission Laser Ultrasonic Propagation Imaging Young-Jun Lee, Hasan Ahmed, Jung-Ryul Lee PII: DOI: Reference:
S0263-8223(19)33098-3 https://doi.org/10.1016/j.compstruct.2020.111871 COST 111871
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
17 August 2019 16 November 2019 2 January 2020
Please cite this article as: Lee, Y-J., Ahmed, H., Lee, J-R., Filament-Wound Composite Pressure Vessel Inspection based on Rotational Through-Transmission Laser Ultrasonic Propagation Imaging, Composite Structures (2020), doi: https://doi.org/10.1016/j.compstruct.2020.111871
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Β© 2020 Elsevier Ltd. All rights reserved.
Filament-Wound Composite Pressure Vessel Inspection based on Rotational Through-Transmission Laser Ultrasonic Propagation Imaging
1Young-Jun 1Department
Lee, 1Hasan Ahmed, 1Jung-Ryul Lee*
of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseonggu, Daejeon, 34141, Republic of Korea
*Corresponding author e-mail: [email protected]
Abstract In this paper, we introduce a rotational through-transmission ultrasonic propagation imaging system to inspect the full thickness of the entire cylindrical section of filament-wound composite overwrapped pressure vessels. This system can achieve high signal-to-noise ratio throughout the cylindrical section of pressure vessels by using a rotational scanning method. Based on the advantages of the laser ultrasonic system that can generate and detect ultrasound waves by using a small-diameter laser beam and making an inspection path in a narrow environment, this system directs the laser beam inside the pressure vessel via a mirror to enable the throughtransmission mode for vessel inspection. A type-3 pressure vessel was inspected using the system developed, which successfully detected two impact damage spots and showed the filament winding direction clearly. The test results show that the system developed is highly reliable and can replace the current hydrostatic sample test for pressure vessel safety certification. Keywords: laser ultrasonic; composite overwrapped pressure vessel; rotational scanning; through-transmission
1. Introduction Pressure vessels are used to hold liquids or gases under high pressure. In some industries, such as the oil and gas industry, where pressure vessels are used in stationary conditions without movement, metal vessels are generally used. On the other hand, in the case of vehicles carrying fuel containers, such as space launchers and flights, filament-wound composite pressure vessels have been used for weight reduction and fuel efficiency. The demand for composite pressure vessels is increasing rapidly owing to their recent use as fuel containers. Pressure vessels are exposed to cyclic loading owing to frequent charging and discharging while in operation, and debonding can occur at any composite layer when subjected to impact damage. Therefore, periodic inspection is essential to ensure the structural integrity of pressure vessels, but it is difficult to inspect composite pressure vessels owing to their structural complexity and low inspection accessibility. Currently, composite pressure vessels must be regularly recertified by visual inspection and a hydrostatic test, which imposes significant costs and out-of-service time [1]. Numerous studies have been conducted to develop reliable inspection and diagnostic methods that can replace the hydrostatic test for composite pressure vessels. Pelkner et al. [2] inspected an artificially aged type-3 pressure vessel using the eddy current method and detected defects in the metallic liner. Bulletti et al. [3] 1
attached two piezopolymer film transducer arrays to the exterior of a composite pressure vessel and conducted damage localization using sensor networking. Only one of the four impact damage spots was localized similarly, and the other three damage spots were not. Degrieck et al. [4] showed that strain measurement is possible under variable and cyclic loading conditions using embedded fiber Bragg grating sensors. Klute et al. [5] embedded optical fibers in a composite pressure vessel and attempted to detect defects by measuring the residual strain after impact test and hydrostatic test using optical frequency domain reflectometry. Dahmene et al. [6] studied the acoustic emission method with eight sensors that could cover the entire vessel and established a criterion based on acoustic emission (AE) events to determine whether a composite pressure vessel is usable after impact damage. More than other methods, the AE test has been studied for composite pressure vessel inspection by many researchers [6β8]. Most previous studies have focused on embedded sensor-based monitoring to determine whether damage has occurred or the vessels are reusable rather than on in-situ inspection based on external sensors to evaluate or characterize defects. Furthermore, most methods currently under development can be used to inspect only near the surface layers of the tank because of difficulties in accessing the inner surface or high vessel thickness. In this study, we developed a rotational through-transmission ultrasonic propagation imaging (R-TT-UPI) system that can inspect the entire thickness of the cylindrical section of pressure vessels. This system is based on laser ultrasonics and can acquire extremely clear ultrasound signals throughout the entire circular cylindrical section using a rotational scanning method. In addition, this system is capable of being used in the throughtransmission mode, which makes it possible to inspect thicker pressure vessels than those inspected using the pulse-echo mode. This paper is organized as follows. In Section 2, we explain the rotational scanning method and algorithm for the inspection of the circular, cylindrical section of pressure vessels. In Section 3, we describe the implementation of the through-transmission mode of inspection and discuss ultrasonic propagation through the metal-composite layers of a composite pressure vessel. In Section 4, we describe the set-up and configuration of the system developed, and in Section 5, we finally present the inspection results for a type-3 pressure vessel using the system developed.
2. Rotational scanning method using laser ultrasonic bulk wave
A laser ultrasonic system is advantageous for the inspection of curved surfaces because it can inspect remotely without any constraint of contact, and its footprint on the target surface is extremely small [9]. Most studies on the inspection of curved surfaces using laser ultrasonics entail the use of guided waves [10β12], which propagate along an out-of-plane direction. However, when we inspect a thick structure using guided waves, only surface waves are excited at a low depth near the surface and not Lamb waves that can inspect the entire thickness [13]. Therefore, bulk waves should be used because guided waves cannot inspect the full thickness of a thick structure. However, unlike that of guided waves, the signal quality of bulk waves is degraded owing to variations in the incidence angle and the standoff distance between the curved surface and sensing laser. Therefore, a rotational scanning method is proposed to solve this problem. The cross section of a pressure vessel is a circle, which is a set of points at the same distance from the center, and the tangential line at each point is perpendicular to the center of the circle. Therefore, if a circular object is rotated around its center, as shown in Fig. 1, the laser beams are always normally incident and the standoff distance is constant.
