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Abstract
During the whole life cycle of solid rocket motor (SRM), shell damage and propellant interface debonding will occur, which will destroy the structural integrity of SRM. Therefore, it is necessary to monitor the SRM health status, and the existing nondestructive testing technology and the designed optical fiber sensor cannot meet the monitoring requirements. In order to solve this problem, this paper uses femtosecond laser direct writing technology to write high contrast short femtosecond grating array. A new packaging method is proposed to enable the sensor array to measure 9000 με. It solves the grating chirp phenomenon caused by stress concentration in the SRM, and breaks through the key technology of fiber optic sensor implantation in the SRM. The shell pressure test and strain monitoring inside the SRM during long-term storage are realized. For the first time, the experiments of tearing and shearing specimens were simulated. Compared with the results of computed tomography, it proves the accuracy and progressiveness of implantable optical fiber sensing technology. Combined with theory and experiment, the problem of SRM life cycle health monitoring is solved.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Solid rocket motors (SRM) are widely used in various missile weapons and aerospace vehicles. It has the advantages of good maneuverability, easy realization of large thrust, high reliability, simple structure, simple launching operation, convenient use and maintenance, etc. As an important component of space launch vehicle power, SRM occupies an important position and plays an important role in the development of space launch vehicle technology in the world [1–3]. Therefore, it is very important to master the state parameters and healthy life of SRM at all time. After research, it is found that there are two aspects that will affect the structural integrity of SRM [4–6].
Composite materials are widely used in the manufacture of SRM cases because of their unique advantages such as strong designability, good fatigue fracture resistance, corrosion resistance and good structural and dimensional stability. But on the other hand, composite materials hold the characteristics of heterogeneity, anisotropy and large process discreteness. Due to the factors of process and operation, the defects such as porosity, inclusion and delamination are not difficult to form in the shell of composite SRM during the production process [7]. Damage such as fracture, delamination, crack and debonding may occur in the composite shell during transportation and storage due to collision, overload, corrosion, environmental aging and other reasons [8,9]. Interfacial debonding usually occurs between the case and the insulation and between the liner and the propellant. The interface debonding between the shell and the insulation layer will be caused by improper curing heating and pressing, poor quality of the binder, storage aging and transportation. The interfacial debonding between the liner and the propellant is mainly caused by aging or excessive stress during storage and transportation [10].
The occurrence of the situations will destroy the integrity of SRM. If the defects and damages can not be found and repaired properly, they will have serious implications on the performance of the missile weapon system [11]. SRM are subjected to temperature changes throughout life cycle. In the process of development, test and use of SRM, there are temperature field changes and heat transfer of propellant in the motor combustion chamber. It is necessary to monitor the combustion of the propellant in the process of SRM test [12]. Therefore, it is necessary to establish a full life cycle state parameter monitoring system for SRM to monitor the thermal state parameters of the motor in real time.
The traditional damage detection methods include industrial computed tomography (CT) technology, ultrasonic detection method, laser holographic detection method, infrared method and so on [13]. COMS sensor array can also be used as a nondestructive testing method for imaging analysis of the monitoring system [14]. However, the SRM CT scanning device is generally relatively large, expensive equipment and high maintenance costs, generally only used in the development and production process. Ultrasonic testing is effective to detect the debonding between solid motor case and insulation layer, but it is not suitable to detect the debonding between the front and rear end of the motor and the cracks inside the propellant. The basic principle of laser holographic testing method is that the displacement change of the surface of the component to be tested is related to whether there are defects in its interior after a certain load is applied to the component, but the ability of this method to detect the defects in the composite shell is low. The remaining several detection methods have some defects and shortcomings in the field use of SRM, such as not being able to carry out real-time online monitoring for a long time, and not being convenient for on-site construction and detection.
