Marc Huger

Rheological properties of Y-Si-Al-O-N glasses — elastic moduli, viscosity and creep

Three oxynitride glasses from the Y-Si-Al-O-N system and differing in their N/O ratio were studied in the 800–1000 °C temperature range. Their viscosities were measured using a threepoint bending test through the glass transition domain. For a given temperature, 4.8 wt % N2 enhances the viscosity by three orders of magnitude in comparison with the corresponding oxide glass. Nitrogen also improves creep resistance. The activation enthalpy for creep, aboveTg, is of the same order as those measured for silicon nitride ceramic (∼900 kJ mol−1). The elastic moduli were determined by ultrasonic techniques, from room temperature up to 1200 °C, which allowed calculation of the free activation enthalpy for viscous flow. Owing to the sharp decrease of shear modulus in the glass transition domain, the free activation enthalpy (∼500 kJ mol−1) greatly differs from the activation enthalpy usually measured in creep studies.

Temperature dependence of Young's modulus in Si3N4-based ceramics: roles of sintering additives and of SiC-particle content

Abstract The temperature dependence of Youngs modulus has been investigated by ultrasonic-echography in the 20–1400 °C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si 3 N 4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920 °C. The transition observed in ceramics is located between 1080 and 1150 °C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor ( b ) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.

Ultrasonic measurement of Young’s modulus MgO/C refractories at high temperature

Abstract The paper deals with the elastic behavior of MgO/C refractories used in BOF at temperatures up to 1400°C in air or inert atmosphere. Measurements have been made by the way of a high temperature ultrasonic technique. Heating-cooling cycles and long time aging in the range 700–1400°C show strong variations of Young’s modulus which have been interpreted with the aid of XRD analysis, SEM observations and EDS analysis. Carbon oxidation and sintering of MgO particles are found to be responsible of the major parts of the measured evolutions. ©

Time delay and phase shift measurements for ultrasonic pulses using autocorrelation methods

At the interface between two media, a monochromatic ultrasonic pulse separates into two waves, a reflected one and a transmitted one. A simple hypothesis is to consider the phase shift, introduced by the reflection, as equal to 0 or π. But, in many cases, it becomes necessary to know the phase shift precisely. This is done by a Fourier analysis of the different signals. This article deals with mathematical methods that can be used for time and/or phase shift measurements. A new method, based on an analysis of the analytic signal of the autocorrelation function, is described and compared to existing phase slope and autocorrelation methods. An application of this method is given in the case of a ‘‘long bar’’ mode propagation at the interface between alumina and glass.

Thermo-elastic behaviour of a natural quartzite: itacolumite

Itacolumite is a particular type of sandstone constituted of quartz grains, with an interlocked microstructure and large intergranular decohesions. This article is devoted to the study of the thermo-elastic behaviour of this material during thermal cycles between 20 and 800 °C. This was made by using an ultrasonic pulse-echo measurement technique of Young’s modulus E, coupled to thermal expansion experimentation and to acoustic emission. An unusual evolution of elastic properties versus temperature is found, with strong irreversible effects around the temperature of the α–β transition of quartz. A damage parameter, representing the fractional number of cracks, is calculated with a Kachanov type formulation, by using the experimental data for itacolumite and the values of E, obtained from literature for an hypothetic ideal quartz polycrystal. The evolution of this parameter versus temperature is explained by internal stresses that are developed by both thermal and elastic effects in quartz grains.

On the glass transition domain in some M-SiAlON (M = Y or Ln) oxynitride glasses

The behaviour in the glass transition domain of some oxynitride glasses has been studied by thermoanalytical methods (dilatometry and differential thermal analysis) and mechanical techniques (creep and ultrasonic measurements of Youngs modulus). The thermoanalytical data are in good agreement with the glass transition domain defined from viscosity data. The sharp decrease of Youngs modulus, that starts at the temperature of the strain point, is compared to results obtained from mechanical spectroscopy by other authors. The difference in the apparent activation energies for viscous flow above and below the temperature of the strain point is used to separate the contribution of the thermal and structural components. The high temperature apparent activation energy of viscosity is in fair agreement with that of the α-relaxation peak described by the formalism of hierarchically correlated molecular mobility.

