Foams and other highly porous materials with a cellular structure are known to have many interesting combinations of physical and mechanical properties. In addition, foaming of shape memory alloys (SMA) greatly improves the set of application possibilities. In this work an open-cell metal foam of a CuZnAl SMA is presented. This foam was produced through a recently proposed process, which consists mainly in molten metal infiltration of a bed of silica-gel particles and the subsequent use of hydrofluoric acid (HF) as solvent. The results showed that very regular open-cell foams could be obtained, having an almost spherical cell morphology and relative densities of approximately 0.3. Microstructural and compositional analyses on foamed specimens showed uniform microstructure of ligaments and the absence of SiO 2 interaction with the metal. In this way the possibility of foaming CuZnAl system through the proposed low-cost process was clearly demonstrated.
Abstract. In the present work an experimental-numerical approach is used to study the thermo-mechanical behaviour of NiTi wire for defining what parameters are the most important in actuator designing. Tests were carried out heating, by an electrical current, a wire having a diameter of 150 m, under constant stresses of 200 MPa. Data concerning strain, applied current and voltage are acquired during the tests by PC while wire temperature is recorded by thermographic system. The numerical code integrates the ordinary differential equation describing the volumetric wire heating and cooling transients assuming that the electrical power is the only external heating source. Temperature distribution in the wire is considered uniform and its change in time depends on wire specific heat, latent heat of the Martensitic-Austenitic transformation, free convection heat exchange coefficient and total emissivity of the surface wire. Comparisons between experimental and numerical results, obtained under different operating conditions, indicate that the wire heating rate mainly depends on the applied electrical power, latent heat of the Martensitic-Austenitic transformation, free convection heat exchange coefficient. In the case of the cooling transient, the capability to decrease the wire temperature to reach the ambient value is strongly effected, once the free convection heat exchange coefficient is defined, by the shape of the curve representing the Austenitic-Martensitic transformation. Simulations of the wire behaviour point out that the time necessary to obtain the maximum strain can be reduce to few ms if current of 10 A is used. Moreover, the strain dependence on time can be tailored by choosing the suitable heating transient.
In this article, an advanced laminated composite is developed, combining the high damping properties of shape memory alloy (SMA) with mechanical properties and light weight of a glass-fiber reinforced polymer. The composite is formed by stacking a glass-fiber reinforced epoxy core between two thin patterned strips of SMA alloy, and two further layers of fiber-glass reinforced epoxy. The bars of the laminated composite\ud were assembled and cured in autoclave. The patterning was designed to enhance the interface adhesion between matrix and SMA inserts and optimally exploit the damping capacity of the SMA thin ribbons. The patterned ribbons of the SMA alloy were cut by means of a pulsed fiber laser source. Damping properties at different amplitudes on full scale samples were investigated at room temperature with a universal testing machine through dynamic tension tests, while temperature dependence was investigated by dynamic mechanical analyses (DMA) on smaller samples. Experimental results were used in conjunction with FEM\ud analysis to optimize the geometry of the inserts. Experimental decay tests on the laminated composite have been carried out to identify the adimensional damping value related to their first flexural mode
In this work an experimental-numerical approach was used to analyze the thermo-mechanical behavior of thin NiTi wires, electrically heated, finalized to defining the influence of both wire position and the operating conditions of the actuator functioning. Tests were carried out on wires having diameters of 80 and 150 lm, loaded by constant stresses of 100 and 200 MPa and characterized by DSC and strain/temperature hysteresis measurements. Two wire positions (horizontal and vertical) were adopted in single cycle tests and designed to obtain different typologies of the heating and cooling transients. In general, the heating time was selected to reach a steady state condition while the cooling time always allowed decreasing the wire temperature to the ambient one. Data concerning strain, applied current and voltage were simultaneously acquired during the tests. Moreover, for the optimization and validation of a numerical model, for the 150 lm wire in diameter was used, its temperature was recorded by IR thermographic system. On the basis of the collected experimental data, a simple model was tested to reproduce the experimental results and data regarding the heat exchange coefficient and wire electrical resistivity dependence on temperature were obtained. The influence of the experimental wire positioning and wire diameter on the free convection coefficient is reported and the results indicate that the heating transient is associated with different convection coefficients depending on the heating modalities.
Hybrid textile structures composed by polyamide (PA), Lycra (or Elastane EL), and NiTi thin wires were manufactured. The fabrics were realized by knitting Lycra (EL) as weft and warp filaments and a coupled PA/NiTi yarn through in-laying technique. Superelastic NiTi wire with diameter of 50 lm in both straight annealed and snake-like form and shape memory with a snake-like memorized form were used. The polyamide filament used coupled with the NiTi wire has a diameter around 140 lm, while the textile structure is composed by various Lycra filaments, characterized by different diameters. The textile structures were realized by a circular machine appropriately equipped and used in order to minimize problems related to the use of the thin NiTi alloy filament. To study the influence of NiTi filaments on mechanical properties, specimens taken from textile fabric were analyzed by using a DMA Q800 TA Instruments, equipped with a tension film clamp, in static and dynamic conditions. Force/strain measurements up to 150% in elongation and Tandelta versus frequency were carried out on fabrics with and without NiTi filaments. Finally, some tests strain recovery under load versus temperature are conducted.
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