In order to solve this problem, it is found that the implantable measurement of optical fiber sensing network can form contact measurement and improve the detection coverage, which has strong engineering application value [15–18]. The application of optical fiber sensors in aerospace and aviation fields such as aircraft has been analyzed and summarized in detail by C. Marques et al. [19]. FBGs and SRM have excellent matrix compatibility. The FBG is small in size and light in weight, and the diameter of the FBG can reach the order of hundreds of microns. Multiple sensing points can be inscribed on the single optical fiber. One sensing point can realize the measurement of multiple physical quantities at the same time, which makes the optical fiber sensor can be used in SRM on a large scale without occupying too much payload. The SRM test phase requires the installation of a large number of sensors, and the removal after the test is not only a large amount of work, but also requires great care to avoid damaging the spacecraft structure. The fiber sensor is light in weight, can be embedded in a structure, and does not need to be removed, so that the workload is greatly reduced.
Fiber sensors are also used in SRM at home and abroad, but they only stay in simulation and laboratory research, and do not solve practical problems [20–24]. For this reason, we have developed an implantable femtosecond FBG sensor array to measure the stress and temperature parameters of the SRM in real time, and to reflect the health status by obtaining a variety of mechanical and thermal physical parameters. The biggest difference from the traditional nondestructive testing method is that it can realize the online real-time monitoring. It can find and determine the location and extent of the damage to the structure in time, and monitor the expansion of the damage area, which is conducive to the discovery of early problems in SRM structure. Thus, remedial measures can be taken in time to reduce the rate of defective products and improve product quality. It provides an effective guarantee for the safe use and maintenance of the SRM, and has broad prospects for development.
2. Model establishment and theoretical analysis
The model is built according to the actual structure and experimental scheme of the developed SRM. The combustion chamber structure of the SRM is composed of shell, thermal insulation layer, liner and propellant, among which the shell and propellant are called adhesive layer. Optical fiber sensors are mainly embedded in the adhesive layer to monitor the mechanical properties of interfacial debonding. The sensors are laid in the adhesive layer along the axial direction of the SRM in the actual experiment. The debonding model of the SRM and the fiber sensor are analyzed theoretically. The finite element simulation of the lateral compression process is performed through the software Ansys to explore the stress distribution on the FBG. A schematic representation of the model and coordinate system used is illustrated in Fig. 1. The pressurized medium consists of the SRM case and the propellant. The thickness t of the contact surface and the material properties are modified from simulation to simulation, and the properties and structure of the SRM case remain constant throughout. Lateral loads are applied vertically to the propellant. Its main principle is that the change of strain and temperature in the SRM will act on the embedded optical fiber sensor, causing the optical characteristic parameters of the transmitted light in the optical fiber sensor to change. The functional relationship between the optical characteristic parameters and the measured physical parameters is established, so that the measurement of the mechanical and thermal parameters in the SRM can be indirectly realized.
The theoretical analysis is based on the mechanical characteristics and the SRM bonding surface. The fiber is considered to be an isotropic medium, and its intrinsic birefringence is not obvious. The core and cladding have the same photoelastic and mechanical property but different refractive indices. The relationship between the refractive index gradient at any point $P(x,y,z)$ in the grating and the strain distribution can be expressed as
The length and width of the propellant are fixed in the simulation process because the research problem is the influence of the propellant to the lateral pressure adjustment. After the test load was applied, the contact area was found to increase at each loading step with increasing deformation. The fiber is compressed in the radial direction, producing a positive longitudinal strain in the axial direction, the magnitude of which is highly dependent on the stiffness, thickness, and contact friction of the contact medium. The simulation shows that the low stiffness propellant can distribute the lateral load with a large contact angle, and make the force on the fiber more uniform. Since the stress is almost uniformly distributed in the radial direction, the birefringence produced by the fiber is not significant. In addition, the uniform distribution of the applied load enhances the axial strain, thereby increasing the sensitivity of the wavelength shift to the applied transverse load. Higher contact friction helps to enhance the wavelength shift during lateral pressure tuning. Under higher contact friction, the applied load can be effectively transferred to the fiber, and the fiber is difficult to slip.
