Carmine Maletta

Fatigue of pseudoelastic NiTi within the stress-induced transformation regime: a modified Coffin–Manson approach

Strain controlled fatigue tests of a pseudoelastic nickel?titanium (NiTi) shape memory alloy (SMA) have been carried out in this investigation. In particular, flat dog-bone shaped specimens, obtained from commercial NiTi sheets, have been analyzed, under pull?pull loading conditions, in two subsequent steps: (i)?material stabilization and (ii)?fatigue life estimation. The first step was carried out to obtain a stable pseudoelastic response of the SMA, i.e. with no residual deformations upon unloading, and it can be regarded as a preliminary processing condition of the alloy. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, a modified Coffin?Manson approach for fatigue life estimation of SMAs is proposed, which takes into account the strain mechanisms involved during repeated stress-induced martensitic transformations.

Fracture Behaviour of Nickel-Titanium Laser Welded Joints

In this study, the effects of Nd:YAG laser welding on the fracture behavior of Ni-rich nickel-titanium sheets are analyzed by experimental investigations. The welding was carried out in open air conditions by using a special shielding/clamping system to avoid the chemical contamination of the molten zone and the formation of hot cracks. Mechanical tests of standard dog bone-shaped and single edge crack specimens were carried out to measure the stress-strain response and the fracture resistance of both the base and the welded materials. Furthermore, scanning electron microscopy observations of the fracture surfaces were carried out in order to better understand the failure mechanisms. Finally, systematic comparative studies between base and laser-welded materials were carried out.

Cohesive Zone Modeling of Mode I Fracture in Adhesive Bonded Joints

This paper deals with the application of Cohesive Zone Model (CZM) concepts to study mode I fracture in adhesive bonded joints. In particular, an intrinsic piece-wise linear cohesive surface relation is used in order to model fracture in a pre-cracked bonded Double Cantilever Beam (DCB) specimen. Finite element implementation of the CZM is accomplished by means of the user element (UEL) feature available in the FE commercial code ABAQUS. The sensitivity of the cohesive zone parameters (i.e. fracture strength and critical energy release rate) in predicting the overall mechanical response is first examined; subsequently, cohesive parameters are tuned comparing numerical simulations of the load-displacement curve with experimental results retrieved from literature.

Stress-Induced Martensitic Transformation in the Crack Tip Region of a NiTi Alloy

The evolution of stress-induced martensitic transformation in front of the crack tip in a NiTi alloy is analyzed in this investigation, by two-dimensional finite element simulations of single edge-crack specimens. In particular, the transformation start and finish contours, i.e., the boundaries of the transformation zone, were obtained by using plasticity concepts, and the effects of the temperature were taken into account by using the Clausius-Clapeyron relation. Furthermore, comparisons between numerical and analytical results, obtained by Irwin’s modified linear elastic fracture mechanics relations, were carried out. These comparisons show that a good agreement in terms of the martensite start and finish sizes is obtained; moreover, the analytic approach could be able to describe the stress field in the crack tip region outside the phase transformation zone, i.e., in the austenitic region, but a proper equation to estimate the effective crack length should be found. To this aim, further studies should be carried out.

A novel fracture mechanics approach for shape memory alloys with trilinear stress–strain behavior

The high values of local stresses arising near the crack tip in Nickel–Titanium based shape memory alloys (SMAs) causes a stress-induced martensitic transformation. Previous studies have demonstrated that this micro-structural evolution plays a significant role in the fracture and fatigue properties of SMAs, as it significantly changes the crack tip stress distribution with respect to common engineering metals. In this investigation a novel analytical model is proposed, which is based on the small scale yielding condition and on modified elastic-plastic fracture mechanics concepts, to predict the extent of the crack tip transformation region together with the resulting stress distribution. In particular, based on the general framework of a recent literature approach, a new model has been developed in order to overcome one of the major limitation of the previous one, i.e. the assumption of constant stress during phase transformation. In fact, the proposed approach uses a trilinear stress–strain constitutive behavior, i.e. it is able to analyze SMAs with not constant transformation stress. The model has been applied to some case studies, in order to analyze the effects of several thermo-mechanical parameters and loading conditions on the crack tip transformation region and stress distribution.

