Franco Furgiuele

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.

A full-field approach for the elastic characterization of anisotropic materials

In the paper a procedure is described whereby the characterization of isotropic and anisotropic laminae is carried out by full-field measurement of the surface rotations under proper flexural loads. The analytic formulation does not necessarily imply a numerical calibration and the elastic constants are determined by integrating the whole experimental data on the surface of the specimen. Measurements were carried out by phase-stepped speckle interferometry using a shearometer based on Michelson design. The experimental instrumentation, including the loading device, and the procedures for the manipulation of experimental data are detailedly described; experimental results obtained on a steel specimen and a composite laminate are also reported.

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.

Influence of Laser Surface Modification on Bonding Strength of Al/Mg Adhesive Joints

Key properties of magnesium alloys, such as the high strength-to-density ratio, are driving the production of lightweight structural components in the automotive and aeronautical industries. Many efforts have been carried out on various aspects of processing and fabrication, but the joining of Mg alloys to dissimilar materials is a subject which attracted much research interest in the last decades. In the present work a preliminary investigation on the strength of Al/Mg (AA6082/AZ31B) single-lap epoxy bonded joints was carried out. To this aim, Mg and Al substrates were laser irradiated using a pulsed ytterbium fiber laser. For comparison, and in order to estimate the beneficial action of the laser surface treatment, single lap joints with grit-blasted substrates were prepared and tested. The interaction between laser treated surfaces and two different epoxy adhesives was also analyzed. Finally, the results and discussion were supported by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) executed on treated and post-failure sample surfaces.

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.

Surface Patterning of Metal Substrates Through Low Power Laser Ablation for Enhanced Adhesive Bonding

The aim of the present work is to study the effect of laser-induced surface patterning on the strength of adhesive joints. Laser ablation was carried out on aluminum alloy (AA6082-T4) and stainless steel (AISI 304) substrates using a pulsed ytterbium fiber laser (LaserPoint YFL 20P). The morphological and elemental modifications induced by the laser ablation were analyzed by means of surface profilometry, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). In addition, surface wettability was analyzed by means of the sessile drop technique. In order to assess the capability of laser ablation to improve mechanical interlocking, cross-sectional areas of the samples taken across the interfacial region were probed under an optical microscope. Finally, mechanical tests were carried out on single-lap shear joints and the fracture surfaces were observed using the SEM. The obtained results showed that low power laser ablation can increase the strength of adhesive joints.

Analysis of Fracture in Aluminum Joints Bonded with a Bi-Component Epoxy Adhesive

Adhesive bonding is a viable alternative to traditional joining systems (e.g., riveting or welding) for a wide class of components belonging to electronic, automotive, and aerospace industries. However, adhesive joints often contain flaws; therefore, the development of such technology requires reliable knowledge of the corresponding fracture properties. In the present paper, the candidate mode I fracture toughness of aluminum/epoxy joints is determined using a double cantilever beam fracture specimen. A proper data reduction scheme for fracture energy calculation has been selected based on the results of a sensitivity analysis. Furthermore, a scanning electron microscope is used in order to explore the locus of failure. Finally, the experimental findings are assessed by means of numerical simulations of crack growth carried out using a cohesive zone model.

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.