Liquid fuel pressure testing and damage monitoring method for composite material pressure container

Abstract

The invention discloses a liquid fuel pressure testing and damage monitoring method for a composite material pressure vessel, and the method carries out the system analysis of the pressure change of the composite material pressure vessel in the liquid fuel filling and pressurization process. Distributed optical fiber sensors and strain gauges are arranged in a stress concentration area, and a time control method and a pressure control method are adopted for real-time monitoring in the two stages of liquid fuel injection and pressurization respectively, so that strain and temperature distribution conditions are accurately captured; and the total leakage rate of the container is detected by combining a packaging method with a helium mass spectrum technology, so that the sealing performance and the structural integrity of the container are evaluated. The numerical simulation and experimental detection combined method provided by the invention provides a brand new thought for performance evaluation and safety monitoring of the composite material pressure vessel under extreme working conditions.

Description

Technical Field

[0001] The present invention relates to a monitoring method, and particularly to a monitoring method for a pressure vessel. Background Art

[0002] Composite pressure vessels are widely used in fields such as aerospace, energy storage, and deep-sea exploration due to their advantages of light weight, high strength, and corrosion resistance. However, in actual working conditions, the containers often face the ultra-low temperature environment of liquid fuels such as liquid nitrogen and liquid oxygen, as well as the complex stress effects during the pressurization process, posing great challenges to their structural safety and reliability. The ultra-low temperature characteristics of liquid fuels not only cause temperature gradients inside the container, generating thermal stress, but may also embrittle the composite matrix, thereby reducing its impact resistance. At the same time, the non-uniform stress field generated during the pressurization process may lead to local stress concentration, inducing damage mechanisms such as interlaminar delamination and fiber fracture. The combined action of these factors is directly related to the stability and safety of the overall system operation, and there is an urgent need to implement comprehensive and real-time monitoring and diagnosis of composite pressure vessels.

[0003] Currently, common structural health monitoring techniques include piezoelectric sensors, strain gauges, image processing techniques, acoustic emission (AE) detection, and fiber optic sensors. However, most of these methods can only obtain data from local areas and are difficult to achieve comprehensive monitoring of the overall structure. Since the occurrence of structural damage is random and difficult to predict, spatial distributed monitoring techniques have more advantages than traditional point-type monitoring methods in damage detection and structural evaluation. Distributed optical fiber sensors (DOFS) can continuously obtain strain and temperature distribution information along the entire length of the sensing optical fiber to achieve monitoring of the global state of the structure. For composite structures, strain distribution data plays an important role in state evaluation and health monitoring. Strain measurement can not only be used to detect structural anomalies, but also identify, classify, locate, and quantitatively analyze damage conditions. For example, the change in strain distribution can effectively detect the propagation of internal cracks in laminates or the creep effect of matrix materials, providing key data support for early damage identification and life prediction of composite structures. Gebhart et al. installed fiber optic sensors on the surface of the test container and analyzed the influence of the container surface profile on the measurement results by correlating the three-dimensional scanned surface and the measurement results. Ahmed et al. proposed the concept of an intelligent space structure, introduced the operating requirements of this intelligent pressure vessel and the intelligent functions achieved by fiber optic sensors. At the same time, a quantitative comparison was made between the fiber Bragg grating sensor (FBG) and the distributed optical fiber sensor of the optical frequency domain reflectometer (OFDR). Qu et al. introduced a strain monitoring method in which optical fibers are embedded along a helical path in a composite cylindrical structure, and the strain data collected by this method is used to evaluate the health state of the structure.

[0004] In summary, integrating intelligent sensors in composite pressure vessels to achieve real-time service condition monitoring has become a hot topic in current research and engineering applications and is gradually developing into a key technical means. However, during the actual operation process, the measurement accuracy of fiber optic sensors may be affected by the complexity of the container's curved surface and structural characteristics, resulting in monitoring errors. Therefore, optimizing the layout path of fiber optic sensors and adopting a reasonable monitoring strategy are crucial for improving measurement accuracy and monitoring reliability. Summary of the Invention

[0005] The purpose of the present invention is to provide an improved monitoring method for composite pressure vessels.

[0006] To this end, some embodiments of the present application propose a method for liquid fuel pressure testing and damage monitoring of a composite pressure vessel, which includes using numerical simulation methods to systematically analyze the stress distribution data obtained from the pressure change during the process of filling the composite pressure vessel with liquid fuel and pressurization to determine the stress concentration area of the composite pressure vessel; arranging distributed fiber optic sensors and strain gauges in the stress concentration area of the composite pressure vessel; using the time control method for real-time monitoring during the liquid fuel injection stage to capture the strain and temperature distribution of the composite pressure vessel; using the pressure control method for real-time monitoring during the pressurization stage to capture the strain and temperature distribution of the composite pressure vessel; and detecting the total leakage rate of the composite pressure vessel by combining the bag method with helium mass spectrometry technology to evaluate its sealing performance and structural integrity.