Three-Point Bending Tests of Zirconia Core/Veneer Ceramics for Dental Restorations

Introduction. The mechanical strength and the surface hardness of commercially available yttrium-doped zirconia were investigated. Furthermore, a comparative study of eight different ceramic veneers, to be used for the production of two-layered all-ceramic restorative systems, was carried out. Materials and Methods. Four types of zirconia specimens were analyzed, according to a standard ISO procedure (ISO 6872). Besides, two-layered zirconia-veneer specimens were prepared for three-point bending tests. Results. A strong effect of the surface roughness on the mechanical strength of zirconia specimens was observed. Finally, a comparative study of eight commercially available veneering ceramics shows different modes of failure between the selected veneers. Conclusion. The results indicate that close attention should be paid to the preparation of zirconia-based crowns and bridges by CAD/CAM process, because surface roughness has an important effect on the mechanical strength of the material. Finally, the results of the mechanical tests on two-layered specimens represent an important support to the choice of the veneering ceramic.

A phenomenological model for superelasticity in NiTi alloys

A phenomenological model to simulate the superelastic effect in nickel–titanium (NiTi) alloys is proposed. The model is able to simulate the stress–strain hysteretic behaviour of the material in a phenomenological way by using the Prandtl–Ishlinskii operator. The parameters of the phenomenological model are identified by simple and efficient numerical procedures, from a set of experimental measurements. The model was developed in the commercial software package Simulink® and is able to simulate complete and incomplete stress-induced martensitic transformations, as well as the effects of temperature on the stress–strain hysteretic behaviour. Systematic comparisons between experimental measurements, carried out for several stress paths, and numerical predictions are presented. The results show good accuracy and the model needs little computational time, which allows its use in real-time applications.

NiTi Belleville washers: Design, manufacturing and testing

The thermomechanical properties of nickel–titanium-based Belleville washers have been analyzed in this investigation, together with their unusual mechanical and functional features, which can be attributed to the reversible phase transformation mechanisms of nickel–titanium alloys. In particular, numerical simulations have been carried out for a preliminary design of the Belleville washer, using a commercial finite element software and a special constitutive model for shape memory alloys. Subsequently, Belleville washers have been manufactured from a commercial pseudoelastic nickel–titanium alloy, by disk cutting and a successive shape setting by a thermomechanical treatment. Finally, the thermomechanical response of the washers, in terms of isothermal force–deflection curve and thermal cycles between phase transition temperatures, has been experimentally analyzed. The results highlighted a marked effect of the temperature on the characteristic curve, as well as good recovery capabilities under both mechanical and thermal cycles. In addition, nickel–titanium Belleville washers exhibit a marked hysteretic behavior, as a consequence of the hysteresis in the stress–strain response of the alloy. Thanks to these features, nickel–titanium Belleville washers can be used as smart elastic elements, that is, with tunable stiffness and damping properties, as well as solid-state actuators, due to their recovery capabilities.

Two-way shape memory effect of a Ti rich NiTi alloy: experimental measurements and numerical simulations

In this paper the two-way shape memory effect (TWSME) of a Ni?51?at.% Ti alloy is investigated and a numerical model is developed, which allows real time simulations of its hysteretic behaviour strain versus temperature. The two-way shape memory effect (TWSME) was induced through a proper thermo-mechanical training, carried out at an increasing number of training cycles and for two values of training deformation. The TWSME was measured under different applied stresses and the hysteretic behaviour in the strain?temperature response was recorded. In order to evaluate the thermal stability of the hysteresis loops the material was subjected to many cycles, by repeated heating and cooling, between Af (austenite finish temperature) and Mf (martensite finish temperature). The numerical method is based on a phenomenological approach and was developed in a Matlab? function, which calculates the model parameters from a set of experimental data, and a Simulink? model, which is efficient enough for use in real time applications. The accuracy of the proposed model was analysed through systematic comparisons between experimental measurements and numerical predictions.

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.