Emanuele Sgambitterra

Temperature dependent fracture properties of shape memory alloys: novel findings and a comprehensive model

Temperature dependent fracture properties of NiTi-based Shape Memory Alloys (SMAs), within the pseudoelastic regime, were analyzed. In particular, the effective Stress Intensity Factor (SIF) was estimated, at different values of the testing temperature, by a fitting of the William’s expansion series, based on Digital Image Correlation (DIC) measurements. It was found that temperature plays an important role on SIF and on critical fast fracture conditions. As a consequence, Linear Elastic Fracture Mechanics (LEFM) approaches are not suitable to predict fracture properties of SMAs, as they do not consider the effects of temperature. On the contrary, good agreements between DIC results and the predictions of an ad-hoc analytical model were observed. In fact, the model takes into account the whole thermo mechanical loading condition, including both mechanical load and temperature. Results revealed that crack tip stress-induced transformations do not represent a toughening effect and this is a completely novel result within the SMA community. Furthremore, it was demonstrated that the analytical model can be actually used to define a temperature independent fracture toughness parameter. Therefore, a new approach is proposed, based on the analytical model, where both mechanical load and temperature are considered as loading parameters in SIF computation.

Modeling and simulation of the thermo-mechanical response of NiTi-based Belleville springs

Nickel–titanium (NiTi) Belleville washers exhibit unique mechanical and functional features due to the reversible stress-induced and/or thermally induced phase transition mechanism of NiTi alloys. In this article, the thermo-mechanical response of such washers has been analyzed by finite element simulations as well as by a special semi-analytical model, which is based on a modified literature approach. The effects of the geometrical configuration of the washers as well as of different thermo-mechanical loading conditions, under stress-induced phase transformation regime, have been analyzed. The results highlighted a marked hysteretic response, in terms of force–deflection curve, due to the hysteresis in the stress–strain behavior of NiTi alloys. In addition, a marked influence of the geometry, as well as of the temperature, has been observed on the thermo-mechanical response of the washer.

Force-Deflection Characteristics of SMA-Based Belleville Springs

The thermo-mechanical properties of Nickel-Titanium based Belleville washers have been analyzed by numerical simulations. In fact, these components exhibit unique mechanical and functional features due to the reversible stress-induced and/or thermally-induced phase transition mechanism of NiTi alloys. The numerical simulations have been carried out by using a commercial finite element software code and a special constitutive model for SMAs. The effects of the geometrical configuration of the washers as well as of the operating temperature, under fully austenitic conditions, have been analyzed. The results highlighted a marked hysteretic response, in terms of force-deflection curve, due to the hysteresis in the stress-strain behavior of NiTi alloys. In addition, a marked influence of the geometry, as well as of the temperature, has been observed on the thermo-mechanical response of the washer, i.e. in terms of both mechanical and functional properties.Copyright

Fatigue assessment of Ti-6Al-4V titanium alloy laser welded joints in absence of filler material by means of full-field techniques

The aim of this research activity was to study the fatigue behavior of laser welded joints of titanium alloy, in which the welding was performed using a laser source and in the absence of filler material, by means of unconventional full field techniques: Digital Image Correlation (DIC), and Infrared Thermography (IRT). The DIC technique allowed evaluating the strain gradients around the welded zone. The IRT technique allowed analyzing the thermal evolution of the welded surface during all the fatigue tests. The fatigue limit estimated using the Thermographic Method corresponds with good approximation to the value obtained from the experimental fatigue tests. The obtained results provided useful information for the development of methods and models to predict the fatigue behavior of welded T-joints in titanium alloy.

A thermo-mechanical model for shape memory alloy–based crank heat engines

A thermo-mechanical model to simulate the mechanical response of a shape memory alloy heat engine has been developed. In such a kind of engine, the thermally induced shape recovery capabilities of shape memory alloys are exploited to obtain mechanical work from low-grade energies, such as warm wastewater, geothermal, and solar sources. As a consequence, these engines represent simple and environmentally friendly solutions, and several architectures have been proposed in the last years. However, none of these devices has been industrialized, mainly due to the lack of robust design tools as well as to several technological and economic issues. To this aim, a simple semi-empirical numerical model has been developed to analyze the mechanical response of a shape memory alloy–based crank heat engine. In particular, the engine output characteristics (e.g. torque, specific power, efficiency) have been studied as a function of several geometrical configurations (e.g. dimensions, number of cranks) as well as of different thermo-physical properties of the heating and cooling thermal sources.

A Thermo-Mechanical Model for SMA-Based Crank Heat Engines

Shape memory alloys (SMA) provide unique functional features and several SMA-based heat engines have been proposed in recent years, exploiting the thermally induced recovery capabilities of such class of materials. These engines represent simple and environmentally-friendly solutions to obtain mechanical energy from low-grade energy sources, such as warm wastewater, geothermal and solar sources. However, despite the scientific interest in last years, no commercial devices have been developed. One of the reasons of these unsuccessful commercial development is the lack of robust design tools. In the proposed work a thermo-mechanical model has been developed, which describes the mechanical response of a SMA-based crank heat engine, as a function of several geometrical configurations, such as the dimension and total number of cranks, as well as for different thermo-physical properties of the heating and cooling thermal sources. In particular, the engine is made of two parallel crankshafts and uses nickel-titanium helical springs operating between warm water and cold air. The model has been developed within the MatLab® Simulink software platform, and several simulations have been carried out to analyze the mechanical response of the engine, in terms of output characteristics (torque, specific power, thermodynamic efficiency) as a function of the aforementioned geometrical and physical parameters.

Fracture mechanics of pseudoelastic NiTi alloys: review of the research activities carried out at University of Calabria

This paper reports a brief review of the research activities on fracture mechanics of nickel-titanium based shape memory alloys carried out at University of Calabria. In fact, this class of metallic alloys show a unusual fracture response due to the reversible stress-induced and thermally phase transition mechanisms occurring in the crack tip region as a consequence of the highly localized stresses. The paper illustrates the main results concerning numerical, analytical and experimental research activities carried out by using commercial NiTi based pseudoelastic alloys. Furthermore, the effect of several thermo-mechanical loading conditions on the fracture properties of NiTi alloys are illustrated.