In a compression test, the fracture in a cast iron specimen would:
🔬 Understanding Material Failure Modes
The way a material fractures under load is a direct indicator of its properties. This question tests the understanding of how brittle materials behave under compression.
Key Property of Cast Iron:
Cast iron is a classic brittle material. This means it is very strong when you compress it, but it is significantly weaker in shear. This weakness in shear is what dictates its failure mode under a compression test.
⚖️ Detailed Analysis of the Failure Mechanism
When a vertical compressive load is applied to a specimen, it creates both compressive stress and shear stress within the material.
- Compressive Stress is maximum on the plane perpendicular (at 90°) to the load.
- Shear Stress is maximum on planes that are at a 45° angle to the load.
Since cast iron's shear strength is much lower than its compressive strength, it will fail where the shear stress is highest. This means the fracture will initiate and propagate along these 45° planes.
(b) Occur along an oblique plane
This is the technically correct answer. The failure occurs due to shear stress, which is maximal on an oblique (inclined) plane, typically at around 45 degrees to the axis of the load.
(c) Occur at right angles to the axis of the specimen
This type of fracture (a clean break perpendicular to the load) is characteristic of a brittle material failing under tension, not compression. In a tension test, the tensile stress is maximum at a right angle to the load, and the material pulls apart.
⚠️ Important Note on Official Answer Keys
Official Answer: It has been noted that in some official examinations, option (c) Occur at right angles to the axis of the specimen has been marked as the correct answer.
Possible Reasoning: This can be confusing. It's possible this answer is considered correct under a simplified view that brittle materials fail perpendicular to the principal stress, which in compression would be along the horizontal plane. However, this ignores the critical role of shear stress in the failure of brittle materials under compression. For engineering and materials science principles, failure along an oblique plane due to shear is the more accurate and accepted explanation.
Exam Strategy: Be aware of this discrepancy. While (b) is technically correct, if you are preparing for a specific exam, it is wise to check previous answer keys to see which convention they follow.