Application Background:
The high-temperature universal testing machine is a key equipment for evaluating the mechanical properties of refractories in high-temperature environments. Refractory materials are usually subjected to the synergistic effect of high temperature, thermal shock and mechanical load, and their thermal flexural strength is directly related to the safety and service life of industrial kilns, metallurgical vessels and ceramic firing structures. The testing machine can apply a bending load to the specimen at a set temperature and record the stress value at breaking, thereby quantifying the flexural resistance of the material at high temperature.
The test follows relevant national standards and international common methods, such as placing a standard-sized cuboid specimen on a three- or four-point bending fixture and loading it at a constant rate until it breaks. Parameters such as heating and heating speed, holding time and atmosphere control should be adjusted according to the characteristics of the material. Formula (1) is used to calculate flexural strength σ_f (in MPa):
σ_f = 3FL / (2b h²) (Equation 1)
where F is the maximum load (N), L is the lower span (mm), b is the width of the specimen (mm), and h is the height of the specimen (mm).
Test process
Before the test, the specimen should be dried or pre-burned to eliminate the interference of moisture and volatiles. During the heating process, the temperature uniformity of the furnace should be controlled within ±5 °C to avoid measurement deviations caused by temperature gradients. The holding time is usually 30 minutes to 2 hours, depending on the thermal conductivity and thickness of the material. The loading rate is generally set to 0.5~2 mm/min, and too fast loading may lead to dynamic effects, making the measured value too high. After the test, the fracture position was recorded and the fracture morphology was observed to assist in judging the failure mode.
The core components of the high-temperature universal testing machine include a high-temperature furnace, a ceramic pressure rod and a heat-resistant fixture. Heat-resistant fixtures are often made of silicon carbide or corundum, and creep and oxidation problems need to be paid attention to above 1500 °C. The atmosphere can be introduced with air, inert gas or reduced atmosphere as needed to simulate real working conditions. Table 1 lists the reference range of thermal flexural strength of common refractory materials.
Table 1 Reference range of thermal flexural strength of common refractories (1200°C)
| Material type | Flexural strength (MPa) |
| Alumina-based refractory bricks | 4.0～8.5 |
| Silica refractory bricks | 2.5～5.0 |
| Silicon carbide refractory | 8.0～12.0 |
| High chromium refractory castable | 6.0～9.5 |
Influencing factors
Flexural strength test results are influenced by multiple factors: the surface condition of the specimen (e.g., roughness, defects), the accuracy of loading and centering, the chemical reaction between the fixture and the specimen at high temperatures, and the temperature field distribution in the high-temperature furnace. In actual operation, the equipment should be calibrated regularly with standard samples to ensure the traceability of the load cell and displacement sensor. In addition, brittle materials may exhibit viscoplastic behavior at high temperatures, resulting in nonlinearity of stress-strain curves, so it is necessary to strictly distinguish between elastic fracture and plastic fracture. For this type of material, a nonlinear correction factor can be introduced on the basis of Equation 1, but it needs to be fitted based on a large number of experimental data.
To reduce random error, at least 5 parallel samples were used for each set of tests, and the mean and standard deviation were calculated. If the specimen shows early fracture at the beginning of loading, check for macroscopic cracks or uneven heating. At the same time, the thermal insulation efficiency of the high-temperature furnace needs to be checked regularly to prevent heat radiation from affecting the accuracy of adjacent components.
Application prospects
The high-temperature universal testing machine plays an irreplaceable role in the field of refractory research and development and quality inspection. By systematically evaluating the thermal flexural strength under different formulations and preparation processes, reliable data can be provided for material selection, process optimization and life prediction. In the future, as test temperatures expand to a higher range (above 1600°C) and the demand for multi-field coupling (thermal-force-atmosphere) increases, equipment will need to further improve high-temperature tightness and load stability. The use of digital twin technology and real-time data acquisition system is expected to realize intelligent monitoring and abnormal warning of the testing process, and promote the development of refractory performance evaluation in the direction of high throughput and high precision.
In summary, this technical solution has been maturely applied in steel, building materials and chemical industries, supporting the safe design of structural materials under high temperature working conditions. Continuous improvement of standardized operation and data processing methods will further highlight its value in engineering materials research.
citation
1. National standard GB/T 3002-2017 on the test method of high-temperature flexural strength of refractory materials.
2. The international standard ISO 5013:2013 is a general requirement for thermal flexural strength testing of refractory materials.
3. Related research review: Analysis of the influence of fixture and atmosphere on measurement results in high-temperature mechanical testing.