Michel Nganbe

In vitro assessment of strength, fatigue durability, and disassembly of Ti6Al4V and CoCrMo necks in modular total hip replacements.

Modularity in total hip replacement offers advantages with regard to biomechanical adjustments and leg lengths. Recently, modular femoral necks were introduced as an added advantage to head modularity permitting further adjustments in femoral version as well as offset and ease of revision. Currently, most necks are made of Ti6Al4V for which cases of in vivo fractures and inseparable neck-stem junctions have been reported. Therefore, we investigated CoCrMo head-Ti6Al4V stem hip replacements with necks made of CoCrMo as an alternative to Ti6Al4V. We compared the two materials with respect to (1) compressive load bearing capacity; (2) fatigue durability; and (3) component distraction. We performed in vitro fatigue-pull-off, microscopy, fatigue durability and compression investigations. The CoCrMo neck showed a load bearing capacity of 18 kN, 38% higher than 13 kN for the Ti6Al4V neck. A fatigue load of 11.2 kN for 1 million cycle failure was achieved with CoCrMo translating into nearly 1000 times longer fatigue life compared to Ti6Al4V necks. The neck-stem distraction force showed large statistical variation and was similar for both neck materials. Overall, the results suggest a superiority of CoCrMo over Ti6Al4V as neck material with regard to mechanical behavior. However, the corrosion behavior was not appropriately assessed and necessitates additional investigations.

Cyclic stress-strain response of the ODS nickel-base, superalloy PM 1000 under variable amplitude loading at high temperatures

Abstract The cyclic stress–strain behaviour of the recently developed oxide dispersion-strengthened nickel-base alloy PM 1000 was studied under constant and variable amplitude loading conditions. Single-step tests with a constant total strain amplitude as well as incremental step tests covering the same amplitude range have been carried out at 1123 and 1273 K. The interaction of the dislocations with the fine, homogeneously distributed oxide dispersoids was found to suppress the formation of dislocation cell structures. Rather, networks with dislocations frequently pinned at the particle/matrix interface have been observed by transmission electron microscopy. However, wavy dislocation slip still contributes to the stress–strain response. Despite the similarity of the resulting microstructures, the cyclic stress–strain curve obtained from constant amplitude tests deviates slightly from the one observed in incremental step tests. While non-Masing behaviour was found for constant amplitude testing, the strong influence of the dispersoids on dislocation mobility in combination with the constancy of dislocation arrangement yields Masing behaviour for the incremental step tests.

Retrieval analysis and in vitro assessment of strength, durability, and distraction of a modular total hip replacement†

We investigated a commercial Co-Cr-alloy head--Ti6Al4V alloy neck and Ti6Al4V stem modular total hip replacement. We assessed the distraction forces after in vitro cycling in bovine serum, fatigue durability, fretting corrosion damage, and load bearing capacity of new implants using fatigue-corrosion, pull-off, scanning electron microscopy, fatigue and compression investigations. In addition, we studied corrosion, fretting damage, and distraction forces on retrievals. For both retrievals and in vitro test samples, the neck-stem interface required the higher distraction force as compared with the head-neck interface. One of 12 retrievals showed strong fretting corrosion at the neck-stem interface which resulted in a high disassembly force of about 16 kN. For in vitro test samples, the neck-stem pull-off force initially increased during cycling and showed a maximum value of 5.704 kN at ∼100,000 cycles, which is equivalent to gait cycles performed in approximately 36 days. Overall, assembly force, initial component settling, and interface corrosion primarily determine the force required to distract the modular components. One million cycles fatigue failure of the neck can be expected at a maximum compression load of -6.5 kN. No component failure was observed during quasistatic compression; rather the neck deformed plastically and the ultimate compression load-bearing capacity was -13 kN.

Plastic anisotropy of textured ODS nickel-base alloy PM 1000

Abstract Strongly textured ODS nickel-base alloy PM 1000 with a pancake grain structure has been deformed in axisymmetric and plane strain compression at 1000°C and a true strain rate of 10 −4 s −1 . The texture has been measured by neutron diffraction. Based on the texture, Taylor factors have been calculated for the different deformation modes. The linear dependence of the strength on the Taylor factor indicates that the plastic anisotropy observed is due to the texture and not to the anisotropy of the grain structure.

Modeling of Thermal Expansion Coefficients of Ni-Based Superalloys Using Artificial Neural Network

The objective of this work is to model the thermal expansion coefficients of various Ni-based superalloys used in gas turbine components. The thermal expansion coefficient is described as a function of temperature, chemical composition including Ni, Cr, Co, Mo, W, Ta, Nb, Al, Ti, B, Zr, and C contents as well as heat treatment including solutionizing and aging. Experimental values are well described and their relative changes well correlated by the model. Because gas turbine engine components operate under severe loading conditions and at high and varying temperatures, the prediction of their thermal expansion coefficient is crucial. The model developed in this work can be useful for design optimizations for minimizing thermo-mechanical stresses between the base alloys and potential protective coatings or adjacent components. It can substantially contribute to improve the performance and service life of gas turbine components.

