Entangled single-wire NiTi material: a porous metal with tunable superelastic and shape memory properties

NiTi porous materials with unprecedented superelasticity and shape memory were manufactured by self-entangling, compacting and heat treating NiTi wires. The versatile processing route used here allows to produce entanglements of either superelastic or ferroelastic wires with tunable mesostructures. Three dimensional (3D) X-ray microtomography shows that the entanglement mesostructure is homogeneous and isotropic. The thermomechanical compressive behavior of the entanglements was studied using optical measurements of the local strain field. At all relative densities investigated here ($\sim$ 25 - 40$\%$), entanglements with superelastic wires exhibit remarkable macroscale superelasticity, even after compressions up to 25$\%$, large damping capacity, discrete memory effect and weak strain-rate and temperature dependencies. Entanglements with ferroelastic wires resemble standard elastoplastic fibrous systems with pronounced residual strain after unloading. However, a full recovery is obtained by heating the samples, demonstrating a large shape memory effect at least up to 16% strain.Comment: 31 pages, 10 figures, submitted to Acta Materiali

Modélisation de l'écoulement de fluides en loi puissance en milieux fibreux anisotropes

L'écoulement de fluide en loi puissance à travers des milieux fibreux anisotropes est étudié en s'appuyant sur les résultats théoriques obtenus par la méthode d'homogénéisation des structures périodiques. Afin de determiner la structure de la loi d'écoulement, des simulations numériques ont été réalisées sur des volumes élémentaires représentatifs d'un milieu fibreux modèle 2D constitué d'un arrangement carré de fibres parallèles de sections circulaires ou elliptiques. Enfin une méthodologie, basée sur l'étude de courbes d'iso-dissipation mécanique ainsi que la théorie de représentation des fonctions tensorielles, est proposée afin de formuler la loi d'écoulement macroscopique. Cette méthodologie est illustrée sur les résultats numériques obtenus

Non-Symmetric Tension-Compression Behaviour of NiTi Alloy

Development of tensorial constitutive equations suitable to model the thermomechanical behaviour of shape memory alloys (SMA) would greatly help engineering design of sophisticated components. While a number of studies have focused on mechanical behaviour under tensile loading, only a few have been done to characterise properties under other stress states. The aim of our study is to report some new experimental results obtained on a equiatomic NiTi. The experimental study was carried out using both uniaxial tension and compression tests which were performed with the same form for the specimens (sheet samples of gauge length 40mm and cross section 5.6mmx2.7mm) in order to avoid any geometrical effect. This choice allows to submit the material to the same prior thermomechanical treatment, i.e. a cold rolling leading to a thickness reduction of 18% followed by an annealing at 430°C for ½ hour. The temperature of the experiments were achieved using a silicon oil bath. A special device was designed in order to avoid buckling during compression. Preliminary tests were performed in order to study the effect of this device which was concluded as negligeable The results show that tension and compression behaviours are not symmetric, especially for superelastic behaviour. Moreover, the amplitude of the one-way memory effect and the slope of the linear variation of the transformation stress with the temperature are compared for these two stress states

Stress state effect on mechanical behaviour of shape memory alloys : Experimental characterisation and modelling

As engineering components utilising the beneficial properties of shape memory alloys (SMA) become geometrically complex and as applications involve combinations of loading states, a full evaluation of the effects of stress state on stress-strain response of these materials becomes critical for the success of these applications. Such evaluation is required to establish reliable constitutive relationship to model the complex thermomechanical behaviour of SMAs. This paper is intended to present a non-exhaustive overview on studies and results dealing with experimental characterisation and modelling of the effects of stress state on mechanical behaviour of shape memory alloys. In that respect, our presentation is mainly focused on one hand on experimental results on superelastic and ferroelastic deformation of polycrystalline alloys and on the other hand on the modelling at the macroscopic level, yielding critical reorientation or transformation constitutive equations

Experimental Study of Mechanical Hysteresis of NiTi During Ferroelastic and Superelastic Deformation

Shape memory alloys are known to exhibit a range of novel thermomechanical behaviour associated with the unique thermoelastic martensitic transformation. Such behaviour includes the superelasticity associated with stress-induced martensitic transformation at relatively high temperatures and the ferroelasticity associated with a martensite reorientation process at low temperatures. Both the stress-induced martensitic transformation and the martensite reorientation are energy-dissipative processes, i.e., hysteretic between the forward and reverse processes. This work was aimed at studying the hysteretic behaviour of the ferroelasticity and superelasticity observed in a polycrystalline NiTi alloy by carrying out simple shear deformation tests through both major and subloop cycles. It was found that subloops are always closed and enclosed inside the major loop and that the stress hysteresis of a subloop is only dependent on the strain amplitude of the subloop, regardless of the position of the subloop inside the major loop

Influence of Elastic Energy on the Unloading Behaviour of NiTi Shape Memory Alloys

A non-linear unloading path of deformed NiTi shape memory alloys was observed during mechanical testing at various temperatures, giving a strain recovery larger than what expected from elasticity. Whilst the nonlinear unloading path at above the As temperature may be rationalised by a simultaneous reverse transformation from the oriented martensite to austenite, the non-linear strain recovery during unloading at below the As temperature must have a different mechanism, because the thermodynamic driving force for a reverse transformation is negative. This second mechanism is ascribed to a 'self-deorientation' process of the fully oriented martensite and the driving force for this process is attributed to the elastic energy stored in the matrix during the forward deformation process

Rheology of highly concentrated planar fiber suspensions

The rheology of highly concentrated fibers suspended in power-law fluids is investigated by upscaling the physics at the fiber scale. A deterministic upscaling technique is used, namely the homogenization method for periodic discrete structures. This micro-macro approach is used to carry out a quantitative study of concentrated fiber suspensions with planar fiber orientation, performing "numerical rheometry experiments" on a set of representative elementary volumes of fiber suspensions. The simulations underline the significant influence of the fiber volume fraction and orientation, as well as of the non-Newtonian properties of the suspending fluid on the resulting macroscopic rheological behavior. The predictions of the model are compared with experimental results obtained on an industrial thermoset short fiber-bundle polymer composite (SMC)