Juho Lee

Frequency tunable vibration and shock isolator using shape memory alloy wire actuator

Launch vehicles and satellites experience severe dynamic loads during flight phases. In particular, pyroshock generated from several separation events could result in malfunctions in the electric components in the launch vehicles. Shock isolators are generally applied in order to attenuate these severe shock environments; however, these isolators could amplify the low-frequency vibration generated by the engine thrust and aerodynamic loads and reduce the payload stability. When the natural frequency of the isolator is increased in order to avoid the low-frequency vibration amplification, sufficient shock attenuation could not be obtained. Thus, the isolators used in launch vehicles need to be designed with trade-offs between the low-frequency vibration amplification and the pyroshock attenuation. This article presents a novel frequency tuning method for the isolator in order to achieve both shock attenuation performance and avoidance of the vibration amplification. Compressed mesh washer isolators using the pseudoelasticity of shape memory alloy were adopted for easier attainment of the frequency tuning with a high performance in the shock attenuation.

A Parametric Study of Ridge-cut Explosive Bolts using Hydrocodes

Explosive bolts are one of pyrotechnic release devices, which are highly reliable and efficient for a built-in release. Among them, ridge-cut explosive bolts which utilize shock wave generated by detonation to separate bolt body produce minimal fragments, little swelling and clean breaks. In this study, separation phenomena of ridge-cut explosive bolts or ridge-cut mechanism are computationally analyzed using Hydrocodes. To analyze separation mechanism of ridge-cut explosive bolts, fluid-structure interactions with complex material modeling are essential. For modeling of high explosives (RDX and PETN), Euler elements with Jones-Wilkins-Lee E.O.S. are utilized. For Lagrange elements of bolt body structures, shock E.O.S., Johnson-Cook strength model, and principal stress failure criteria are used. From the computational analysis of the author’s explosive bolt model, computational analysis framework is verified and perfected with tuned failure criteria. Practical design improvements are also suggested based on a parametric study. Some design parameters, such as explosive weights, ridge angle, and ridge position, are chosen that might affect the separation reliability; and analysis is carried out for several designs. The results of this study provide useful information to avoid unnecessary separation experiments related with design parameters.

Pyroshock Prediction of Ridge-Cut Explosive Bolts Using Hydrocodes

Pyrotechnic release devices such as explosive bolts are prevalent for many applications due to their merits: high reliability, high power-to-weight ratio, reasonable cost, and more. However, pyroshock generated by an explosive event can cause failures in electric components. Although pyroshock propagations are relatively well understood through many numerical and experimental studies, the prediction of pyroshock generation is still a very difficult problem. This study proposes a numerical method for predicting the pyroshock of a ridge-cut explosive bolt using a commercial hydrocode (ANSYS AUTODYN). A numerical model is established by integrating fluid-structure interaction and complex material models for high explosives and metals, including high explosive detonation, shock wave transmission and propagation, and stress wave propagation. To verify the proposed numerical scheme, pyroshock measurement experiments of the ridge-cut explosive bolts with two types of surrounding structures are performed using laser Doppler vibrometers (LDVs). The numerical analysis results provide accurate prediction in both the time (acceleration) and frequency domains (maximax shock response spectra). In maximax shock response spectra, the peaks due to vibration modes of the structures are observed in both the experimental and numerical results. The numerical analysis also helps to identify the pyroshock generation source and the propagation routes.

Limit-Cycle Oscillation Suppression of Bioinspired Ornithopter: Wind Tunnel Testing

This study experimentally shows that an oscillatory behavior observed in a trim flight of an ornithopter has a stable limit-cycle oscillation (LCO) characteristics and that the magnitude of the LCO in body pitch dynamics can be suppressed by active tail motion. A free flight of the tested ornithopter is emulated in the wind tunnel using a specially devised tether that provides the minimal mechanical interference to the flight of ornithopter. Due to the symmetric wing motion in forward trim flight, the longitudinal flight dynamics is more focused than the lateral one. The non-contact type sensors are used to measure the time histories of the flight state variables such as wing and tail motions, body pitch angle, and altitude. The tail motion for the pitch LCO reduction is achieved by two actuators: 1) Servo motor for the rigid-body motion of the tail elevation angle, and 2) Macro-Fiber Composite strain actuator for the elastic deformation of the tail camber. The performances of the LCO suppressions are compared in the root-mean-square-error sense and the harmonically activated in-phase tail motion linked to wing motion is observed to effectively reduce the pitch LCO.© 2011 ASME

Flutter Experiment Equipment Design with Compliant Mechanism

This paper deals with a development of 2-DOF flutter experiment equipment which represents a 2-DOF typical section model. For a conventional 2-DOF flutter experiment equipment, it is hard to observe flutter boundary clearly due to the complexity of the experiment equipment. To refine our flutter experiment equipment system, a compliant mechanism based torsional spring is used. Well-designed extruded aluminum pipe works as a torsional spring. SolidWorks and ANSYS are used for modeling, analysis and design of the torsional spring. With this designed torsional spring, the 2-DOF flutter experiment equipment is developed and wind tunnel tests are performed. Clear flutter boundary which is estimated by classical flutter analysis is observed in the experiments.