[0007] In some embodiments, the numerical simulation method includes establishing a finite element model of the composite pressure vessel and analyzing the finite element model in a fully thermo-mechanical coupling manner to determine the stress concentration area.

[0008] In some embodiments, the liquid fuel is liquid nitrogen, and the liquid fuel pressure test of the composite pressure vessel includes: filling the composite pressure vessel with liquid nitrogen to a preset liquid level, maintaining the filling pressure within a preset range, and monitoring the temperature uniformity; before the formal pressure loading test, conducting a pre-pressure test to ensure system stability and evaluate the preliminary response; after completing the pre-test, gradually filling helium to a preset pressure, stabilizing the pressure and then discharging the liquid nitrogen; purging the inside of the pressure vessel with nitrogen to ensure cleanliness and complete dryness, and eliminating the influence of condensation and pollutants.

[0009] In some embodiments, the liquid fuel is liquid nitrogen, and the damage monitoring of the composite pressure vessel includes: collecting initial baseline data of the distributed optical fiber sensors and strain gauges for calibrating zero drift and environmental effects; using a time control method to monitor the composite pressure vessel during the process of filling the composite pressure vessel to a preset liquid level, recording measurement data of the distributed optical fiber sensors and strain gauges at preset time intervals; after the liquid nitrogen filling is completed, re-sampling the data of the distributed optical fiber sensors to eliminate drift caused by temperature changes and ensure the accuracy of subsequent pressure monitoring data; before formal pressure loading, performing a pre-pressurization test, collecting data from each distributed optical fiber sensor and plotting a distribution curve, and saving the original data for comparison; after the pre-pressurization test is completed, re-sampling the data of the distributed optical fiber sensors; using a pressure control method to monitor the composite pressure vessel during pressurization; during the helium filling and pressurization process, synchronously recording data changes of the distributed optical fiber sensors and strain gauges at preset pressure intervals; and performing final data acquisition after the pressure returns to zero to ensure that the strain and temperature evolution of the entire pressurization-unloading cycle is fully recorded, completing the health monitoring process.

[0010] In some embodiments, the finite element analysis adopts a fully thermal-mechanical coupling analysis method, and uses ABAQUS software to model a composite pressure vessel composed of an outer cylinder wall formed by carbon fiber reinforced plastic (CFRP), a head formed by CFRP, and a head cover plate formed by structural steel. The applied loads include: a temperature load of -196°C applied to the entire inner surface of the composite pressure vessel; an internal pressure of 0.84 MPa applied to the composite pressure vessel; a gravity load along the negative Z axis; and a hydrostatic pressure based on the density of the liquid fuel applied to the composite pressure vessel.

[0011] In some embodiments, the stress concentration area includes the junction between the outer cylinder wall and the adhesive ring, the outer curved surface of the head, and the connection area between the head and the head cover plate.

[0012] In some embodiments, the distributed optical fiber sensor layout scheme includes: laying out four distributed optical fiber sensors on the upper and lower boundaries of each adhesive ring of the outer cylinder wall, the middle of the outer cylinder wall and the axial direction; laying out a distributed optical fiber sensor on each outer curved surface in the head; dividing eight monitoring areas at 45° intervals along the circumferential direction, and laying out monitoring points at 0°, 45°, 135° and 225°; and laying out two additional monitoring points in the corner area of the distributed optical fiber sensor to reduce measurement errors.

[0013] In some embodiments, the strain gauge layout scheme includes: dividing the composite pressure vessel into four monitoring areas, namely Area A, Area B, Area C, and Area D, with eight strain gauges arranged in each monitoring area; arranging eight strain gauges longitudinally, six strain gauges are arranged on each of the heads, and the rest are arranged on the 45° and 225° meridians; each monitoring area is equipped with an independent wireless strain collector, and a compensator made of the same material as the composite pressure vessel is connected for strain compensation.

[0014] In some embodiments, the time control method includes recording the measurement data of the distributed optical fiber sensor and the strain gauges at 5-minute intervals during the process of filling liquid nitrogen to 95% of the liquid level; maintaining a filling pressure of 0.10 - 0.15 MPa during the liquid nitrogen filling process, and after the filling is completed, filling helium to increase the pressure to 0.40 MPa and stabilizing the pressure for 10 minutes; the pressure control method includes recording the data of the distributed optical fiber sensor and the strain gauges every time the pressure increases by 0.10 MPa during the process of increasing the helium pressure to 0.40 MPa, and performing final data acquisition after the pressure returns to zero; the total leakage rate detection includes purging the inside of the pressure vessel with nitrogen at 0.06 - 0.08 MPa after the test is completed, and then filling helium at 0.40 - 0.42 MPa, and measuring the leakage rate using the bag method combined with the helium mass spectrometry technique.

