Dominique Perreux

Tensile fracture of soft and hard PZT

Power applications generate high stresses which can damage piezoceramic components. In this study tensile fracture of several types of PZT (hard/soft) is investigated. After validation of the specimen geometry by means of numerical simulation, samples are led to failure using a specific device. Weibull law parameters enable the characterisation of the tensile strength distribution and highlight clear differences between soft and hard ceramics. A fractographic approach emphasises the specificities of the fracture mode and the fracture origin for each type of samples.

Evaluation of the durability of composite tidal turbine blades

The long-term reliability of tidal turbines is critical if these structures are to be cost effective. Optimized design requires a combination of material durability models and structural analyses. Composites are a natural choice for turbine blades, but there are few data available to predict material behaviour under coupled environmental and cycling loading. The present study addresses this problem, by introducing a multi-level framework for turbine blade qualification. At the material scale, static and cyclic tests have been performed, both in air and in sea water. The influence of ageing in sea water on fatigue performance is then quantified, and much lower fatigue lives are measured after ageing. At a higher level, flume tank tests have been performed on three-blade tidal turbines. Strain gauging of blades has provided data to compare with numerical models.

A meso–macro finite element modelling of laminate structures: Part I: time-independent behaviour

Abstract A meso–macro modelling is proposed for laminates made of unidirectional layers of a polymer matrix reinforced with long fibres. The behaviour of a layer is expressed through elasticity coupled to damage and plasticity. The anisotropic damage is completely described by a single scalar variable and its evolution law is specified from the principle of maximum dissipation. The effective undamaged space concept associated with the strain equivalence principle is used for the plastic analysis. The flow rules are defined within the context of the generalised standard materials assuming the existence of a dissipation pseudo-potential and a plastic criterion expressed as scalar valued functions of the effective stresses and the thermodynamic force associated with a non-linear kinematic hardening. The integration of the layer behaviour through the thickness is obtained within a Kirchhoff shell element. The constitutive equations of the layer are numerically integrated using an unconditionally stable algorithm consisting of a multi-level iterative scheme. A consistent integration of the equilibrium equations is performed to assure a quadratic convergence of the global solution scheme. A methodology is also proposed for the evaluation of the inter-laminar shear stresses in a consistent manner with the stresses and internal variables computation algorithm. The proposed formulation has been implemented in the finite element code CASTEM2000 in order to test its validity. The obtained results have been compared with the available experimental ones in the case of progressive repeated loading tests by applying pure traction, pure internal pressure and internal pressure with end effect on a tube.

Mechanical behavior of syntactic foams for deep sea thermally insulated pipeline

Ultra Deep offshore oil exploitation (down to 3000 meters depth) presents new challenges to offshore engineering and operating companies. Flow assurance and particularly the selection of insulation materials to be applied to pipe lines are of primary importance, and are the focus of much industry interest for deepwater applications. Polymeric and composite materials, particularly syntactic foams, are now widely used for this application, so the understanding of their behavior under extreme conditions is essential. These materials, applied as a thick coating (up to 10-15 cm), are subjected in service to: - high hydrostatic compression (up to 30 MPa) - severe thermal gradients (from 4°C at the outer surface to 150°C at the inner wall), and to high bending and shear stresses during installation. Damageable behavior of syntactic foam under service conditions has been observed previously [1] and may strongly affect the long term reliability of the system (loss of thermal properties).This study is a part of a larger project aiming to model the in-service behavior of these structures. For this purpose it is important to identify the constituent mechanical properties correctly [2, 3]. A series of tests has been developed to address this point, which includes: - hydrostatic compression - shear loading using a modified Arcan fixture This paper will describe the different test methods and present results obtained for different types of syntactic foams.

