Characteristics Of Pressure Measurements Using PVDF Gauge And The Dynamic Response Of Metallic Sandwich Panels Subjected To Proximity Underwater Explosion

In recent decades, metallic sandwich structures have been widely used in protection, aerospace, architectural and ocean engineering for its excellent physical and mechanical characteristics such as extra light, high specific stiffness and high efficient energy absorption. Sandwich structures with metallic cores achieve sufficiently good performance under various different service conditions, and studies on its mechanical behavior subjected to quasi-static and dynamic loadings have been one of the research focuses for the past decades. However, on account of diversity and complexity of structural styles and dynamic loadings, there are still several problems in this subject. At present, studies on its dynamic mechanical response are mostly limited to impact and air blast loadings. Metallic sandwich structure, as its excellent physical and mechanical performance, should have a promising application prospect in ocean engineering and ship industry, while published results on its dynamic behavior and failure mechanism subjected to underwater explosion loading is very limited. In addition, pressure gauges using PVDF piezoelectric film as the sensing element have been widely applied in impact/blast pressure measurements with its large piezoelectric constant, wide frequency response range and high SNR (signal-to-noise ratio). By the way, it is easy in fabrication and cheap compared to other sensing elements. Pressure gauges using PVDF film is convenient to be applied on structural surface and sandwiched between compartments without disturbance on the structural mechanical response. Even so, there are still several problems on the instability of its sensitivity coefficient and its application techniques, especially for blast pressure measurements at a fluid-structure interface. In this study, experimental, computational and analytical investigations were conducted on numbers of peripherally clamped sandwich panels with aluminum honeycomb cores subjected to proximity underwater blast loading. Besides, for the purpose of blast pressure measurements at the fluid-structure interface, pressure measuring characteristics of sandwich PVDF gauge at several typical interfaces between different mediums were analyzed and summarized.The sensitivity coefficient of sandwich PVDF pressure gauge is studied through a series of calibration experiments by using SHPB apparatus. It was found that internal unevenness caused by production process and contact condition between the gauge and pressure bars mainly account for the instability of the pressure gauge sensitivity. And in addition, the internal evenness was mainly consisted of stress concentration and shear effects which were due to electrode-leads and the uncertainty of stressed area of piezoelectric film between incident and transmitted pressure bar. Based on improvements on pressure gauge thickness and contact condition between the gauge and the pressure bars, the sensitivity coefficient is steadily fitted by K=24.7pC/N. It can thus be seen that the actual measurement condition should be considered in the calibration and application of sandwich PVDF pressure gauge. Therefore, several typical pressure measurement tests were conducted to investigate pressure measurement characteristics at solid-solid and fluid-structure interfaces. The measurement characteristics were analyzed from three aspects, that is, transverse effect of the piezoelectric film, contact condition at the interface and the physical properties difference of mediums locating on both sides of the interface. And that, the physical properties difference were further described by acoustic impedance and compressibility difference of mediums on both sides of the measuring interface. Afterwards, in order to expand application range of PVDF pressure gauge in submarine measurements, a new type underwater blast pressure sensor used PVDF film as sensitive element was designed and manufactured. Results of tests on the pressure sensor show that the PVDF typed sensor enough to measure underwater explosion pressure-time history. Sensitivity coefficient of the pressure sensor was calibrated and well fitted by K=13.84pC/N. Result of underwater explosion calibration test further prove that the actual measurement condition should be considered in the calibration and application of PVDF pressure gauge.To investigate blast resistance of metallic sandwich panels which include structural response and the of secondary shock wave strength, a series of tests on sandwich panels consisted of two aluminum alloy face-sheets and an aluminum alloy honeycomb core subjected to underwater explosion were carried out. In the analysis, blast resistance of metallic sandwich panels was assessed by the structure resist deformation ability which is characterized by the back face-sheet deflection and shock pressure attenuation performance which is represented by the secondary shock wave intensity. Different deformation and failure modes of face-sheets and cores were identified in the analysis. It was found that specimens with thicker face-sheet thickness produced smaller deflection and the pressure attenuation performance also shows a positive relationship with face-sheet thickness. By adopting a higher honeycomb core, back face-sheet deflection and secondary shock wave intensity were simultaneously reduced. The back face-sheet deflection was decreased by using thicker foil while the secondary shock wave intensity was increased. The back face-sheet deflection is not linearly dependent on cell size of cores. When the cell size was the single variable factor, by using cores with larger cell, more energy was absorbed in the compression process thereby the structure produces smaller deformation. However, when the cell size is large enough, the core is too easy to be compressed compacted which lead to larger deformation on back face-sheet. In conclusion, the secondary Shockwave intensity is