[0015] In some embodiments, before the liquid nitrogen filling and pressurization test, the following preprocessing steps are performed: collecting the initial reference data of the distributed optical fiber sensor and the strain gauges to calibrate the zero drift; performing a pre-pressure test of 0.10 MPa, collecting data and plotting the distribution curve; after the liquid nitrogen filling is completed and the data trial collection is completed, zeroing the data of the distributed optical fiber sensor respectively.

[0016] The beneficial effects of the present invention include: through numerical simulation, the present invention systematically analyzes the global and local stress responses of the composite pressure vessel during the liquid fuel filling and pressurization process, and can optimize the layout scheme of the distributed optical fiber sensor and the strain gauges accordingly. Subsequently, the time control method and the pressure control method are used to monitor the stress and temperature changes of the pressure vessel under different working conditions, and finally accurate monitoring results can be obtained.

[0017] The optimized layout path of the distributed optical fiber sensor and the strain gauges of the present invention can achieve full coverage of the key weak areas of the composite pressure vessel. Through the data cross-validation of the distributed optical fiber sensor and the strain gauges, the accuracy of strain measurement can be effectively improved, ensuring the reliability of the monitoring results.

[0018] During the monitoring process of the present invention, a strategy combining the time control method and the pressure control method is adopted to gradually capture the dynamic changes in the service state of the composite pressure vessel.

[0019] In some embodiments of the present invention, pressure test production and multiple zero point calibrations were carried out before the test, thus laying a solid foundation for subsequent data collection. Description of the Drawings

[0020] FIG. 1(a) is a schematic structural view of the outer cylindrical wall of a composite material pressure vessel model according to an embodiment of the present application;

[0021] FIG. 1(b) is a schematic structural view of the head of a composite material pressure vessel model according to an embodiment of the present application;

[0022] FIG. 1(c) is a schematic structural view of the head cover plate of a composite material pressure vessel model according to an embodiment of the present application;

[0023] FIG. 1(d) is a schematic overall assembly view of a composite material pressure vessel model according to an embodiment of the present application;

[0024] FIG. 2(a) is a schematic mesh view of the outer cylindrical wall of a composite material pressure vessel model according to an embodiment of the present application;

[0025] FIG. 2(b) is a schematic network view of the mesh of the head of a composite material pressure vessel model according to an embodiment of the present application;

[0026] FIG. 2(c) is a schematic mesh view of the head cover plate of the outer cylindrical wall of a composite material pressure vessel model according to an embodiment of the present application;

[0027] FIG. 2(d) is a schematic mesh view of the bonded area of a composite material pressure vessel model according to an embodiment of the present application;

[0028] FIG. 3(a) is a maximum displacement nephogram of the overall response of a composite material pressure vessel according to an embodiment of the present application,

[0029] FIG. 3(b) is a maximum stress nephogram of the overall response of a composite material pressure vessel according to an embodiment of the present application.

[0030] FIG. 4(a) is a schematic diagram of the distribution of the main stress components and the maximum principal stress of the outer cylindrical wall of a composite material pressure vessel according to an embodiment of the present application, axial direction.

[0031] FIG. 4(b) is a schematic diagram of the distribution of the main stress components and the maximum principal stress of the outer cylindrical wall of a composite material pressure vessel according to an embodiment of the present application, circumferential direction.

[0032] Figure 5 is a schematic diagram of the maximum principal stress distribution of the head and cover plate of a composite material pressure vessel according to an embodiment of the present application.

[0033] Figure 6Schematic diagram of distributed optical fiber sensor layout on the outer cylindrical wall section of a composite pressure vessel according to an embodiment of the present application.

[0034] Figure 7 Schematic diagram of distributed optical fiber sensor layout on the upper and middle heads of a composite pressure vessel according to an embodiment of the present application.

[0035] Figure 8 Information diagram of strain gauge layout area of a composite pressure vessel according to an embodiment of the present application. Detailed implementation manners

[0036] The following combines the accompanying drawings to detail the specific implementation manners and beneficial effects of the technical solutions of the present application.

[0037] 1. Finite element model and simulation results

[0038] The present application uses the commercial finite element software ABAQUS to perform linear thermo-mechanical coupling analysis on the composite pressure vessel, aiming to evaluate its structural strength and predict potential stress concentration areas. Through this analysis, the strength check of relevant specimens can be effectively completed, and a theoretical basis can be provided for the optimal layout position of sensors in the experiment, thereby improving the monitoring accuracy and structural safety.

