Enhanced properties of magnetorheological fluids: Effect of pressure

Magnetorheological fluids are extensively used in the industrial world to produce dissipative systems in an easily adjustable or even self-adaptive way. Sometimes their intrinsic rheological properties fail to meet system requirements in terms of available forces or yield stress for a given design space. In technical literature, previous works show a dependency of the shear strength of magnetorheological fluids on the internal pressure of the fluid, called squeeze strengthen effect. This work aims at the experimental validation of the behaviour of the magnetorheological fluids in both flow and shear modes under a given compressive state. Two specific ad hoc experimental test rigs are used for the campaign. The systems are designed in order to apply the magnetic field and the pressure at the same time and the tests are carried out following a design of experiment method. The magnetic parts of the system are designed with the help of a magnetic finite element simulation software, then the experiments are performed and the results are collected. The output is analysed through an analysis of variance approach, a statistical procedure that shows the influence of multiple variables on the system outputs. The outcome of the experimental tests confirms the beneficial effect of the pressure in both flow and shear modes, with performances up to three times compared with the datasheet values, where no pressure is considered

Experimental Validation of a Novel Magnetorheological Damper with Internal Pressure Control

In the present article, we have investigated the behaviour of magnetorheological fluids under a hydrostatic pressure of up to 40 bar.We have designed, manufactured and tested a magnetorheological damper with a novel architecture, which provides the control of the internal pressure. The pressurewas regulated by means of an additional apparatus connected to the damper that acts on the fluid volume. The magnetorheological damper was tested under sinusoidal inputs and with several values for the magnetic field and internal pressure. The results show that the new architecture is able to work without a volume compensator and bear high pressures. On the one hand, the influence of the hydrostatic pressure on the yield stress of the magnetorheological fluids is not strong, probably because the ferromagnetic particles cannot arrange themselves into thicker columns. On the other hand, the benefits of the pressure on the behaviour of the magnetorheological damper are useful in terms of preventing cavitation

SISTEMA DI CARATTERIZZAZIONE PER FLUIDI MAGNETOREOLOGICI: EFFETTO DELLA PRESSIONE IN MODALITA’ SCORRIMENTO

Il presente lavoro descrive l’ apparecchiatura sperimentale per la caratterizzazione di fluidi magnetoreologici (MR) in modalità scorrimento, sottoposti a pressione. Lavori di letteratura riportano indicazioni sull’effetto della pressione sui fluidi MR a taglio, ma oiché molti dispositivi commerciali lavorano in modalità scorrimento è interessante investigare questo aspetto. La progettazione del sistema è sviluppata in tre fasi: progetto del sistema meccanico, progetto del circuito magnetico, progetto della campagna sperimentale. L’attrezzatura effettua la misura della tensione di snervamento apparente del fluido MR in funzione di pressione, campo magnetico e velocità del fluido. Il circuito magnetico, progettato con un software FEM è stato verificato con un gaussmetro, evidenziando un ottimo accordo numerico-sperimentale. Il sistema sviluppato si pone come strumento per valutare se la pressione del fluido MR possa essere usata come strumento per potenziare gli attuali sistemi semiattivi basati su fluidi MR, come smorzatori e dissipatori lineari

Mounting of accelerometers with structural adhesives: experimental characterization of the dynamic response

The use of accelerometers to monitor the vibrations of either complex machinery or simple components involves some considerations about the mounting of the sensor to the structure. Different types of mounting solutions are commonly used, but in all cases they can be classified in one of these categories: stud mounting, screw mounting, adhesive mounting, magnetic mounting, and probe sensing. Indeed, each of them has a specific field of application depending on e.g. the mounting surface conditions, the temperature, the accessibility to the specific mounting point, etc. The choice of the mounting solution has an important effect on the accuracy of the usable frequency response of the accelerometer, since the higher the stiffness of the fixing, the higher the low-pass frequency limit of the mounting. This article specifically focuses on adhesive mounting of accelerometers, which includes a great number of different products from the temporary adhesives like the beeswax to the permanent ones like cyanoacrylate polymers. Among the variety of commercial adhesives, three specific products have been experimentally compared to assess their transmissivity and the results are reported in this article. A two-component methylmethacrylate (HBM X60), a modified silane (Terostat 737), and a cyanoacrylate (Loctite 454) adhesive have been used to join two aluminum bases, one connected to an accelerometer and the other to the head of electromagnetic shaker. A design of experiment (DOE) approach was used to test the system at several levels of amplitude and frequency of the external sinusoidal excitation supplied by the shaker

