Gérald Camus

Development of damage in a 2D woven C/SiC composite under mechanical loading: I. Mechanical characterization

Abstract An investigation has been undertaken to determine the damage mechanisms and the associated mechanical response of a 2D reinforced composite of carbon fibers in an SiC CVI-processed matrix subjected to uniaxial tensile and compressive loadings at room temperature. Under tension loading, an extended non-linear stress/strain response was evidenced and related to a multi-stage development of damage involving transverse matrix microcracking, bundle/matrix and inter-bundle debonding as well as thermal residual stress release. This tensile behavior proved to be damageable-elastic with respect to a fictitious thermalstress-free origin of the stress/strain axis lying in the compression domain. In compression, after an initial stage involving closure of the thermal microcracks present from processing, the composite displayed a linear-elastic behavior until failure. The extent of damage over the material was characterized quantitatively at the microscale by the decrease of the average transverse microcrack spacing and at the macroscale by the decrease of both the longitudinal Youngs modulus and the in-plane Poissons ratio.

Modelling of the mechanical behavior and damage processes of fibrous ceramic matrix composites: application to a 2-D SiC/SiC

A continuum damage mechanics constitutive model is developed to describe the mechanical behavior of fiber-reinforced ceramic matrix composites submitted to complex multiaxial loadings. This model relies on the use of a phenomenological internal damage variable defined as the change of the compliance tensor induced by any given loading. Microstructural observations of the damage entities provide qualitative data allowing us to formulate physically founded simplifying hypotheses. The evolution laws of scalar damage variables derived from the components of the compliance tensor are established within a classical thermodynamic framework, using coupled multicriteria expressed in the space of the associated thermodynamic forces. Damage deactivation processes related to the unilateral crack closure effect observed under compressive loadings are introduced through the definition of effective compliance tensor components. This approach is applied to a 2-D woven SiC/SiC composite processed by chemical vapor infiltration (CVI). Major advantages and drawbacks of the proposed approach as observed from various validation tests are discussed.

Oxidation effects on the mechanical properties of 2D woven C/SiC composites

Abstract The effects of oxidizing treatments on the room-temperature tensile properties of 2D woven C/SiC composites partly protected with a SiC seal-coating have been investigated between 700°C and 1400°C, for durations corresponding to a maximum of 6% relative mass loss. Results have shown three main mechanisms of composite property degradation corresponding to three well differentiated morphological changes. Oxidation at low temperatures ( 1100/dgC), ageing treatments led to a limited decrease of the tensile properties owing to a combined notch/overloading effect taking place only at high stress levels from the superficial degradation of the composite reinforcement. Both decreasing the oxygen content of the environment (i.e. from pure oxygen to dry air) or using unstabilized materials (i.e. unaged at 1600°C under argon prior to the oxidizing treatments) resulted in a better oxidation resistance.

A constitutive model for the mechanical behavior of a 3D C/C composite

A constitutive model is developed to describe the mechanical behavior of a 3D stitched carbon/carbon composite subjected to complex multiaxial loadings. This model lies within the framework of Continuum Damage Mechanics (CDM) for the description of damage accumulation, and the approach to plasticity for the description of inelastic residual strains induced by the onset of damage. Microstructural observations of the various damage entities provide qualitative data allowing us to formulate physically based simplifying hypotheses. The evolution laws of scalar internal damage variables derived from the components of the compliance tensor are established within a classical thermodynamic framework, using coupled multicriteria expressed in the space of the associated thermodynamic forces. The various processes involved in the non-linear behavior observed under compressive loadings are introduced through the definition of an effective compliance tensor increment. Major advantages and drawbacks of the proposed approach as observed from various validation tests are discussed.

