Introduction
Fracture toughness is a measure of solid materials resistance to fracture, or the amount of energy required to propagate a crack when stress is applied. It is an important mechanical strength property which can be used to define the behavior of materials under dynamic loading, fatigue and fracture. The fracture toughness of graphite is of particular interest as it is very brittle and exhibits relatively low strength.
Types of Graphite
Graphite is a form of carbon which exists in two major varieties – amorphous and crystalline. Amorphous graphite has a disorganized, relatively unstructured form. It is less ordered than crystalline graphite and is generally used as a lubricant. Crystalline graphite, on the other hand, is composed of flake graphite flakes with a highly ordered structure produced by annealing. It is more resistant to fracture than amorphous graphite and is often used in applications where a higher strength material is required.
Measures of Fracture Toughness
There are a number of different ways that the fracture toughness of graphite can be measured. Some of the most common include 3-point bend test, 4-point bend test, and tear propagation test. The 3-point bend test uses two fixed points at the sides of a sample and a single load point at the bottom center. The sample is then bent until it fractures, and the energy to fracture is measured. The 4-point bend test is similar but uses four fixed points. The tear propagation test is a destructive test which is used to measure the energy needed to propagate a crack.
Effect of Temperature on Fracture Toughness
The fracture toughness of graphite is affected by temperature. As temperature increases, the amount of energy required to cause a crack in the sample increases. This increase in energy is due to the increased strength of the material at higher temperatures. Conversely, as temperature decreases, the amount of energy needed to fracture the sample decreases.
Effect of Structure on Fracture Toughness
The structure of graphite can have a considerable effect on its fracture toughness. For example, crystalline graphite has a higher fracture toughness than amorphous graphite due to its highly ordered structure and greater strength. Similarly, the size of the graphite flakes in the sample can have an effect. Smaller flakes are less ordered and more brittle, resulting in lower fracture toughness.
Conclusion
Graphite is a very brittle material, although it does exhibit some degree of fracture toughness. The fracture toughness of graphite can be affected by factors such as temperature, structure, and size of the graphite flakes. It is an important property for applications requiring higher strength materials, such as in the aerospace industry.
