This article is part of the research topic “Mechanical properties of energy absorption in modern materials and structures”, view all 7 articles
In order to ensure the reliable performance of the hard penetration fuze on the battlefield, this paper focuses on the study of the protective lining of the fuze. The main factors leading to the failure of the fuze are summarized, and a simplified model of the projectile penetration into the target is established. Based on the stress wave propagation theory, the calculated conditions of the yield strength parameters of the fuze body material are derived. The ANSYS dynamic simulation software is used to conduct the modal analysis of the projectile and determine the vibration shape and low-pass filter frequency of the projectile. Static compression tests on various rubber materials (nitrile rubber, fluororubber, silicone rubber and natural rubber) are carried out, resulting in the stress-strain curves and constitutive model parameters. Marshall hammer tests are conducted on rubber pads of different materials and thicknesses to check the reliability of the simulation results and the filtering capability of rubber. The study shows that natural rubber has the best protective effect when used with a 2 mm thick rubber pad for protection; Nitrile rubber provides the best protective effect when using a 6 mm thick rubber pad as protection. Under an impact load of 13 teeth, the best protective effect is provided by a 2 mm thick natural rubber pad, and the best protective effect is provided by a 6 mm thick silicone rubber pad; under an impact load of 15 teeth, the best protective effect is provided by 2 mm thick fluororubber, and the best protective effect is provided by 6 mm thick silicone rubber; Under an impact load of 17 teeth, the best protective effect is provided by 2 mm thick natural rubber, and the best protective effect is provided by 6 mm thick fluororubber. The research results provide a basis for developing a method of protection against a penetrating fuse of solid targets.
With the development of modern warfare, the survivability of military bases in various countries has been continuously improved, mainly due to the strengthening of buildings or the installation of shelters on military facilities. Conventional ammunition is ineffective against strong and complex defensive structures. In order to achieve better lethality, ammunition with programmable layers, cavity recognition, environment recognition and other functions has become the subject of research in various countries (Dan Jian and Li Jianjun 2014; Zhang Jianjun 2018; Liu Jianjun et al. 2023). The penetration process of a hard target has sharply nonlinear characteristics under strong impact loads. Under high-speed penetration, the overload signal experienced by the fuse is more complex than the overload signal experienced by the projectile. The overload peak of the fuse is larger than the overload peak of the projectile, and the overload pulse width of the fuse is shorter. Under the influence of short-term high-intensity pulses, the fuse body is subject to deformation, which leads to problems such as the breakdown of the printed circuit board and the loss of the microcircuit pin, making it impossible to accurately detect and detonate the fuse. It is necessary to take protective measures to prevent the fuse from being triggered.
In response to the problem of failure of electronic components inside the fuse under high impact G-forces, scholars at home and abroad have conducted a series of theoretical and experimental studies on protection technology for hard penetration fuses (Manjesh and Manas, 2022). A magnetorheological impact vibration isolation system was used to replace the traditional vibration isolation system, and it was found that this method could effectively reduce the G-force (Taherishargh et al., 2014). A new type of perlite-aluminum foam composite foam was obtained, which was found to have high energy absorption efficiency (Xu and Huang, 2012). Considering the shortcomings of the existing buffering mechanism of elastomeric plastic materials, a stress wave attenuation mechanism was proposed, and theoretical calculations and simulations confirmed that the filtering pad mechanism is more reasonable than the buffering mechanism. The simulation results show that materials with lower impedance have better filtering properties (Chen Jianjun and Niu Jianjun, 2021). The problems existing in the theory of multi-layer lining protection in fuse protection design are pointed out. A model of projectile penetration into the multi-layer cushion structure is established. Simulation tests show that the currently widely used multi-layer cushion protection structure has good protection effect against concrete penetration, but is prone to functional failure when penetrating metal (Li et al., 2016). Three different buffer structures were designed, and experiments showed that the buffer performance of the two-layer structure with an ordered impedance combination is better than that of the inverse impedance combination. For the three-layer buffer structure, the buffer performance of the impedance fluctuation combination is better than that of the ordered combination. It has been reported in the literature that foam metal materials are suitable as energy absorption cushions due to their large deformation (Stamenkovic and Krstic 2017; Ashu et al. 2020; Su et al. 2020; Fan et al. 2022; Wang et al. 2023). However, metal materials are only suitable for single impact. In the case of multiple impacts, their buffer effect may not meet the protection requirements and cause the fuse to operate (Ashu et al. 2020; Luo et al. 2021; Adam et al. 2022; Jiang et al. 2022; Wei et al. 2022). Rubber is the most common protection method, and its function is to attenuate the transmitted stress wave reaching the fuse (Rose et al. 2023). Currently, the choice of the type and thickness of rubber in engineering protection has no theoretical or informational basis and is based solely on engineering experience.