[0039] 1.1 Modeling method

[0040] The composite pressure vessel can be composed of a carbon fiber reinforced plastic (referred to as CFRP) cylindrical outer wall 10, three elliptical CFRP heads, namely the upper head 21, the middle head 22, and the lower head 23, and three structural steel head covers, namely the upper head cover 31, the middle head cover 32, and the lower head cover 33; among them, the upper head 21, the middle head 22, and the lower head 23 are hermetically connected to the outer wall 10 by adhesive bonding, and the adhesive bonding areas between the upper head 21, the middle head 22, and the lower head 23 and the outer wall are defined as adhesive bonding rings, namely the upper adhesive bonding ring 11, the middle adhesive bonding ring 12, and the lower adhesive bonding ring 13. The upper head cover and the upper head, the middle head cover and the middle head, and the lower head cover and the lower head are hermetically connected by screwing (omitted) to form two sealed chambers for storing liquid nitrogen. Schematic diagrams of the outer wall 10, each head, and each cover are shown in Figures 1(a), 1(b), and 1(c), and a schematic diagram of the overall assembly of the composite pressure vessel is shown in Figure 1(d). The forces on the two sealed chambers separated by the middle head of the composite pressure vessel are almost the same, so only one of the chambers is selected for key analysis, that is, the boxed area in Figures 3(a), 3(b), 4(a), and 4(b).

[0041] ABAQUS thermal-mechanical coupling analysis function includes two analysis methods, namely "sequential thermal-mechanical coupling" and "fully coupled thermal-mechanical coupling". The two analysis methods are equivalent, but the sequential thermal-mechanical coupling analysis has lower requirements for computer computing power. For the finite element model of the composite pressure vessel established in this application, the sequential thermal-mechanical coupling analysis cannot be stably achieved through experiments. Therefore, the fully coupled thermal-mechanical coupling method is finally adopted for analysis on the workstation.

[0042] ABAQUS is used to perform geometric modeling and mesh generation on the composite pressure vessel. The mesh generation and element types of each component are shown in Figure 2. Among them, the outer cylinder wall is meshed with two layers of bilinear traditional shell elements, and the total number of elements is 224,664. Each head, each head cover plate, and the bonding area are meshed with trilinear C3D8T temperature-displacement coupled solid elements, and the total number of elements is 24,564×3, 24,284×3, and 163,944×3 respectively. The total number of elements in the entire co-bottom storage tank model is approximately 860,000.

[0043] Combined with the actual working conditions, referring to Figure 2(a), one end of the outer cylinder wall of the composite pressure vessel is fixed, and the following temperature loads and mechanical loads are applied to the composite pressure vessel model, taking the liquid nitrogen tank as an example:

[0044] A. Temperature load:

[0045] A1: Apply a temperature load of -196°C to the entire inner surface of the liquid nitrogen tank;

[0046] B. Mechanical load:

[0047] B1: Apply an internal pressure of 0.84 MPa (safety factor is 2) inside the liquid nitrogen tank;

[0048] B2: Apply a gravity load of 1g in the negative Z-axis direction to the entire model, where the gravitational acceleration is taken as

[0049] g = 9800 mm / s 2 ; where the Z-axis is the long axis direction of the liquid nitrogen tank.

[0050] B3: Apply the corresponding hydrostatic pressure according to the reference Z-axis height in the liquid nitrogen tank based on the density of liquid nitrogen under the above temperature and pressure load conditions.

[0051] In the specific ABAQUS operation, both the "axial pressure" and "hydrostatic pressure" in the above loads are applied through a reference point (abbreviation: RP) established at the geometric center of end A of the outer cylinder wall.

[0052] 1.2 Result analysis

[0053] Figure 3 shows the overall displacement amplitude and maximum principal stress (both positive and negative) analysis results for the composite pressure vessel. The boxed area represents the area of focus. Figure 2 shows that the peak maximum displacement amplitude of the composite pressure vessel is approximately 5.27 mm; the peak maximum tensile principal stress is approximately 325 MPa, and the peak maximum compressive principal stress is approximately 264.5 MPa.

[0054] As can be seen from Figure 4, the peak values of the maximum tensile principal stress and the maximum compressive principal stress of the outer cylinder wall are located on both sides of the second bonding area, with magnitudes of 325.4 MPa and -264.5 MPa, respectively, and both are in the axial direction; however, this is a stress concentration phenomenon near the bonding area. Upon inspection, the maximum principal stress (absolute value) in most areas of the outer cylinder wall is lower than 260 MPa.

