Characteristics Of Plate Waves Induced By Hypervelocity Impact And Onboard Monitoring Technique For Detection Of Impact On Spacecraft By Space Debris

With the increase of space activities, space debris environment has deteriorated. Impact events on spacecraft by space debris became a nonnegligible risk factor. The mitigation, avoid, protection technique have been developed to ensure the security of spacecrafts. In addition to these measures, a new concept about onboard monitoring system based on acoustic emission (AE) technology to locate position of impact point and evaluate effects of impact in real time also presented. AE technology has been successfully applied into the industrial fields as a practical non-destructive test means, a series of source location and identification techniques have been developed. However these techniques are usually based on the type of source and propagation characteristics of wave. Current knowledge on AE event induced by hypervelocity impact (HVI) is not enough and a deep investigation is necessary. The characteristics of the AE source induced by HVI and the waveforms in far fields are studied in this dissertation.Due to the complexity of load technique and requirement for high fidelity of waveforms, some basic work about collecting AE signals in HVI cases should be done as the foundation of whole work. An experimental platform was build for study on AE in HVI, on which aluminum balls were projected to aluminum alloy targets with two stages light gas gun and signals in far filed were collected by a high damp, high resonance frequency piezoelectricity transducer. For restoration of original waveforms from the collected signals, the sensibility of transducer was evaluated using lead break AE source and dynamics finite element method (DFEA) simulation. The results indicated that N182 sensor is sensitive to the out-plane velocity rather than the displacement mentioned in some literatures. And the approximate local sensitivity in the cases was also been evaluated. To make up the shortage of the experimental means, Lagrange solver was also applied in simulation of HVI to obtain the far field out-plane velocity signals. Waveforms restorated from the original signals in experiments and velocity waveforms from numerical simulation cases under same cases were compared and the results indicate their consistency, so both experiment and simulation can provide effective AE signals.In order to meet the demand of AE source location and identification technique, the knowledge on mode characteristics of AE signals induced by HVI is necessary. With experimental and numerical means, AE signals in cases ofΦ1.5~10.0mm, 0.05~8km/s aluminum projectiles impacting aluminum alloy targets with thickness of 5mm were obtained. With wavelet transform, plate modes of signals were identified; S0, A0 and S2 were found to be main modes. A concept of amplitude characteristic parameter (ACP) was presented for quantified the amplitude of every mode. The relationship between the impact velocity, size of projectile and ACP of modes were explored respectively. The results indicated that ACP can characterize quantificationally amplitude of mode to some extent; before the limits of trajectory, ACP of A0 mode is proportional to the mass and impact velocity of projectile and ACP of S0, S2 modes are proportional to impact velocity, as well. Under comparative higher velocity, the antiphase of A0 mode waveforms causes change of sign of ACP.Location of impact source is the basic function of monitoring system, and applicable location method must be built based on the characteristics of AE signals. Location method using arrival time adopted in case of HVI consists of arrival time algorithm and location algorithm: 1st arrival time was determined by modified threshold algorithm based the stronger head waveform in AE signals, and 2ed arrival time was determined by wavelet transform algorithm based on plate modes of signals; a new least variance algorithm was presented on base of least square method as location algorithm. Location experiments for HVI were performed with these algorithms mentioned above, the results indicate: more accurate arrival time can be obtained with modified threshold algorithm than wavelet transform algorithm, and least variance method is steady and accurate in the cases, location of HVI source can be solved with these two algorithms.The research on the wave source mechanism is the advanced fields of the AE technique. Based on the characteristics of AE signals, the generation mechanism of AE in HVI case is studied. Analyzing the course of HVI, AE signals is thought as elastic stress wave in far field induced by the impact load. A model of normal and radial impact was built on the basis of these assumptions. The generation mechanism of the main mode of AE signals was interpreted as following: A0 mode is induced by the normal impact force and its ACP is approximately proportional to the normal impulse; the mode of S0 and S2 is induced by the radial impact force and the ACP of S0 is approximately proportional to the radial impact impulse (time integral of radial force) under low impact velocity. With model of normal impulse, negative normal impulse was interpreted as the cause of antiphase phenomena of A0 mode waveforms.The results and conclusions in this dissertation are valuable reference for developing monitoring technique for detection of impact on spacecraft by space debris. The results on location can be directly adopted in monitoring system and the results on the relationship between wave mode and impact parameters are foundation for source identification in onboard monitoring technique.