This paper fills the research gap of rubber protection and analyzes the theory of elastic wave reflection and transmission at the interfaces of different media. In combination with the elastic wave analysis of the rubber-fuze projectile structures with different cross-sectional areas, different rubber layers are added on both sides of the fuze shell for protection. Based on the dynamic simulation of the projectile penetration of a reinforced concrete target, the effects of different rubber pads, thickness and target condition on the fuze overload were studied. The modal analysis of the projectile body is carried out to determine the first-order axial vibration frequency of the projectile body, and the fuze overload curve is filtered accordingly. Combining the numerical simulation results with the experimental design, a Marshall hammer is used to apply the impact load to the accelerometer. The protective properties of rubber of different thicknesses and impact loads were verified through the tests.
When a high-velocity armor-piercing projectile hits the target, the fuse is subjected to the voltage transmitted by the projectile and propagates through the fuse as a voltage wave. When these voltage waves reach the fuse board, the fluctuations can cause solder joints to come off, connecting wires to break, and batteries to discharge, ultimately causing the fuse to fail, as shown in Figure 1.
Figure 1. Analysis of fuse failure during penetration. (A) Deformation of the recovery system housing. (B) Chip separation.
When the high-speed impact occurs, the projectile and fuse will be subjected to strong dynamic loads, and the resulting stresses will be transmitted to different parts in the form of different waves. When the stress does not exceed the yield strength of the material, the generated stress wave is mainly an elastic wave; when the stress does not exceed the yield strength of the material, the generated stress wave is mainly a plastic wave. This paper mainly discusses the situation where the applied overload stress does not exceed the yield strength of the material, that is, the propagation of elastic waves at the interface of different media. The propagation of elastic stress waves at the interfaces of variable cross-section obeys Newton’s third law of motion, that is, the stresses on both sides of the interface are equal. Thus, it can be determined that:
In the formula, ΔvI is the velocity of the incident wave of the particle, ΔvR is the velocity of the reflected wave of the particle, ΔvT is the velocity of the transmitted wave of the particle, ΔσI is the incident wave, ΔσR is the reflected wave, ΔσT is the transmitted wave, A1 is the first cross-sectional area, A2 is the second cross-sectional area, and all parameters in the formula are vectors. Using the principle of conservation of wave front momentum, we can obtain:
In the formula, ρ0C0A is the generalized wave impedance, n is the ratio of the wave impedances of two different media, R is the reflection coefficient, and T is the transmission coefficient. Both coefficients are completely determined by the ratio of the wave impedance n.
For simplicity, consider the case where the characteristic impedances ρ0C0 on both sides of the interface are equal, so that the ratio of characteristic impedances is n = A1/A2. From Equation 5, it is clear that the sign of the transmitted wave is always the same as that of the incident wave. The sign of the reflected wave depends on the positive or negative value of F, and the value of F is related to the relative sizes of A1 and A2. When the stress wave propagates from the smaller contact area to the larger contact area (A1 < A2, i.e. n < 1), T A1/A2 = 2n/1 + n < 1, resulting in the reflected wave and the incident wave having the same sign, and the transmitted wave being smaller than the incident wave. On the contrary, when the stress wave propagates from a larger contact area to a smaller contact area (A1 > A2, i.e. n > 1), T A1/A2 = 2n/1 + n > 1, as a result of which the reflected wave and the incident wave have opposite signs, and the transmitted wave is larger than the incident wave. Therefore, with the same wave resistance, the transmission of the shock wave from the end with a smaller cross-section to the end with a larger cross-section can still play a damping and buffering role.
