Christian Hochard

Monitoring of Laminated Composites Delamination Based on Electro-Mechanical Impedance Measurement

This paper paves the way for a multi-damage monitoring system for composite structures. The final goal of this work being to design a monitoring system able to predict the damage in the composite structure under static and fatigue loading. This system will be based on the simultaneous use of measurement and damage predicting models. We describe the first step of this work, which consists in the use of electro-mechanical impedance measurement to detect delamination. An electro-mechanical model of a beam into which delamination has been introduced is presented and compared with experimental results. The influence of geometrical parameters is studied and future improvements of our method are outlined.

Optimisation of hybrid high-modulus/high-strength carbon fibre reinforced plastic composite drive shafts

This study deals with the optimisation of hybrid composite drive shafts operating at subcritical or supercritical speeds, using a genetic algorithm. A formulation for the flexural vibrations of a composite drive shaft mounted on viscoelastic supports including shear effects is developed. In particular, an analytic stability criterion is developed to ensure the integrity of the system in the supercritical regime. Then it is shown that the torsional strength can be computed with the maximum stress criterion. A shell method is developed for computing drive shaft torsional buckling. The optimisation of a helicopter tail rotor driveline is then performed. In particular, original hybrid shafts consisting of high-modulus and high-strength carbon fibre reinforced epoxy plies were studied. The solutions obtained using the method presented here made it possible to greatly decrease the number of shafts and the weight of the driveline under subcritical conditions, and even more under supercritical conditions. This study yielded some general rules for designing an optimum composite shaft without any need for optimisation algorithms.

Femoral neck fracture prediction by anisotropic yield criteria

The fracture risk due to osteoporosis, is undertaken with Dual-Energy X-ray Absorptiometry (DEXA) which is an average of bone mineral density measurement, without taking into account the bone structure. The objective of this study was an experimental test to solicit the human proximal femurs by a physiological configuration (one leg stance phase of walking). For this, transversely isotropic finite element models were developed from CT scan acquisition. The failure load assessment was insured by anisotropic yield behaviour criteria based on distortion energy criterion (Hill’s criterion) and taking into account the difference between tension and compression yield properties (Tsai–Wu’s criterion). The results found in this study showed the significance part of anisotropic yield behaviour of bone on proximal femur.

IMPROVED FEMORAL NECK FRACTURE PREDICTIONS USING ANISOTROPIC FAILURE CRITERIA MODELS

Finite element models are widely used to assess long bone strength, implant stability and other clinical problems. In most of the models presented so far in the literature, the bone is taken to be isotropic, and the occurrence of failure is predicted by defining a threshold von Mises stress. However, human bone is found to show orthotropic behavior. Studies so far have focused only on the use of anisotropic criteria in orthotropic models designed to predict the occurrence of human femur failure. The aim of this study was therefore to investigate how specific finite element models for human femora combined with composite failure theories could be used to improve failure predictions in vitro. For this purpose, nine human proximal femora were tested mechanically up to failure under the loading conditions present during the one-leg stance phase in walking. Specific finite element models using various materials to represent the bone were generated for each femur. First, the bone material was modeled in the form of an isotropic brittle material, and the von Mises criterion was used to predict the occurrence of fracture. Second, the bone was modeled as a transversely isotropic brittle material with asymmetric strength characteristics, and the occurrence of fracture was predicted using the Hill and the Tsai–Wu criteria. The results obtained here show that the transversely isotropic model combined with Tsai–Wu and Hill criteria accurately predicted the fracture load (values of R2 = 0.94 and SEE = 10.3% were obtained with the Tsai–Wu criteria and R2 = 0.82 and SEE = 22.9% were obtained with the Hill criteria), while the isotropic model combined with the von Mises criterion overestimated the fracture load, although a good correlation was generally observed with the experimental results (R2 = 0.77, SEE = 30.6%).

Hybrid tape-spring for deployable hexapod

In this work we present a new concept of a hybrid tape-spring for the deployment of space structures. The control of its unreeling, needed to avoid any risk of damage, is achieved thanks to a local heating of thermoplastic layers stuck on its surface. The lay-up properties and the geometry are chosen by minimizing the strain energy of the coiled state. The feasibility of the concept is proved experimentally and numerically thanks to a rst ABAQUS FEM model describing the coiling and the unreeling of the tape-spring.

Numerical simulation of an osteoporotic femur: Before and after total hip arthroplasty

Bone remodelling adapts bone geometry and properties under supported loadings. This optimization process is deteriorated by metabolic diseases like osteoporosis which involves femoral neck fractures and implies Total Hip Arthroplasty. Two finite element models are developed to evaluate the stress distribution within osteoporotic human femur bone tissue, and its influence on the stem stability. The geometries of human femur and prosthesis are obtained by helicoid scanner acquisition. The cortical bone was separated from the trabecular bone by apparent density threshold. The results obtained for osteoporotic femur show that the degradation of trabecular architecture causes high stresses in the anteroinferior zone of the cortical bone. For the femur with hip prosthesis, high stresses weak the bone tissue in the lateral zone of the proximal dyaphisis and in the medial zone of the distal part at the end of the stem.

Numerical implementation of a non local criterion to describe the failure of laminates with stress concentrations

A non local ply scale criterion was previously developed for predicting the failure of balanced woven ply structures with stress concentrations. This non local criterion was based on the mean values determined over a Fracture Characteristic Volume (FCV). A behavioural model was developed from a classical Continuum Damage Mechanics (CDM) model. The FCV approach and the CDM behavioural model were implemented in ABAQUS and applied to the case of unbalanced woven ply. Comparisons are made between the experimental data and the modelling predictions obtained on plates with open holes, notches and saw cuts. The numerical implementation of this approach is studied in this paper.

FEMORAL NECK FAILURE PREDICTION: EXPERIMENTAL AND NUMERICAL STUDY

Osteoporosis is a worldwide health problem with about 1700 fractures per day only in Europe (World Health Organisation). Hip fractures are the more recurrent consequences of this disease. The fracture risk screening is usually undertaken with DualEnergy X-ray Absorptiometry (DEXA), with the fact that only the integral bone mass and the area bone density can be measured, without taking into account the bone structure. In order to improve the assessment of fracture risk, many non–invasive methods were developed. These techniques, based on densitometric properties of human bone [Lockmuller, 2002], geometric and structural engineering properties of the proximal femur [Beck, 1990; Keyak, 2000], have little precisions for estimating fracture load and identifying subjects with high fracture. Our aim was to investigate the use of anisotropic yield criteria to predict the femoral neck failure. For this, the strength of human femurs was measured by experimental test and finite element analysis.

Experimental analysis of proximal femur fracture

Osteoporosis is a worldwide health problem with about 1700 fractures per day only in Europe (http://www.euro. who.int/HEN/Syntheses/osteoporosis/20060504_1). Related to age, this disease weakens the bone structure by deterioration of the trabecular architecture (Hajjar and Kamel 2004) and also decreases the cortical envelop width and increases its porosity (Bell et al. 1999). Hip fractures are the more recurrent consequences of osteoporosis, and are the cause of morbidity and increase the rate of mortality. In spite of the important incidence on the public health, its screening remains unutilized because of its cost and also, because of the technique used (DEXA). Our aim was to develop an experimental test in order to load the human proximal femurs by a physiological solicitation (one leg stance phase of walking) to retrieve the clinical osteoporosis fractures and analysis the contribution of both cortical envelop and trabecular bone in the resistance of femur structure.