Analytical Modeling of an Oblique Edge Crack in Rolling Contact Fatigue

Surface cracks represent a frequent cause of damage and even failure in rolling contacts, observed in gears, cams, rails, and so on. In the literature, different approaches have been applied to describe the crack behaviour by means of Fracture Mechanics parameters, such as the stress intensity factors (SIFs) and the J-integral. In this paper, a general procedure for dealing with plane problems is presented, which is based on Linear Elastic Fracture Mechanics hypotheses. It combines the Weight Function Method for evaluating the SIFs in a loading cycle with the Kolosov-Muskhelishvili complex variable approach for estimating the nominal stress field. In this way, a completely analytical procedure can be applied for a general loading condition, assuming that the real geometry can be simplified in a half-plane with an oblique edge crack. As test case, a travelling load has been considered representing a combination of three contributions: Hertzian pressure distribution, traction force due to friction, and pressurization of the crack faces. A comparison with literature results proved that the proposed approach can be an efficient tool for SIFs estimation and crack growth description

Influence of the wear partition factor on wear evolution modelling of sliding surfaces

Wear of engineering components is crucial to assess their performance during all their service life. Numerical wear models are a promising tool, cheaper and quicker than experimental tests, both to investigate wear effects and to compare design solutions. However, frequently, numerical models assume that only one body gets worn or both elements undergo the same volume loss. This study proposes a generalization of the Archard wear law, introducing the concept of wear partition factor  to take into account a different wear behaviour of the rubbing elements of a coupling. The proposed approach is applied to the case of a cylinder sliding over a plane with different stroke amplitudes st. A numerical wear model has been developed in Abaqus®, exploiting the UMESHMOTION routine. Implementation procedures are described and discussed along with the model convergence. Twenty combinations of  and st were simulated covering the cases both of unilateral/bilateral wear and fretting/sliding wear. Results provide important indications on the evolution of wear volumes, wear profiles and contact variables with travelled distance, revealing the remarkable role of . The present study aims to an improved understanding and modelling of sliding wear evolution thus clarifying some critical issues slightly discussed by the literature

Biotribology of artificial hip joints

Hip arthroplasty can be considered one of the major successes of orthopedic surgery, with more than 350000 replacements performed every year in the United States with a constantly increasing rate. The main limitations to the lifespan of these devices are due to tribological aspects, in particular the wear of mating surfaces, which implies a loss of matter and modification of surface geometry. However, wear is a complex phenomenon, also involving lubrication and friction. The present paper deals with the tribological performance of hip implants and is organized in to three main sections. Firstly, the basic elements of tribology are presented, from contact mechanics of ball-in-socket joints to ultra high molecular weight polyethylene wear laws. Some fundamental equations are also reported, with the aim of providing the reader with some simple tools for tribological investigations. In the second section, the focus moves to artificial hip joints, defining materials and geometrical properties and discussing their friction, lubrication and wear characteristics. In particular, the features of different couplings, from metal-on-plastic to metal-on-metal and ceramic-on-ceramic, are discussed as well as the role of the head radius and clearance. How friction, lubrication and wear are interconnected and most of all how they are specific for each loading and kinematic condition is highlighted. Thus, the significant differences in patients and their lifestyles account for the high dispersion of clinical data. Furthermore, such consideration has raised a new discussion on the most suitable in vitro tests for hip implants as simplified gait cycles can be too far from effective implant working conditions. In the third section, the trends of hip implants in the years from 2003 to 2012 provided by the National Joint Registry of England, Wales and Northern Ireland are summarized and commented on in a discussion

A novel approach to the estimation and application of the wear coefficient of metal-on-metal hip implants

A novel approach is proposed to estimate and model the wear of metal-on-metal hip implants. The approach is based on two distinct wear coefficients for the head and cup, derived from separate measurements on the two components. This is in contrast to the usual assumption that a single wear coefficient (k) is valid for both bodies. Actually, the head and cup do not wear equally; thus, assuming equal wear leads to predictive errors. Additionally, in most papers, k is chosen considering only implant materials while neglecting geometry and testing conditions. It is suggested that experimental procedures designed for hip implants should measure the head and cup volume losses separately and that wear maps should be provided to validate numerical models

Irresponsabilità Sociale di Impresa nelle Economie Emergenti

L'elaborato proposto cerca di rispondere a molti quesiti in tema di irresponsabilità sociale. Quest'ultimo è un tema molto dibattuto negli ultimi anni proprio perché difficilmente si riesce a dare una definizione completa ed esaustiva. Nella prima parte viene analizzato il concetto di Irresponsabilità sociale, come è nato, come si è sviluppato, come si misura (anche attraverso alcuni esempi). Nella seconda parte vengono spiegate le metodologie di reperimento delle informazioni e dei dati, al fine di analizzare un campione di 172 imprese che hanno commesso abusi nei paesi in via di sviluppo. Segue una descrizione del database, caso per caso, degli abusi commessi da queste imprese. Nella terza parte viene analizzata l'interazione tra alcune variabili indipendenti, come settore di appartenenza, paese nel quale hanno la sede, se l'impresa è pubblica o privata, e altre variabili dipendenti presenti nel database, come soggetti coinvolti, numero di abusi per azienda, ecc. Per fare ciò si ricorre al metodo ANOVA, il quale ci permette di vedere la significatività delle variabili prese in considerazione e di definire alcune probabili linee giuda