The fuse system consists of structures such as the projectile, the main charge, the fuse and the bottom plug. The fuse structure mainly includes filling materials, printed circuit boards and mechanical safety components. The signal measured by the fuse board reflects the overload that the fuse system is subjected to during the penetration process. The strong dynamic load that the projectile experiences when hitting the target is transmitted through the projectile-main charge-filler-fuze-filler-bottom plug structure. The signal transmission process is very complex, and the signal overload during the transmission process will be affected by various factors. The analysis of the impact of the projectile on the target is similar to the problem of a finite length rod hitting a rigid wall (Zhu Zhijun et al., 2022). The moment of impact of the projectile on the target can be simplified by assimilating it to the process shown in Figure 2. This study does not consider the plastic compression waves caused by the deformation of the target and the warhead, and focuses only on the transmission of elastic waves to the fuse after the projectile hits the target. In the elastic region, the stress experienced by the object is the yield strength, which is shown below:
Where VY is the particle flow velocity, ρ0 is the particle density, and C0 is the velocity of propagation of the stress wave in the object.
According to the theory of reflection and transmission of elastic waves on interfaces of variable cross-section, during reflection and transmission of an elastic stress wave at the interface between the front buffer cushion and the striker, interface 1 can be defined as:
Where T1 and R1 are the transmittance and reflectance coefficients at a given interface, respectively, and so on for subsequent interfaces.
When the stress wave penetrates through interface 1, the incident wave at interface 2 is the transmitted wave at interface 1, that is, (σI)2=(σT)1. At this time, the magnitude of the reflected wave generated at interface 2 relative to the incident wave is calculated as follows:
Similarly, the magnitude of the reflected wave generated at interface 3 relative to the incident wave is expressed as follows:
The magnitude of the reflected wave generated at interface 4 relative to the incident wave is calculated as follows:
The magnitude of the transmitted wave generated by the stress wave reflected at interface 4 on interface 3 can be obtained using the following formula:
To avoid plastic failure of the fuse body, the stress on the fuse body must be less than the yield strength σY, which leads to the following condition:
By combining conditions ①, ② and ③, the setting parameters of the fuse shell protective material can be determined.
To simulate the actual overload condition of the projectile penetration, a solid model of each projectile component was created using the 3D modeling software Solidworks, as shown in Figure 4. The projectile length is 720 mm, the diameter is 110 mm, and the aspect ratio is 6.5. The simulation model includes the imitation propellant charge, the entire fuse body, and the front and rear buffer pads inside the projectile body. The lower plug at the tail end of the projectile body is rigidly connected. ANSYS/Workbench software is used to conduct the modal analysis of the projectile. Since the modal analysis only focuses on the natural frequency and vibration mode of the entire projectile and does not consider the failure between the contacting materials, the contact between the materials inside the projectile is set as a bonded contact. Silicone rubber is taken as an example of the elastic model of the rubber material. The material parameters used in the simulation model of each projectile component are shown in Table 1. Both the projectile body and the reinforced concrete target are axisymmetric models, and the missile guidance system penetrates the reinforced concrete target perpendicularly. In order to reduce the calculation time and cost, this paper uses a 1/4 3D geometric model to simulate the missile guidance model and the reinforced concrete target, and then combines them in the Hypermesh software. Except for the warhead mesh, the standard mesh size of other meshes is about 5 mm, and the minimum mesh size of the missile structure is a regular hexagon with a side of 2 mm. The mesh division results of the missile guidance structure are shown in Figure 3.
Due to the irregular three-dimensional structure of the warhead head, a special grid division was performed on it. The division result is shown in Figure 4 below.
Figure 4. (A) Details of the warhead mesh. (B) Separation of the single-layer target plate and steel mesh.
