Jui-Pin Hung

The computer simulation of wear behavior appearing in total hip prosthesis

Computer algorithms are proposed for the estimation of wear appearing in artificial hip joints using finite element analysis based on the modified Archards wear law, contact features and an analogue wear process. A pin-on-disk plate experiment is reconstructed to assess the efficiency and validity of the algorithms proposed here. Through the successful verification of wear depth and volume loss of the pin-on-disk plate as well as the artificial hip joint, the current algorithms provide significant agreement with experiments, clinical measurements and numerical calculations and are shown to be both valid and feasible. Further investigation into the effect of femoral heads with various sizes suggests that the larger femoral head may induce larger wear volume but gives a smaller wear depth and that wear depth and volume loss are apparently nonlinearly related to the femoral head diameter. It is shown that the current algorithms are useful and helpful in understanding wear behavior for alternative or new designs of artificial hip joints and even for other analogous structures.

Computer simulation on fatigue behavior of cemented hip prostheses: a physiological model.

This paper is concerned with the investigation on the fatigue failure of implant fixation by numerical approaches. A computer algorithm based on finite element analysis and continuum damage mechanics was proposed to quantify the fatigue damage rate of cement mantle under physiological conditions. In examining the interfacial debonding effect, the interface elements were introduced at cement-stem interfaces and calibrated with the increase of loading cycles. Current results reveal that the major sites for failure initiation are in the proximal anterior-medial regions and at the distal prosthesis tip, which clearly demonstrate the same failure scenario as observed in clinical studies. Such fatigue failures not only result in the corruption of cement-stem interfaces, but also greatly affect the cement stress distribution and the damage rate in subsequent loading cycles. Another significant result is that the predicted damage rate increases steadily with gait cycles. This trend in damage development is consistent with the findings obtained from fatigue tests available in literature. It is anticipated that presented methodology can serve as a pre-clinical validation of cemented hip prostheses.

The effect of contact interface on dynamic characteristics of composite structures

In this project, nonlinear characteristics on the rolling interface of a linear guide were studied by the finite element analysis and experimental verification. Contact of the ball/surface rolling interface in the rolling guides was simulated as a three-dimensional membrane element without thickness. By introducing Hertzian contact theory and applying proper normal/shear stiffness to such contact elements in the overall finite element model, dynamic behaviors of linear guides affected by preload were thus investigated. In the finite element procedure, three contact models, 1D point-to-point, 2D point-to-point and 3D surface-to-surface, were sequentially introduced for purpose of verification with experiments. As a validation in this project, vibrational experiments on linear guides with different preloads were conducted and related frequency spectrums were derived. Both the finite element and the experimental results reveal that the natural frequency of a linear guide increases with the increment of the preload. In addition, the dynamic characteristics predicted by finite element analysis agree well with those measured from instrumental experiments. The proposal of current study may provide an alternate and reliable way for understanding of the dynamic characteristic of the rolling contact components in machine design field.

A COMPARATIVE STUDY ON WEAR BEHAVIOR OF HIP PROSTHESIS BY FINITE ELEMENT SIMULATION

A numerical approach was proposed to investigate the wear behavior occurred in the artificial hip joints in this paper. In the numerical simulations, the wear coefficients taken from pin-on-disk tests were introduced into the wear analysis model to assess the wear rates of polyethylene acetabular cups against metallic or ceramic femoral heads. For the established material combinations, different values of polyethylene wear rates were obtained respectively, which were not necessarily the realistic one as expected in vivo but could be confirmed after further discussion on the wear mechanism involved in wear tests. Current results indicated that the polyethylene/ceramic couples represented better wear performances than the polyethylene/metal couples. Furthermore, the ratio of wear rates for polyethylene cups against alumina and the metallic femoral heads was 0.5, which agreed well with that deduced from clinical studies or laboratory hip simulators. It is obvious that these comparable wear behaviors observed from clinics or laboratory studies also can be found by means of the numerical simulation.

Effects of interfacial debonding on fatigue damage of cemented hip prostheses

Abstract Clinical studies on retrieved cement mantles have pointed out that the cemented hip prostheses failed after long‐term use due to debonding at the cement‐stem interface and local fractures in the cement mantles. These were linked to fatigue damages of cement mantles proved by fatigue experiments. In this paper, a numerical approach based on finite element analysis and continuum damage mechanics is proposed to investigate the fatigue behavior of cement mantles during gait cycles. Results reveal that the major sites for failure initiation are at the proximal medial regions and at the distal prostheses tip. Such fatigue failures not only result in the corruption of cementstem interfaces, but also greatly affect the stress distribution and damage rate of the proximal cement mantles in subsequent loading cycles. The interfacial debonding rate increases from 2.5% to 15% with gait loadings from five to twenty million cycles. Meanwhile, owing to the partial debonding of interface, the cement stresses on the remaining regions increase by 91% to 871% when compared with those generated with a fully bonded interface, which in turn accelerates the fatigue damage accumulation rate of the cement mantle from 5.99 % to 21.5%.