Build on the above analysis, the three-dimensional model is established. The upper part and the lower part are three-dimensional solid units of the SRM shell and the propellant. The material properties are isotropic. Young's modulus 210 GPa. The Poisson's ratio is 0.3. The middle thin layer is a three-dimensional viscoelastic element. The debonded layers are three-dimensional viscoelastic elements, which interact with each other by surface forces. In the simulation parameter setting, its Young modulus is 150 Gpa. The mesh controls how the attribute selects the sweep. The simulation was performed in Abaqus software with a shell elastic modulus of 53304 MPa and a Poisson's ratio of 0.3. The elastic modulus of the propellant is 200 MPa, and the Poisson's ratio is 0.485. The boundary condition is to fix the two sides of the right side and apply 5 mm displacement to the two sides of the left end respectively. Fig. 2 shows the stress distribution of the model of the debonding interface. The model of debonding can clearly show the situation of strain generation at different positions [25]. Because the elastic modulus of shell and propellant is 266 times different, the deformation of propellant is more obvious when the same displacement boundary condition is applied. The bending moment and strain of the fixed face generated by tension are the largest.
Let there be n + 1 intermediate layers. ${r_i}$ is the outer diameter of the i-th intermediate layer. ${r_g}$ and ${r_m}$ are the outer diameter of the fiber and the inner diameter of the matrix, respectively. G is the shear modulus of elasticity of each intermediate layer. The following expression can be obtained
By analogy, for each intermediate layer:
The average strain transfer efficiency is expressed as
Because the material of the adhesive layer is different from the material of the optical fiber sensor, the stress and strain transfer coupling characteristics of the adhesive layer and the optical fiber sensor are further analyzed from the perspective of actual experiments. The accuracy of the measured strain is ensured. The length of the sensor and the thickness of the encapsulation layer and the adhesive layer will affect the strain transfer efficiency. The effect of fiber strain transfer mode is shown in Fig. 3. The longer the length of the sensor and the smaller the thickness of the intermediate layer, the greater the average strain transfer rate. When the length of the strain sensor is short, the thickness of the middle layer of the sensor has a significant effect on the average strain transfer rate. When the thickness of the interlayer is constant, the longer the length of the strain sensor is, the greater the average strain transfer rate is. The strain coupling characteristic at the center of the sensor is the largest, and gradually decreases towards both ends of the sensor. This decreasing relationship can play a guiding role in the measurement of strain at different points. It provides theoretical guidance for developing the measuring range of the sensor. Since the strain of the middle section of the fiber is uniform, the FBG with the middle length of 5 mm is selected as the average strain value. According to the formula, the average strain transfer efficiency is 0.781. When there is deformation or damage in the composite shell, the sensor strain will change. When the adhesive surface of propellant and shell is debonded, the sensor on the adhesive surface will feel the strain. The change of the sensor strain will cause the change of the central wavelength of the grating, thus realizing the monitoring of the change of the two states.
3. Experimental test and result analysis
Figure 4 shows the components of the experimental system. Grating sensor arrays are implanted in both parts of the SRM. Part of the interface between the propellant and the case and part in the composite case of the SRM. And that grate sensor array is connected to the terminal demodulation display part through a signal transmission optical cable. Through the data processing and analysis of the upper computer software, the online monitoring of the SRM health status is achieved.