Lightweight mechanical metamaterials designed using hierarchical truss elements

Rotating unit systems constitute one of the main classes of auxetic metamaterials. In this work, a new design procedure for lightweight auxetic systems based on this deformation mechanism is proposed through the implementation of a hierarchical triangular truss network in place of a full block of material for the rotating component of the system. Using numerical simulations in conjunction with experimental tests on 3D printed prototypes, the mechanical properties of six types of auxetic structures, which include a range of rotating polygons and chiral honeycombs, were analysed under the application of small tensile loads. The results obtained show that there is almost no difference in the Poisson's ratios obtained from the regular, full structures and the ones made from triangular truss systems despite the latter, in some cases, being 80% lighter than the former. This indicates that these systems could be ideal candidates for implementation in applications requiring lightweight auxetic metamaterial systems such as in the aerospace industry

Evaluation of polymeric 3D printed adhesively bonded joints: effect of joint morphology and mechanical interlocking

Purpose The purpose of this paper is to evaluate and exploit the combination of additive manufacturing polymeric technology and structural adhesives. The main advantage is to expand the maximum dimension of the 3D printed parts, which is typically limited, by joining the parts with structural adhesive, without losing strength and stiffness and keeping the major asset of polymeric 3 D printing: freedom of shape of the system and low cost of parts. Design/methodology/approach The materials used in the paper are the following. The adhesive considered is a commercial inexpensive acrylic, quite similar to superglue, applicable with almost no surface preparation and fast curing, as time constraint is one of the key problems that affects industrial adhesive applications. The 3D printed parts were in acrylonitrile butadiene styrene (ABS), obtained with a Fortus 250mc FDM machine, from Stratasys. The work first compares flat overlap joint with joints designed to permit mechanical interlocking of the adherends and then to a monolithic component with the same geometry. Single lap, joggle lap and double lap joints are the configurations experimentally characterized following a design of experiment approach. Findings The results show a failure in the substrate, due to the low strength of the polymeric adherends for the first batch of typical bonded configurations, single lap, joggle lap and double lap. The central bonded area, with an increased global thickness, never does fail, and the adhesive is able to transfer the load both with and without mechanical interlocking. An additional set of scarf joints was also tested to promote adhesive failure as well as to retrieve the adhesive strength in this application. The results shows that bonding of polymeric AM parts is able to express its full potential compared with a monolithic solution even though the joint fails prematurely in the adherend due to the bending stresses and the notches present in the lap joints. Research limitations/implications Because of the 3D printed polymeric material adopted, the results may be generalized only when the elastic properties of the adherends and of the adhesive are similar, so it is not possible to extend the findings of the work to metallic additive manufactured components. Practical implications The paper shows that the adhesives are feasible way to expand the potentiality of 3 D printed equipment to obtain larger parts with equivalent mechanical properties. The paper also shows that the scarf joint, which fails in the adhesive first, can be used to extract information about the adhesive strength, useful for the designers which have to combine adhesive and additive manufactured polymeric parts. Originality/value To the best of the researchers’ knowledge, there are scarce quantitative information in technical literature about the performance of additive manufactured parts in combination with structural adhesives and this work provides an insight on this interesting subject. This manuscript provides a feasible way of using rapid prototyping techniques in combination with adhesive bonding to fully exploit the additive manufacturing capability and to create large and cost-effective 3 D printed parts

Effect of Temperature on the Dynamic Response of Adhesively Mounted Accelerometers

