Gruppo Italiano Frattura (IGF)
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Contact between the components of a knee prosthesis: numerical and experimental study
No full textThe aim of this work is the analysis of the contact area in a knee prosthesis using two different approaches. In particular, the interface between the femoral component and the polyethylene insert has been studied both numerically and experimentally. The interest in studying the contact area is related to the fact that the wear of the polyethylene insert, due to the high contact pressures, represents one of the major causes of failure of the total knee prosthesis. The possibility to evaluate the contact area at different loads and mutual position between femur and tibia is, therefore, of fundamental importance to study the service life of a prosthesis and to improve its performance. The finite element numerical approach has required the acquisition, through reverse engineering, and CAD modelling of the prosthetic components. Then the FEM simulations have been developed considering two different load conditions. In order to compare the calculated data, the same load configurations have been used for experimental tests based on ultrasonic method. In this case, some preliminary tests were required to calibrate the system depending on the particular characteristics of materials, geometries and surface finish of the prosthesis.The results show a good correlation between the data obtained with the two different approaches and, consequently, a good level of reliability of the procedures developed for the numerical and experimental evaluation of the contact area. The numerical procedure can be used to determine the area for different angles and loads, but especially in the design phase. The ultrasonic technique can be used to validate the numerical data
Application of martensitic SMA alloys as passive dampers of GFRP laminated composites
No full textThis paper describes the application of SMA (Shape Memory Alloy) materials to enhance thepassive damping of GFRP (Glass Fiber Reinforced Plastic) laminated composite. The SMA has been embeddedas reinforcement in the GFRP laminated composite and a SMA/GFRP hybrid composite has been obtained.Two SMA alloys have been studied as reinforcement and characterized by thermo-mechanical tests. Thearchitecture of the hybrid composite has been numerically optimized in order to enhance the structural dampingof the host GFRP laminated, without significant changes of the specific weight and of the flexural stiffness. Thedesign and the resultant high damping material are interesting and will be useful in general for applicationsrelated to passive damping. The application to a new designed lateral horn of railway collector of the Italianhigh speed trains is discussed
Design and characterization of a fractal-inspired multi-frequency piezoelectric energy converter
No full textA promising harvesting technique, in terms of simplicity and efficiency, is the conversion ofambient kinetic energy through piezoelectric materials. This work aims to design and investigate a piezoelectricconverter conform to a fractal-inspired, multi-frequency structure previously presented by the author. Aphysical prototype of the converter is built and experimentally examined, up to 120 Hz, in terms of modalresponse and power output. Three eigenfrequencies are registered and the power output is particularly good atthe fundamental eigenfrequency. Also the effect of the resistive load applied to the converter is investigated
Compliant actuation based on dielectric elastomers for a force-feedback device: modeling and experimental evaluation
No full textThanks to their large power densities, low costs and shock-insensitivity, Dielectric Elastomers (DE)seem to be a promising technology for the implementation of light and compact force-feedback devices such as,for instance, haptic interfaces. Nonetheless, the development of these kinds of DE-based systems is not trivialowing to the relevant dissipative phenomena that affect the DE when subjected to rapidly changingdeformations. In this context, the present paper addresses the development of a force feedback controller foran agonist-antagonist linear actuator composed of a couple of conically-shaped DE films and a compliantmechanism behaving as a negative-rate bias spring. The actuator is firstly modeled accounting for the viscohyperelasticnature of the DE material. The model is then linearized and employed for the design of a forcecontroller. The controller employs a position sensor, which determines the actuator configuration, and a forcesensor, which measures the interaction force that the actuator exchanges with the environment. In addition, anoptimum full-state observer is also implemented, which enables both accurate estimation of the time-dependentbehavior of the elastomeric material and adequate suppression of the sensor measurement noise. Preliminaryexperimental results are provided to validate the proposed actuator-controller architectur
Infrared thermography study of the fatigue crack propagation
