High temperature flexural strength and high temperature volume stability
High temperature flexural strength and high temperature volume stability are important physical characteristics of materials that aid in the design and performance of structural components. Both characteristics can be evaluated through a variety of tests, including fatigue cracking, thermal shock, porosity and strength measurements. High temperature flexural strength is determined from flexural tests performed on a sample material at increasing temperatures; the curvature of the sample is also measured to determine its resistance to flexure. Thermal shock tests involve rapidly heating and cooling a material to determine if its structure is resistant to rapid temperature changes. Finally, porosity and strength measurements involve working a sample material at various temperatures and measuring its long-term strength and resistance to deformation.
High temperature flexural strength is primarily determined by the composition of a materials microstructure. At high temperatures, bonds between the materials microscopic particles become weakened, leading to plastic deformation and loss of strength. This plastic deformation is most commonly caused by twin boundaries, or the boundaries between crystallographic grain boundaries. Additionally, heterogeneous grain structures, such as bimodal grain distributions, can also reduce strength by promoting localized stress concentrations.
High temperature volume stability is evaluated through a variety of tests, including ASTM D4648-12 and ASTM D4648-13. ASTM D4648-12 evaluates a materials resistance to thermal expansion and contraction, while ASTM D4648-13 evaluates its dimensional stability. The tests are performed by heating the sample material to a specified temperature and then measuring the linear shrinkage. The results of these tests can be compared to similar tests performed at different temperatures to identify any abnormal behavior. For example, a material may become very olivine or undergo crystallization at certain temperatures.
Finally, both high temperature flexural strength and high temperature volume stability can be improved through the introduction of additives. For example, addition of graphite, boron nitride, and molybdenum disilicide can improve both properties. In general, the addition of any material that has a higher thermal expansion coefficient than the base material can help improve flexural strength. Furthermore, some materials, such as plastics, can also be blended together to create a modified material that has improved high temperature flexural strength and volume stability.
High temperature flexural strength and high temperature volume stability are important characteristics of materials and components used in structural applications. By evaluating the composition of a materials microstructure and performing tests to identify any abnormal behavior, engineers can determine if the material has the necessary properties for a given application. Furthermore, addition of certain additives, such as graphite and boron nitride, can further improve the materials properties.
