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Brittle materials are very sensitive in nature. They easily break up or fail when they are exposed to sudden shock loads. Brittle materials fail by fracture rather than yielding. A good example for the brittle material would be cast iron.
Brittle fracture occurring due to a tension force is due to the normal tensile stress created by the corresponding force. A material failing under brittle compression is mainly due to a combination of compressive stress and shear stress. Before we proceed into the various theories let us look into the various types of brittle materials. They are
1. Even Brittle Materials: The even materials are called so because their tensile strength equals the compressive strength of the material. A good example for this would be the fully hardened tool steel.
2. Uneven Brittle Materials: Brittle materials normally tend to have compressive strength which is very much greater than their tensile strengths. Typical grey cast iron possesses this type of property and hence forms a good example for Uneven Brittle materials.
We have come across Mohr’s circle diagram in many cases. Click here to see the Mohr’s circle diagram for an Even and Uneven material. This will tend to give you a clear picture on the difference between an Even and Uneven material.
The main theory that contributes for the failure of brittle materials under tension is the Modified Mohr theory. This applies mainly to the uneven brittle materials failing during static loading. The theory is based on calculating a common “Effective Stress" value which takes into consideration all types of stresses acting on a body like tensile stress, shear stress, etc.
The Mohr’s effective stress is calculated on the basis of the three principal stresses. For the material to sustain failure, the effective stress value should be lesser than the tensile strength of the material and not the yield strength of the material. This is how the factor of safety is calculated.
Further brittle materials come under a special category of Fracture mechanics for failure. This is applicable for both ductile and brittle materials. We will look in detail into this in the forthcoming articles.