Mechanical Engineering - Strength of Materials - Discussion

Oblique plane is the correct answer. I agree.

The angle can be specified as 45 degrees for a brittle material.

A brittle material will always fail along tge plane which is normal to the stress axis for uniaxial tension as it follows the maximum normal stress condition.

Option C is correct. I too agree.

For tensile test it will in the right angle plane but for compression it fail in oblique plane normally 45deg.

In case of brittle material, the failure due to compression will be shear crack failure at 45degree which is an oblique plane so the answer is B.

Cast iron is very strong in compression but weak in shear. When a cast iron specimen is subjected to a compression test, the perpendicular cross-section bears the direct compressive stress. But a plane inclined at 45 degree to the normal plane has shear stresses due to resolved compression force.

Thus this plane is subjected to shear stress. As said earlier, cast iron is weak in shear. So failure occurs along this inclined or oblique plane.