The F.T.-CIREP Test was designed to simulate the degradation of refractories by the action of low intensity and repetitive thermal shocks. Such thermal fatigue conditions prevail in most high temperature industrial confinement units.
The novel equipment used for this test is composed of an electric furnance, a water cooled copper plate linked to a thermostatic bath, a push/pull actuator and a microcomputer. Once the furnace has reached its desired temperature, the specimens (preferably a pair) are placed in a holder, the test parameters are introduced into the control programand the test is started. The computer automatically moves the specimens in and out of the furnace for a given number of cycles. When the specimens are pulled out of the furnace, they are moved onto the copper plate for thermal shock. Typically, the complete thermal cycle is 42 min duration, of which 30 min is heating and 12 min is cooling. The duration of the moving operation is a little less than a second and its variance is in the range of a tenth of a second
Typically, 25mm x 25mm x 150mm sample bars are used. The specimens are cut from bricks or pre-fired cast blocks and are rectified on their four principal facets. Specimens are dried at 110°C for 24 hours prior to testing.
The typical testing procedure uses four pairs of specimens. After drying, each specimen is weighted and its dimensions (length, thickness and width) are measured. The initial elastic properties are also measured non-destructively.
Each pair of specimens is submitted to thirty five shock cycles at different temperature drop amplitude: 200°C, 400°C, 600°C and 800°C. The residual stiffness and the weight are measured seven times during a test at predetermined cycle numbers: 0,1,2,3,5,8 and 35. With these parameters, it takes 24 hours to test a pair of specimens and four days to complete the characterization.
After the test, the damage evolution curves (DEC) are plotted on a log-log scale. On such a scale, all data points obtained at a given temperature drop amplitude fall into a straight line. The DEC are fitted with the following equation:
where N is the number of cycles and m and b are the curve parameters which vary as a function of the temperature drop amplitude, and depend on the material. Material rating is made according to the value of the m and b parameters, as well as their evolution with repect to the temperature drop amplitude.
Fig. 1: Set-up used for thermal fatigue resistance measurement of refractories with automatic monitoring of samples lost of elastic properties during the test.
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