The modal analysis was performed using ANSYS/Workbench software, which yielded the natural frequency and corresponding vibration mode of the projectile. Since the projectile is unconstrained and in a state of free vibration, the first six vibration modes are rigid body vibrations with a natural frequency of zero. Starting with the seventh vibration mode, the vibration modes include compression, tension, torsion, and bending. Table 2 lists the natural frequencies and vibration modes of the projectile for the 7th through 12th modes, and Figure 5 shows the vibration modes of these modes.
During the penetration process, most of the G-force signal experienced by the projectile is caused by the axial G-force. Since the bending and torsional modes have little effect on the axial G-force measurement during the penetration process of the projectile, the lowest mode of the compression and extension modes should be used as the cutoff frequency of the signal filtering. According to Figure 5 and Table 2, it can be concluded that the collected G-force signal should be filtered at a frequency of about 3482.8 kHz to eliminate the influence of high-frequency oscillations on the collected G-force signal.
The AGX-V series universal testing machine manufactured by Shimadzu Corporation of Japan was used to conduct compression tests on four rubber materials (nitrile rubber, fluororubber, silicone rubber and natural rubber). One specimen was taken for each type of rubber, and the compression test standard was in accordance with ISO 13314:2011 “Metallic materials – Ductility tests – Compression tests on porous and cellular metals”. In order to obtain reliable stress-strain curves, the rubber material was uniformly cut into cubes of 10 mm × 10 mm × 10 mm and placed on the testing platform, as shown in Figure 6. The loading rate applied to the material was 2 mm/min, and the strain value was set to 0.9.
Using the static structural analysis function of Workbench software, select the engineering data option. Using the engineering data option, the stress-strain test data of four materials are imported into the hyperelastic test data. The 5-parameter Mooney-Rivlin constitutive model in hyperelastic material is selected. After curve fitting, the parameters C10, C01, C11, C02, C20 and other parameters of each rubber can be obtained. Input these parameters into the constitutive model *MAT_H-HYPERELASTIC_RUBBER in the k-file. For the specific parameters, see Table 3. In the table, RO is the density, PR is the Poisson’s ratio, and C is the rubber material constant.
The projectile penetration process can be divided into three stages: the pitting stage, the tunneling stage, and the target destruction stage (exit stage). In the pitting stage, the detonator overload gradually increases from zero until the first peak appears; in the tunneling stage, the detonator overload remains in a certain range and continues to fluctuate; In the target plate destruction stage, the detonator overload continues to decrease until it reaches zero. In order to study the overload of the printed circuit board inside the detonator during projectile penetration under the action of various protective gaskets, this paper investigated the overload conditions of the detonator under the protection of nitrile rubber, fluororubber, silicone rubber, and natural rubber with a thickness of 2 mm, respectively. LS-Prepost software was used to extract the data and obtain the overload curve of the printed circuit board inside the detonator during projectile penetration into the C60 target with an initial velocity of 900 m/s. In addition, the Fourier transform is applied to the obtained curve to obtain its frequency curve, as shown in FIG. 7.
As can be seen from the figure, the overload of the detonator protected by natural rubber is the smallest at the moment of impact, and it is at least 47,600 g; then comes the overload of nitrile rubber, which is 62,100 g; the overload of fluororubber is 68,000 g; the overload of silicone rubber is the smallest, and the overload of the detonator board is 68,400 g. After the pitting stage, the penetration enters the tunneling stage, and the overload stabilizes at a certain value. Due to the failure of the grid on the target plate and other reasons, the overload signal will be disrupted, as a result of which the overload of the detonator will continue to fluctuate, and the oscillation amplitude may exceed the peak overload at the moment of penetration. The oscillation amplitude of the detonator protected by natural rubber and nitrile rubber is smaller, the oscillation amplitude of the detonator protected by fluororubber is larger, and the oscillation amplitude of the detonator protected by silicone rubber is the largest. At the stage after the target collapse from 1.08 m to 1.48 m, the overload of the initiator gradually decreased and then fluctuated around zero. After the projectile leaves the target, due to the residual elastic potential energy inside the rubber material, silicone rubber has a higher internal elastic potential energy than other rubbers. As shown in the figure, after the projectile left the target, the overload on the detonator board caused obvious oscillations that lasted about 1.5 meters and caused significant damage to the detonator.
