Effect of pressure on the physical properties of magnetorheological fluids

To date, several applications of magnetorheological (MR) fluids are present in the industrial world, nonetheless system requirements often needs better material properties. In technical literature a previous work shows that MR fluids exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluid based devices are rotary devices, this paper investigates the behaviour of MR fluids under pressure when a rotation is applied to shear the fluid. The system is designed in order to apply both the magnetic field and the pressure and follows a Design of Experiment approach. The experimental apparatus comprises a cylinder in which a piston is used both to apply the pressure and to shear the fluid. The magnetic circuit is designed to provide a nearly constant induction field in the MR fluid. The experimental apparatus measures the torque as a function of the variables considered and the yield shear stress is computed. The analysis of the results shows that there is a positive interaction between magnetic field and pressure, which enhances the MR fluid performances more than twice

Effect of pressure on the physical properties of magnetorheological fluids

To date, several applications of magnetorheological (MR) fluids are present in the industrial world,nonetheless system requirements often needs better material properties. In technical literature a previous workshows that MR fluids exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluidbased devices are rotary devices, this paper investigates the behaviour of MR fluids under pressure when arotation is applied to shear the fluid. The system is designed in order to apply both the magnetic field and thepressure and follows a Design of Experiment approach. The experimental apparatus comprises a cylinder inwhich a piston is used both to apply the pressure and to shear the fluid. The magnetic circuit is designed toprovide a nearly constant induction field in the MR fluid. The experimental apparatus measures the torque as afunction of the variables considered and the yield shear stress is computed. The analysis of the results showsthat there is a positive interaction between magnetic field and pressure, which enhances the MR fluidperformances more than twice

Stress states and moment rates of a two-asperity fault in the presence of viscoelastic relaxation

Abstract. A fault containing two asperities with different strengths is considered. The fault is embedded in a shear zone subject to a constant strain rate by the motions of adjacent tectonic plates. The fault is modelled as a discrete dynamical system where the average values of stress, friction and slip on each asperity are considered. The state of the fault is described by three variables: the slip deficits of the asperities and the viscoelastic deformation. The system has four dynamic modes, for which analytical solutions are calculated. The relationship between the state of the fault before a seismic event and the sequence of slipping modes in the event is enlightened. Since the moment rate depends on the number and sequence of slipping modes, the knowledge of the source function of an earthquake constrains the orbit of the system in the phase space. If the source functions of a larger number of consecutive earthquakes were known, the orbit could be constrained more and more and its evolution could be predicted with a smaller uncertainty. The model is applied to the 1964 Alaska earthquake, which was the effect of the failure of two asperities and for which a remarkable post-seismic relaxation has been observed in the subsequent decades. The evolution of the system after the 1964 event depends on the state from which the event was originated, that is constrained by the observed moment rate. The possible durations of the interseismic interval and the possible moment rates of the next earthquake are calculated as functions of the initial state

Functional fatigue of NiTi Shape Memory wires for a range of end loadings and constraints

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermalactivation (functional fatigue) is central to the rational design of smart actuators based on these materials. Testresults on SMAs under functional fatigue are scarce in the technical literature and the few data available aremainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased byelastic springs or by another SMA element, their stress state is far from constant in operation. The mismatchbetween actual working conditions and laboratory arrangements leads to suboptimal designs and underminesthe prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Fourtest procedures are planned, covering most of the typical situations occurring in practice: constant-stress,constant-strain, constant-stress with limited maximum strain and linear stress-strain variation with limitedmaximum strain. The paper describes the experimental apparatus specifically designed to implement the fourloading conditions and presents fatigue results obtained from commercial NiTi wires tested under all thoseprotocols

Functional fatigue of shape memory wires under constant-stress and constant-strain loading conditions

Abstract Shape memory alloys (SMAs) are increasingly used for the construction of simple solid-state actuators characterized by outstanding power density. The rational design of these actuators requires reliable data on the fatigue strength of the alloy under cyclic thermal activation (functional fatigue). The technical literature shows scanty test results for SMAs under functional fatigue. Furthermore, the few data available are mainly limited to the condition of constant stress applied to the material. Sincethe SMA elements used within actuators are normally biased by conventional springs or by another SMA element, their stress condition is far from constant in operation. The disagreement between actual working conditions and laboratory conditions leadsto suboptimal designs and undermines the prediction of the life of the actuator. This paper aims at bridging the gap between experiment and reality. Four characteristic test conditions are envisioned, covering most of the actual situations occurring inpractice: constant-stress, constant-strain, constant-stress with controlled maximum strain and cyclic-stress with controlled maximum strain. The paper presents the experimental apparatus specifically designed to implement the four test conditions. Fatigue results on a commercial NiTi wire (0.15 mm diameter) tested under constant-stress and constant-strain loading are also presented and discussed

Modellazione efficiente agli elementi finiti per l’analisi a collasso di strutture incollate complesse

Il lavoro verifica l’applicabilità di un modello semplificato agli elementi finiti per l’analisi a collasso post elastico di strutture incollate complesse in parete sottile. Al fine di superare le limitazioni dei modelli di letteratura come l’uso di elementi speciali, il lavoro sfrutta un modello ridotto già presentato dagli autori in campo elastico. Tale modello è basato sulla rappresentazione degli aderendi mediante elementi semistrutturali (piastre o gusci) e dell’adesivo per mezzo di speciali elementi coesivi. La continuità strutturale tra aderendi e adesivo è ottenuta mediante vincoli interni (tied mesh) che accomunano i gradi di libertà dei nodi mutuamente affacciati di aderendi ed adesivo. La struttura analizzata è un simulacro di incollaggio industriale e produce nella strato adesivo una sollecitazione complessa, analizzabile solo con modelli numerici. Si considera una struttura tubolare in parete sottile a sezione quadrata, fatta di due spezzoni posti testa a testa e incollati con fazzoletti di lamiera sui quattro lati. La struttura è sottoposta a flessione a tre punti fino al cedimento e la zona incollata posta disassata rispetto al punto di applicazione del carico riceve una sollecitazione indiretta. I risultati dell’analisi FEM, confrontati direttamente con le curve sperimentali forza-spostamento, evidenziano una buona accuratezza del metodo, in termini di rigidezza, forza massima e comportamento post elastico della struttura, accompagnati da ridotte dimensioni del modello e tempi di calcolo molto contenuti. Grazie a questi vantaggi, la procedura si presta ad effettuare l’analisi di strutture incollate complesse, altrimenti ingestibili se affrontate con una modellazione agli elementi finiti tradizionale

Optimum Mechanical Design of Binary Actuators Based on Shape Memory Alloys

This chapter describes the optimum mechanical design of shape memory based actuators. The authors show how to exploit the Shape Memory Alloy (SMA) to design silent, compact and light binary actuators. Two simple mechanical models are considered to describe the SMA behaviour and design equations are provided for two classes of actuators. First SMA actuators are analyzed and designed on the basis of the backup element needed to recover the stroke. Second SMA actuators are improved by adding a compensator system to enhance the output mechanical response, especially in terms of available stroke. Useful design procedures are provided to help the engineer in the synthesis of SMA actuators. Starting from the design specifications, a step by step procedure is built to define the mechanical dimension of the SMA active elements, of the backup system and of the compensator