[0055] Figure 5 The maximum principal stress analysis results for the head and head cover are presented. Because "temperature strain" does not directly generate stress, its contribution to the stress of the head cover is ultimately small. Consequently, all three head covers are ultimately in a conventional mixed stress state, where tension and compression coexist. The peak values of the maximum tensile principal stress and the maximum compressive principal stress are both located near the connection between the upper head cover and the upper head, reaching 268.9 MPa and 154.5 MPa, respectively. The peak value of the maximum tensile principal stress for the head is located near the connection between the lower head and the lower head cover, reaching 235.3 MPa; the peak value of the maximum compressive principal stress is located near the connection between the middle head and the middle head cover, reaching 218.6 MPa.

[0056] Finite element model analysis results show that the critical stress-bearing areas of composite pressure vessels are primarily concentrated at the junctions between the adhesive rings and the outer cylinder wall, as well as on the outer curved surfaces of the end caps. Based on these analysis results, this application optimizes the placement of fiber optic sensors and strain gauges in these critical stress-bearing areas to improve monitoring accuracy and enhance the ability to perceive the structural stress state.

[0057] 2. Sensor layout

[0058] Based on the conclusions drawn previously, this section outlines the layout of distributed optical fibers and strain gauges within the outer wall section, upper and middle end caps of the composite pressure vessel (circled in the box). The types and quantities of monitoring equipment and sensors used in this experiment are summarized as follows:

[0059] Two fiber optic devices: LUNA-ODiSI-A and LUNA-ODiSI-B (hereinafter referred to as LUNA-A and LUNA-B);

[0060] A strain gauge acquisition device: Donghua Testing DH3819;

[0061] Types and quantities of sensors: 6 distributed optical fiber sensors and 36 low-temperature strain gauges, among which 32 are used for strain monitoring and 4 are used for temperature compensation.

[0062] 2.1 Layout scheme of distributed optical fiber sensors

[0063] In the structure of the outer cylinder wall, the range of 310 mm to 500 mm corresponds to the lower bonding ring 13, the range of 950 mm to 1140 mm corresponds to the upper bonding ring 11, and the position of 650 mm is the midline of the outer cylinder wall. Based on the above analysis results and the above key positions, 4 distributed optical fiber sensors are arranged at the upper and lower boundaries of the bonding rings of the outer cylinder wall, the middle part of the outer cylinder wall and the axial direction in this application to achieve precise monitoring of the stress and temperature distribution of the pressure vessel structure. For example, for the examples in Fig. 3(a) and Fig. 3(b), distributed optical fiber sensors are arranged at the upper and lower boundaries of the upper bonding ring 11 and the upper and lower boundaries of the middle bonding ring.

[0064] Considering that the liquid nitrogen filling amount of the composite pressure vessel only reaches 95% of the liquid level during the experiment and the liquid surface does not cover the upper boundary of the upper bonding ring, the distributed optical fiber sensor of the upper bonding ring can be arranged at its lower boundary to ensure that the monitoring area matches the range of liquid nitrogen action, thereby improving the effectiveness and accuracy of measurement. In addition, to optimize the monitoring layout scheme, the entire straight cylinder section is divided into 8 monitoring areas at 45° intervals along the circumferential direction, and multiple monitoring points are arranged along the distributed optical fiber sensor in the directions of 0°, 45°, 135° and 225°.

[0065] Since strain singularities are likely to occur at the corners of the optical fiber sensors, 2 additional monitoring points are arranged in the corner area of the sensors to reduce measurement errors and improve data reliability. After completing the reference acquisition, the specific positions of each monitoring point are finally determined and marked in Figure 5 the standard. Table 1 and Figure 6 respectively show the layout information and schematic diagram of the distributed optical fiber sensors on the outer cylinder wall.

[0066] Table 1 Layout information of optical fiber sensors on the outer cylinder wall

[0067]

[0068] According to the finite element analysis results, one distributed optical fiber sensor is arranged on the outer surface curves of the upper head and the middle head of the composite pressure vessel, and the laying path is as Figure 6 shown. Similar to the outer cylinder wall, the entire upper head or middle head is divided into 8 monitoring areas at 45° intervals along the circumferential direction, 1 measuring point is arranged every 45°, and 2 additional monitoring points are arranged in the corner area of the sensor to reduce measurement errors and improve data reliability. Figure 6 The specific positions of each monitoring point finally determined after completing the reference acquisition are marked. Table 2 andFigure 7 It respectively shows the layout information and schematic diagrams of the distributed optical fiber sensors on the upper head and the middle head.

[0069] Table 2 Layout Information of Optical Fiber Sensors on the Upper Head and the Middle Head

[0070]

[0071] 2.2 Strain Gauge Layout Scheme

[0072] In practical applications, the measurement accuracy of the distributed optical fiber sensor may be affected by the surface quality of the composite pressure vessel and the change of environmental temperature. To ensure the accuracy of the strain data at the measuring points, this application uses strain gauges as an auxiliary verification means.