2
Figure 1. Schematic of rotational scanning method
Figure 2. Algorithm of rotational scanning method Figure 2 shows the algorithm of the rotational scanning method. The user first inputs the outer radius of the target cross section, length to be inspected, rotation speed, and the number of triggers per revolution. The system then provides the inspection interval in angle and length scale, and shows the number of scanning lines required to inspect the entire cylindrical section and this is equal to the total number of revolutions. When an inspection is started, the rotation stage rotates and generates triggers at every angular scan interval, and the triggers are then transmitted to the excitation and sensing lasers and DAQ to generate and capture the ultrasound wave 3
signal at a single scanning point. This rotational scanning of each scanning point is repeated until one revolution of the rotation stage is completed. Thereafter, the linear stage moves according to the linear scan interval, and the rotational scanning step is repeated. The inspection is terminated when the number of inspected lines reaches the desired number of scanning lines.
3. Through-transmission mode of inspection of metal-composite layers of composite pressure vessel
The most commonly used inspection modes in ultrasonic testing are the pulse-echo and through-transmission modes. The notable difference between the two inspection modes is the plane on which the excitation and sensing take place. In the pulse-echo mode, the excitation and sensing are carried out on the same plane, and therefore it is advantageous when only one side of the structure is accessible. On the other hand, in the throughtransmission mode, the ultrasound waves should be excited on one side of the structure and detected on the other side, and so the drawback of this method is that both the surfaces of the structure must be accessible. However, the through-transmission mode is more advantageous than the pulse-echo mode when a thick structure is to be inspected. The amplitude of the ultrasound wave (A) is exponentially attenuated as travel distance (z) increases, as expressed in Equation 1: π΄ = π΄0π β β π§,
(1)
where π΄0 is the unattenuated amplitude of the ultrasound wave, and Ξ± is the attenuation coefficient of the wave traveling in the z-direction. In the pulse-echo mode, the travel distance of an ultrasound wave is twice the thickness of the structure, whereas in the through-transmission mode, the travel distance is equal to the thickness of the structure and, hence, is half the travel distance in the pulse-echo mode, so that the attenuation of the ultrasound waves can be drastically reduced. This makes it possible to use the through-transmission mode to inspect thick structures that cannot be inspected using the pulse-echo mode. Pressure vessels are intended to store gases and liquids at high pressure without leakage; therefore, all the sections are perfectly closed except for a boss on one side for fuel injection. Consequently, the inspection accessibility is limited, and most researchers have tried to inspect or monitor the exterior of the pressure vessel. The laser ultrasonic system, on the other hand, uses a laser beam with a small diameter in the sub-millimeter range for ultrasound excitation and detection. The laser has good directivity and a low divergence angle, and the diameters of the laser beams can be easily adjusted using optical components. Therefore, if a small-diameter laser beam is propagated into a pressure vessel, it is possible to achieve the through-transmission mode of inspection. The procedure to implement the through-transmission mode in a pressure vessel is explained in detail in Section 4. Composite pressure vessels are manufactured by winding continuous fiber filaments on a metallic or plastic liner. Hence, bulk wave, which propagates in the through-thickness direction, can be transmitted through metalcomposite or plastic-composite layers. When ultrasound wave propagates through different media, it is partially reflected and transmitted at the interface. The transmission and reflection ratios of the ultrasound wave at the interface depend on the acoustic impedance of the two adjacent media. The acoustic impedance is expressed as: π = π Γ πΆ = π/π,
(2)
where Ο and C are the density and speed of sound, respectively, and P and V are pressure and velocity, respectively. The reflection and transmission coefficients (R and T, respectively) can be calculated using the acoustic impedance of the two media, which can be expressed in various terms such as pressure, intensity, and velocity (or displacement), as shown in Equation 3 [14]: ππ
π2πΆ2 β π1πΆ1
π2 β π1
ππ‘
2π2πΆ2
2π2
Pressure: π π = ππ = π2πΆ2 + π1πΆ1 = π2 + π1 , ππ = ππ = π1πΆ1 + π2πΆ2 = π1 + π2 4
(3-1)
πΌπ
π2π /2π2πΆ2
πΌπ‘
π2π‘ /2π2πΆ2
π1πΆ1
Intensity: π πΌ = πΌπ = π2/2π πΆ = π 2π , ππΌ = πΌπ = π2/2π πΆ = π2πΆ2π2π π
ππ/π1πΆ1
1 1
π
ππ‘/π2πΆ2
π1πΆ1
(3-2)
1 1
2π1πΆ1
2π1
Velocity: π π = ππ/π1πΆ1 = π π, ππ = ππ/π1πΆ1 = π2πΆ2ππ = π1πΆ1 + π2πΆ2 = π1 + π2
,
(3-3)
where the subscripts i, r, and t refer to incidence, reflection, and transmittance, respectively. Generally, the ultrasound wave is detected by measuring the microscopic movement of a surface caused by the propagation of the wave inside the structure and not by directly measuring the wave. In this study, a laser Doppler vibrometer (LDV), which measures the velocity of objects, was used to detect the ultrasound waves. Therefore, the expression in terms of velocity, as presented in Equation 3-3, should be applied. From this equation, it can be seen that the transmission coefficient is greater when an ultrasound wave propagates from a medium with high acoustic impedance to one with low acoustic impedance. We conducted an experiment to observe how the amplitudes of ultrasound waves differ according to the direction of propagation through metal-composite layers. The amplitudes of the ultrasound waves were measured by changing the excitation and detection sides of the specimen, which is the composite bonded to an aluminum plate, as shown in Fig. 3(a). This structural type can be easily found in the case of composite patch repair of a metallic structure. The test specimen was stacked with 2 mm cross ply composites on a 2 mm thick aluminum plate. Table 1 shows the acoustic impedances of the aluminum and composite. The acoustic impedance of aluminum is about thrice that of the composite. The transmission coefficient according to the ultrasound wave propagation is 0.45 when an ultrasound wave propagates from the composite to aluminum and 1.55 when the wave propagates from the aluminum to composite. It can be expected that the amplitude of an ultrasound wave propagating from aluminum to composite is 3.44 times greater than that of a wave in the opposite direction. The peak-to-peak amplitude of the ultrasound signal in each direction of propagation was measured under the same conditions except the excitation and sensing sides. Ultrasound generation and sensing were carried out using Q-switched Nd:YAG laser and LDV, respectively, and the signal was filtered within 50 kHz to 1 MHz. In each propagation case, ultrasound signals at five different points were acquired and the signal at each point was averaged 128 times. The amplitude measured in each propagation direction is plotted using an error bar plot, as seen in Fig. 3(b). As expected in the calculation of acoustic impedance, when the ultrasound wave propagates from aluminum to composite, the signal amplitude is 3.51 times greater than that in the opposite direction of propagation. From this investigation, we can conclude that an ultrasound wave of greater amplitude can be obtained by excitation on the interior aluminum liner and sensing on the exterior composite in the inspection of type-3 pressure vessels.