Estimation of Wear Factors of MoM Hip Implants from Simulator Tests

Nowadays wear is recognized as one of the main concern of metal-on-metal (MoM) hip implants, causing osteolysis and the release of dangerous metallic ions. Numerical wear simulations of hip implants are an attractive tool to investigate and predict long-term wear at low cost. A few wear models have been proposed in the literature for MoM bearings [1-3], all based on the Archard wear law, as adhesion and abrasion are considered the main wear mechanisms. The reliability of such models mainly depends on a dimensional wear factor k whose evaluation/choice is actually a critical issue. Indeed k depends on many factors, such as lubrication regime, bearing materials and geometry, loading and kinematic conditions and thus can vary during a wear test/implant lifetime. This complex scenario is simplified in wear simulations which typically assume two constant values of k - equal for head and cup - one higher for the initial running-in phase (kri) and the other lower for the steady state phase (kss), according to experimental observations (Fig. 1). Such k values are generally estimated by matching numerical and experimental wear volumes obtained by hip joint simulator tests [2-3], sometimes even simulating conditions different from the experimental ones [3]. For these reasons and because of a complete neglect of the comparison between numerical and experimental wear maps, the validity of the wear models can be disputable

A Simple Modular Test Rig for Measuring Static and Dynamic Friction

In this paper an apparatus for investigations on both static and dynamic friction is described. The apparatus is simple but very modular and can be used in two different configurations: (i) controlling the displacement of an upper specimen connected by an elastic wire to a moving load cell; (ii) controlling the displacement of a lower specimen connected to a moving plate. The capabilities of the apparatus are shown through some preliminary tests that also highlight the several factors that can affect the friction coefficient estimation

A comparative study of wear laws for soft-on-hard hip implants using a mathematical wear model

Wear of UHMWPE acetabular components is the most serious threat to the long-term success of hip replacements. Consequently numerical and experimental wear simulations are of great interest. The present study proposes a mathematical wear model and compares the most recent wear laws, based on the so-called cross-shear (CS) effect. Simulations highlighted the variability of wear predictions with the wear factors/laws. Moreover a sensitivity analysis underlined that the kinematic conditions affect volumetric/linear wear more than the loading ones. This study confirms the importance of the CS in wear predictions even if some critical issues are still open, requiring further investigations

Fracture Healing Assessment Based on Impact Testing: In Vitro Simulation and Monitoring of the Healing Process of a Tibial Fracture with External Fixator

In clinical practice, bone healing is monitored with X-rays and manipulation. Its assessment is thus subjective, depending on the skills of the operator. Alternative and quantitative approaches have been proposed, generally based on the estimation of bone stiffness, which is known to increase with the healing process. The present study investigates the application of experimental modal analysis to fracture healing assessment focusing on fractures treated with an external fixator. The aim is to ascertain the capability of this approach to detect changes in the bone-callus stiffness as variations in the resonant frequencies despite the presence of the fixator, which might hide the bone response. In vitro tests were performed on a tibia phantom where the healing process was simulated creating three different types of callus surrogates, using glue and resin. The resonant frequencies of the phantom with screwed pins and of the phantom with the complete fixator were estimated. Results confirm an increase in the frequencies as the simulated bone-callus stiffness increases, encouraging the application of experimental modal analysis to fracture healing monitoring. This approach can offer remarkable advantages with respect to the actual standards: being non-invasive and quantitative, it would allow a more frequent healing monitoring

Numerical and experimental investigations for the evaluation of the wear coefficient of reverse total shoulder prostheses

In thepresentstudy,numericalandexperimentalwearinvestigationsonreversetotal shoulder arthroplasties(RTSAs)werecombinedinordertoestimatespecific wear coefficients, currentlynotavailableintheliterature.Awearmodelpreviouslydeveloped by theauthorsformetal-on-plastichipimplantswasadaptedtoRTSAsandappliedina double direction: firstly, toevaluatespecific wearcoefficients forRTSAsfromexperimental results andsecondly,topredictweardistribution.Inbothcases,theArchardwearlaw(AR) and thewearlawofUHMWPE(PE)wereconsidered,assumingfourdifferent k functions. The resultsindicatedthatboththewearlawspredicthigherwearcoefficients forRTSA with respecttohipimplants,particularlytheARlaw,with k values higherthantwofoldthe hip ones.Suchdifferencescansignificantly affectpredictivewearmodelresultsforRTSA, when non-specific wearcoefficients areused.Moreover,thewearmapssimulatedwiththe two lawsaremarkedlydifferent,althoughprovidingthesamewearvolume.Ahigherwear depth (þ51%) isobtainedwiththeARlaw,locatedatthedomeofthecup,whilewiththePE law themostwornregionisclosetotheedge.Takingadvantageofthelineartrendof experimental volumelosses,thewearcoefficientsobtainedwiththeARlawshouldbe valid despitehavingneglectedthegeometryupdateinthemodel