C. Olagnon

Thermal shock and thermal fatigue study of ceramic materials on a newly developed ascending thermal shock test equipment

Abstract A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples.

Creep behaviour of alumina, zirconia and zirconia-toughened alumina

Creep behaviour of various alumina, zirconia and zirconia-toughened alumina is investigated. A very large scale of creep rates and creep behaviours is observed. Creep rate depends on the grain size, on the purity and on the composition of the glassy phase present in grain boundaries. Cavitation and microcracking by grain boundary sliding have been identified as the main creep mechanisms.

Thermal shock and thermal fatigue study of alumina

Preliminary results on the thermal shock and thermal fatigue behaviour of alumina studied in a newly designed novel and simple test equipment are reported. The top surface of a test sample was heated by an oxy-hydrogen flame while the opposite surface was cooled to generate temperature and thermal stress gradients. The maximum stressed zone and the effect of the thickness of the sample on critical temperature difference (TC) were studied by conducting actual experiments on plain and indented alumina specimens and also by modelling temperature and thermal stress distribution in the sample using a finite element (FE) software. It was observed that the maximum stress was experienced near the periphery of the top surface and TC was more for thicker samples. Thermal fatigue study was conducted by varying temperature difference of the fatigue cycles and also by inducing different crack lengths in the sample. It was observed that the fatigue life sharply decreased with increase in initial crack length or by increasing the temperature difference of the fatigue cycles. The acoustic emission (AE) signals corresponding to formation and growth of large number of micro-cracks were observed. # 2002 Elsevier Science Ltd. All rights reserved.

Room temperature quasi-brittle behaviour of an aluminous refractory concrete after firing

Abstract The study deals with the thermomechanical behaviour of aluminous refractory concretes used in steel plants. In the temperature range where plasticity is negligible, the material presents a pronounced non linear behaviour both in uniaxial tensile and compressive modes, with a quasi brittle fracture. The behaviour is characterised by damage, firstly diffuse, then more localised, which finally leads to the formation of a macrocrack. The threshold of damage is very low in tension (≈ 5% of the maximum load) and higher in compression (≈ 40% of the maximum load). An approach, in terms of composite material, is proposed to analyse the damage behaviour. The concrete is, therefore, considered as a composite material, formed by coarse aggregates embedded in a fine matrix. The different mechanical behaviour observed between the matrix alone and the concrete can be explained by two phenomena : a structural temperature-independent effect and a temperature-dependent effect. Finally, the authors show that the real tensile behaviour of the refractory concrete, observed in the present work, is far from the description generally used in classical damage models.

Experimental characterisation of high temperature creep resistance of mullite

Abstract The creep behaviour of mullite has been studied at temperatures between 1100 °C and 1450 °C. The two standard stages, primary and steady state are observed. The steady state strain creep rate may be represented by the standard relationship: e = Aσ n exp (− Q RT ) with a stress exponent value n equal to about unity and an activation energy Q of about 410 kJ mol at low temperatures and up to 731 kJ mol for temperatures above 1300 °C. At high stress or temperature specimens failed. SEM observations of the fracture surface revealed that SCG played a major role in the failure process. The experimental lifetime values were compared to a simulation conducted by integration of V-KI laws determined by Double Torsion technique at 1200 and 1300 °C. A fair agreement is observed suggesting that the creep duration is limited by SCG which causes the material failure, as assumed by Lange.

Acoustic emission monitoring of damage evaluation in ceramics submitted to thermal shock

In studies conducted on two ceramic materials, namely a porous alumina and a dense zirconia, it is shown that a simple acoustic emission experiment can provide valuable information on the processes of thermal shock degradation in the materials.

Crack propagation behaviour in mullite at high temperatures by double-torsion technique

Abstract The double-torsion technique has been used to study subcritical crack growth in mullite at room temperature (RT) and above. The crack velocity was evaluated by measuring a change in crack length during a prescribed time under a constant applied load. Threshold values of stress intensity factor, below which no propagation occurs, of 1.5 and 1.6 MPa m 1 2 were found at RT and high temperature, respectively. Considering a power law, measured stress exponents of 24, 35, 18 and 10 at RT, 1100, 1200 and 1300 °C were obtained. A change of crack growth mechanisms is observed around 1200 °C, with cleavage crack growth occurring below 1200 °C and viscous intergranular crack growth dominating at higher temperatures.

High-temperature deformation of mullite and analysis of creep curves

The creep behavior of mullite was studied under different stresses and in the temperature range 1200-1450 °C, and an analysis of creep curves was proposed. The study of creep behavior of mullite at high temperatures clearly indicates that this material exhibits concurrent creep and slow crack growth. An effective transition stress exists at each temperature. The analysis takes account of the total creep curve; in particular, the primary and stationary stages. It is now possible to determine by extrapolation the steady-state creep rate for specimens that break in the transient domain during tests. Thus, one can verify the influence of the stress on the steady-state creep rate over a wide stress range. On the other hand, this analysis clearly indicates the existence of two values of the activation energy around 1300 °C; this suggests a change of creep mechanism at this temperature.

Experimental Analysis of Glassy Polymers Fracture Using a Double Notch Four Point-Bending Method

ABSTRACT A twin notch specimen under four point bending is designed to analyse the mechanisms and the properties of glassy polymers fracture. The two notches are submitted to an identical bending moment so that one will fail and provides a measure of the toughness while the other serves as a snap-shot of the strain fields prior to unstable crack propagation. The evolution of the toughness with the loading rate and the influence of the notch radius is analyzed for both PMMA and PC.