Table 1. Acoustic impedance and density of, and speed of sound in, aluminum and composite Material
Z (kg/m2s)
Ο (kg/m3)
C (m/s)
Aluminum
17.01 Γ 106
2.7 Γ 103
6.3 Γ 103
CFRP
4.96 Γ 106
1.6 Γ 103
3.1 Γ 103
5
Figure 3. Ultrasound wave amplitude comparison with respect to propagation direction: (a) specimen, (b) amplitude comparison in error bar plot
4. System set-up Figure 4(a) shows the overall configuration of the R-TT-UPI system. To inspect the entire cylindrical section, either the pressure vessel or the inspection devices, namely the excitation and sensing lasers, must move linearly while the pressure vessel continuously rotates. In the R-TT-UPI system, the pressure vessel is in motion, and the excitation and sensing lasers are located at fixed positions outside the pressure vessel. Two shafts are connected to each boss of the pressure vessel and are inserted into the bearing supports equipped with a jig bearing. The bearing supports sustain the weight of the pressure vessel to reduce the burden on the rotation stage and minimize eccentricity during continuous rotation. The rotation stage that facilitates the rotation of the pressure vessel was connected to a single shaft, as shown in Fig. 4(b), by assembling a lathe chuck. The pressure vessel, bearing supports, and rotation stage are placed on a linear stage and moved in the horizontal direction with continuous rotation throughout the inspection.
6
Figure 4. Rotational through-transmission ultrasonic propagation imaging system: (a) overall configuration, (b) rotation stage connection with lathe chuck and shaft, (c) alignment between excitation laser and pipe, (d) 45ο° mirror at the end of pipe and alignment with sensing laser As previously mentioned in Section 3, during the inspection of a type-3 pressure vessel with fiber filaments wound on an aluminum liner, an ultrasound wave of greater amplitude can be obtained by exciting the aluminum liner and detecting at the external composite layer. To excite an aluminum liner inside a pressure vessel, the excitation laser must be inserted into the vessel through an open boss and then refracted perpendicular to the liner. A slender pipe was placed for the propagation of the laser beam into the vessel, and the excitation laser was aligned with the center of the pipe, as shown in Fig. 4(c). A mirror mounted at 45ο° was assembled at the end of the pipe to reflect the laser beam into the inner surface of the vessel, as shown in Fig. 4(d). The sensing laser was then aligned to detect the excitation point correctly.
5. Inspection result for type-3 pressure vessel 5.1. Test structure and previous inspection result using G-UPI A type-3 pressure vessel was inspected to test the performance of the inspection system developed. The composite pressure vessel was used to store the liquid fuel of a space launcher, and its shape and dimensions are given in Fig. 5(a). A sectional view of the layers is provided in Fig. 5(b). On the aluminum seamless liner with a minimum thickness of 3 mm, composite filaments were wound in the sequence of 1 mm thickness in the hoop direction, 2.5 mm thickness in the helical direction of Β± 9ο°, and 1.5 mm thickness in the hoop direction. Glass fibers were wound in the middle of the cylindrical section, the glass fiber layer was 0.2 mm thick, and the 7
vessel was coated with epoxy layer. As shown in Figs. 6(a) and 6(b), hexagon bolt head- and ball-shaped impact masses were used to make impact damage spots on the pressure vessel. The weight and potential energy were 1 kg and 16.5 J, respectively, for the bolt head-shaped impact mass, and 1.09 kg and 16.1 J, respectively, for the ball-shaped impact mass. The pictures of the impact locations are provided in Figs. 6(c) and 6(d). The shape of the impact damage spot created by the bolt head-shaped mass can be clearly observed in Fig. 6(c), while the location of the impact damage made by the ball-shaped mass is barely visible, as can be seen in Fig. 6(d).
Figure 5. Test structure: (a) shape and dimensions, (b) lay-up
Figure 6. Impact masses and damage locations: (a) bolt head-shaped impact mass, (b) ball-shaped impact mass, (c) bolt head-shaped damage location, (d) ball-shaped damage location 8
Lee et al. [15] inspected a composite pressure vessel with a guided wave ultrasonic propagation imaging (GUPI) system. The G-UPI system is a partial, noncontact laser ultrasonic inspection system that generates ultrasound waves at a high speed using a laser mirror scanner and detects the guided wave with a contact type sensor attached to the target structure at a fixed point. The inspection results were postprocessed using various visualization algorithms, and the variable time window amplitude mapping (VTWAM) method showed the best performance. Figure 7 shows the VTWAM results for the two impact damage locations. The sizes of the damage spots evaluated from these results were 15Γ13 mm2 in the case of bolt head-shaped mass drop and 25Γ15 mm2 in the case of the ball-shaped mass drop.