[0073] This application divides the composite pressure vessel into four monitoring areas, namely Area A, Area B, Area C, and Area D. 8 strain gauges are arranged in each area to enhance the comprehensiveness of monitoring and the reliability of data. As shown in Figure 8 the figure. Among them, 8 strain gauges are arranged longitudinally, 6 are arranged on the upper head and the middle head respectively, and the rest are arranged on the 45° and 225° meridians respectively to ensure the effective monitoring of stress changes in different directions.

[0074] In addition, each monitoring area is equipped with an independent wireless strain collector for data collection and transmission. For strain compensation, the compensation port of each collector is connected to a compensating piece made of the same material as the composite pressure vessel to eliminate the influence of environmental factors on the measurement data. All strain data is finally received and processed by the data receiving terminal to achieve accurate assessment of the structural state of the container. Figure 8 It details the layout area information of the strain gauges, and Tables 3 to 6 list the detailed layout positions of the strain gauges in different monitoring areas.

[0075] Table 3 Specific Information of Strain Gauges in Area A

[0076]

[0077] Table 4 Specific Information of Strain Gauges in Area B

[0078]

[0079] Table 5 Specific Information of Strain Gauges in Area C

[0080]

[0081]

[0082] Table 6 Specific Information of Strain Gauges in Area D

[0083]

[0084] 3. Testing and monitoring process

[0085] 3.1 Liquid nitrogen test of composite pressure vessels

[0086] During the liquid nitrogen pressure test of a composite pressure vessel, liquid nitrogen is added to the pressure vessel to approximately 95% of the liquid level, and a pressure of 0.10-0.15 MPa is maintained during the filling process. Once the filling is complete, helium is added to raise the tank pressure to the operating pressure, i.e., 0.40 MPa, and maintained for a certain period of time to observe the stability of the structural performance. The filling time, tank pressure during the filling process, appearance, and raw data of abnormal noise are recorded. The main process steps are as follows:

[0087] Step 11: Check the status of the test equipment to ensure that the instruments, pipelines, sensors and other equipment are complete and operating normally;

[0088] Step 12: According to the boosting requirements, correctly connect the relevant pipes and test interfaces to ensure good sealing;

[0089] Step 13: Fill the composite pressure vessel with liquid nitrogen to 95% of the liquid level, maintain the filling pressure within the range of 0.10 to 0.15 MPa, and monitor the temperature uniformity;

[0090] Step 14: Before the formal loading pressure test, conduct a 0.10 MPa pressurization pre-test to ensure system stability and evaluate the initial response;

[0091] Step 15: After completing the pre-test, fill the chamber with helium to 0.40 MPa, maintain the pressure for 10 minutes, and then drain the liquid nitrogen.

[0092] Step 16: Use 0.06-0.08 MPa nitrogen to purge the inside of the pressure vessel to ensure it is clean and completely dry, eliminating the effects of condensation and contaminants;

[0093] Step 17: Fill the pressure vessel under test with 0.40-0.42 MPa helium and perform a total leak rate test using a helium mass spectrometer using the bagging method. Record the leak rate data to evaluate the sealing and structural integrity.

[0094] 3.2 Monitoring process of composite pressure vessels

[0095] In order to monitor the strain changes and surface temperature changes of composite pressure vessels during the liquid nitrogen filling and pressurization process in real time, this embodiment adopts a combined monitoring solution of optical fiber sensors and strain gauges. According to the above experimental process, the monitoring process steps during the liquid nitrogen filling process and the container pressurization process are as follows:

[0096] Step 21: Baseline data acquisition: Before the test begins, collect initial baseline data of the fiber optic sensor and strain gauge to calibrate zero drift and environmental effects;

[0097] Step 22: Monitoring during liquid nitrogen filling: During the process of filling the composite pressure vessel to 95% liquid level with liquid nitrogen, the time control method is adopted for monitoring. The measurement data of the fiber optic sensor and the strain gauge are recorded at 5-minute intervals, and the evolution trend of the strain with temperature change is observed.

[0098] Step 23: Resetting the reference after liquid nitrogen filling is completed: After the liquid nitrogen filling is completed, the fiber optic sensor is zeroed again to eliminate the drift caused by temperature change and ensure the accuracy of subsequent pressure monitoring data.

[0099] Step 24: Preliminary data acquisition before the formal pressure test: Before the formal pressure loading, a preliminary pressure test of 0.10 MPa is carried out. The data of each fiber optic sensor are collected and the distribution curve is plotted, and the original data are saved for comparison.

[0100] Step 25: Resetting the reference after the preliminary data acquisition is completed: After the preliminary data acquisition is completed, the fiber optic sensor is zeroed again.