By performing the Fourier transform on the overload signal in FIG. 7, we obtain the overload frequency domain curve of the pyrotechnic device, as shown in FIG. 8. During the penetration process, the natural frequency of the projectile has a significant effect on the overload of the pyrotechnic device. It can be seen from the figure that near the natural frequency of the projectile of about 3.4 kHz, natural rubber has the best filtering effect, followed by nitrile rubber, fluororubber has a weak effect, and silicone rubber has the worst effect. In the higher frequency range, the filtering properties of different rubbers differ. Nitrile rubber has the best filtering effect at a frequency of 14.5 kHz, and fluororubber at frequencies of 6.95 kHz and 10.3 kHz.
Figure 9 shows the collection result of the PCB overload signal after being processed by the db6 third-order fundamental wavelet transform. The analysis result is consistent with the raw overload signal. The filtering property of natural rubber is the best, and the peak overload is 32800g. After that, the overload is kept within a certain range, and the oscillation amplitude gradually decreases. The maximum penetration overload of silicone rubber is 47200g. The oscillation amplitude is large in the penetration stage, and reaches 23000g after penetration, and then the oscillation amplitude gradually decreases. The projectile velocity time curve is obtained by the post-processing software, as shown in Figure 10. It can be seen from the figure that after the projectile penetrates the C60 reinforced concrete plate at an initial velocity of 900m/s, the residual velocity is about 585m/s.
In summary, under these conditions, the use of a 2mm thick natural rubber shock absorber layer provides the most effective protection with virtually no vibration during and after penetration. This is because natural rubber has high mechanical strength, good bending fatigue strength, low hysteresis losses and self-reinforcement properties.
Figure 11 shows the overload signal of the fuse internal board. It can be seen from the figure that at the moment of target penetration, the protective effect of the four rubber materials on the fuse is approximately the same, and the peak overload at the moment of penetration is about 52900 g. After the projectile passes the initial penetration stage, it enters the penetration channel stage. At this stage, the overload is stabilized at a certain value. However, due to factors such as the failure of the target network, the overload signal is distorted, causing the fuse overload to continue to fluctuate, and the amplitude may exceed the peak overload during the penetration process. At the penetration channel stage, the oscillation with the largest peak value occurs at the fuse protected by silicone rubber. The distance from 1.08 to 1.48 m corresponds to the stage where the projectile penetrates the target and the target is destroyed. At this stage, the overload of the fuse gradually decreases and eventually fluctuates around the zero point. After the projectile left the target, the overload signal of the natural rubber fuse showed obvious oscillations of about 0.5 meters with a large amplitude.
By performing the Fourier transform on the overload signal shown in FIG. 11, we obtain the overload frequency domain curve of the fuse shown in FIG. 12. During the penetration process, the natural frequency of the projectile has a significant effect on the overload of the fuse. It can be seen from the figure that near the natural frequency of the projectile of about 3.4 kHz, nitrile rubber has the best filtering effect, followed by silicone rubber, then fluororubber, and natural rubber has the worst filtering effect at this frequency. In the high frequency range, there are great differences between different rubber materials, among which silicone rubber has the best filtering effect in the region of 10.3 kHz and 14.5 kHz.
FIG. 13 is the overload signal of the printed circuit board after processing the collected data with the third-order Daubechies wavelet transform (db6). The signal analysis result is consistent with the overload signal before filtering. Among them, the maximum overload of the fluororubber-protected fuse is about 30,000g, and there is no obvious oscillation phenomenon after the impact; while when using the fuse protected by natural rubber, although the overload at the moment of impact is slightly lower than that of the fuse, there is oscillation after the impact. The curve of the projectile velocity versus time is obtained by the post-processing software, as shown in Figure 14. It can be seen from the figure that after the projectile hit the C60 reinforced concrete target with an initial velocity of 900 m/s, there are obvious differences in the projectile velocity under the protection of four different buffer materials. The maximum difference is 16 m/s, and the average residual velocity is 573 m/s.
