Influencia de la geometría en los máximos de las tensiones en ajustes por interferencia con agujeros ranurados

Memoria D. Samuel Sol rzano Barruso” (USAL, Grant FS/21-2012 and Grant 463AC06).Elastic stress distributions in interference fits match the values predicted by pressure cylinders theory only if the two joined parts have the same length. In fact, a heavy stress concentration in the radial component appears at the edges of the contact zone being more intense in the case of the hub. Many methods have been proposed for relieving the stress peaks. In order to investigate how the stress concentrations are reduced, in this paper, different stress analysis in interference fits with grooved hubs were carried out using the finite element method.The authors wish to acknowledge the financial support provided by the Spanish Institution “Memoria D. Samuel Sol rzano Barruso” (USAL, Grant FS/21-2012 and Grant 463AC06)

Wear reduction in modular taper-trunnion hip prostheses using straight cylindrical stems

Evidence suggests that fretting wear at the taper-trunnion (head-stem) junction of modular metallic total hip replacement (THR) implants contributes to high rates of metallosis and reduced durability. Currently in THR, tapered stems are employed which are press-fitted by impaction into the ball-head. Any pull-out movement between the two components will relieve the pressure between the components rendering them loose and susceptible to fretting. To ameliorate this problem, this thesis proposes a new design, straight cylindrical stems interference fitted into the ball head. In an interference joint, the pressure between the components does not relax due to gross outward movement, thus maintaining resistance to fretting. The proposed hypothesis is investigated by numerical simulation of wear due to normal walking activity using 3D finite element (FE) models of tapered and straight stem taper trunnion joints. The forces required to separate the components of taper-trunnion joints were determined by experimental testing of tapered stems with different impaction forces and straight stem joints fitted with different interference fits. The tests show that the pull-out forces for straight stem joints are significantly higher even at low levels of interference. Comparison of the pull-out test results with FE pull-out simulations was used to estimate the friction between the components, for use in the wear assessment simulations. Finite element analysis (FEA) of taper-trunnion joints with straight stems with different interference fits is performed to determine the relationship for volumetric wear under normal gait loading and the interference fit values. These are compared to volumetric wear estimated by FEA for tapered stem joints with different impaction forces. The results show that even with small values of interference fit, the wear in the proposed straight stem joints is significantly lower. Hence, interference fits eliminate the sliding motion between the modular parts thereby reducing the fretting wear. Therefore, this thesis recommends further research towards the feasibility and strategies for the implementation of the interference fits in modular hip implants in the operating theatre institu. The implementation of the proposed new design with straight stem taper-trunnion joints in THR can significantly reduce wear leading to increased durability of the prosthetics and reduced metallosis in patients thereby returning active life to all patients needing hip arthroplasty

Optimisation des assemblages par interférence soumis à des sollicitations de fatigue.