3.1 .Design of Optical Fiber Sensor
Femtosecond lasers have ultrashort pulse width, ultra-high peak power and unique physical properties. When femtosecond laser is used to act on the fiber, the pulse laser can penetrate the surface area of the fiber to process at a specific exposure position. The femtosecond laser point by point direct writing technology can efficiently and flexibly manufacture high- performance FBG. Write directly through the coating layer without damaging the strength of the optical fiber itself. The nonlinear optical effects of femtosecond lasers can produce extremely stable and non-erasable gratings as sensing devices. Firstly, keeping the femtosecond laser still, placing the cleaned optical fiber on a precise electric displacement platform, and controlling the electric platform to move along the axial direction of the optical fiber through a program; Finally, the fiber is moved at a constant speed, so that the femtosecond laser pulse induces a series of refractive index modulation regions in the center of the fiber core through the multi-photon absorption effect, thereby forming the FBG. The spacing of the refractive index modulation region is the period of the grating, which is mainly determined by the pulse frequency and the moving speed of the fiber. A single femtosecond grating with different reflectivity and period can be obtained by controlling the energy of the femtosecond laser and the spacing of the modulation domain. After the writing of one grating is completed, the laser baffle is closed, the optical fiber is moved to the starting position of the next grating to be written by driving the precise electric displacement platform, and the laser baffle is opened to continue writing the next grating. These steps are repeated to inscribe the femtosecond grating array. The length of each FBG is 3 mm. Stability of FBG itself reaches 1000 ℃. The mechanical strength of optical fiber is greater than 200 kpsi. The fiber with multipoint grating is structurally packaged, as shown in Figure 5. The single FBG sensor channel comprises of a high-temperature resistant joint, a tensile and bending-resistant polyimide-coated optical fiber, a double-sleeve packaged grating sensing unit and a pigtail fiber protective sleeve. Among them, the high temperature resistant joint can withstand the high temperature of 230 ℃, and it can withstand the curing temperature of the SRM shell. The maximum curing temperature of composite shell is 180 ℃. The material of the tensile and bend-resistant optical fiber is G657A. The coating layer is a polyimide coating and can withstand a temperature of 350 ℃. The grating sensing unit is packaged by a silicon rubber sleeve and a polyurethane sleeve. The gap between the sleeve and the optical fiber is sealed by high temperature sealant to prevent resin from entering the sleeve and causing the optical fiber to become brittle and easy to be broken. Packaging and coating can solve the chirp phenomenon of the FBG sensor.
The pigtail protective sleeve is composed of high-temperature heat shrinkable tube, Teflon sleeve and metal woven mesh. The direct length of Teflon sleeve is 0.8 mm. The direct value of metal braided pipe is 0.9 mm, as shown in Fig. 6.
The strain of the designed sensor is tested, and the results are shown in Fig. 7(a). The results show that the sensor can measure the strain range to at least 9000 με. The sensitivity of the sensor has a good linear response, and the linearity is up to 0.999, which shows that the measurement error caused by the linear demodulation of the sensor is very small. It can be optimized by improving the femtosecond grating technology and collecting more experimental data. The substrate design, sensor design, and fabrication are the same for all implanted sensors, so the sensing performance of the sensors is the same. The sensor designed in this manuscript has the following characteristics compared with the same type.The strain measurement range of femtosecond grating is large, the contrast is high and the length of grating area is short. The repeatability of the sensor is tested, as shown in Fig. 7(b). It can be seen from the figure that in the four repeatability experiments, the sensor shows a high degree of repeatability. The measurement error caused by the repetition process is very small. A new packaging method is adopted for the grating to prevent the spectral chirp caused by the stress concentration of the SRM structure. The designed sensor has higher mechanical strength. The problem of implanting sensor in SRM structure is solved. According to the reported types of optical fiber sensors, there are interferometric sensors, distributed optical fiber sensors and FBG sensors [26,27]. These sensors have high sensitivity, but they are only used in the laboratory, not combined with practical applications.
Fig. 7. Measurement of sensor strain test range, (a) Grating strain response test, (b) Sensor repeatability test.
3.2 SRM structural integrity simulation experiment
The stress on the bonding surface of the SRM will be more complex in reality, and the propellant will creep in the long-term process. The force on the optical fiber sensor is also a complex force coupling. In order to verify whether the designed FBG sensor can correctly reflect the force of the measured object under the axial force, radial force and bending. The experiments of shearing, tearing and side-wall mounting of FBG were carried out. In the shear test, the SRM shell and grain are simulated as three-dimensional solid elements. The material property is isotropic, with Young's modulus of 210 GPa and Poisson's ratio of 0.3. The boundary condition is that the left end is fully constrained, and the right end gives a displacement of 5 mm in the negative direction of the z-axis. Mises stress is shown in Fig. 8(a), and the maximum stress is 3.14 MPa. Shear test verifies whether the FBG can accurately monitor the whole process of debonding of the bonding interface during the shear process of the specimen. Figure 8(b) shows the layout of optical fiber for shear test.