In summary, under these working conditions, the best protective effect is provided by the 6 mm thick rubber pad in combination with nitrile rubber. The original signal did not show obvious fluctuations during and after the armor penetration. This is due to the fact that nitrile rubber is an amorphous rubber with a slightly low elasticity. In general, the energy absorption of a special energy-absorbing structure is determined by the plasticity of energy dissipation under compressive loads. The four types of rubber showed stable performance under impact loads, and all of them underwent plastic deformation. Nitrile rubber is stronger and therefore absorbs more kinetic energy during penetration.
In this study, the Marshall hammer impact test method is used to test the buffer protection performance of various buffer materials under different impact overloads. The Marshall hammer structure is shown in FIG. 15 and consists of a handle, a hammer head, steel shots, a counterweight, and other components. Due to the different weights of the counterweights in the Marshall hammer test device with the same number of teeth, differences in the impact force applied to the experimental specimens occur. The impact overload is calibrated according to the number of teeth of the Marshall hammer required for the experiment. The sensitivity of the sensor used in the experiment is 0.8 μV/g, and the gain is 70 times. The calibration results are shown in Table 4.
In this study, a piezoresistive accelerometer was used to collect the overload signals. The basic principle is that the change in resistance at the moment of impact causes a change in voltage. These voltage changes are then used to calculate the change in voltage, which in turn indicates the overload signal experienced by the sensor. The data acquisition device consists of an acceleration sensor, a threaded cap, a counterweight, a test material, and a threaded base. The physical schematic diagram of the components of the data acquisition device is shown in Figure 16.
Figure 19 is the voltage change curve of the acceleration sensor protected by rubber pads of different thicknesses of 2 mm under the impact of 13-tooth impact load. The red solid line represents nitrile rubber, the blue dotted line represents fluororubber, the green dotted line represents silicone rubber, and the purple dotted line represents natural rubber. Under the impact load of 13 teeth with a rubber pad of 2 mm thickness, it can be seen that the natural rubber has the best protective performance, with a peak overload value of 8703 g and the smallest oscillation. The silicone rubber has the worst protective performance, with a peak overload value of 9885 g and obvious oscillation. Although the protective performance of nitrile rubber is lower than that of natural rubber, there is no oscillation signal after the impact. Table 5 provides summary data for different rubber materials under the same experimental conditions.
Figure 20 shows the acceleration sensor voltage change curve under the impact load of 13 teeth and the protection conditions of rubber pads with different thicknesses of 6 mm. As in the previous example, different rubber materials are represented by different colors. The rubber pad with a thickness of 6 mm has the best protective property. Silicone rubber has the best protective property with a peak overload value of 5550 g, but the fluctuation is large. Natural rubber has the worst protective property with a peak overload value of 8834 g, but the fluctuation is small. Fluororubber has slightly worse protective property than silicone rubber, but the fluctuation after impact is small. Table 6 shows the summary data of different rubber materials under the same experimental conditions.
Figure 21 shows the change curve of the acceleration sensor voltage under a 15-tooth impact load when using different 2mm-thick rubber pads for protection. The red solid line indicates that nitrile rubber is used as the buffer pad, the blue dotted line indicates that fluororubber is used as the buffer pad, the green dotted line indicates that silicone rubber is used as the buffer pad, and the purple dotted line indicates that natural rubber is used as the buffer pad. It can be seen that under the 15-tooth impact load when the 2mm-thick rubber pad is used, the fluororubber has the best protective performance, with a peak overload value of 11,724g but with obvious fluctuation; The silicone rubber has the worst protective performance, with a peak overload value of 14,351g and large fluctuation; The protective properties of nitrile rubber are lower than those of fluororubber, but the vibrations after impact are smaller; the sensor protected by natural rubber did not vibrate. Table 7 shows the summary data for different rubbers under the same experimental conditions.