Figure 9. VTWAM results using G-UPI: (a) bolt head-shaped damage, (b) ball shaped-damage [15]
5.2. Inspection result using R-TT-UPI system We inspected a composite pressure vessel using the R-TT-UPI system. The aluminum liner inside the vessel was excited using a Q-switched laser with a wavelength of 1064 nm and pulse energy of 4 mJ. The LDV using a continuous HeNe laser with a wavelength of 633 nm was placed on the exterior of the composite pressure vessel to detect the ultrasound waves. The ultrasound wave signals received were acquired through a bandpass filter in the 50β250 kHz range. A total of 2250 pulsed laser beams were emitted per revolution at an inspection interval of 0.47 mm. The speed of the rotation stage was 20ο°/s and the pulse repetition rate was 125 Hz. The inspection result was visualized using the ultrasonic wave propagation imaging (UWPI) algorithm [16], which is the basic visualization algorithm of UPI systems. Figure 9 shows the inspection results obtained using the R-TT-UPI system. As shown in Fig. 9(a), an area of 1068 mmΓ330 mm was inspected, and the cutaway part made for other tests was covered with a white paper. In Fig. 9(b), the circular delamination defects caused by the bolt head- and ball-shaped impact masses were clearly detected, and both defects were evaluated to be about 50 mm in diameter. In the previous G-UPI inspection results given in Fig. 8, the defect sizes were evaluated as 15Γ13 mm2 in the case of the bolt headshaped mass and 25Γ15 mm2 in the case of the ball-shaped mass. However, based on the inspection results obtained using R-TT-UPI, the actual delamination defects are found to be larger than the sizes evaluated by GUPI. Also, half-elliptical defects were detected around the boundaries of the upper and lower edges of the cutaway. This part is presumed to be the debonding defects between the aluminum liner and composite layers caused during the mechanical cutting. Besides, in the inspection results shown in Fig. 9(c), composite layers in 9
a helical direction at the center of the layer were clearly observed. From this observation, it is expected that this system can be used to check manufacturing defects and in-service damage during regular inspections.
Figure 3. Type-3 composite pressure vessel inspection: (a) scanning area, (b) UWPI inspection result at 10 ΞΌs, (c) UWPI inspection result at 16.6 ΞΌs The inspectable flaw size, which is same as the minimum detectable defect size, was assessed for the quantitative evaluation of system performance in this application. The ultrasound signals of 160 scanning points near the ball-shaped drop damage, as indicated in Fig. 10(a), were selected, and the magnitudes of the selected signals at 10 ΞΌs, which is identical to the time slice of the inspection result shown in Fig. 9(b), were plotted in Fig. 10(b). In Fig. 10(b), the signal magnitudes in the left and right intact areas are relatively high while those in the central defective area are low owing to the discontinuity caused by the delamination defect. Both boundaries between the intact and defective areas, as highlighted by solid red lines, show sharp slopes, which remain almost similar even if the defect becomes smaller. If the defect size continuously decreases, both boundaries get closer to each other and eventually merge, as shown in Fig. 11. This condition can be determined as a critical condition that an inspector can distinguish as a defect. The inspectable flaw size can be assessed using the full width at half maximum (FWHM) value from the signal distribution of the merged boundary at this critical condition [17,18]. The FWHM of the boundary is 4890.5, as indicated by the dashed line in Fig. 11. For a conservative assessment, the two adjacent points on the left and right boundaries above the FWHM are the 42th and 48th scan points, and the distance between these two points is six times the inspection interval. Therefore, with the inspection condition applied to the inspection of the type-3 pressure vessel in this study, the inspectable flaw size is defined as 2.82 mm, i.e., six times the inspection interval (0.47 mm).
10
Figure 4. Ultrasound wave signals from intact and defective areas: (a) signal extracted location, (b) signal magnitude of 160 points
Figure 5. Signal magnitude at critical condition (both boundaries merged)
6. Conclusion An R-TT-UPI system was developed for the inspection of a filament-wound composite pressure vessel. The R-TT-UPI system uses a rotational scanning method to achieve high signal-to-noise ratio (SNR) throughout the circular cylindrical section of the composite pressure vessel. The proof-of-concept test with an aluminum pipe specimen showed that the SNR was maintained at a high level throughout the inspection area in the rotational scanning method but significantly decreased in the raster scanning method. The R-TT-UPI system enabled the through-transmission mode of inspection by propagating laser beams to the interior of the pressure vessel through the boss, and this made it possible to inspect thicker pressure vessels. A type-3 pressure vessel was inspected using the system developed, and the inspection result was compared with the result of a previous inspection using a G-UPI system. The system developed successfully detected two impact damage spots, and the inspection result showed that the defect size had been underestimated in a previous inspection using G-UPI. Since the UWPI inspection can reveal even the fiber winding direction, the system developed can be used for quality control in manufacturing process as well as for inspection in operational fields. From these results, it 11
can be concluded that the R-TT-UPI system can be used as a reliable inspection system for a composite pressure vessel and replace the hydrostatic test for the safety certification of such vessels. Acknowledgments
This research was supported by Hyundai NGV. References [1] [2]
R. P. Gangloff and B. P. Somerday, Gaseous hydrogen embrittlement of materials in energy technologies: the problem, its characterisation and effects on particular alloy classes. Elsevier, 2012. M. Pelkner, R. Casperson, R. Pohl, D. Munzke, and B. Becker, βEddy current testing of composite pressure vessels,β Int. J. Appl. Electromagn. Mech., vol. Preprint, no. Preprint, pp. 1β6, Dec. 2018. A. Bulletti, P. Giannelli, M. Calzolai, and L. Capineri, βAn Integrated Acousto/Ultrasonic Structural Health Monitoring System for Composite Pressure Vessels,β IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 63, no. 6, pp. 864β873, Jun. 2016. J. Degrieck, W. De Waele, and P. Verleysen, βMonitoring of fibre reinforced composites with embedded optical fibre Bragg sensors, with application to filament wound pressure vessels,β NDT&E Int., vol. 34, no. 4, pp. 289β296, Jun. 2001. S. M. Klute, D. R. Metrey, N. Garg, N. Aida, and A. Rahim, βin-Situ Structural Health Monitoring of CompositeOverwrapped Pressure Vessels,β Luna Innovations Incorporated, 2016. F. Dahmene et al., βUse of Acoustic Emission for Inspection of Composite Pressure Vessels Subjected To Mechanical Impact.β in 33rd European conference on acoustic emission testing, Senlis, France, 2018. H. Y. Chou, A. P. Mouritz, M. K. Bannister, and A. R. Bunsell, βAcoustic emission analysis of composite pressure vessels under constant and cyclic pressure,β Compos. Part A Appl. Sci. Manuf., vol. 70, pp. 111β120, Mar. 2015. M. Toughiry, βExamination Of The Nondestructive Evaluation Of Composite Gas Cylinders FINAL REPORT,β The Nondestructive Testing Information Analysis Center, 2002. T. Kundu, Ultrasonic and Electromagnetic NDE for Structure and Material Characterization. CRC Press, 2018. H. Nishino, K. Yoshida, H. Cho, and M. Takemoto, βPropagation phenomena of wideband guided waves in a bended pipe,β Ultrasonics, vol. 44, no. SUPPL., pp. e1139βe1143, Dec. 2006. J. R. Lee, J. Takatsubo, N. Toyama, and D. H. Kang, βHealth monitoring of complex curved structures using an ultrasonic wavefield propagation imaging system,β Meas. Sci. Technol., vol. 18, no. 12, pp. 3816β3824, Dec. 2007. C. Lee and S. Park, βDamage visualization of pipeline structures using laser-induced ultrasonic waves,β Structural Health Monitoring., vol. 14, no. 5, pp. 475β488, Sep. 2015. J. L. Rose, Ultrasonic guided waves in solid media. Cambridge University Press, 2014. L. W. Schmerr, Fundamentals of ultrasonic nondestructive evaluation. Springer, 2016. J.-R. Lee, H. Jeong, T. T. Chung, H. Shin, and J. Park, βDamage Visualization of Filament Wound Composite Hydrogen Fuel Tank Using Ultrasonic Propagation Imager,β Compos. Res., vol. 28, no. 4, pp. 143β147, Aug. 2015. J.-R. Lee, C. C. Chia, C.-Y. Park, and H. Jeong, "Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Algorithm," Optics & Laser Technology, vol. 44, pp. 1507-1515, 2012. G. C. Wetsel and F. A. McDonald, βResolution and definition in photothermal imaging,β J. Appl. Phys., vol. 56, no. 11, pp. 3081β3085, Dec. 1984. D. P. Almond and S. K. Lau, βDefect sizing by transient thermography. I. An analytical treatment,β J. Phys. D. Appl. Phys., vol. 27, no. 5, pp. 1063β1069, May 1994.