RÉSUMÉ Ce mémoire présente une méthode d’optimisation des assemblages par interférence soumis à des sollicitations de fatigue. Le phénomène de fatigue par fretting réduit la vie en fatigue des assemblages par interférences à l’interface de contact. Il existe toutefois des configurations géométriques qui atténuent ce phénomène. De plus, des résultats expérimentaux montrent que l’ajout d’une rainure circulaire au bord du contact permet de diminuer considérablement l’effet de la fatigue par fretting. Par contre, cet ajout introduit une importante concentration de contrainte dans l’arbre et peut provoquer la rupture par fatigue normale. Les essais expérimentaux montrent qu’il existe une zone de transition dans la localisation de la rupture à l’interface de contact et dans la rainure. Toutefois, ils démontrent également qu’il existe un compromis de paramètres, favorable à la vie en fatigue de l’assemblage. Une méthodologie de calcul par éléments finis des assemblages par interférences sollicités en fatigue est présentée dans la revue de littérature. En comparant les résultats obtenus par la méthode des éléments finis et les essais expérimentaux, il a été possible d’établir l’existence d’un facteur de réduction de vie en fatigue causée par le fretting (kfret). Bien qu’il soit possible de simuler avec précisions la vie en fatigue de l’assemblage, il n’existe pas de méthode qui permet d’obtenir directement la valeur des paramètres selon des critères de conception spécifiques. La plastification du matériau et la présence d’éléments de contact nécessitent une modélisation par éléments finis non linéaires. Par conséquent, son temps de calcul est considérablement élevé. Le couplage entre le modèle d’éléments finis et un algorithme d’optimisation devient irréaliste compte tenu du nombre de simulations nécessaires. La méthode d’optimisation doit donc être conçue en utilisant un nombre restreint de simulations. Afin de répondre à cette problématique, un algorithme d’optimisation séquentielle multi-objectifs (OSMO) a été développé. Cette méthode consiste à optimiser une fonction d’interpolation d’un plan d’expériences. La totalité de l’algorithme a été réalisée à partir des modules Hypermath, Hyperstudy et Sampling du logiciel Altair Hyperworks V10. La méthode du Krigeage a été utilisée pour interpoler le plan d’expériences. Le plan utilisé est le Latin Hypercube Optimal.----------ABSTRACT This thesis presents a methodology for designing interference fit assemblies subjected to fatigue load. Due to the assembly, a fretting-fatigue phenomenon was found responsible of reduction of the fatigue life inside the contact. However, there are geometric configurations that contribute to reduce its impact on the life of the assembly. Experimental tests have shown that the addition of a fillet, circular groove or hub overhang improves the fatigue life of interference fit assemblies. It was also determinate that in contact, failure is caused by fretting fatigue while it is caused by normal fatigue in the fillet or groove. As a result of the competing maximum equivalent stresses in contact and in the fillet, a shift in the location of fatigue failure has been observed in both experimental tests and finite elements simulation. A methodology for fatigue strength calculation of interference fit assemblies subjected to fatigue load is presented. A comparison between calculations by finite element analysis and experimental tests has show the existence of fatigue strength reduction factor (kfret). While it’s possible to calculate the fatigue strength with precision, there is actually no method to directly obtain the parameters values according to design criteria. Plastic deformation and contact element needs non-linear calculations of finite element model. Therefore, obtaining the optimal solution with a classic procedure may be time consuming, hence inefficient. In order to address this problem, a Sequential Approximate Multi-Objective Optimization (SAMOO) algorithm was developed in this work. This method consists of optimizing an interpolation function from design of experiments. Entire algorithm was programmed from modules Hypermath, HyperStudy Sampling of Altair HyperWorks V11 suite. The kriging method was used to interpolate the Optimal Latin Hypercube design of experiments. The multi-objective objective genetic algorithm (MOGA) from Hyperstudy is used to generate the Pareto Front of the problem

Modelling and Experimental Analysis of Fretting Fatigue in Complete and Bolted Contacts

Fretting is the micrometer-level relative movement between contacting surfaces that can lead to fretting fatigue and fretting wear in practical connections in machine components. Fretting has a clear tendency to nucleate cracks that can continue to grow as a result of the cyclic loads in a component, leading to premature failure. The lack of fundamental knowledge of the fretting mechanism and a universal model poses challenges for the design of modern machine components having contacts under high loading.This thesis comprises seven publications. Its purpose is to study the effect of various design parameters on the fretting fatigue behavior in practical connections and also to apply the Digital Image Correlation method to fretting contact in order to measure displacement fields. Both experimental and modelling methods were employed to study complete and bolted contacts. The material used was self-mated quenched and tempered steel. A complete contact fretting test device was developed that had a large contact area and transverse loading resembling practical connections. Numerous fatigue tests were carried out. The Finite Element Method was used to analyze the contacts in greater detail.Fretting significantly decreased the fatigue life in complete and bolted contacts. Increasing contact normal load decreased life in both types of contact while rounding of the sharp contact edge did not extend fatigue life. Fatigue life decreased when the amplitude of cyclic loading of the test specimen was increased. In complete contact tests, the cracking point was at the contact edge whereas in the bolted joints the area of fretting damage and cracking point was away from the geometric stress concentration (bolt hole) and corresponded to the distribution of frictional energy dissipation. Shot peening and nitriding were particularly effective in increasing fatigue life in sharp ended contacts. Cyclic relative displacement fields close to the contact interface were measured using the Digital Image Correlation method so that bulk compliances were minimized. These displacement measurements were successfully compared with numerical results. The modelling results agreed with the experiments in terms of cracking prediction and contact quantities

Aspects of fretting fatigue finite element modelling

Fretting fatigue is a type of failure that may affect various mechanical components, such as bolted or dovetail joints, press-fitted shafts, couplings, and ropes. Due to its importance, many researchers have carried out experimental tests and analytical and numerical modelling, so that the phenomena that govern the failure process can be understood or appropriately modelled. Consequently, the performance of systems subjected to fretting fatigue can be predicted and improved. This paper discusses different aspects related to the finite element modelling of fretting fatigue. It presents common experimental configurations and the analytical solutions for cylindrical contact. Then, it discusses aspects of fretting fatigue crack initiation, such as crack location, orientation, and length, as well as stress averaging approaches. Then, it deals with the propagation stage; crack face interaction, orientation criteria, and crack growth rate are discussed. Lastly, additional aspects of recent research on fretting fatigue are reviewed: out-of-phase loading, cohesive zone modelling, wear effects, heterogeneity, and crystal orientation. Fretting fatigue is a phenomenon not well understood, and much more research is needed so that its understanding is increased and proper criteria and laws may be available for different cases