This paper focuses on the effect of temperature on the frequency response function (FRF) of three different structural adhesives; namely a two component methylmethacrylate (HBM X60), a modified silane (Terostat 939) and a cyanoacrylate (Loctite 454). The structural adhesives are commonly used in vibration analysis to mount accelerometers on structures or machines. The stiffness of the adhesive can influence the response function on large frequency band, affecting the proportional excitation between the structure and the accelerometer. In the “system structure + adhesive + accelerometer”, the adhesive may acts like a filter between the source and the sink of vibrations. A variation of the dynamic response of the filter could lead to an erroneous analysis. The authors already investigated the relation between the frequency response function and operating conditions of the test. This paper expands the research by considering the temperature effect in order to depict a complete picture of the adhesive behavior on dynamic response of an accelerometer. A design of experiments (DOE) approach was used to test two bonded aluminum bases at different levels of temperature and frequency of the external sinusoidal excitation, supplied by an electromagnetic shaker. The results clearly demonstrate that the adhesive is not able to change the system response, therefore the signal transmission is good in the entire range of temperature regardless the adhesive chosen

Characterization Of Commercial Magnetorheological Fluids At High Shear Rate: Influence Of The Gap

This paper reports the experimental tests on the behaviour of a commercial MR fluid at high shear rates and the effect of the gap. Three gaps were considered at multiple magnetic fields and shear rates. From an extended set of almost two hundred experimental flow curves, a set of parameters for the apparent viscosity are retrieved by using the Ostwald de Waele model for non-Newtonian fluids. It is possible to simplify the parameter correlation by making the following considerations: the consistency of the model depends only on the magnetic field, the flow index depends on the fluid type and the gap shows an important effect only at null or very low magnetic fields. This lead to a simple and useful model, especially in the design phase of a MR based product. During the off state, with no applied field, it is possible to use a standard viscous model. During the active state, with high magnetic field, a strong non-Newtonian nature becomes prevalent over the viscous one even at very high shear rate; the magnetic field dominates the apparent viscosity change, while the gap does not play any relevant role on the system behaviour. This simple assumption allows the designer to dimension the gap only considering the non-active state, as in standard viscous systems, and taking into account only the magnetic effect in the active state, where the gap does not change the proposed fluid model

Stress concentrations in skew pressurized holes: A numerical analysis

This work provides a numerical analysis of the stress concentration factor in an elastic solid containing non-aligned and non-concurrent circular holes subjected to an internal pressure. Using Finite Element Analysis, a variety of systems with a range of geometric configurations and loading conditions were simulated and the trends observed were analysed using a qualitative and statistical analysis in order to determine the correlation between these factors and the maximum stress concentration in these systems. Furthermore, simple empirical models were calibrated on the simulation results and used to plot reference graphs which may be employed to predict the stress concentration factor of these systems according to several geometric parameters and loading conditions. The numerical results and the empirical models presented here also show good agreement with previously derived analytical results based on 2D models and are expected to provide a useful tool for the designer of such systems especially for fatigue problems in fluid-power systems, where a straightforward evaluation of the stress concentration factor in pressurized hole systems is needed

Progettazione di uno smorzatore a fluido magnetoreologico con controllo della pressione

Il presente lavoro riguarda la progettazione di uno smorzatore a fluido magnetoreologico (MR) che prevede la possibilità di controllarne la pressione interna. Precedenti studi degli autori, hanno evidenziato che la tensione di primo distacco τB di un fluido MR aumenta in funzione non solo dell'intensità del campo magnetico, ma anche della pressione alla quale si trova. Poiché l'aumento del campo magnetico è limitato da considerazioni quali la potenza dissipata e la saturazione magnetica, regolare in maniera attiva la pressione, consente di potenziare le prestazioni di tali sistemi in modo semplice ed efficiente. Il lavoro descrive la progettazione di dettaglio delle principali funzionalità del sistema: dimensionamento del circuito magnetico, progettazione del sistema idraulico e sviluppo del sistema di controllo della pressione. L’approccio è prevalentemente analitico, al fine di fornire le equazioni necessarie per un dimensionamento di massima del sistema. La metodologia sviluppata fornisce una traccia per un corretto approccio alla progettazione di un sistema smorzante a fluido MR sulla base delle caratteristiche di forza e corsa desiderate e degli ingombri disponibili