[3]
[4]
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Author statement
12
Young-Jun Lee: Investigation, Formal analysis, Writing-Original Draft, Hasan Ahmed: Software, Jung-Ryul Lee: Conceptualization, Methodology, Supervision, Writing- Review & Editing
[19]
Declaration of interests
β The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
βThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
JR Lee on the behalf of authors
[20]
13
S0263-8223(19)33098-3 https://doi.org/10.1016/j.compstruct.2020.111871 COST 111871
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
17 August 2019 16 November 2019 2 January 2020
Please cite this article as: Lee, Y-J., Ahmed, H., Lee, J-R., Filament-Wound Composite Pressure Vessel Inspection based on Rotational Through-Transmission Laser Ultrasonic Propagation Imaging, Composite Structures (2020), doi: https://doi.org/10.1016/j.compstruct.2020.111871
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Β© 2020 Elsevier Ltd. All rights reserved.
Filament-Wound Composite Pressure Vessel Inspection based on Rotational Through-Transmission Laser Ultrasonic Propagation Imaging
1Young-Jun 1Department
Lee, 1Hasan Ahmed, 1Jung-Ryul Lee*
of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseonggu, Daejeon, 34141, Republic of Korea
*Corresponding author e-mail: [email protected]
Abstract In this paper, we introduce a rotational through-transmission ultrasonic propagation imaging system to inspect the full thickness of the entire cylindrical section of filament-wound composite overwrapped pressure vessels. This system can achieve high signal-to-noise ratio throughout the cylindrical section of pressure vessels by using a rotational scanning method. Based on the advantages of the laser ultrasonic system that can generate and detect ultrasound waves by using a small-diameter laser beam and making an inspection path in a narrow environment, this system directs the laser beam inside the pressure vessel via a mirror to enable the throughtransmission mode for vessel inspection. A type-3 pressure vessel was inspected using the system developed, which successfully detected two impact damage spots and showed the filament winding direction clearly. The test results show that the system developed is highly reliable and can replace the current hydrostatic sample test for pressure vessel safety certification. Keywords: laser ultrasonic; composite overwrapped pressure vessel; rotational scanning; through-transmission
1. Introduction Pressure vessels are used to hold liquids or gases under high pressure. In some industries, such as the oil and gas industry, where pressure vessels are used in stationary conditions without movement, metal vessels are generally used. On the other hand, in the case of vehicles carrying fuel containers, such as space launchers and flights, filament-wound composite pressure vessels have been used for weight reduction and fuel efficiency. The demand for composite pressure vessels is increasing rapidly owing to their recent use as fuel containers. Pressure vessels are exposed to cyclic loading owing to frequent charging and discharging while in operation, and debonding can occur at any composite layer when subjected to impact damage. Therefore, periodic inspection is essential to ensure the structural integrity of pressure vessels, but it is difficult to inspect composite pressure vessels owing to their structural complexity and low inspection accessibility. Currently, composite pressure vessels must be regularly recertified by visual inspection and a hydrostatic test, which imposes significant costs and out-of-service time [1]. Numerous studies have been conducted to develop reliable inspection and diagnostic methods that can replace the hydrostatic test for composite pressure vessels. Pelkner et al. [2] inspected an artificially aged type-3 pressure vessel using the eddy current method and detected defects in the metallic liner. Bulletti et al. [3] 1
attached two piezopolymer film transducer arrays to the exterior of a composite pressure vessel and conducted damage localization using sensor networking. Only one of the four impact damage spots was localized similarly, and the other three damage spots were not. Degrieck et al. [4] showed that strain measurement is possible under variable and cyclic loading conditions using embedded fiber Bragg grating sensors. Klute et al. [5] embedded optical fibers in a composite pressure vessel and attempted to detect defects by measuring the residual strain after impact test and hydrostatic test using optical frequency domain reflectometry. Dahmene et al. [6] studied the acoustic emission method with eight sensors that could cover the entire vessel and established a criterion based on acoustic emission (AE) events to determine whether a composite pressure vessel is usable after impact damage. More than other methods, the AE test has been studied for composite pressure vessel inspection by many researchers [6β8]. Most previous studies have focused on embedded sensor-based monitoring to determine whether damage has occurred or the vessels are reusable rather than on in-situ inspection based on external sensors to evaluate or characterize defects. Furthermore, most methods currently under development can be used to inspect only near the surface layers of the tank because of difficulties in accessing the inner surface or high vessel thickness. In this study, we developed a rotational through-transmission ultrasonic propagation imaging (R-TT-UPI) system that can inspect the entire thickness of the cylindrical section of pressure vessels. This system is based on laser ultrasonics and can acquire extremely clear ultrasound signals throughout the entire circular cylindrical section using a rotational scanning method. In addition, this system is capable of being used in the throughtransmission mode, which makes it possible to inspect thicker pressure vessels than those inspected using the pulse-echo mode. This paper is organized as follows. In Section 2, we explain the rotational scanning method and algorithm for the inspection of the circular, cylindrical section of pressure vessels. In Section 3, we describe the implementation of the through-transmission mode of inspection and discuss ultrasonic propagation through the metal-composite layers of a composite pressure vessel. In Section 4, we describe the set-up and configuration of the system developed, and in Section 5, we finally present the inspection results for a type-3 pressure vessel using the system developed.