dominantly affected by core density while the structural energy absorption and dynamic response is more leaded by the key design parameters. On the other hand, face-sheets and cores deformation/failure modes were identified based on the experimental observations. The front face-sheets of specimens with thinner skins, lighter cores and subjected to a closer shock produces more complicated deformation modes. The front face-sheets mainly produce localized failure at its central zone, flower-shaped failure at the peripheral zone and global plastic deformation on the whole exposed area. As for the back face-sheets, all the panels show spherical global deformation. Also, several failure modes were observed on tested cores. Honeycomb cores produced the similar global deformation as the back face-sheet and progressive compression, while the compressed length decreased from the central zone to its outskirts. As the core was compressed compacted, it was likely to produce out-plane penetration and tensile failure. Finally, sandwich panels after blast tests were compared with monolithic aluminum plates which have equal quality. It was found that by using metallic sandwich structure, the secondary shock wave was obviously decreased. Results show that metallic sandwich panels have better blast resistance than monolithic panels.According to strain measurement tests for metallic sandwich panels subjected to proximity underwater explosion, dynamic response of face-sheets were further analyzed. And then, air blast with consistent charge mass and shock distance on sandwich panels were carried out to investigate the difference of deformation and failure mode between air and submarine blast loading. Results show that, for the front face-sheet, at the preliminary stage of structural response, deformation at the central zone is mainly caused by bending moment while deformation of the peripheral zone is transitorily caused by bending moment and then convert to be dominated by membrane force. Deformation and failure near the exposed area border were mainly attributed to bending moment produced by clamped boundary. It is meant that bending moment effect on the deformation and failure of central zone and the boundary is greater than that at the intermediate zone.For the back face-sheet, the deformation and failure of central zone and the boundary were both dominated by bending moment and membrane force while the deformation is more decided by membrane force at the intermediate zone. It is observed that there are distinct shock strains at initial stage of the strain signals of both front and back face-sheets which was argued to be caused by shock wave. Amplitude of shock strain is increased by decreasing the face-sheet thickness. Moreover, shock strain of the back face-sheet appears before strain signal which corresponds to the bending moment. It means that, structural dynamic response described by the three decoupling stages of Fleck model is relatively conservative for sandwich panels with weak intensity. According to experimental observation, sandwich panels after underwater explosion tests mainly produced global plastic deformation and failure mode of the cores is dominated by progressive compression.For the air blast tests with consistent charge mass and shock distance, the front face-sheet produced flower-shaped tearing failure at the central zone while failure mode of the core was dominated by strong transverse compression failure around the tearing zone. This means that damage degree of sandwich panels caused by proximity air blast is more intensity than samples subjected to underwater explosion with same experimental setup. It is argued that the difference of deformation and failure modes could be attributed to the different shock wave propagation and fluid-structure interaction process between air blast and underwater explosion. On the one hand, dynamic loading caused by close-in air blast is more localized and transient than underwater explosion loadings. On the other hand, momentum and energy loaded on the structure by shock wave is equivalent to bubble loading for underwater explosion while there is distinct difference between the time scale of shockwave and bubble loading. Therefore, the fluid-structure interaction mechanism is different between air blast and underwater explosion, sandwich panels subjected to underwater explosion trend to produce global deformation.Based on the experiments, corresponding finite element simulations have been conducted using commercial LS-DYNA software. The underwater explosion loading process and dynamic response of sandwich panels and absorption energy distribution were investigated in the simulation work. Effect of face-sheet thickness, core density and height on structural deformation and energy absorption law was investigated in the simulation of model with solid cores divided by hexahedral element. And the simulated results show good uniformity with the experiments. Results show that the total absorption energy of the structure and the proportion of energy absorbed by cores both decreased by using thicker face-sheets. Sandwich panels with higher cores absorbed less energy while its resist deformation ability and energy absorption proportion of the cores were both enhanced. By adopting denser cores, the total absorption energy of the structure decreased while the proportion of energy absorbed by the cores in the total energy increased. In the simulation of honeycomb cores modeled by shell elements, several failure modes of the core were observed such as progressive compression, compacted, large plastic deformation and transverse compression. It was also argued that the out-plane compression buckling and the rotation of cells should account for the interlayer failure between the core and back face-sheet.Based on the above research, it is recommended that the blast resistance assessment criteria of sandwich panels should simultaneously combine its resist deformation ability, shock wave pressure attenuation capacity and energy absorption capacity of structural components.