Thermoplastic carbon fibre-reinforced polymer recycling with electrodynamical fragmentation: From cradle to cradle

The end of life of carbon fibre-reinforced polymer (CFRP) structures represents a major challenge to the aerospace industry, as new European regulations are demanding recycling solutions that can be complicated and expensive to apply. This study aims to address new practical ways to recycle CFRP materials. CFRP materials with a polyether ether ketone (PEEK) matrix were fragmented via electrodynamical fragmentation, which exhibits several benefits compared to mechanical shredding processes, especially for composites commonly found in the aerospace industry. The fragments are characterized and reused to produce new CFRP aerospace parts. Structural testing of recycled composite parts revealed a 17% decrease of the mechanical properties compared to the novel material. The combination of these manufacturing and recycling techniques closes the cradle to cradle loop of thermoplastic CFRP.

Mechanical behaviour of an epoxy-glass composite under photo-oxidation

Abstract In previous papers the photo-degradation of an epoxy resin (DGEBA) cross-linked with the hardener methyltetrahydrophthalic anhydride (MTHPA) in a tubular stratified glass-epoxy composite has been studied. The photo-chemical evolution was analysed by FT-IR spectrophotometry with attenuated total reflection (ATR) and by X-ray analysis. The analyses confirm the presence of a thin photo-oxidation layer at the surface of the material, which was shown by measurement of the organic matrixs ablation. After the establishment period, the layer and the ablation of the organic matrix evolve spatially at a constant rate. Despite an ablation of about 15–30 μm of the organic matrix, stress tests made on the samples artificially photo-aged for 1000 h or naturally for 2 years did not show any weakening in the material. This proves that photochemical degradation is superficial. For that reason, we designed a tension device that can function inside the artificial photo-ageing cell. Nevertheless, we wanted to verify if the combined action of mechanical stress, photo-oxidative constraints and temperature provoke a weakening in the material by synergy. We show that the effect of the mechanical stress can be analysed on a time scale significantly inferior to that of the physico-chemical stress.

Uniaxial Electromechanical Behavior of a Soft PZT: Experiments and Modeling

The aim of this work is to collect a database from uniaxial experiments and model the material behavior. A soft PZT is tested in compression; the influence of stress rate seems to be negligible and the material accommodates to cyclic mechanical loadings. These responses are simulated by means of a phenomenological model taking into account the coupling between remnant strain and polarization thanks to a mechanically induced depolarization function.

A Contribution to Time-Dependent Damage Modeling of Composite Structures

The paper presents a new damage model for predicting stiffness loss due to creep loading and cyclic fatigue. The model, developed within a continuum damage mechanics framework, is based on the idea of a time-dependent damage spectrum, some elements of which occur rapidly and others slowly. The use of this spectrum allows a single damage kinematic to model creep and fatigue damage and to take into account the effect of stress amplitude, R ratio, and frequency. The evolution equations are based on similar equation than the one describing the viscoelasticity model and are relatively easy to implement. The new model is compared to the experimental results on carbon fiber/epoxy tubes. Quasi-static, creep and fatigue tests are performed on filament-wound tubular specimens to characterize the elastic, viscoelastic and plastic behavior of the composite material. Varying amounts of damage are observed and discussed depending on stress level and R ratio. The experimental work aims to develop and validate the damage model for predicting stiffness loss due to creep loading and cyclic fatigue.

Towards the Development of Filament Wound Composite Structures Submitted to Very High Internal Pressure, Based on Complex Geometry Shapes

Industrial applications, especially composite structures bearing high internal pressure, and fabricated using the filament winding process face certain difficulties like the reinforcement of complex shapes, as well as the correct placement of fibers over the surface of a mandrel. In some cases the definition of the manufacturing parameters respond more to cost or time criteria rather than engineering standards, reducing largely the advantages of the said manufacturing process. In order to overcome these obstacles, this research aims to propose a solution that permits to fabricate complex shapes with the desired winding angles at a certain region of complex-shaped mandrels. A numerical tool that simulates the placement of fiber tows over the surface of complex geometries is developed and validated by means of the fabrication of convex and concave composite structures using detachable mandrels. Previous results show that it is feasible to wind complex geometries with good accuracy.Copyright