The military aircrafts receive many external threats on the battlefield. Those threats can cause serious battle damage by external impacts. Hydrodynamic Ram(HRAM) is a typical combat damage that occurs when a high speed projectile passes through a fluid in the structure and explodes. The fluid-filled structure is expanded by the pressure waves and expansion of the cavity in the fluid, and the damage is applied to the entire structure. The HRAM phenomenon occurs in aircraft wing structures with internal fuel tanks. HRAM can generate approximately 690 bar (10000 psi) of pressure in the fuel tank. It causes great damage to the fuel tanks and connection structures. Thus, the testing as well as analysis for the aircraft structural damage by HRAM are essential for the airframe survivability design.
In this paper, we performed experiments to confirm the dynamic strains on the structures subjected to hydrodynamic ram using strain gauges and the piezopolymer film(PVDF;Polyvinylidene fluoride) sensors without charge amplifier. First, PVDF sensors calibration test was performed to determine the capacitance of the entire circuit. Second, an HRAM test of a fluid-filled cylindrical PET vessel was performed using two different diameter steel ball projectiles. Through the HRAM test of the cylindrical PET vessel, the pressure of the fluid, the surface strain of the structure, and the behavior of the projectiles in the fluid by HRAM were examined. The PVDF sensors were attached to the vessel surface along with strain gages to measure the dynamic strain placed on the structure by the HRAM phenomenon. Finally, an HRAM test of a composite T-Joint was performed using a Ram simulator. The dynamic strains of composite T-Joint were measured using the strain gauges and PVDF sensors. The short circuit voltage signal of the PVDF sensors, which were without charge amplifiers, was converted to strain signals and compared with the signals of the strain gauge attached to the same locations.
The conclusions are as follows,
(1) In the PVDF sensors calibration test, when the number of parallel connections increased, it was confirmed that the actual strain and PVDF sensor signals become similar.
(2) In the HRAM test of a fluid-filled cylindrical PET vessel, It was found that both PVDF sensor signals without charge amplifiers and strain gage signals have similar waveforms at 0~0.8ms. The strain calculated by PVDF sensors and the strain gage signals shows an erratic trend from 0.8 ms. The structural behavior of fluid-filled structures are very complex and these surface strains of PET vessel depends on not only dynamic pressure generated by projectile but also structural vibration behavior due to the fluid-structure interaction. When initial velocity of the projectiles increased, a increase of fluid pressure and structure deformation were confirmed in the same projectiles. When size of the projectiles increased, a increase of fluid pressure and structure deformation were confirmed in the same initial velocity.
(3) In the HRAM test of a composite T-Joint, the dynamic strains on the web was measured by strain gauges and PVDF sensors. Bending in the web was removed as the average value of strain gauges and PVDF sensors. The dynamic strain of the web was compared with the results of the HRAM test and static tensile test. The high frequency dynamic strains of the web in HRAM test were observed compared to those of the web in static test.
In the future, the test should be performed using PVDF sensors with charge amplifier to measure the dynamic strains of structures due to the HRAM phenomenon for the comparison with the correct dynamic strains and also in order to measure the dynamic strains with the wide frequency band of a structure subjected to HRAM using strain gauges and PVDF sensors.