2. Rotational scanning method using laser ultrasonic bulk wave
A laser ultrasonic system is advantageous for the inspection of curved surfaces because it can inspect remotely without any constraint of contact, and its footprint on the target surface is extremely small [9]. Most studies on the inspection of curved surfaces using laser ultrasonics entail the use of guided waves [10β12], which propagate along an out-of-plane direction. However, when we inspect a thick structure using guided waves, only surface waves are excited at a low depth near the surface and not Lamb waves that can inspect the entire thickness [13]. Therefore, bulk waves should be used because guided waves cannot inspect the full thickness of a thick structure. However, unlike that of guided waves, the signal quality of bulk waves is degraded owing to variations in the incidence angle and the standoff distance between the curved surface and sensing laser. Therefore, a rotational scanning method is proposed to solve this problem. The cross section of a pressure vessel is a circle, which is a set of points at the same distance from the center, and the tangential line at each point is perpendicular to the center of the circle. Therefore, if a circular object is rotated around its center, as shown in Fig. 1, the laser beams are always normally incident and the standoff distance is constant.
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Figure 1. Schematic of rotational scanning method
Figure 2. Algorithm of rotational scanning method Figure 2 shows the algorithm of the rotational scanning method. The user first inputs the outer radius of the target cross section, length to be inspected, rotation speed, and the number of triggers per revolution. The system then provides the inspection interval in angle and length scale, and shows the number of scanning lines required to inspect the entire cylindrical section and this is equal to the total number of revolutions. When an inspection is started, the rotation stage rotates and generates triggers at every angular scan interval, and the triggers are then transmitted to the excitation and sensing lasers and DAQ to generate and capture the ultrasound wave 3
signal at a single scanning point. This rotational scanning of each scanning point is repeated until one revolution of the rotation stage is completed. Thereafter, the linear stage moves according to the linear scan interval, and the rotational scanning step is repeated. The inspection is terminated when the number of inspected lines reaches the desired number of scanning lines.
3. Through-transmission mode of inspection of metal-composite layers of composite pressure vessel
The most commonly used inspection modes in ultrasonic testing are the pulse-echo and through-transmission modes. The notable difference between the two inspection modes is the plane on which the excitation and sensing take place. In the pulse-echo mode, the excitation and sensing are carried out on the same plane, and therefore it is advantageous when only one side of the structure is accessible. On the other hand, in the throughtransmission mode, the ultrasound waves should be excited on one side of the structure and detected on the other side, and so the drawback of this method is that both the surfaces of the structure must be accessible. However, the through-transmission mode is more advantageous than the pulse-echo mode when a thick structure is to be inspected. The amplitude of the ultrasound wave (A) is exponentially attenuated as travel distance (z) increases, as expressed in Equation 1: π΄ = π΄0π β β π§,
(1)
where π΄0 is the unattenuated amplitude of the ultrasound wave, and Ξ± is the attenuation coefficient of the wave traveling in the z-direction. In the pulse-echo mode, the travel distance of an ultrasound wave is twice the thickness of the structure, whereas in the through-transmission mode, the travel distance is equal to the thickness of the structure and, hence, is half the travel distance in the pulse-echo mode, so that the attenuation of the ultrasound waves can be drastically reduced. This makes it possible to use the through-transmission mode to inspect thick structures that cannot be inspected using the pulse-echo mode. Pressure vessels are intended to store gases and liquids at high pressure without leakage; therefore, all the sections are perfectly closed except for a boss on one side for fuel injection. Consequently, the inspection accessibility is limited, and most researchers have tried to inspect or monitor the exterior of the pressure vessel. The laser ultrasonic system, on the other hand, uses a laser beam with a small diameter in the sub-millimeter range for ultrasound excitation and detection. The laser has good directivity and a low divergence angle, and the diameters of the laser beams can be easily adjusted using optical components. Therefore, if a small-diameter laser beam is propagated into a pressure vessel, it is possible to achieve the through-transmission mode of inspection. The procedure to implement the through-transmission mode in a pressure vessel is explained in detail in Section 4. Composite pressure vessels are manufactured by winding continuous fiber filaments on a metallic or plastic liner. Hence, bulk wave, which propagates in the through-thickness direction, can be transmitted through metalcomposite or plastic-composite layers. When ultrasound wave propagates through different media, it is partially reflected and transmitted at the interface. The transmission and reflection ratios of the ultrasound wave at the interface depend on the acoustic impedance of the two adjacent media. The acoustic impedance is expressed as: π = π Γ πΆ = π/π,
(2)
where Ο and C are the density and speed of sound, respectively, and P and V are pressure and velocity, respectively. The reflection and transmission coefficients (R and T, respectively) can be calculated using the acoustic impedance of the two media, which can be expressed in various terms such as pressure, intensity, and velocity (or displacement), as shown in Equation 3 [14]: ππ
π2πΆ2 β π1πΆ1
π2 β π1
ππ‘
2π2πΆ2
2π2
Pressure: π π = ππ = π2πΆ2 + π1πΆ1 = π2 + π1 , ππ = ππ = π1πΆ1 + π2πΆ2 = π1 + π2 4
(3-1)
πΌπ
π2π /2π2πΆ2
πΌπ‘
π2π‘ /2π2πΆ2
π1πΆ1
Intensity: π πΌ = πΌπ = π2/2π πΆ = π 2π , ππΌ = πΌπ = π2/2π πΆ = π2πΆ2π2π π
ππ/π1πΆ1
1 1
π
ππ‘/π2πΆ2
π1πΆ1
(3-2)
1 1
2π1πΆ1
2π1
Velocity: π π = ππ/π1πΆ1 = π π, ππ = ππ/π1πΆ1 = π2πΆ2ππ = π1πΆ1 + π2πΆ2 = π1 + π2
,
(3-3)