[0101] Step 26: Monitoring during the helium pressurization process: The pressure control method is adopted to monitor the composite pressure vessel during the pressurization process. During the process of filling helium and pressurizing to 0.40 MPa, every time the pressure increases by 0.10 MPa, the data changes of the fiber optic sensor and the strain gauge are synchronously recorded. After the pressure returns to zero, the final data acquisition is carried out to ensure the complete recording of the strain and temperature evolution during the entire pressurization - unloading cycle and complete the health monitoring process.

[0102] Similarly, when the same steps are used for the test with liquid oxygen as the liquid fuel, similar results can be obtained.

[0103] The signals from each distributed fiber optic sensor and the strain gauge in each monitoring area can be compensated by the compensator respectively and then collected by the wireless strain collector through its access interface, and received and processed by the data receiving terminal.

[0104] The ultra-low temperature characteristics of liquid fuels such as liquid nitrogen and liquid oxygen may cause damage to the structural integrity of the composite pressure vessel, and the complex stress evolution during the pressurization process further accelerates its fatigue aging and shortens its service life. Therefore, the real-time monitoring of the pressure vessel under low temperature and high pressure conditions is crucial.

[0105] In this application, through numerical simulation, the global and local stress responses of the composite pressure vessel during the liquid nitrogen filling and pressurization processes are systematically analyzed, and accordingly, the layout scheme of the distributed fiber optic sensor and the strain gauge is optimized. Subsequently, the time control method and the pressure control method are used to monitor the stress and temperature changes of the pressure vessel under different working conditions, and finally the following conclusions can be obtained:

[0106] The peaks of the maximum tensile principal stress and the maximum compressive principal stress on the outer cylinder wall are located on the right side of the upper end of the second bonding zone and the right side of the lower end of the third bonding zone respectively. The directions are both axial and appear in the area around the bonding ring. The maximum principal stress (absolute value) in most areas of the outer cylinder wall is lower than 260 MPa.

[0107] Finally, all three head covers are in a conventional stress state where tensile and compressive stresses coexist. The peaks of the maximum tensile principal stress and the maximum compressive principal stress are both located near the connection area between the upper head cover and the upper head. The peak of the maximum tensile principal stress of the head is located near the connection position between the lower head and the lower head cover.

[0108] The optimized layout paths of the fiber optic sensors and strain gauges can achieve full coverage of the key weak areas of the composite pressure vessel. Through the data cross-validation of the distributed fiber optic sensors and strain gauges, the accuracy of strain measurement can be effectively improved, ensuring the reliability of the monitoring results.

[0109] During the experimental monitoring process, a strategy combining the time control method and the pressure control method was adopted to gradually capture the dynamic changes in the service state of the composite pressure vessel. Before the test, pressure test extraction and multiple zero calibrations were carried out first, thus laying a solid foundation for subsequent data collection.

[0110] As described above, only the preferred specific embodiments of the present invention are provided. These specific embodiments are all different implementation manners based on the overall concept of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for liquid fuel pressure testing and damage monitoring of a composite pressure vessel, characterized in that: By using numerical simulation method, the stress distribution data obtained from systematic analysis of the pressure change of the composite pressure vessel during the liquid fuel filling and pressurization processes is used to determine the stress concentration area of the composite pressure vessel; Distributed optical fiber sensors and strain gauges are arranged in the stress concentration area of the composite pressure vessel; During the liquid fuel injection stage, the time control method is used for real-time monitoring to capture the strain and temperature distribution of the composite pressure vessel; During the pressurization stage, the pressure control method is used for real-time monitoring to capture the strain and temperature distribution of the composite pressure vessel; And The total leak rate of the composite pressure vessel is detected by the bag method combined with helium mass spectrometry technology to evaluate its sealing performance and structural integrity.

2. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 1, wherein: The numerical simulation method includes establishing a finite element model of the composite pressure vessel and analyzing the finite element model in a fully thermo-mechanical coupling manner to determine the stress concentration area.

3. The method for liquid fuel pressure testing and damage monitoring of a composite pressure vessel according to claim 1, characterized in that: The liquid fuel is liquid nitrogen, and the liquid fuel pressure testing of the composite pressure vessel includes: Filling liquid nitrogen into the composite pressure vessel to a preset liquid level, maintaining the filling pressure within a preset range, and monitoring the temperature uniformity; Before the formal pressure loading test, a pre-pressurization test is carried out to ensure system stability and evaluate the initial response; After the pre-test is completed, helium is gradually filled to a preset pressure, and the liquid nitrogen is discharged after stabilizing the pressure; The inside of the pressure vessel is purged with nitrogen to ensure cleanliness and complete dryness, and eliminate the influence of condensation and contaminants.

