The Influence of Assembly Force on the Material Loss at the Metallic Head-Neck Junction of Hip Implants Subjected to Cyclic Fretting Wear

The impaction force required to assemble the head and stem components of hip implants is proven to play a major role in the mechanics of the taper junction. However, it is not clear if the assembly force could have an effect on fretting wear, which normally occurs at the junction. In this study, an adaptive finite element model was developed for a CoCr/CoCr head-neck junction with an angular mismatch of 0.01° in order to simulate the fretting wear process and predict the material loss under various assembly forces and over a high number of gait cycles. The junction was assembled with 2, 3, 4, and 5 kN and then subjected to 1,025,000 cycles of normal walking gait loading. The findings showed that material removal due to fretting wear increased when raising the assembly force. High assembly forces induced greater contact pressures over larger contact regions at the interface, which, in turn, resulted in more material loss and wear damage to the surface when compared to lower assembly forces. Although a high assembly force (greater than 4 kN) can further improve the initial strength and stability of the taper junction, it appears that it also increases the degree of fretting wear. Further studies are needed to investigate the assembly force in the other taper designs, angular mismatches, and material combinations

An experimental investigation on crack effect on the mechanical behavior and energy absorption of thin-walled tubes

Energy absorption capacity and collapse of cylindrical and square thin-walled aluminum tubes with a crack shaped trigger under axial compression are studied in this paper. Furthermore, the effects of length, angle, location and situation of cracks on the mechanical behavior of tubes are investigated. The results of this research show that the cracks change the collapse processes and folding modes; this effects are greater for the cylindrical tubes; the maximum load is reduced between 4.92% and 31.33% for cylindrical and between 2.55% and 18.52% for square tubes; the cracks increase the crush force efficiency up to 67.03% and 31.06%, and absorbed energy up to 30.45% and 30.16% for cylindrical and square tubes, respectively. The maximum load for all of the cracked tubes is less than that of intact tubes and increasing the crack angle from 0 degrees to 45 degrees decreases the maximum load and from 45 degrees to 60 degrees increases it. Finally, parallel cracks are more effective than perpendicular cracks

An adaptive finite element simulation of fretting wear damage at the head-neck taper junction of total hip replacement: the role of taper angle mismatch

An adaptive finite element simulation was developed to predict fretting wear in a head-neck taper junction of hip joint implant through a two dimensional (2D) model and based on the Archard wear equation. This model represents the most critical section of the head-neck junction which was identified from a 3D model of the junction subjected to one cycle of level gait loading. The 2D model was then used to investigate the effect of angular mismatch between the head and neck components on the material loss and fretting wear process over 4 million gait cycles of walking. Generally, junctions with distal angular mismatches showed a better resistance to fretting wear. The largest area loss in the neck after 4 million cycles of loading was 1.86E-02\ua0mm which was found in the junction with a proximal mismatch angle of 0.124°. While, the minimum lost area (4.30E-03\ua0mm) was found in the junction with a distal angular mismatch of 0.024°. Contact stress, amplitude of sliding and contact length were found as the key parameters that can influence the amount of material loss and the process of fretting wear damage. These parameters vary over the fretting wear cycles and are highly dependent on the type and magnitude of the taper angle mismatch. This study also showed that lost area does not have a linear relationship with the mismatch angle of taper junctions

Determination of combined hardening material parameters under strain controlled cyclic loading by using the genetic algorithm method

In this paper, experimental and numerical investigations on mechanical behaviors of SS304 stainless steel under fully reversed strain-controlled, relaxation, ratcheting and multiple step strain-controlled cyclic loading have been performed. The kinematic and isotropic hardening theories based on the Chaboche model are used to predict the plastic behavior. An iterative method is utilized to analyze the mechanical behavior under cyclic loading conditions based on the Chaboche hardening model. A set of kinematic and isotropic parameters was obtained by using the genetic algorithm optimization approach. In order to analyze the effectiveness of this optimization procedure, numerical and experimental results for an SS304 stainless steel are compared. Finally, the results of this research show that by using the material parameters optimized based on the strain-controlled and relaxation data, good agreement with the experimental data for ratcheting is achieved

Ratcheting behavior of cylindrical pipes based on the Chaboche kinematic hardening rule

In this study, cyclic loading behavior of thick cylindrical pipes are described. Effects of internal pressure level and axial strain amplitude on the ratcheting rate under different types of loading histories are investigated. The kinematic hardening theory based on the Chaboche model is used to predict the plastic behavior of the structures. An iterative method is developed to analyze the structural behavior under cyclic loading conditions based on the Chaboche kinematic hardening model

Effects of buckling initiators on mechanical behavior of thin-walled square tubes subjected to oblique loading

Thin-walled structures usually collapse in Eulerian buckling mode under oblique loads. Energy absorption capacity and crush force efficiency of the structure in this type of collapse are low. Collapse initiators are used to improve these properties. In this research, effect of collapse initiators on energy absorption characteristics of square tubes under oblique quasi-static loads is investigated both experimentally and numerically. Initiators are in the form of cuttings on the tube corners. Results show that collapse initiators in most of the specimens change deformation mode from general buckling to progressive buckling and decrease considerably the peak load; therefore increase crush force efficiency. Furthermore, effect of location and number of initiators is studied. There is good agreement between the numerical results and data from experiments. (C) 2012 Elsevier Ltd. All rights reserved

An exponential material model for prediction of the flow curves of several AZ series magnesium alloys in tension and compression

This paper is concerned with flow behaviors of several magnesium alloys, such as AZ31, AZ80 and AZ81, in tension and compression. The experiments were performed at elevated temperatures and for various strain rates. In order to eliminate the effect of inhomogeneous deformation in tensile and compression tests, the Bridgeman's and numerical correction factors were respectively employed. A two-section exponential mathematical model was also utilized for prediction of flow stresses of different magnesium alloys in tension and compression. Moreover, based on the compressive flow model proposed, the peak stress and the relevant true strain could be estimated. The true stress and strain of the necking point can also be predicted using the corresponding relations. It was found that the flow behaviors estimated by the exponential flow model were encouragingly in very good agreement with experimental findings

Investigation of tension and compression behavior of AZ80 magnesium alloy

Magnesium alloy deformation should be carried out at high temperature and it is essential to investigate the deformation behavior of these alloys at high temperature. In this paper, practical tests are conducted on AZ80 alloy which includes tension and compression tests at high temperature and different strain rates. As this alloy is sensitive to temperature and strain rate, tension tests are difficult to carry out. The traditional compression test should be conducted in zero-friction condition but such a condition is impossible to prepare. Therefore, bulge correction factor and numerical correction factor are used to eliminate the friction effect which exists between the surfaces. Tensile tests are also carried out on the standard specimens at high temperature and different strain rates. The effect of necking phenomenon is corrected using Bridgman correction factor and is simulated using finite element software. Also T-shape compression test is used to valuate friction parameter at high temperatures. (c) 2013 Elsevier Ltd. All rights reserved