where the subscripts i, r, and t refer to incidence, reflection, and transmittance, respectively. Generally, the ultrasound wave is detected by measuring the microscopic movement of a surface caused by the propagation of the wave inside the structure and not by directly measuring the wave. In this study, a laser Doppler vibrometer (LDV), which measures the velocity of objects, was used to detect the ultrasound waves. Therefore, the expression in terms of velocity, as presented in Equation 3-3, should be applied. From this equation, it can be seen that the transmission coefficient is greater when an ultrasound wave propagates from a medium with high acoustic impedance to one with low acoustic impedance. We conducted an experiment to observe how the amplitudes of ultrasound waves differ according to the direction of propagation through metal-composite layers. The amplitudes of the ultrasound waves were measured by changing the excitation and detection sides of the specimen, which is the composite bonded to an aluminum plate, as shown in Fig. 3(a). This structural type can be easily found in the case of composite patch repair of a metallic structure. The test specimen was stacked with 2 mm cross ply composites on a 2 mm thick aluminum plate. Table 1 shows the acoustic impedances of the aluminum and composite. The acoustic impedance of aluminum is about thrice that of the composite. The transmission coefficient according to the ultrasound wave propagation is 0.45 when an ultrasound wave propagates from the composite to aluminum and 1.55 when the wave propagates from the aluminum to composite. It can be expected that the amplitude of an ultrasound wave propagating from aluminum to composite is 3.44 times greater than that of a wave in the opposite direction. The peak-to-peak amplitude of the ultrasound signal in each direction of propagation was measured under the same conditions except the excitation and sensing sides. Ultrasound generation and sensing were carried out using Q-switched Nd:YAG laser and LDV, respectively, and the signal was filtered within 50 kHz to 1 MHz. In each propagation case, ultrasound signals at five different points were acquired and the signal at each point was averaged 128 times. The amplitude measured in each propagation direction is plotted using an error bar plot, as seen in Fig. 3(b). As expected in the calculation of acoustic impedance, when the ultrasound wave propagates from aluminum to composite, the signal amplitude is 3.51 times greater than that in the opposite direction of propagation. From this investigation, we can conclude that an ultrasound wave of greater amplitude can be obtained by excitation on the interior aluminum liner and sensing on the exterior composite in the inspection of type-3 pressure vessels.
Table 1. Acoustic impedance and density of, and speed of sound in, aluminum and composite Material
Z (kg/m2s)
Ο (kg/m3)
C (m/s)
Aluminum
17.01 Γ 106
2.7 Γ 103
6.3 Γ 103
CFRP
4.96 Γ 106
1.6 Γ 103
3.1 Γ 103
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Figure 3. Ultrasound wave amplitude comparison with respect to propagation direction: (a) specimen, (b) amplitude comparison in error bar plot
4. System set-up Figure 4(a) shows the overall configuration of the R-TT-UPI system. To inspect the entire cylindrical section, either the pressure vessel or the inspection devices, namely the excitation and sensing lasers, must move linearly while the pressure vessel continuously rotates. In the R-TT-UPI system, the pressure vessel is in motion, and the excitation and sensing lasers are located at fixed positions outside the pressure vessel. Two shafts are connected to each boss of the pressure vessel and are inserted into the bearing supports equipped with a jig bearing. The bearing supports sustain the weight of the pressure vessel to reduce the burden on the rotation stage and minimize eccentricity during continuous rotation. The rotation stage that facilitates the rotation of the pressure vessel was connected to a single shaft, as shown in Fig. 4(b), by assembling a lathe chuck. The pressure vessel, bearing supports, and rotation stage are placed on a linear stage and moved in the horizontal direction with continuous rotation throughout the inspection.
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Figure 4. Rotational through-transmission ultrasonic propagation imaging system: (a) overall configuration, (b) rotation stage connection with lathe chuck and shaft, (c) alignment between excitation laser and pipe, (d) 45ο° mirror at the end of pipe and alignment with sensing laser As previously mentioned in Section 3, during the inspection of a type-3 pressure vessel with fiber filaments wound on an aluminum liner, an ultrasound wave of greater amplitude can be obtained by exciting the aluminum liner and detecting at the external composite layer. To excite an aluminum liner inside a pressure vessel, the excitation laser must be inserted into the vessel through an open boss and then refracted perpendicular to the liner. A slender pipe was placed for the propagation of the laser beam into the vessel, and the excitation laser was aligned with the center of the pipe, as shown in Fig. 4(c). A mirror mounted at 45ο° was assembled at the end of the pipe to reflect the laser beam into the inner surface of the vessel, as shown in Fig. 4(d). The sensing laser was then aligned to detect the excitation point correctly.
5. Inspection result for type-3 pressure vessel 5.1. Test structure and previous inspection result using G-UPI A type-3 pressure vessel was inspected to test the performance of the inspection system developed. The composite pressure vessel was used to store the liquid fuel of a space launcher, and its shape and dimensions are given in Fig. 5(a). A sectional view of the layers is provided in Fig. 5(b). On the aluminum seamless liner with a minimum thickness of 3 mm, composite filaments were wound in the sequence of 1 mm thickness in the hoop direction, 2.5 mm thickness in the helical direction of Β± 9ο°, and 1.5 mm thickness in the hoop direction. Glass fibers were wound in the middle of the cylindrical section, the glass fiber layer was 0.2 mm thick, and the 7
vessel was coated with epoxy layer. As shown in Figs. 6(a) and 6(b), hexagon bolt head- and ball-shaped impact masses were used to make impact damage spots on the pressure vessel. The weight and potential energy were 1 kg and 16.5 J, respectively, for the bolt head-shaped impact mass, and 1.09 kg and 16.1 J, respectively, for the ball-shaped impact mass. The pictures of the impact locations are provided in Figs. 6(c) and 6(d). The shape of the impact damage spot created by the bolt head-shaped mass can be clearly observed in Fig. 6(c), while the location of the impact damage made by the ball-shaped mass is barely visible, as can be seen in Fig. 6(d).
