A multi-functional (sensing -actuating) greenhouse ventilation window heated/cooled naturally by convection was designed to overcome different industry challenges in terms of designing smart applications. This ventilation window design includes a three-pulley system to reduce the load on the NiTi actuator and enhance its long-life time. In addition, using the NiTi actuator allows energy saving due to natural phase transformation induction (i.e. convection) and high force generation compared to the small NiTi wire mass. Structural analysis was used to determine the force generated in the “C-shaped” NiTi wire after loading. Transient thermal and structural analysis also was used to investigate the strain rate effects on the shape memory response of “C shaped” NiTi alloy element under different thermomechanical loadings and boundary conditions. Two types of loading have been applied isothermally or at adiabatic conditions. The results showed a significant effect of the high loading rates on increasing the stress plateau which is caused by the corresponding shift in the transformation temperatures. As a result, it could be expected that the actuator life time could be reduced when a rapid, as opposed to a slow loading rate, is adopted. In addition, the dynamic loading of the NiTi leads to a decrease of the recoverable strain. Experimental work was done to validate the simulation model by testing a commercial NiTi sample dynamically and compare the macroscopic displacement during mechanical loading and the strain recovery process.

Alloy NiTi with nearequiatomic composition of nickel and titan belongs to a group of metal materials with a shape memory effect (Shape memory alloys). NiTi alloys are a guite attractive materials not only as practical shape memory alloys with hight strenght and ductility but also as those exhibiting unique physical properties. The production of these matrerials is complying with chemical composition. Final charakteristics of alloy are influenced by these bounderies and also by mechanical-heat treatment. This work deal with DSC evaluation of homogenity of ingot structure of NiTi alloy containing 50,8 at.% Ni. The alloy was melted in Y2O3 cricible. In the end the results of DSC method are compared to the microstructure of alloy obtained by SEM and TEM methods.

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Thesis (M.Sc. (Physics)) -- University of Limpopo, 2021 / First-principles density functional theory has been used to study the stabilities of binary TiPt, TiTa, TiNi and TiCo potential shape memory alloys. Furthermore, ternary alloys Ti50Pt50-xMx with V, Ta, Hf and quaternary Ti50(PtCo)50-xTax systems were also investigated. The structural, electronic and mechanical properties were deduced to mimic the stabilities of these alloys. Furthermore, their vibrational stability, x-ray diffraction and temperature dependence have been examined. The structures were subjected to full geometry optimization to obtain equilibrium lattice constants. It was found that the equilibrium lattice parameters for all the binary systems are in good agreement with experimental results to within 5%. The heats of formation (ΔHf) were calculated to determine the thermodynamic stability of the B2 TiM systems. It was revealed that TiPt is the most energetically favourable (most stable) whereas TiTa is the least favourable due to high ΔHf value (less stable). In addition, electronic properties suggest that TiPt, TiNi and TiCo systems are stable with TiTa being the least favourable consistent with the ΔHf. The elastic properties were also calculated to mimic the mechanical stability of these alloys. TiNi, TiCo and TiTa were found to be mechanical stable whereas TiPt is unstable. This behaviour is consistent with the phonon dispersion curves for TiPt and TiCo. TiCo structure, in particular is the most stable in line with the predicted phonon dispersion. The effect of alloying on Ti50Pt50-xMx (M = V, Ta, Hf) ternary system was carried out using the supercell approach. It was observed that the lattice parameters decrease minimally with an increase in V and increases with an increase in Ta and Hf content. The structures ii become thermodynamically less stable with an increase in V, Ta and Hf content, as depicted by heats of formation. The shear modulus (C′) of Ti50Pt50-xMx increases with an increase in M (V, Ta and Hf) concentration suggesting mechanical stability of these alloys. This has been confirmed from the phonon curves where the phonon soft modes are reduced and tend to disappear with increasing content of the alloying elements. Thus the results suggest that the V, Ta and Hf addition reduces the transformation temperatures of the TiPt alloy as indicated by its higher shear modulus C′. Furthermore, it was observed that the lattice parameters of the quaternary system decrease with an increase in Ta content in the system. Thus ΔHf of the B2 and B19 Ti50Pt43.75-xCo6.25Tax and B19 Ti50Pt31.25-xCo18.75Tax alloy system showed that the 6.25 at.% Ta addition is energetically most favourable (ΔHf<0). The DOS behaviour confirms that the 6.25 at.% Ta as least favourable whereas for B19, the 6.25 at.% Ta is most favourable. The elastic constants for B19 and B2 show the positive shear modulus (mechanical stability). Moreover, the phonon dispersions and phonon density of states for the B2 and B19 Ti50Pt43.75-xCo6.25Tax and Ti50Pt31.25-xCo18.75Tax were calculated and are consistent with the elastic constant. The LAMMPS code was employed to investigate the temperature dependence of the B19 Ti50Pt43.75-xCo6.25Tax and Ti50Pt31.25-xCo18.75Tax structures. The martensitic to austenite transformation temperature decreases with an increase in Ta concentration. Temperature variations of the XRD patterns for the B19 are in reasonable agreement with predicted lattice parameters. / National Research Foundation (NRF) and Titanium centre of competence (TiCoC)

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Concrete is considerably stronger in compression than it is in tension. When cracks form in concrete members, the flexural stiffness of the member decreases and the deflection increases which increases the overall size of the member. Prestressing concrete remedies this problem by inducing a compressive stress in the concrete thereby reducing the net tension in the member and increasing the load required to crack the member. Traditional prestressing is generally limited to large, straight members. During the last decade, shape memory alloys (SMA) have become more prevalent in engineering and civil engineering applications. The shape memory effect refers to the contraction of the SMA when it is heated to its austenite phase. When a prestrain is induced in the SMA, it can be recovered when it goes through the phase change. Nitinol, a NiTi shape memory alloy was used in this research. Thin, steel cables were also tested to provide a comparison. Two different Nitinol alloys were studied in this research. The alloy M wires were elongated to 8% stain while the alloy X wires were prestrained by the manufacturer. The wires were cast into thin concrete beams and once cured, the beams were heated and a phase change from martensite to austenite occurred in the Nitinol. As a result, the Nitinol contracted and compressed the concrete. The SMA fibers are randomly oriented and allow prestressing to occur along all three axis. This is ideal for thin, curved specimens. Third-point bending tests showed that the SMA fibers prestressed the concrete and upon reheating the cracked specimens, the shape memory effect provides partial crack closure.

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