Effects of hip implant modular neck material and assembly method on fatigue life and distraction force

Hip implant neck fractures and adverse tissue reactions associated with fretting‐corrosion damage at modular interfaces are a major source of concern. Therefore, there is an urgent clinical need to develop accurate in vitro test procedures to better understand, predict and prevent in vivo implant failures. This study aimed to simulate in vivo fatigue fracture and distraction of modular necks in an in vitro setting, and to assess the effects of neck material (Ti6Al4V vs. CoCrMo) and assembly method (hand vs. impact) on the fatigue life and distraction of the necks. Fatigue tests were performed on the cementless PROFEMUR® Total Hip Modular Neck System under two different loads and number of cycles: 2.3 kN for 5 million cycles, and 7.0 kN for 1.3 million cycles. The developed in vitro simulation setup successfully reproduced in vivo modular neck fracture mode and location. Neck failure occurred at the neck–stem taper and the fracture ran from the distal lateral neck surface to the proximal medial entry point of the neck into the stem. None of the necks failed under the 2.3 kN load. However, all hand‐assembled Ti6Al4V necks failed under the 7.0 kN load. In contrast, none of the hand‐assembled CoCrMo necks and impact‐assembled necks (Ti6Al4V or CoCrMo) failed under this higher load. In conclusion, Ti6Al4V necks were more susceptible to fatigue failure than CoCrMo necks. In addition, impact assembly substantially improved the fatigue life of Ti6Al4V necks and also led to overall higher distraction forces for both neck materials. Overall, this study shows that the material and assembly method can affect the fatigue strength of modular necks. Finally, improper implant assembly during surgery may result in diminished modular neck survivability and increased failure rates.

A Deformation Mechanism Map for the 1.23Cr-1.2Mo-0.26V Rotor Steel and Its Verification Using Neural Networks

A deformation mechanism map is constructed for the 1.23Cr-1.2Mo-0.26V rotor steel as a function of temperature, stress, and strain rate using published creep test results and the current understanding of time dependent deformation mechanisms operative in complex engineering alloys. Instead of diffusional creep, grain boundary sliding (GBS) accommodated by different deformation processes is considered dominant at lower strain rates. The GBS dominated region is further sub-divided into two parts, where GBS is accommodated by wedge type cracking at temperatures below 0.5T/Tm and the accommodation process changes to creep cavitation at temperatures above 0.5T/Tm. The map is verified using experimental data and artificial neural network modeling. The proposed artificial neural network model is capable of predicting the dominance of different deformation mechanisms in 1.23Cr-1.2Mo-0.26V steel over a wide range of stress and temperature. This modeling procedure can potentially be used to construct or expand deformation mechanism maps for other engineering alloys.

Dependence of mechanical strength on grain structure in the γ' and oxide dispersion-strengthened nickelbase superalloy PM 3030

Abstract The mechanical strength of the oxide dispersion-strengthened (ODS) nickelbase superalloy PM 3030 during deformation at constant strain rate (CSR) is discussed with special emphasis placed on its dependence on grain structure. In the fine-grain state the material shows a very high strength at low temperatures due to Hall – Petch type strengthening. However, the 0.2% offset yield strength σ0.2 falls off sharply at temperatures above 700 °C. Increase of grain size by isothermal annealing leads to a reduction of σ0.2 at low temperatures, but also to an increase of creep resistance at higher temperatures. Coarse and elongated grain structures essentially eliminate grain-boundary sliding and reduce diffusion-controlled void formation and, therefore, exhibit a superior strength level at elevated temperatures. The dependence of σ0.2 on grain structure results in a cross-over of the σ0.2 vs. T-curves within a narrow temperature range where all grain variants exhibit similar mechanical strengths. In accord...

Modeling of ?' Precipitate Size of IN738LC Using LevenbergMarquardt Backpropagation Neural Network

The γ’ precipitate size of IN738LC is predicted using a Levenberg–Marquard back propagation neural network in matlab toolbox. A cast polycrystalline Ni based super alloy IN738LC (a gas turbine material) is considered and the γ’ precipitate size is described as a function of 5 variables (solutionizing temperature, solutionizing duration, ageing temperature, ageing duration, and cooling method (furnace cooling, water quenching, induction cooling, salt bath cooling, accelerated air cooling and oil quenching). The model converges very well and accurately predicts the ’ precipitate size. Because first stage gas turbine blades operate at very high and varying temperatures for extended period of time, the prediction of their ’ precipitate size is crucial as ’ precipitate morphology is responsible for most high temperature properties. The model developed in this work can be useful for predicting creep and other mocrstuctural properties at high temperatures.

Study of mechanical properties of micromachined dental implants

ABSTRACT Advancements in technologies and techniques within the dental industry have given rise to new and effective tooth replacement procedures for damages resulting from causes such as trauma or aging. While these treatments are widely available for patients, they remain highly expensive, preventing patients from much-needed dental care. The elevated cost of dental implants is in part associated with their components that are mainly available through third-party companies at a premium cost. To be cost effective, dental laboratories are exploring the option of producing their own dental implant components, and are therefore acquiring knowledge of manufacturing techniques and quality assurance expertise to produce quality components. In order to ensure high quality and reliability, the fabricated components must be tested and benchmarked against current implants on the market. The present study examines the micro machining process of dental implants, specifically for the abutments and screws, and its impact on the mechanical properties of the components. To achieve this, dental implant abutment and screw prototypes were fabricated, experimentally tested, and compared. The impact of different machining processes on the mechanical properties of the implants was comparatively determined and analysed. The fabricated implant testing results show coherent mechanical properties displayed by good hardness, and material microstructures similar to market components, indicating a high level of prototype quality.