Mathematical Model for the Separation Behavior of Low-Shock Separation Bolts

High-explosive separators used to separate structures in space systems and launch vehicles have many advantages, including low cost, high reliability, and high operating energy. However, there are also critical disadvantages such as a large pyroshock and the potential for debris. Low-shock separation devices address these disadvantages. The pressure-cartridge-type device, covered in this study, is attractive because of its high load capacity and high reliability. Its separation behavior, however, is complicated and not understood well because the separation happens within a few milliseconds through complex mechanical interaction. In this Paper, a mathematical model is established to predict the separation behavior of a ball-type separation bolt. The model consists of simultaneous differential equations that include a combustion model and equations of motion involving interactions between various components. Using the model, the effects of two representative design parameters, the coefficient of friction a...

Pyroshock Measurement and Characteristic Analysis of Explosive Bolt and Pyrotechnic Initiator

Pyroshock produced by the pyrotechnic devices can induce failures in nearby electronic devices. To handle and mitigate pyroshock inducing problems, appropriate measurement of pyroshock is essential. In this study, pyroshock measurement technique is established using laser Dopper vibrometers (LDVs) and shock accelerometers. Pyroshock produced by the explosive bolts and the pyrotechnic initiators under various environments is measured. The characteristics of pyroshock including the effects of supporting structures, propagation form on thin plate, sensor (contact and non-contact) types are discussed.

Nondestructive evaluation of pyroshock propagation using hydrocodes

Pyroshock or pyrotechnic shock generated by explosive events of pyrotechnic devices can induce fatal failures in electronic payloads. Therefore, understanding and estimation of pyroshock propagation through complex structures are necessary. However, an experimental approach using real pyrotechnic devices is quite burdensome because pyrotechnic devices can damage test structures and newly manufactured test structures are necessary for each experiment. Besides, pyrotechnic experiments are quite expensive, time-consuming, and dangerous. Consequently, nondestructive evaluation (NDE) of pyroshock propagation without using real pyrotechnic devices is necessary. In this study, nondestructive evaluation technique for pyroshock propagation estimation using hydrocodes is proposed. First, pyroshock propagation is numerically analyzed using AUTODYN, a commercial hydrocodes. Hydrocodes can handle stress wave propagation including elastic, plastic, and shock wave in the time domain. Test structures are modeled and pyroshock time history is applied to where the pyroshock propagation originates. Numerical NDE results of pyroshock propagation on test structures are analyzed in terms of acceleration time histories and acceleration shock response spectra (SRS) results. To verify the proposed numerical methodology, impact tests using airsoft gun are performed. The numerical analysis results for the impact tests are compared with experimental results and they show good agreements. The proposed numerical techniques enable us to nondestructively characterize pyroshock propagation.

Design of frequency-tunable mesh washer isolators using shape memory alloy actuators

This article introduces a novel frequency-tunable isolator that uses shape memory alloy wires as actuators and as an isolation material. The isolation material is a compressed mesh washer using the pseudoelasticity of the shape memory alloy. Frequency tuning of the isolator can easily be achieved using a simple electric circuit. Two improved models of frequency-tunable isolator, based on the authors’ previous model, were proposed and fabricated. This article presents detailed design procedures for adaptive shock isolators for launch vehicles that are able to achieve both shock attenuation performance and avoidance of vibration amplification. Launch vehicles experience a severe dynamic environment during the flight phase. In particular, pyroshock generated from several separation events could result in malfunctions of electrical components. Moreover, low-frequency vibration (<100 Hz) at the maximum dynamic-pressure phase could reduce the structural integrity of payloads. With this system, the resonant frequencies of the isolators are selectively controlled in two states using an adaptive mechanical system with compression of the isolation materials. Successful designs for the isolators and various test results regarding frequency tuning are presented.

Frequency Tunable Isolator Based on Shape Memory Alloy for Effective Shock and Vibration Suppression

A Shock and vibration isolator is widely used due to its simplicity and effectiveness. It attenuates vibration energy when the external excitation frequency is more than about Display Formula2 times its natural frequency, while the vibration around its natural frequency is generally amplified. However, an exciting frequency often varies so that it is difficult to avoid the vibration amplification. In particular, when these amplification phenomena occur in the low frequency domain, induced large vibration displacements degrade the structural integrity.This paper introduces a novel frequency tunable isolator proposed by the present authors. The isolator uses SMA wires as actuator as well as the isolation materials. The isolator material is a compressed mesh washer isolator using the pseudoelasticity of SMA. Frequency tune of the isolator can be easily achieved through a simple electric circuit. Thus, this isolator can be widely applied to various vibration and shock environments such as in aircrafts and motor vehicles. Particularly, the detail design procedure is presented here for the adaptive shock isolator for launch vehicle in order to achieve both shock attenuation performance and avoidance of the vibration amplification. Launch vehicles experience severe dynamic environment during the flight phase. Specially, pyroshock generated from the several separation events could result in malfunctions of electric components and low frequency vibration below 100 Hz at the maximum dynamic pressure phase could reduce the structural integrity of payload. The resonant frequency of the isolator is selectively controlled in two modes by using an adaptive mechanical system with compressing the isolation materials. The isolator was successfully designed and various test results with frequency tuning are presented, in this paper.Copyright