The compressive strength of brittle materials is _______ its tensile strength.

Concept:

Modes of failure

• Since brittle material is strong in compression but weak in tension whereas its shearing strength is in between its compressive strength and tensile strength.
• Ductile material is approximately equal to strong tension and compression but weak in shear.

Case-1: Brittle material in torsion

• Since brittle materials have minimum tensile strength. Hence, brittle materials fail in tension.
• Hence brittle material subjected to torsion fails at 45° plane (Helicoidal failure).

Case-2: Ductile material in torsion

• Since, ductile materials are weak in shear. Hence ductile materials failure occurs due to principle shear stress.
• In torsion test maximum shear stress is in the direction perpendicular to longitudinal axis. Hence, ductile failure plane is torsion will be perpendicular to longitudinal axis.

Cast iron is a brittle material. So failure occurs as per case 5.

Case-3: Brittle material under tension

• Brittle materials are very weak in tension. Brittle materials fail due to the separation of particles along the surface which is at 90° to the direction of load. The failure surface is rough.

Case-4: Brittle material under compression

• Since brittle material is strong in compression, therefore, failure is due to shear. The plane to failure is at 45° from the axis of the shaft.
• The compressive strength of brittle materials is greater than its tensile strength so option 3 is correct.

Case-5: Ductile material in tension

• Since ductile material strong in tension, therefore failure is due to shear, the plane of failure is at 45° from the axis of the shaft, such failure is known as cup and cone failure.

Case-6: Ductile material in compression

• Short compression members fail in compression yielding. The failure plane is at 90°, to the compressive load. In compression yielding, bulging of material occurs.