Figure 5. Test structure: (a) shape and dimensions, (b) lay-up
Figure 6. Impact masses and damage locations: (a) bolt head-shaped impact mass, (b) ball-shaped impact mass, (c) bolt head-shaped damage location, (d) ball-shaped damage location 8
Lee et al. [15] inspected a composite pressure vessel with a guided wave ultrasonic propagation imaging (GUPI) system. The G-UPI system is a partial, noncontact laser ultrasonic inspection system that generates ultrasound waves at a high speed using a laser mirror scanner and detects the guided wave with a contact type sensor attached to the target structure at a fixed point. The inspection results were postprocessed using various visualization algorithms, and the variable time window amplitude mapping (VTWAM) method showed the best performance. Figure 7 shows the VTWAM results for the two impact damage locations. The sizes of the damage spots evaluated from these results were 15Γ13 mm2 in the case of bolt head-shaped mass drop and 25Γ15 mm2 in the case of the ball-shaped mass drop.
Figure 9. VTWAM results using G-UPI: (a) bolt head-shaped damage, (b) ball shaped-damage [15]
5.2. Inspection result using R-TT-UPI system We inspected a composite pressure vessel using the R-TT-UPI system. The aluminum liner inside the vessel was excited using a Q-switched laser with a wavelength of 1064 nm and pulse energy of 4 mJ. The LDV using a continuous HeNe laser with a wavelength of 633 nm was placed on the exterior of the composite pressure vessel to detect the ultrasound waves. The ultrasound wave signals received were acquired through a bandpass filter in the 50β250 kHz range. A total of 2250 pulsed laser beams were emitted per revolution at an inspection interval of 0.47 mm. The speed of the rotation stage was 20ο°/s and the pulse repetition rate was 125 Hz. The inspection result was visualized using the ultrasonic wave propagation imaging (UWPI) algorithm [16], which is the basic visualization algorithm of UPI systems. Figure 9 shows the inspection results obtained using the R-TT-UPI system. As shown in Fig. 9(a), an area of 1068 mmΓ330 mm was inspected, and the cutaway part made for other tests was covered with a white paper. In Fig. 9(b), the circular delamination defects caused by the bolt head- and ball-shaped impact masses were clearly detected, and both defects were evaluated to be about 50 mm in diameter. In the previous G-UPI inspection results given in Fig. 8, the defect sizes were evaluated as 15Γ13 mm2 in the case of the bolt headshaped mass and 25Γ15 mm2 in the case of the ball-shaped mass. However, based on the inspection results obtained using R-TT-UPI, the actual delamination defects are found to be larger than the sizes evaluated by GUPI. Also, half-elliptical defects were detected around the boundaries of the upper and lower edges of the cutaway. This part is presumed to be the debonding defects between the aluminum liner and composite layers caused during the mechanical cutting. Besides, in the inspection results shown in Fig. 9(c), composite layers in 9
a helical direction at the center of the layer were clearly observed. From this observation, it is expected that this system can be used to check manufacturing defects and in-service damage during regular inspections.
Figure 3. Type-3 composite pressure vessel inspection: (a) scanning area, (b) UWPI inspection result at 10 ΞΌs, (c) UWPI inspection result at 16.6 ΞΌs The inspectable flaw size, which is same as the minimum detectable defect size, was assessed for the quantitative evaluation of system performance in this application. The ultrasound signals of 160 scanning points near the ball-shaped drop damage, as indicated in Fig. 10(a), were selected, and the magnitudes of the selected signals at 10 ΞΌs, which is identical to the time slice of the inspection result shown in Fig. 9(b), were plotted in Fig. 10(b). In Fig. 10(b), the signal magnitudes in the left and right intact areas are relatively high while those in the central defective area are low owing to the discontinuity caused by the delamination defect. Both boundaries between the intact and defective areas, as highlighted by solid red lines, show sharp slopes, which remain almost similar even if the defect becomes smaller. If the defect size continuously decreases, both boundaries get closer to each other and eventually merge, as shown in Fig. 11. This condition can be determined as a critical condition that an inspector can distinguish as a defect. The inspectable flaw size can be assessed using the full width at half maximum (FWHM) value from the signal distribution of the merged boundary at this critical condition [17,18]. The FWHM of the boundary is 4890.5, as indicated by the dashed line in Fig. 11. For a conservative assessment, the two adjacent points on the left and right boundaries above the FWHM are the 42th and 48th scan points, and the distance between these two points is six times the inspection interval. Therefore, with the inspection condition applied to the inspection of the type-3 pressure vessel in this study, the inspectable flaw size is defined as 2.82 mm, i.e., six times the inspection interval (0.47 mm).
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Figure 4. Ultrasound wave signals from intact and defective areas: (a) signal extracted location, (b) signal magnitude of 160 points
Figure 5. Signal magnitude at critical condition (both boundaries merged)
6. Conclusion An R-TT-UPI system was developed for the inspection of a filament-wound composite pressure vessel. The R-TT-UPI system uses a rotational scanning method to achieve high signal-to-noise ratio (SNR) throughout the circular cylindrical section of the composite pressure vessel. The proof-of-concept test with an aluminum pipe specimen showed that the SNR was maintained at a high level throughout the inspection area in the rotational scanning method but significantly decreased in the raster scanning method. The R-TT-UPI system enabled the through-transmission mode of inspection by propagating laser beams to the interior of the pressure vessel through the boss, and this made it possible to inspect thicker pressure vessels. A type-3 pressure vessel was inspected using the system developed, and the inspection result was compared with the result of a previous inspection using a G-UPI system. The system developed successfully detected two impact damage spots, and the inspection result showed that the defect size had been underestimated in a previous inspection using G-UPI. Since the UWPI inspection can reveal even the fiber winding direction, the system developed can be used for quality control in manufacturing process as well as for inspection in operational fields. From these results, it 11
can be concluded that the R-TT-UPI system can be used as a reliable inspection system for a composite pressure vessel and replace the hydrostatic test for the safety certification of such vessels. Acknowledgments
This research was supported by Hyundai NGV. References [1] [2]
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Author statement
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Young-Jun Lee: Investigation, Formal analysis, Writing-Original Draft, Hasan Ahmed: Software, Jung-Ryul Lee: Conceptualization, Methodology, Supervision, Writing- Review & Editing
[19]
Declaration of interests
β The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
βThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
JR Lee on the behalf of authors
[20]
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