Effect of Antibiotic Additions on the Mechanical Properties of Bone Cement

Clinical studies have proved that artificial joints may fail under prolonged gait load, which failure mechanism includes mechanical loosening and infectious loosening. Infectious loosening can be prevented by avoiding osteomyelitis, caused by bacterial infection arising from the marrow cavity, which affects the fixation function of the bone handle. As a result, use of bone cement containing various antibiotics has become an important method for prevention and treatment of infection after artificial joint replacement. This study was aimed to investigate the mechanical properties of bone cement after the addition of antibiotics through the mechanical tests. With the measurements we can then assessed the variations of mechanical strength with the dosage of antibiotics. The results showed that the dose of antibiotics directly affected the compression strength and elastic modulus of antibiotic bone cement. When the antibiotics was added more than 4.8 wt %, the cement strength was obviously affected and reduced, by 27%, indicating that during artificial hip joint replacement, the dose of antibiotics should be concerned, in order to avoid affecting the strength of bone cement and stability of the entire implant.

Chaos and Nonlinear Dynamic Analysis of Porous Air Bearing System

The chaos and nonlinear dynamic behaviors of porous air bearing system are studied by a hybrid numerical method combining the finite difference method (FDM) and differential transformation method (DTM). The numerical results are verified by two different schemes including hybrid method and FDM and the current analytical results are found to be in good agreement. Furthermore, the results reveal the changes which take place in the dynamic behavior of the bearing system as the rotor mass is increased. From the dynamic responses of the rotor center, they reveal complex dynamic behaviors including periodic, sub-harmonic motion and chaos. The results of this study provide an understanding of the nonlinear dynamic behavior of PAB systems characterized by different rotor masses. Specifically, the results have shown that system exists chaotic motion over the ranges of rotor mass 10.66≤ Mr<13.7kg. The proposed method and results provide an effective means of gaining insights into the porous air bearing systems.

3D Quasi-Dynamic Load Contact Analysis of Conjugate Spur Gear

In order to provide a profitable and cost-effective way in maintaining a machine, it is necessary to prepare an economical way to provide service manipulation is to prepare standard and stable spare parts, replacements, and consumables in stock. The problem of accurate prediction of load, deformation and stresses in 3D contacts is then revisited. Most research has used one, two or three pairs of meshing teeth to simulate the mating process. It is insufficient to describe the entire process of three contact zones during each mating pair. By implementing a combined 3D face-contact and FEM, contact stress analysis between two spur gear teeth was considered in eleven different contact positions during a full mating process. The proposed approach provides a complete and effective solution of the contact problem in quasi-dynamic way. Instead of applying more fixed boundary conditions to constrain and simplify material behavior in 2D models, point and line contact of tooth surfaces are substituted by a face contact model of teeth in this article.

Tool-Path Generation for Conical Groove of Cylindrical Cams by Small-Sized Cutting Tools

Instead of 2D expanding diagram method, this paper presents a new regenerating method for cutter location paths of using smaller tools to produce cylindrical cams. According to the expected motions, cams are put to use in various applications in mechanism. For a cylindrical cam, the roller follower operates in a groove cut on the periphery of an end mill with the diameter same as the roller. By using the conventional method, full-sized cutting tools, will restrict the flexibility of choosing cutting tools for wide roller guide. The manufacture of cylindrical cams is complicate and precise work that depends on the generating method and types of machine tools employed. Since the guiding curve cannot be offset exactly along the cylindrical surface, this leads to some approximating problems. Though the tool-paths generation by using the same size tools as rollers is applied in practice, the study of NC program by unequal tools is not available to meet high precision requirement. This proposed 3D offset-based generating method can regenerate tool-paths for standard cutting tools instead of larger ones and implemented on computerized CAM system. Examples with wider grooves are demonstrated to prove its effectiveness.

Computational Modeling of Surface Fracture of Polyethylene Acetabular Cup in Total Artificial Hip Replacement

In this paper, a crack analysis model based on finite element method and virtual crack extension technique was proposed to investigate the occurrence of surface fracture of polyethylene acetabular cup under gait loadings. To this, a simplified hip joint model was created for facture analysis. The stress intensity factor (SIF) at crack site was estimated and used to evaluate the propagation of the surface crack. Current results show that under normal gait loading, the SIF at crack tip within polyethylene cup was predicted to be lower than the fatigue threshold of polyethylene material. However, under the heel strike instant, the crack tip SIF exceeds the fracture strength of polyethylene subject to gamma radiation, which may drive the crack to propagate to final fracture. Overall, the presented analysis model has demonstrated the probability of severe surface damage occurring in polyethylene cup under impact walking conditions. This provides a valuable reference to the improvement of the mechanical properties or design of bearing materials in clinical orthopedic application.