Ehsan Askari

A Computational Analysis of Squeaking Hip Prostheses

A ceramic-on-ceramic (CoC) hip prosthesis with clearance is modeled as a multibody dynamics system for the purpose of studying hip squeaking. A continuous contact force model provides the intrajoint forces developed at the hip joint. Friction effects due to the relative motion are also considered. A FFT analysis of the audible sounds from CoC hip acceleration is carried out to analyze hip squeaking. The effects of friction, hip implant size, and the head initial position on hip squeaking and the trajectory of femoral head are analyzed and discussed. It was shown that the causes of hip squeaking are stick/slip, friction-induced vibration, and the femoral head angular speed and force changes. [DOI: 10.1115/1.4028109]

A particle swarm-based algorithm for optimization of multi-layered and graded dental ceramics

The thermal residual stresses (TRSs) generated owing to the cooling down from the processing temperature in layered ceramic systems can lead to crack formation as well as influence the bending stress distribution and the strength of the structure. The purpose of this study is to minimize the thermal residual and bending stresses in dental ceramics to enhance their strength as well as to prevent the structure failure. Analytical parametric models are developed to evaluate thermal residual stresses in zirconia-porcelain multi-layered and graded discs and to simulate the piston-on-ring test. To identify optimal designs of zirconia-based dental restorations, a particle swarm optimizer is also developed. The thickness of each interlayer and compositional distribution are referred to as design variables. The effect of layers number constituting the interlayer between two based materials on the performance of graded prosthetic systems is also investigated. The developed methodology is validated against results available in literature and a finite element model constructed in the present study. Three different cases are considered to determine the optimal design of graded prosthesis based on minimizing (a) TRSs; (b) bending stresses; and (c) both TRS and bending stresses. It is demonstrated that each layer thickness and composition profile have important contributions into the resulting stress field and magnitude.

Fluid-structure coupling of concentric double FGM shells with different lengths

The aim of this study is to develop a semi-analytical method to investigate fluid-structure coupling of concentric double shells with different lengths and elastic behaviours. Co-axial shells constitute a cylindrical circular container and a baffle submerged inside the stored fluid. The container shell is made of functionally graded materials with mechanical properties changing through its thickness continuously. The baffle made of steel is fixed along its top edge and submerged inside fluid such that its lower edge freely moves. The developed approach is verified using a commercial finite element computer code. Although the model is presented for a specific case in the present work, it can be generalized to investigate coupling of shellplate structures via fluid. It is shown that the coupling between concentric shells occurs only when they vibrate in a same circumferential mode number, n. It is also revealed that the normalized vibration amplitude of the inner shell is about the same as that of the outer shell, for narrower radial gaps. Moreover, the natural frequencies of the fluid-coupled system gradually decrease and converge to the certain values as the gradient index increases.

Wear Prediction of Ceramic-on-Ceramic Artificial Hip Joints

Wear can influence the lifetime and performance of implants and has been found to be a key factor in primary failure of artificial hip joints. The present study aims to present a spatial multibody dynamic model to predict wear in ceramic-on-ceramic hip implants. The problem was formulated by developing a spatial multibody dynamic model of a hip prosthesis taking three-dimensional physiological loading and motion of the human body into account. Then, the Archard wear model was integrated into the dynamic calculation of the hip implant to predict wear. Additionally, geometries of the cup and head were updated throughout the simulation to generate a more realistic wear simulation. The results were validated against current literature. Finally it was illustrated that friction-induced vibration caused excessive wear of hip implant components.