# Wedge-Splitting Test on Carbon-Containing Refracories at High Temperatures
## 3 Device  
The wedge splitting test device (universal testing device with displacement measurement) was developed in close cooperation with the company ZwickRoell Testing Systems GmbH (Furstenfeld, Austria). An existing universal testing device with a rigid frame and two rotating spindles were equipped with several components in order to perform the wedge-splitting test at high temperatures. A schematic drawing of the device is illustrated in Figure 2. The load cell is integrated in the lower piston and the maximum load capacity is 300 kN at room temperature. The vertical load rate is set to 0.5 mm/min for the wedge-splitting test.  
In this device, a gas-tight furnace was installed (Figure 2, Figure 3). The furnace has two windows (front and rear) in order to measure the displacement during the test. The maximum testing temperature is 1500 \({}^{\circ}\)C. In order to protect the carbon in the samples from oxidation, the furnace chamber can be purged with an inert gas. The purging rate depends on the material and on the testing temperature  
Figure 2: High-temperature testing device (courtesy of Messphysik).  
and can be adjusted between 8 and 70 L/h. The adjustment of the purging rate was done with a valve at the digital flowmeter.  
For the displacement measurement during the wedge-splitting test, the device was equipped with two laser-speckle extensometers. One is situated in the front and the other one in the rear of the furnace. The sample was illuminated with green laser light (wavelength: 532 nm), creating a speckled pattern on the sample surface. The movement of the speckled pattern was recorded with two full-screen digital cameras. In each evaluation area, a displacement measurement of the speckled pattern was performed. The increase in the distance between the measurement points is calculated. It is called horizontal displacement in this paper [7].  
The setup of the furnace including the laser-speckle extensometers is shown in Figure 4 (all units in mm).  
Each extensometer has two lasers and two cameras. The green lines in Figure 4 show the course of the laser beam. The distance of the measuring points was set to 40 mm. This initial measurement length can be adjusted according to the sample geometry. The distance between the position of the camera and the sample surface is 380 mm.  
The advantage of a laser-speckle-extensometer compared to a mechanical extensometer is that the sample and the extensometer fingers are not in contact. Contacting extensometer fingers could be damaged during the cracking of the sample, furthermore, a possible deformation of the extensometers could influence the results. An advantage compared to a standard laser extensometer is that an application of measuring marks to increase the contrast is not necessary because of the characteristic speckled pattern. The resolution of the strain measurement is 0.11 \(\upmu\)m according to the specifications from ZwickRoell. The scattering of the measurement signal during a high-temperature test was below 3 \(\upmu\)m. This value was determined together with the company ZwickRoell in the course of preliminary investigations. The horizontal displacement was measured in the front and rear side, the average value was used for the evaluation. Figure 5 shows the configuration after a test.  
Figure 4: Furnace with two laser-speckle extensometers (courtesy of Messphysik).  
Figure 3: Gas-tight furnace (courtesy of Messphysik).  
The alignment of the rolls and the wedge is essential to achieve symmetrical displacement on the front and rear side of the sample.