Design and control of a vibration isolator using a biased magnetorheological elastomer

The objective of this work is to explore the capability of a Semi-Active (SA) elastomer and control techniques in the area of shock and vibration isolation. Typical passive isolation methods have short comings in meeting competing objectives. A specific problem is isolating electronic packages mounted to military vehicle walls from shock. Often passive elastomer based isolators are used. The ideal solution for shock isolation is a soft lightly damped isolator. However a soft lightly damped isolator will cause excessive sway during normal driving conditions. Further, vehicle dynamics during normal driving conditions are typically in the range of a few hertz, presenting the possibility of a lightly damped soft system experiencing severe resonance. As a result most elastomer based isolators have significant damping, which decreases their ability to isolate shock. Active systems are able to theoretically reach a optimal compromise between shock isolation and sway, however for several reasons active systems are not practical. SA systems combine the benefits of passive systems, primarily cost and low actuator power input, with the capability of varying system parameters in real-time with performance indexes nearing that of active systems; This work investigates an interesting SA elastomer, a magnetorheological elastomer (MRE), that is able to change its properties with the application of a external magnetic field. Methods of controlling the field to achieve a desired response is discussed. Finally experimental data is presented of a MRE based device using a SA control scheme to isolate a payload from shock and vibration

A Tunable Vibration Isolator Using a Magnetorheological Elastomer With a Field Induced Modulus Bias

Magnetorheological Elastomers (MRE) are composed of a ferromagnetic filler, micron sized iron particles, in an elastomer matrix. When a magnetic field is applied to an MRE, the iron particles develop a dipole interaction energy, which results in the material displaying a field dependent modulus. MRE materials have received attention in the last decade due in part to their potential application in semi-active vibration isolators. However, compared to MR fluid dampers, few applications of MRE materials have been developed, and no commercial devices are available. This paper describes the development of an MRE based isolator. Unique to this design is the introduction of a field induced modulus bias via a permanent magnet, which can be offset with a current input to the electromagnetic control coil. If the field bias is not significant enough to saturate the iron particles then an appropriately directed input current can also further increase the field induced modulus. Such a Biased Magnetorheological Elastomer (B-MRE) could be useful for applications where the designer wishes to decrease the system stiffness, something that has not been addressed by other MRE based devices

VOID NUCLEATION AND GROWTH AT GRAIN BOUNDARIES IN COPPER BICRYSTALS WITH SURFACE PERTURBATIONS

ABSTRACT Material failure on the microstructural level is important in determining macroscale behavior. When a material is subjected to dynamic (shock) loading conditions, damage and deformation patterns due to spall failure can provide a basis for connecting micro-to macroscale behavior. By analyzing deformation patterns at and around interfaces and boundaries that are representative of those found in engineering materials at high strain rates, we can develop stronger structures that can withstand impact collisions and rapid crack propagation. The addition of surface perturbations to one side of the samples provides insight on how strain localization occurs during the shock loading process and how the rippled release wave interacts with the boundary. Copper bicrystal samples were grown from two single crystal seeds using the vertical Bridgeman technique. A photolithography process was developed to create periodic surface perturbations on one side of the samples. The square wave ripples had a 150 µm wavelength and 5 µm amplitude. The bicrystals were shocked using laser ablation on the perturbation side at the Trident laser at Los Alamos National Laboratory and monitored using a VISAR (velocity interferometer systems for any reflector) and TIDI (transient imaging displacement interferometry) system. Shock pressures used were around 8 -10 GPa. Targets measured 5 mm in diameter and 100 microns thick. The orientations of the grains were [001] and [111] along the shock direction with a 50° misorientation angle for the boundary, which was aligned parallel to the shock direction. Samples were soft recovered and cross-sectioned to perform quantitative characterization of damage using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM) to gather information on the characteristics of the grain boundary and its surroundings, with emphasis on how the rippled surfaces and material anisotropy affected strain localization and spallation, initial results show that damage indeed localized at the grain boundary and that surface perturbations led to heterogeneity of spall damage distribution in the grain bulks. INTRODUCTION Shock loading can lead to several failure modes, including spallation. The process of spallation is caused by the superposition of tensile release waves created when the shock front reaches the free surface. The tensile stress produced by this superposition can exceed the strength of the material, inducing void nucleation, growth, coalescence, and separation. Pressure, pulse duration and shape of the shock wave have an effect on how voids form, as well as material variables such as constitutive properties, microstructure, and anisotropy [1 -5]. The purpose of this work is to explore the effect the square wave ripples have on strain localization, and spallation in copper bicystals. By observing single boundaries in copper bicrystals, the kinetics of nucleation and growth of damage at the boundary can be studied. The addition of surface perturbations provides insight on how strain localization occurs during the shock loading process via its effects on the development of hydrodynamic instabilities during compression and as fiducials to monitor the deformation behavior close to the boundary as compared to the grain bulks. Also, the presence of these surface perturbations leads to spatial heterogeneities on the release waves, which can then interact with the boundary in novel ways. Terminated twins and grain boundaries with misorientation angles in the 25° to 50° and 55° to 60° range ar