4. The method for liquid fuel pressure testing and damage monitoring of a composite pressure vessel according to claim 1, characterized in that: The liquid fuel is liquid nitrogen, and the damage monitoring of the composite pressure vessel includes: Collecting the initial reference data of the distributed optical fiber sensors and strain gauges for calibrating zero drift and environmental influence; During the process of filling the composite pressure vessel to the preset liquid level, the time control method is used for monitoring work, and the measurement data of the distributed optical fiber sensors and strain gauges are recorded at preset time intervals; After the liquid nitrogen filling is completed, the distributed optical fiber sensors are zeroed again to eliminate the drift caused by temperature change and ensure the accuracy of subsequent pressure monitoring data; Before the formal pressure loading, a pre-pressurization test is carried out, the data of each distributed optical fiber sensor are collected and a distribution curve is plotted, and the original data are saved for comparison; After the pre-pressurization test is completed, the distributed optical fiber sensors are zeroed again; The pressure control method is used to monitor the composite material pressure vessel during the pressurization process; during the helium filling and pressurization process, at preset pressure intervals, the data changes of the distributed optical fiber sensor and the strain gauge are synchronously recorded; after the pressure returns to zero, final data acquisition is carried out to ensure complete recording of the strain and temperature evolution during the entire pressurization - unloading cycle, and the health monitoring process is completed.

5. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 1, wherein: The finite element analysis adopts a fully thermo - mechanical coupling analysis method. The composite material pressure vessel composed of an outer cylinder wall formed by carbon fiber reinforced plastic, a head formed by carbon fiber reinforced plastic, and a head cover plate formed by structural steel is modeled using ABAQUS software. The applied loads include: a temperature load of - 196 °C applied to all the inner surfaces of the composite material pressure vessel; an internal pressure of 0.84 MPa applied inside the composite material pressure vessel; a gravitational load in the negative Z - axis direction; and a hydrostatic pressure based on the density of liquid fuel applied inside the composite material pressure vessel.

6. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 1, characterized in that The stress concentration regions include the junction of the outer cylinder wall and the bonding ring, the outer surface curve of the head, and the connection region between the head and the head cover plate.

7. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 1, characterized in that, The layout scheme of the distributed optical fiber sensor includes: arranging four distributed optical fiber sensors on the upper and lower boundaries of each bonding ring of the outer cylinder wall, in the middle of the outer cylinder wall, and in the axial direction; arranging one distributed optical fiber sensor on each outer surface curve of the head; dividing eight monitoring regions at 45° intervals along the circumferential direction, and arranging monitoring points in the directions of 0°, 45°, 135°, and 225°; additionally arranging two monitoring points in the corner regions of the distributed optical fiber sensor to reduce measurement errors.

8. The method for liquid fuel pressure testing and damage monitoring of a composite pressure vessel according to claim 7, wherein The layout scheme of the strain gauge includes: dividing the composite material pressure vessel into four monitoring regions, namely Region A, Region B, Region C, and Region D, and arranging eight strain gauges in each monitoring region; arranging eight strain gauges longitudinally, six on each head, and the rest on the 45° and 225° meridians; each monitoring region is equipped with an independent wireless strain collector, and a compensator made of the same material as the composite material pressure vessel is connected for strain compensation.

9. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 3, characterized in that The time control method includes recording the measurement data of the distributed optical fiber sensor and the strain gauge at 5 - minute intervals during the process of filling liquid nitrogen to 95% of the liquid level; maintaining a filling pressure of 0.10 - 0.15 MPa during the liquid nitrogen filling process, and after filling, increasing the pressure to 0.40 MPa by filling helium and stabilizing the pressure for 10 minutes; the pressure control method includes recording the data of the distributed optical fiber sensor and the strain gauge every time the pressure increases by 0.10 MPa during the process of helium pressurization to 0.40 MPa, and carrying out final data acquisition after the pressure returns to zero; the total leakage rate detection includes purging the inside of the pressure vessel with nitrogen at 0.06 - 0.08 MPa after the test, and then filling with helium at 0.40 - 0.42 MPa, and measuring the leakage rate using the bag method combined with helium mass spectrometry technology.

10. The method for liquid fuel pressure testing and damage monitoring of the composite material pressure vessel according to claim 9, characterized in that, Before the liquid nitrogen filling and pressurization test, perform the following preprocessing steps: collect the initial reference data of the distributed optical fiber sensor and the strain gauge to calibrate the zero drift; conduct a pre-pressurization test at 0.10 MPa, collect data and plot the distribution curve; after the liquid nitrogen filling is completed and the data trial collection is completed, respectively zero the data of the distributed optical fiber sensor.

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