R. El Guerjouma

Creep Ruptures in Heterogeneous Materials

We present creep experiments on fiber composite materials with different controlled heterogeneity. All samples exhibit a power-law relaxation of the strain rate in the primary creep regime (Andrades law) followed by a power-law acceleration up to rupture. We discover that the rupture time is proportional to the duration of the primary creep regime, showing the interplay between the two regimes and offering a method of rupture prediction. These experimental results are rationalized by a mean-field model of representative elements with nonlinear viscoelastic rheology and with a large heterogeneity of strengths.

Andrade and critical time-to-failure laws in fiber-matrix composites: Experiments and model

Abstract We present creep experiments on fiber composite materials. Recorded strain rates and acoustic emission (AE) rates exhibit both a power-law relaxation in the primary creep regime and a power-law acceleration before global failure. In particular, we observe time-to-failure power-laws in the tertiary regime for acoustic emissions over four decades in time. We also discover correlations between some characteristics of the primary creep (exponent of the power-law and duration) and the time to failure of the samples. This result indicates that the tertiary regime is dependent on the relaxation and damage processes that occur in the primary regime and suggests a method for predicting the time to failure based on the early time recording of the strain rate or AE rate. We consider a simple model of representative elements, interacting via democratic load sharing, with a large heterogeneity of strengths. Each element consists of a non-linear dashpot in parallel with a spring. This model recovers the experimental observations of the strain rate as a function of time.

Elastic moduli of a 2.5d Cf/SiC composite : experimental and theoretical estimates

This study concerns the characterisation of the elastic properties of a long-fibre-reinforced ceramic-matrix composite. Seven of the nine independent elastic constants of a woven 2.5 D carbon-fibre reinforced SiC ceramic matrix have been measured by an ultrasonic technique associated with a numerical optimisation process. The elastic moduli are recovered by minimising the square deviation between measured and theoretical velocities. The ultrasonic measurements are discussed with the approximation of continuum mechanics, in regard to the wavelength and the size of the microstructural details of the material. For comparison, an estimate of the elastic moduli in all directions is performed with an Eshelby-based model, assuming the composite as a two-dimensional tow-reinforced matrix containing voids. The effective moduli for the tow composite structure are estimated from a first homogenisation step. Volume fractions of tow and matrix cracks slightly opened are taken into account in the microstructural description of the composite from experimental data on mean crack orientations, quantities and shapes. The specific effect of neglecting waviness is estimated from finite-element calculations. In the limit of the uncertainties on the phase moduli, the estimates of elastic moduli are in agreement with the available measured ones. As relevant, estimates are thus provided for the missing measurements.

Characterization of austenitic stainless steel welds for ultrasonic NDT

Electricite de France has started a study in collaboration with the Metallurgical and Materials Physics Study Group (GEMPPM) of INSA-Lyon, to evaluate the effect of metallurgical structures of austenitic stainless steel welds on wave propagation for application to ultrasonic nondestructive testing. Experimentally, the anisotropic and heterogeneous characteristics of austenitic welds together with a coarse-grained structure (elongated and oriented grains) lead to the following phenomena: scattering, attenuation, skewing, splitting, and divergence of the ultrasonic beam. To study and predict these phenomena, simulation studies are most helpful. The theory of wave propagation into anisotropic and homogeneous media already allows the prediction of beam skewing and divergence effects. When considering the more complicated case of heterogeneous anisotropic structures, simulation studies require realistic descriptions of the various kind of weld structures which can be encountered. The present paper discusses th...

Characterisation of anisotropic elastic constants of continuous alumina fibre reinforced aluminium matrix composite processed by medium pressure infiltration

The anisotropic elastic properties of an Al metal matrix composite unidirectionally reinforced with continuous α-alumina fibres fabricated, by a medium pressure infiltration technique, has been characterised using ultrasonic bulk and surface waves, tensile tests and microstructural analysis. The ultrasonic data are analysed under the continuum mechanics approximation. Elastic anisotropy is studied by varying the direction of propagation of ultrasonic waves. An optimisation process is used to recover all the volumic effective elastic moduli from bulk ultrasonic velocities. Two local (sub-surface) shear effective moduli are determined from the surface waves velocities. To discuss the elastic anisotropy of the material in relation with its microstructure and the manufacturing process, microstructural observations are correlated to the global and local ultrasonic evaluations and to the mechanical characterisation conducted in and out of symmetry axis.

Non destructive ultrasonic evaluation of fibrous metal matrix composites

An iterative numerical elastic constants optimization method associated to a classical ultrasonic immersion device is used for the entire characterization of two kinds of metal matrix composites. The usual methods of optimization are generally based on Newtons algorithm. Sometimes this algorithm converges towards relative minima that gives elastic constants which do not correspond to the physical reality of the material. As a remedy, we have developed a numerical analysis technique based on a different algorithm allowing a better convergence to the unique global solution. Using the Levenberg-Marquard algorithm, the methodology to recover the elastic constant, consists in minimizing the square deviation between calculated velocities and the experimental ones measured under variable incidence from a computer controlled ultrasonic immersion device.

Optimal determination of the elastic constants of woven 2D SiC/SiC composite materials

For homogeneous materials, the ultrasonic immersion method, associated with a numerical optimization process mostly based on Newtons algorithm, allows the determination of elastic constants for various synthetic and natural composite materials. Nevertheless, a principal limitation of the existing optimization procedure occurs when the considered material is at the limit of the homogeneous hypothesis. Such is the case of the woven bidirectional SiC matrix and SiC fibre composite material. In this study, we have developed two numerical methods for the determination of the elastic constants of the 2D SiC/SiC composite material (2D SiC/SiC). The first one is based on Newtons algorithm: the elastic constants are obtained by minimizing the square deviation between experimental and calculated velocities. The second method is based on the Levenberg–Marquardt algorithm. We show that these algorithms give the same results in the case of homogeneous anisotropic composite materials. For the 2D SiC/SiC composite material, the two methods, using the same measured velocities, give different sets of elastic constants. We then note that the Levenberg–Marquardt algorithm enables a better convergence towards a global set of elastic constants in good agreement with the elastic properties, which can be measured using classical quasi-static methods.

Anisotropic damage evaluation in polymer fiber composites under hygrothermal aging by means of ultrasonic techniques

Ultrasonics is a suitable technique for nondestructive evaluation of structural materials degradation. Although the most common objective of ultrasonic testing is the detection and location of overt flaws, ultrasonics can be used to identify microstructural factors that alter strength and performance. However, while single large defects can be individually detected and characterized, widely dispersed discontinuities are impossible to resolve and only their global effects on bulk properties can be observed and measured. The aim of this paper is, using an immersion technique, to characterize damage progression of a glass fiber epoxy matrix composite during hygrothermal aging by means of attenuation frequency-dependence measured in normal incidence and longitudinal and transversal ultrasonic velocities measured as function of the propagation direction. The anisotropic elastic constants of the material are recovered from the ultrasonic velocities using an optimization process. Effects of hygrothermal aging on...