FBG sensors with different lengths are pasted on different test pieces. The adhesive uses specimen curing adhesive, which can ensure better compatibility between the sensor and the shear specimen. After the sensor is packaged in the shear test piece, it needs to be put into the incubator for curing, and the test piece needs to be fixed with a clamp before curing. During the test, the tensile machine pulls the test piece at the speed of 25 mm/min, as shown in Fig. 9(a). In the meanwhile, the FBG demodulator collects the FBG data at all times. The acquisition frequency is 1KHz. The results of the pull-off test show that the stress change monitored by the FBG sensor is obvious during the pull-off process, which can reflect the force change trend of the specimen. And that chirp phenomenon of the grate does not appear in the shear process. Fig. 9(b) shows that the strain change trend monitored by the sensor is the same as that of the tensile machine. It can monitor the shear force. The action time of the applied force can be optimized and adjusted by the speed and size of the tension machine. The sensor monitoring result curve and the tension machine application curve show a high degree of consistency. It shows that the sensor has high accuracy in monitoring the SRM.
Fig. 9. Comparison between the actual applied force and the strain monitored by the sensor. (a) Relationship between monitoring force and time of tension machine, (b)Relationship between strain and time of optical fiber sensor.
In the pull off test, the FBG is arranged radially along the bonding surface of the test piece in the middle of the two test pieces, as shown in Fig. 10(a). The 3 mm FBGs capable of measuring large strain is used. The assembly is shown in Fig. 10(b). The propellant and the adhesive layer are constrained by binding. It is constrained by the facet-to-face interaction to reach the same constraint state as the experiment. Heating and solidify the structural members pasted with gratings. After complete curing, longitudinal tensile and shear tests are performed on the structural members with optical fiber sensors. During the experiment, the reflected signal of the sensor is monitored at all times. The pulling machine operates at a speed of 50 mm/s during the pulling process. The experimental results demonstrated that the magnitude of the stress and strain detected by the optical fiber sensor is in good agreement with the magnitude of the force loaded in the tensile process. It shows that the optical fiber sensor can effectively reflect the force on the bonding interface. The results are shown in Fig. 10(c) and Fig. 10(d). When the tension reaches the tenth second, the force felt by the test piece is the largest, and then the force felt by the test piece will gradually decrease. The curve is not completely symmetrical, and the duration of the reduction process is shorter than the duration of the continuous increase of the force. The tenth second is the result of the experimental test, which is related to the speed of the tensile machine.
Fig. 10. (a) FBG pulls off the test piece. (b) Mechanical distribution of detached specimen. (c) Relationship between force and time measured by tension machine, (d) FBG measures the relationship between strain and time.
FBGs embedded in the composite case of SRM is also tested under hydrostatic pressure. Fused FC fiber connectors for all fiber paths. Use the optical fiber test pen to detect the channel condition. The lead-out joint of the optical fiber is wound and adhered on the surface shell of the SRM, so that the optical fiber probe is directly connected with the demodulator when acquiring signals [28]. During the hydrostatic test, the data of sensors in different channels were recorded. During the measurement process, the water pressure changes from small to large, and the time stability test is carried out at different water pressures. The spectral energy has a certain attenuation when the pressure increases, but the spectral shape has almost no change, indicating that the spectrum has no distortion.
Observe the spectral on the demodulator and start the hydrostatic pressure test. Fig. 11 (a) shows the process of applying low pressure, and the FBG sensor can accurately reflect the change of the internal pressure of the composite shell. When the maximum pressure of the hydrostatic test is 9.7 Mpa, the grating signal is not lost, and the signal can still be responded in real time. Fig. 11 (b) shows the signal of the SRM during the pressure maintaining process. When the pressure does not vary, the sensing signal is relatively stable. Figure 11 (c) shows the depressurization process. As the pressure drops, the stress and strain sensed by the sensor gradually decreases, which can well reflect the trend of pressure drop and rise. The change of the composite shell caused by internal water pressure can be truly reflected by the limited number of FBG sensor points.