CFC/Cu bond damage in actively cooled plasma facing components

Carbon fibre composite (CFC) armours have been successfully used for actively cooled plasma facing components (PFCs) of the Tore Supra (TS) tokamak. They were also selected for the divertor of the stellarator W7-X under construction and for the vertical target of the ITER divertor. In TS and W7-X a flat tile design for heat fluxes of 10 MW m−2 has been chosen. To predict the lifetime of such PFCs, it is necessary to analyse the damage mechanisms and to model the damage propagation when the component is exposed to thermal cycling loads. Work has been performed to identify a constitutive law for the CFC and parameters to model crack propagation from the edge singularity. The aim is to predict damage rates and to propose geometric or material improvements to increase the strength and the lifetime of the interfacial bond. For ITER a tube-in-tile concept (monoblock), designed to sustain heat fluxes up to 20 MW m−2, has been developed. The optimization of the CFC/Cu bond, proposed for flat tiles, could be adopted for the monoblock concept.

Damage prediction of carbon fibre composite armoured actively cooled plasma-facing components under cycling heat loads

In order to predict the lifetime of carbon fibre composite (CFC) armoured plasma-facing components in magnetic fusion devices, it is necessary to analyse the damage mechanisms and to model the damage propagation under cycling heat loads. At Tore Supra studies have been launched to better understand the damage process of the armoured flat tile elements of the actively cooled toroidal pump limiter, leading to the characterization of the damageable mechanical behaviour of the used N11 CFC material and of the CFC/Cu bond. Up until now the calculations have shown damage developing in the CFC (within the zone submitted to high shear stress) and in the bond (from the free edge of the CFC/Cu interface). Damage is due to manufacturing shear stresses and does not evolve under heat due to stress relaxation.For the ITER divertor, NB31 material has been characterized and the characterization of NB41 is in progress. Finite element calculations show again the development of CFC damage in the high shear stress zones after manufacturing. Stresses also decrease under heat flux so the damage does not evolve. The characterization of the CFC/Cu bond is more complex due to the monoblock geometry, which leads to more scattered stresses.These calculations allow the fabrication difficulties to be better understood and will help to analyse future high heat flux tests on various mock-ups.

Characterisation and Modelling of the Non Linear Mechanical Behaviour of SEPCARB[registered trademark] Pr NB41 Carbon/Carbon Composite

Abstract Design and optimization of a plasma facing component calls for dedicated constitutive laws which can handle the non linear behaviour typical of composite materials when applying this class of materials. This paper depicts the different phases of characterization and modelling which are required. Compared to a classical elastic analysis, this approach provides a more realistic estimate of the stress distribution within a thermally loaded PFC as demonstrated by finite element computations performed on a plasma facing component with a macrobloc® geometry.

Modeling of Simultaneous Oxidation and Volatilization Phenomena along a Crack in a Self-healing Multi- constituent Material

Multilayer ceramic matrix composites with self-healing capacities have been developed for high-temperature aeronautical applications. However, the use of these sophisticated materials leads to complex thermo-chemical behavior which requires a proper analysis. Previous models have allowed to determine the quantity of oxygen reaching the fiber at the end of a crack, in a dry atmosphere. The focus of this study was to extend this model to the case of humid atmospheres, for the presence of moisture can lead to the volatilization of the healing oxide. Applications of the model were conducted on a material composed of successive layers of SiC and B4C. Changes of the crack wall surface could then be evaluated, as well as the healing or recession behavior, and the evolution of the oxygen concentration. Moreover, the introduction of a uniform equivalent averaged material allowed estimations of the consumption of all constituents, and in particular of carbon interphase, over durations representative of aeronautical service lives.

Influence of the Cooling Temperature on the Mechanical Behavior of an Oxidized Self-Healing Ceramic Matrix Composite

Composites made of ceramic for both fibers and matrix are aimed at being used in aerospace applications, which means being submitted to mechanical stresses at high temperatures in oxidizing and corrosive environments for long durations. The oxidation/corrosion resistance of these materials is linked to their ability to self-heal by creating an oxide phase which can restrict the access of the oxygen into the bulk of the material. During thermomechanical cycles, the distribution of O2 inside the material as well as the distribution and the state of the liquid oxides will undergo some modifications. A specific test has been conceived and performed in order to evidence the influence of the viscosity of this oxide on the mechanical properties of the material. Results have shown that the stiffening which is observed seems to be proportional to the increase in the viscosity of the sealing oxide, in conjunction with the decrease in the temperature.