Bernard Forest

Environmental effect on fretting of metallic materials for orthopaedic implants

Abstract The environment of orthopaedic implants sometimes induces vibrations at the contact of the modular prostheses components. These microdisplacements contribute to the total failure of the implant. The necessary optimisation of orthopaedic device life requires a better knowledge of the damages induced by fretting corrosion. The aim of this paper is to describe the damage mechanism at the head–neck contact of a total hip joint with a neck in Ti–6Al–4V alloy and a head in austenitic stainless steel (AISI 316L SS). Simple tests were performed at the ambient air and in an artificial physiologic medium in order to reveal the damage induced by the physiological medium. The presence of a solution containing chloride ions activates a localised corrosion phenomenon which leads to the modification of the displacement accommodation regime. The consequences of electrochemical phenomena on this evolution are discussed. A quantitative energetic approach shows the lubricant effect of the artificial physiological environment. The different materials are ranked according to their wear rates. The result shows that fretting regime, accumulated dissipated energy and corrosion activated at the interface are interdependent.

Fretting corrosion of materials for orthopaedic implants : a study of a metal/polymer contact in an artificial physiological medium

The fretting corrosion behaviour of a 316L SS flat against a PMMA counterface has been investigated in an artificial physiological medium. A specific device has been used to visualize the in situ degradation at the contact interface. Simultaneous analysis of the coefficient of friction and free corrosion potential has shown four distinct stages during fretting experiments. An energy-oriented approach to the fretting process was conducted in tandem with measurement of wear. This method has shown a linear progression in the wear volume of the samples as a function of the interfacial energy dissipated during fretting. The presence of chlorides contributes to a considerable acceleration of the degradation of the stainless steel surface. This process was explained by a mechanism related to crevice corrosion activated by friction.

Two-dimensional finite element simulation of fracture and fatigue behaviours of alumina microstructures for hip prosthesis.

This paper describes a two-dimensional (2D) finite element simulation for fracture and fatigue behaviours of pure alumina microstructures such as those found at hip prostheses. Finite element models are developed using actual Al2O3 microstructures and a bilinear cohesive zone law. Simulation conditions are similar to those found at a slip zone in a dry contact between a femoral head and an acetabular cup of hip prosthesis. Contact stresses are imposed to generate cracks in the models. Magnitudes of imposed stresses are higher than those found at the microscopic scale. Effects of microstructures and contact stresses are investigated in terms of crack formation. In addition, fatigue behaviour of the microstructure is determined by performing simulations under cyclic loading conditions. It is shown that crack density observed in a microstructure increases with increasing magnitude of applied contact stress. Moreover, crack density increases linearly with respect to the number of fatigue cycles within a given contact stress range. Meanwhile, as applied contact stress increases, number of cycles to failure decreases gradually. Finally, this proposed finite element simulation offers an effective method for identifying fracture and fatigue behaviours of a microstructure provided that microstructure images are available.

Finite Element Modelling of Shock-Induced Damages on Ceramic Hip Prostheses

The aim of this work was to simulate the behaviour of hip prostheses under mechanical shocks. When hip joint is replaced by prosthesis, during the swing phase of the leg, a microseparation between the prosthetic head and the cup could occur. Two different sizes of femoral heads were studied: 28 and 32 mm diameter, made, respectively, in alumina and zirconia. The shock-induced stress was determined numerically using finite element analysis (FEA), Abaqus software. The influence of inclination, force, material, and microseparation was studied. In addition, an algorithm was developed from a probabilistic model, Todinovs approach, to predict lifetime of head and cup. Simulations showed maximum tensile stresses were reached on the cups surfaces near to rim. The worst case was the cup-head mounted at 30∘. All simulations and tests showed bulk zirconia had a greater resistance to shocks than bulk alumina. The probability of failure could be bigger than 0.9 when a porosity greater than 0.7% vol. is present in the material. Simulating results showed good agreement with experimental results. The tests and simulations are promising for predicting the lifetime of ceramic prostheses.

Fracture simulation for zirconia toughened alumina microstructure

Purpose – The purpose of this paper is to describe finite element modelling for fracture and fatigue behaviour of zirconia toughened alumina microstructures.Design/methodology/approach – A two‐dimensional finite element model is developed with an actual Al2O3‐10 vol% ZrO2 microstructure. A bilinear, time‐independent cohesive zone law is implemented for describing fracture behaviour of grain boundaries. Simulation conditions are similar to those found at contact between a head and a cup of hip prosthesis. Residual stresses arisen from the mismatch of thermal coefficient between grains are determined. Then, effects of a micro‐void and contact stress magnitude are investigated with models containing residual stresses. For the purpose of simulating fatigue behaviour, cyclic loadings are applied to the models.Findings – Results show that crack density is gradually increased with increasing magnitude of contact stress or number of fatigue cycles. It is also identified that a micro‐void brings about the increase...