No full textThe work is devoted to the experimental study of heat dissipation process caused by fatigue crack propagation. To investigate a spatial and time temperature evolution at the crack tip set of experiments was carried out using specimens with pre-grown centered fatigue crack. An original mathematical algorithm for experimental data treatment was developed to obtain a power of heat source caused by plastic deformation at crack tip. The algorithm includes spatial-time filtration and relative motion compensation procedures. Based on the results of mathematical data treatment, we proposed a way to estimate the values of J-integral and stress intensity factor for cracks with pronounced the plastic zone
Functional fatigue of NiTi Shape Memory wires for a range of end loadings and constraints
No full textThe availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermalactivation (functional fatigue) is central to the rational design of smart actuators based on these materials. Testresults on SMAs under functional fatigue are scarce in the technical literature and the few data available aremainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased byelastic springs or by another SMA element, their stress state is far from constant in operation. The mismatchbetween actual working conditions and laboratory arrangements leads to suboptimal designs and underminesthe prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Fourtest procedures are planned, covering most of the typical situations occurring in practice: constant-stress,constant-strain, constant-stress with limited maximum strain and linear stress-strain variation with limitedmaximum strain. The paper describes the experimental apparatus specifically designed to implement the fourloading conditions and presents fatigue results obtained from commercial NiTi wires tested under all thoseprotocols
Wear surface damage of a Stainless Steel EN 3358 aeronautical component subjected to sliding
No full textThe present paper describes the failure analysis of an aircraft component subjected to severalepisodes of in service failure, resulted in loss of the aircraft safety. Modern aircrafts are provided withmechanical systems which have the task to open not pressurized hatches during landing. The components ofsuch systems are subject to considerable mechanical stresses in harsh environment (presence of moisture andpollutants, significant and sudden temperature variations). The system is constituted by a sliding piston, arelated nipple and by a locking system consisting of 4 steel spheres which are forced into a countersinkmachined on the piston when the hatches is open. The whole system is activated by a preloaded spring. Themachined parts, nipple and piston, are made of EN3358 steel (X3CrNiMo13-8-2), a precipitation hardeningstainless steel with very low content of carbon often used in the aerospace. The samples provided by themanufacturer present different types of damage all referable to phenomena relative to the sliding of the pistoninside the nipple. The present paper describes the different damage observed and the microstructure of thematerial, then are reported the results obtained from the characterization of the material of the samples bymeans of optical and electronic microscopy, carried out to define the mechanisms involved in the systemseizure. In order to define the primary cause of failure and to propose solutions to be adopted, also analyzingthe criticality of using this PH stainless steel for this application, the results of different tests were comparedwith system design and working data
Interference fit effect on holed single plates loaded with tension-tension stresses
No full textThis paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numericalinvestigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation witha commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion
A numerical approach for the analysis of deformable journal bearings
No full textThis paper presents a numerical approach for the analysis of hydrodynamic radial journal bearings. The effect of shaft and housing elastic deformation on pressure distribution within oil film is investigated. An iterative algorithm that couples Reynolds equation with a plane finite elements structural model is solved. Temperature and pressure effects on viscosity are also included with the Vogel-Barus model. The deformed lubrication gap and the overall stress state were calculated. Numerical results are presented with reference to atypical journal bearing configuration at two different inlet oil temperatures. Obtained results show the great influence of elastic deformation of bearing components on oil pressure distribution, compared with results for ideally rigid components obtained by Raimondi and Boyd solution
Fracture mechanics of pseudoelastic NiTi alloys: review of the research activities carried out at University of Calabria
No full textThis paper reports a brief review of the research activities on fracture mechanics of nickel-titaniumbased shape memory alloys carried out at University of Calabria. In fact, this class of metallic alloys show aunusual fracture response due to the reversible stress-induced and thermally phase transition mechanismsoccurring in the crack tip region as a consequence of the highly localized stresses. The paper illustrates the mainresults concerning numerical, analytical and experimental research activities carried out by using commercialNiTi based pseudoelastic alloys. Furthermore, the effect of several thermo-mechanical loading conditions onthe fracture properties of NiTi alloys are illustrated