Recent studies have shown that conventional meshes comprising pure titanium and its alloys can be used to assist the recovery of bone fractures in various parts of the human body, such as the face, jaw, skull and knee. In anticipation of an improved efficiency for these applications and other applications, this work analyses the thermomechanical behaviour of a type of metallic mesh that is fabricated with NiTi shape memory alloys (SMAs), which are smart metals that exhibit functional properties, such as the shape memory effect (SME) and superelasticity (SE). The development of NiTi SMA meshes with different cell designs, good mechanical strength and recoverable deformation to replace titanium meshes and enhance biomedical applications (as well as applications in other fields) can be considered a current technological challenge. In this framework, this study aims to perform the fabrication and mechanical characterization of NiTi SMA meshes produced by investment casting with three different cell geometries (circular, hexagonal and square) in two states (as cast and heat treated). The obtained results show that the manufactured meshes present functional properties even in the as-cast state, as thermoelastic phase transformation and deformation recovery on the order of 6% is demonstrated. The results show that between the heat treatment and mesh cell geometry, the latter factor is the most influential factor in the mechanical behaviour of the meshes. A brief numerical simulation of the tensile behaviour of the meshes is used to deepen the analysis of the influence of cell geometry on their mechanical behaviour. Overall, as-cast meshes with circular and square cells present a high stiffness under tension and bending. The produced meshes show enough thermomechanical features to enhance biomedical applications. These results support the replacement of conventional titanium alloys, which do not possess functional properties, with NiTi SMAs.

The mechanical behavior of nickel-titanium shape memory alloys (NiTi SMA) presents unique features that benefit the bone fracture healing process: matching elastic modulus ranges, the ability to recover large deformations, and mechanical dissipation capacity. In this sense, this work proposes a new manufacturing process for NiTi SMA bone implants, particularly bone plates: additive manufacturing assisted investment casting (IC). This process delivers near-net NiTi SMA parts with either simple or complex shapes, also allowing a high degree of customization. Four identical NiTi SMA bone plates were manufactured through IC and characterized. Four-point bending tests were performed on the bone plate prototypes to verify their mechanical and fatigue behaviors. The NiTi bone plates have an elastic modulus of the order of 49 GPa at 37 °C and recovered all deformations upon unloading when submitted to four-point bending deflections up to 4 mm. The fatigue life was between 10 ³ and >10 ⁵ , depending on the deflection amplitude. Our next work with this technology will address aspects of cytotoxicity and implantation of the part in animals.

This work evaluates the effect of welding energy and cycling strain amplitude on fatigue behavior of welded joints obtained by micro Tungsten Inert Gas spots in NiTi SMA superelastic wires. For this purpose, a Dynamic Mechanical Analyzer operating in single cantilever bending mode is employed. The equipment monitors the evolution of the force level to maintain constant the strain amplitude as cycling proceeds. Fatigue lives of as-received and welded NiTi wires are compared for different strain amplitudes applied during mechanical cycling. In the evaluated range of strain amplitude, as-received NiTi wires showed fatigue lives between 10 ³ and 10 ⁴ cycles. The fatigue life of the welded joints is lower, although it remains in the same range observed for the as-received wires.

The mechanical loading frequency affects the functional properties of shape memory alloys (SMA). Thus, it is crucial to study its effect for the successful use of these materials in dynamic applications. Based on the superelastic cyclic behavior, this work presents an experimental methodology for the determination of the critical frequency of the self-heating of a NiTi Belleville conical spring. For this, cyclic compressive tests were carried out using a universal testing machine with loading frequencies ranging from 0.5 Hz to 10 Hz. The temperature variation during the cyclic tests was monitored using a micro thermocouple glued to the NiTi Belleville spring. Numerical simulations of the spring under quasi-static loadings were performed to assist the analysis. From the experimental methodology applied to the Belleville spring, a self-heating frequency of 1.7 Hz was identified. The self-heating is caused by the latent heat accumulation generated by successive cycles of stress-induced phase transformation in the material. At 2.0 Hz, an increase of 1.2 °C in the average temperature of the SMA device was verified between 1st and 128th superelastic cycles. At 10 Hz, the average temperature increase reached 7.9 °C and caused a 10% increase in the stiffness and 25% decrease in the viscous damping factor. Finally, predicted results of the force as a function of the loading frequency were obtained.