# Wedge-Splitting Test on Carbon-Containing Refracories at High Temperatures
## 1 Introduction  
The wedge-splitting test is performed on notched prismatic specimens that enable stable crack formation for relatively large specimens [1; 2]. The reasons are the action of the wedge and the relatively high fracture surface to sample volume ratio. The specimen length is 100 mm, the height is also 100 mm, and the thickness 75 mm. A three-dimensional schematic representation of the sample including the load transmission parts is shown in Figure 1. The wedge-splitting test (WST) at room temperature is performed with a loading rate of 0.5 mm/min. During the test, a video extensometer detects the horizontal displacement at the measuring points. The displacement is measured at the height where the rollers contact the load transmission elements. The sample rests on a linear support (\(3\times 3\times 90\) mm\({}^{3}\)) to allow for the free vertical displacement of the sample except for the support. The linear support, as well as the wedge and the rollers, are made of corundum to withstand the testing temperatures. From the results, the specific fracture energy \(G_{f^{\prime}}\) and the nominal notch tensile strength \(\sigma_{NT}\), can be calculated directly from the load-displacement curve (Equations (2) and (3)). The horizontal force \(F_{H}\), which is needed for Equations (2) and (3), is calculated from the geometry of the wedge and the measured vertical force \(F_{V}\) (Equation (1)):  
\[F_{H}=\frac{F_{V}}{2 tan\frac{\alpha}{2}} \tag{1}\]  
In this equation, \(\alpha\) is the wedge angle (10\({}^{\circ}\)). \[G_{f^{\prime}}=\frac{1}{A}\int_{0}^{\delta}F_{H}d\delta_{H} \tag{2}\]  
The specific fracture energy \(G_{f^{\prime}}\) is the area below the load-displacement curve divided by the ligament area A. The measured horizontal displacement is defined with \(\delta_{H}\), and the horizontal displacement at 15% of the maximum load is \(\delta\). Here \(G_{f^{\prime}}\) (Equation (2)) is the specific fracture energy directly evaluated from the load-displacement curve. This parameter includes only the major part of the specific fracture energy because a premature termination of the test is necessary to avoid the contact of the wedge with the ligament. This means that the WST is stopped before the force reaches zero. The nominal notch tensile strength \(\sigma_{NT}\) is calculated according to Equation (3):  
\[\sigma_{NT}=\frac{F_{H,max}}{bh}\bigg{(}1+\frac{6y}{h}\bigg{)} \tag{3}\]  
Here, \(F_{H,max}\) is the maximum horizontal load, \(b\) and \(h\) are the width and the height of the ligament, and \(y\) is the vertical distance from the loading position to the center of the ligament. The simplest possibility for the displacement measurement is to make use of the crosshead travel. Unfortunately, this is not accurate because deformations of the testing machine influence the results as  
Figure 1: Schematic representation of the wedge-splitting test. well as the adjustment of the load transmission parts during load application. Out of these reasons, it is advantageous to measure the displacement directly on the sample. At elevated temperatures, one possibility is to measure the displacement with an optical dilatometer. Here a halogen lamp illuminates the sample on one side and on the other side the shadow cast by the sample against the light source is recorded by a high-resolution complementary metal-oxide-semiconductor (CMOS) camera [3]. With this method, the displacement is measured on one side of the sample only. Although the performance of the wedge splitting test at room temperature with optical displacement measurement is state of the art [4], at elevated temperatures, the optical measurement is far more difficult due to the radiation and convection in the furnace chamber. High-temperature testing above 600 \({}^{\circ}\)C of carbon-containing refractories shall be carried out in non-oxidizing atmospheres. Otherwise, the oxidation of carbon \(2C_{(g)}+O2_{(g)}=2 CO_{(g)}\) takes place [5], and the microstructure and the properties will be changed significantly [6].