Ultrasonic investigation of the time-dependent damage in a 2D SiC/SiC composite under static loading

Abstract A time dependence of damage under static tensile loading is observed at room temperature in a 2D SiC/C/SiC composite by means of ultrasonic measurements of uniaxial Young’s modulus, associated with acoustic emission and strain measurements. During ageing under a constant stress above the threshold level necessary for the first crack initiation in the matrix, a significant decrease of Young’s modulus with time occurs, correlated with acoustic emission, which indicates a damage evolution. Additionally it is found that both elastic and inelastic strains increase with time. These static fatigue effects depend on the loading conditions (loading rate and maximum stress level) and lead either to an equilibrium state or to delayed failure for sufficiently high stresses. An interfacial shear stress dependence upon the load level is the most probable mechanism to explain these phenomena.

Refinement of digital image correlation technique to investigate the fracture behaviour of refractory materials

Refractory materials exhibit a heterogeneous microstructure consisting in coarse aggregates surrounded by fine grains that form an aggregate/matrix composite. This heterogeneous microstructure often leads to a complex mechanical behaviour during loading. This paper is devoted to the study, thanks to an optical method, Digital Image Correlation (DIC), of the fracture behaviour of two industrial refractory materials in relation with their microstructure resulting from both the chosen constituents and the sintering process. The aim is here, specifically, to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. DIC is indeed a helpful and effective tool, in the topic of experimental mechanics, for the measurement of deformation in a planar sample surface. This non-contact optical method directly provides full-field displacements by comparing the digital images of the sample surface obtained before and during loading. In the present study, DIC has been improved to take into account the occurrence of cracks and performed so as to better identify the early stage of the cracking behaviour. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex and a discontinuity of displacement should be taken into account. Then each subset which crosses a crack can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening).

ULTRASONIC MEASUREMENT OF YOUNG'S MODULUS IN DOG-BONE-SHAPED SAMPLES SUBJECTED TO A TENSILE STRESS

Thermostructural applications of fibrous ceramic composites require the knowledge of their behavior in severe service conditions (high temperature, high stress, oxidizing atmosphere). Previous results have shown that ultrasonic measurements of Young’s modulus are particularly suitable to monitor oxidation mechanisms at high temperature in these materials. Such measurements in materials subjected to tensile stress are possible by modeling the ultrasonic wave propagation in ‘‘dog‐bone’’ specimens. The tensile specimens are modeled as a ‘‘slender bar’’ with two heads assumed to be thin plates (thickness less than the wave length). The frequency analysis of the signal allowed the determination of the optimal dimensions of the specimen and the calculation of time corrections in order to evaluate the real ultrasonic propagation time t0 in the material. The ultrasonic wave velocity and the Young’s modulus are then derived from the knowledge of t0.

Microstructural effects associated to CTE mismatch for enhancing the thermal shock resistance of refractories

This work is devoted to the study of thermomechanical properties of several industrial and model refractory materials in relation with the evolution of their microstructure during thermal treatments. The aim is, in particular, to highlight the role of thermal expansion mismatches existing between phases which can induce damage at local scale. The resulting network of microcracks is well known to improve thermal shock resistance of materials, since it usually involves a significant decrease in elastic properties. Moreover, this network of microcracks can strongly affect the thermal expansion at low temperature and the stress/strain behaviour in tension. Even if these two last aspects are not so much documented in the literature, they certainly also constitute key points for the improvement of the thermal shock resistance of refractory materials. Evolution of damage during thermal cycling has been monitored by a specific ultrasonic device at high temperature. Beyond its influence on Youngs modulus, this damage also allows to decrease the thermal expansion and to improve the non-linear character of the stress/strain curves determined in tension. The large increase in strain to rupture, which results from this non-linearity, is of great interest for thermal shock application.