Fig. 11. (a)Wavelength response of the sensor during water pressure increase, (b) Sensor signal in stable pressure state,(c) Composite case pressure reduction process sensor signal.
3.3 FBG sensor deployment in the SRM
The optical fiber sensor is mainly embedded in the composite shell in the above water pressure test. After the hydrostatic test, the optical fiber sensor laid on the bonding surface of the propellant and the shell was bonded. In order to further monitor the debonding of the propellant interface, the sensor is pasted and deployed inside the shell. The sensor is arranged along the axial direction of the SRM shell, and a certain prestress is applied to the sensor in the pasting process, so that the sensor can more accurately measure strain change caused by debonding. In this monitoring test, there are 12 optical fibers in total, of which 4 are used in the composite shell for hydrostatic test and shell damage judgment. Six fibers are located at the thermal insulation/propellant interface of the SRM shell, and the distribution position of the optical fibers is shown in Fig. 12 (a) and (b).
Fig. 12. (a) Arrangement of front leading-out end of FBG, (b) Arrangement of rear leading-out end of FBG.
Two fibers are located at the shell/insulation interface. The take-off points are at the front and rear covers of the SRM. The number of effective fibers leading out of each lead-out point is different. Each fiber is positioned at a different angle to the SRM. Taking 0 degree as the reference point, they are 45°, 90°, 135°, 180°, 225°, 270°, 315° and 360° respectively. From the lead-out end to the tail end of the fiber, the wavelengths of the corresponding sensors are sequentially from short wavelength to long wavelength. The location of each sensor in the figure can be clearly found. Figure 13 shows the layout and lead-out of optical fiber sensor.
3.4 Long period health monitoring
The propellant-loaded SRM was monitored for long-term health for 135 days. Figure 14 show the sensor data of the sensor within 135 days of monitoring. The stress and strain sensed by the sensor continues to change with time. From the monitoring results of different channel sensors, it can be seen that the strain at the position of the sensor inside the SRM shell increases with time, and the strain monitored at different stages is different. The strain sensed by the sensor implanted in the shell is not the same as the strain sensed by the sensor at the propellant interface. The strain change detected by the sensor is different in different quadrants of the SRM. The maximum strain change in propellant is from 0 to 275 με. Compared with the previous debonding tool simulation test, this is mainly caused by the internal gravity of the propellant or the change of local load, which does not reach the debonding threshold. Therefore, according to the monitoring data of the sensor, the SRM of this model did not debonding.
In order to further verify the correctness of the monitoring results of the sensor and analyze the monitoring accuracy, CT nondestructive testing is carried out on the SRM implanted with the sensor. CT slices were taken along the radial direction of the SRM case. The slice position is within the range of 168mm∼ 668 mm from the end face of the front skirt. Sections were taken every 100 mm, and partial sections are shown in Fig. 15 (a), 15 (b), and 15 (c). The results show that there are no abnormal phenomena such as interface defects or debonding.
According to the specific laying position of the sensors, all the implanted sensors positions were radiographed. Radiography shall be carried out at the front and rear head sections and the transition section. Wherein, in the column section, the laying position of the 12 fiber strings. Focus on taking pictures at the position of the point of the FBG sensor, taking pictures by rotating left and right3°, 6°and 9° respectively. Fig. 16 shows a specific case of a radiographic image of a part of the fiber laying position, and the rest of the sensors are photographed similarly. The results of ray analysis show that there is no interface debonding and obvious defects in the position where the optical fiber sensor array is implanted. The radiographic condition corresponds exactly to the condition monitored by the 12 fiber sensors, indicating that the detection accuracy of the 12 implanted optical fiber sensors is 100%.