Figure 22 shows the acceleration sensor voltage curve for a 15-tooth impact load protected by rubber pads of varying thicknesses of 6 mm. As in the image above, the color coding remains unchanged. In the case of a 6 mm thick rubber pad, silicone rubber has the best protective performance with a peak overload value of 9606 g and smaller fluctuations. Fluororubber has the worst protective performance, with a peak overload value of 10311 g and larger fluctuations. Table 8 summarizes the data for the different rubbers under the same experimental conditions.
Figure 23 shows the acceleration sensor voltage curve for a 17-tooth impact load using rubber pads of different thicknesses of 2 mm for protection. The color coding scheme corresponds to the previous diagram. For a 17-tooth impact load, natural rubber provided the best protection using a rubber pad of 2 mm thickness, with a peak overload value of 16,880 g and no noticeable fluctuations. The worst performance was shown by fluororubber rubber: the peak overload value was 18,949 g, and significant fluctuations were observed after the impact. Table 9 shows a summary of the different rubbers under the same experimental conditions.
Figure 24 shows the acceleration sensor voltage curve for a 17-tooth impact load using different 6 mm thick rubber pads for protection. The color coding corresponds to the rubber types mentioned earlier. For a 6 mm thick rubber pad, fluororubber provides the best protection with a peak overload value of 13,300 g, but with significant variations. Nitrile rubber provides slightly less protection than silicone rubber, with a peak overload value of 18,029 g and less variations. Table 10 summarizes the protection of the different rubbers under the same experimental conditions.
When comparing the four rubbers horizontally, it can be seen that fluororubber has the worst protective effect regardless of its thickness, and other rubbers show different protective performance under different penetration overloads or thicknesses. When using a 2mm thick gasket, regardless of the overload value, natural rubber has the best comprehensive protective performance. This is because natural rubber has the best comprehensive performance, with relatively high elasticity, strength and impact strength. When using a 4mm thick gasket, silicone rubber should be used for protection when the overload is less than 60,000g, and nitrile rubber should be used for protection when the overload is more than 60,000g, because nitrile rubber has the highest impact strength. When using a 6mm thick gasket for protection, silicone rubber should be used if the overload is less than 50,000g, and nitrile rubber should be used if the overload is greater than 50,000g. If the penetration overload is less than 50,000g, 4mm thick silicone rubber is the best choice. On the contrary, 2mm thick nitrile rubber should be used.
In general, the peak overload of silicone rubber is the smallest, but after the impact overload is over, an obvious vibration signal appears in the acceleration sensor protected by silicone rubber, which is consistent with the phenomenon in the simulation; the overload peak of natural rubber is slightly lower than that of silicone rubber, but there is no obvious vibration signal after the impact; Fluororubber and nitrile rubber have either a high peak overload problem or obvious vibration after the impact. Experiments have confirmed that the rubber used in the simulation has different protective properties in different situations. Therefore, when using rubber as a protective material, one cannot rely only on the “soft” protective property of the material, because the “hard” protective property of the material is unsatisfactory. Even if the same material penetrates different targets, its protective property will still change significantly.
This study focuses on the problem of high detonator overload during penetration into hard targets, the analysis of the main failure modes of the detonator, and the theory of the interface stress wave propagation of the projectile-detonator system. The overload signal of the printed circuit board inside the detonator is studied under different conditions and thicknesses of the protective rubber layers. In addition, the simulation results are verified by laboratory tests using a Marshall hammer. The main conclusions of the study are as follows:
1) Most of the overload signals generated by projectile penetration are due to axial overload. Filtering the collected signals at a frequency of about 3482.8 kHz can effectively eliminate the influence of high-frequency oscillations on the collection of overload signals.
2) When using 2mm thick rubber pads for protection, natural rubber has the best protective effect, and the overload of the detonator board can reach about 68400g; When using 6mm thick rubber pads for protection, nitrile rubber has the best protective effect.