Speaker gender, child age, syntax, and the prosody of parentese

Parentese is the speech-language register adults switch to when talking to children. It regulates arousal, communicates affect, and its segment-marking prosody is thought to facilitate language learning. This "didactic prosody" is studied in 57 male and female parentese speakers addressing to children aged between 1 month and 5 years

Extending fatigue life of aircraft fuselage structures using laser-peening

Fatigue of airframe structures is a constant challenge to aircraft manufacturers when designing, maintaining and repairing new and aging metallic components. Laser-Peening (LP) is a highly flexible and controllable surface treatment and relatively new to manufacturers of large civil aircraft which demonstrated that it can extend the fatigue and crack growth life in aluminium alloys by introducing deep compressive Residual Stresses (RS). Currently there is no application of LP to any components of large civil aircraft. The aim of this research was to demonstrate and explore different LP strategies that can produce significant extension of the fatigue and crack growth performance of aircraft fuselage structures using Laser-Peening. Two representative samples made from 2000 series aluminium alloy were designed to represent features of the fuselage: A Centre Cracked Tension (CCT) panel made of 1.6 mm thick 2524-T3 represented the fuselage skin. Single overlap Lap-Joints (LJ) of 2.5 mm thick 2024-T3 aluminium with titanium Hi-Lok bolts arrayed in 5 columns and 3 rows embodied longitudinal LJ of aircraft fuselages. Both test samples were laser-peened without protective coating (LPwC) using a range of LP strategies in which LP process parameters and spatial arrangements of laser-peened areas were systematically varied. RS fields were measured before fatigue testing under constant amplitude loading. RS measurements used Incremental Centre Hole Drilling (ICHD) and X-ray and Neutron diffraction techniques. Laser-peening produced peak compressive RS of 200 – 350 MPa and compression stress penetration depths between 700-1000 μm. These values are superior to RS profiles induced by Shot-Peening. The value of peak compression stress and penetration depth depends on LP process parameters and on the LP layout. The latter defines the location and size of the laser-peened areas. A study of the effect of different LP strategies to establish the most effective LP treatment to enhance crack growth life of fuselage skins was performed using a Finite Element based crack growth model. The model was first used to introduce balanced RS fields into a cracked CCT sample. The effective stress intensity factor range (ΔKeff) and effective R-ratios (Reff) were then calculated as the crack tip progressed through the sample. Subsequently, fatigue crack growth rates and lives were computed using Walker’s empirical crack growth law. The accuracy of the model was demonstrated by comparison with crack growth test results from laser-peened CCT-samples. Results of the parameter study showed that an increase in the level of compression within the LPS increased life most significantly. Increased width of peen stripe increased the life while increasing the distance of the stripe from the starting position of the crack tip decreased the life. Four different LP strategies were applied to LJ samples. Subsequent fatigue testing demonstrated fatigue life improvements of between 1.14 to 3.54, depending on the LP strategy. The LP layout was identified as a key parameter determining the fatigue life. It was found that when small LP areas were used, to leave as much elastic material as possible between the peened areas, larger compressive stresses and minimised balancing tensile stresses were produced. Observations of fatigue fractures on joint samples showed that crack initiation occurred remote from the fastener holes, either in regions of fretting fatigue in peened areas or in regions of balancing tensile stress adjacent to peen boundaries. Optimum fatigue lives occurred when both fracture types occurred in the same sample. Striation spacing measurement and analysis showed that compressive residual stresses had little or no effect on fatigue growth rates at crack lengths < 600 µm. The majority of fatigue life extension was achieved during initiation and crack growth < 600 μm. The obtained results established evidence of how aircraft fuselage structures made of conventional 2000 series aluminium-copper alloys can be effectively laser-peened to produced fatigue life improvements and also of how to avoid any detrimental reductions in fatigue life which can also occur when LP is applied randomly. The generated research conclusions are applicable to other metals, geometries and components

Numerical modelling techniques for fretting fatigue crack initiation and propagation

status: publishe