3.5 SRM temperature cycle monitoring experiment
The temperature cycling test was carried out, and the strain of the SRM was measured at different temperatures. The cycle range of temperature is 0℃ to 30℃. The process of heating and cooling is linear. The whole temperature cycle monitoring time is 122 hours. A constant temperature process was carried out for 10 hours in the high and low temperature stages. When the temperature rises, the shell temperature of the SRM first increases, and then the propellant temperature gradually increases. When cooling, the shell temperature of the SRM decreases, and then the propellant temperature gradually decreases. Figure 17 (a) shows the curve of ambient temperature control time.
During the long cycle of SRM temperature, the strain change inside the SRM is monitored. Because the sensor will be affected by both external temperature and strain in this process, it is necessary to eliminate the changes caused by temperature when processing experimental data. Figure 17(b) shows the strain change monitored by a typical sensor in one of the channels during the temperature cycle. The ordinate represents the magnitude of the strain, and the abscissa represents the time period, which is divided into two heating stages, two cooling stages, and four constant temperature holding stages.
From the experimental data monitored by the sensor, it can be seen that the stress and strain monitored by the sensor increase gradually with the increase of the ambient temperature of the SRM, and the strain change rate has a certain linear relationship. When the SRM is in the heat preservation stage, the strain value monitored by the sensor is also in the stable stage. Although there is fluctuation, the fluctuation is small, and the fluctuation is within the range of several microstrains. In the cooling stage of the SRM, the strain monitored by the sensor is also decreasing, and the rate of decrease is also linear. Sensors in different channels of the optical fiber sensor have the same trend of strain change and temperature cycle. It shows that the sensor can accurately measure the stress and strain changes of the composite case and propellant interface during the temperature cycle test. Because different sensors are implanted in different positions in the SRM. The sensor with large strain change is located at the bonding interface between the propellant and the shell, and the sensor with a small strain change is located in the sensor in the composite shell of the SRM.
3.6 Strain monitoring during SRM test run
The ignition test of the SRM embedded with optical fiber sensor was carried out. The test mainly monitors the internal and interface strain of the SRM case during ignition, as shown in Fig. 18(a). According to the obtained spectral change condition of the sensor, the data is further processed and analyzed to obtain the change condition of the strain of the sensor. By monitoring the signals of different channel sensors in the process of vehicle setting, it can be found that the signals of sensors are relatively stable before vehicle setting. At the beginning of the ignition, the strain detected by the sensor increases sharply. The strain monitored by different channel sensors varies from 0 microstrain to thousands of microstrains.
The minimum variation is the variation of 2500 με, and the maximum variation can reach the variation of 3500 με, as shown in Fig. 18(b). This shows that during the ignition test, the pressure generated inside the SRM shell increases sharply, resulting in a sharp change in the strain monitored by the sensor inside the SRM. Before the test run, the sensor signals of 12 channels can work normally, and after the test run, the signals monitored by the sensors of all channels are measured. The results show that 8 channels of the 12 grating strings can work normally, and the remaining 4 channels are burned out by high temperature in the process of ignition and combustion due to the uneven coating of the grain interface liner.
The SRM has passed the test phase, and the sensor array embedded in the composite shell can still work. The load monitoring of the SRM shell is studied experimentally. Apply different loads on the surface of the composite shell, and monitor the load through the sensor. The experimental results are shown in the Fig. 19. The monitoring results show that the sensor can accurately measure the external load in the composite shell, and shows good stability.
4. Conclusion
In order to detect deformation damage and interface debonding of composite shell in the process of storage, transportation and test run of SRM, the implantable distributed fiber sensing technology was studied. It breaks through the design, coating and packaging of high strength and large strain sensors. We have overcome the technical difficulties of the SRM shell and interface optical fiber implantation. The problems of compatibility between the sensor and the implanted body, decoupling of multiple physical parameters, highly reliable extraction and long-term survival are mainly solved. Combining theory with experiment, the problem of health monitoring of SRM is solved. It provides reliable data for the study of structural integrity and health management of SRM. The sensing technology can also be applied to the condition monitoring of aircraft, hypersonic vehicles, satellite shells and so on.
Funding
Equipment Development Department support project (30409); Aerospace equipment pre-research project (20204511032); National Natural Science Foundation of China (62275008).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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