Cast iron strengths — comparison of compressive and tensile values For cast iron, the compressive strength is generally __________ its tensile strength.

Introduction / Context:
Cast irons are widely used where compressive loads dominate, such as machine bases, columns, and engine blocks. Their graphite morphology strongly affects behavior in tension versus compression. Understanding this asymmetry is crucial for safe design.

Given Data / Assumptions:

• Grey cast iron contains graphite flakes; ductile iron contains nodular graphite.
• Fracture in tension is promoted by graphite acting as stress concentrators.

Concept / Approach:
Graphite flakes are crack initiators under tensile stress, greatly reducing tensile strength and toughness. Under compressive loading, the flakes do not open cracks in the same way; the matrix carries load effectively, so compressive strength is substantially higher than tensile strength. Even in ductile iron, compressive strength remains higher than tensile strength for the same reasons.

Step-by-Step Solution:
Identify effect of graphite on tension → sharp notches → low tensile strength.Identify effect on compression → cracks do not propagate easily → higher compressive strength.Therefore, compressive strength is greater than tensile strength.

Verification / Alternative check:
Typical data: grey iron tensile strength ~150–300 MPa, compressive strength can be 3–4 times higher; ductile iron also shows higher compressive than tensile strength.

Why Other Options Are Wrong:

• Equal to/less than: contradict standard mechanical property tables.
• “Approximately half of” or “exactly twice”: arbitrary ratios; the relationship is “significantly greater,” not a fixed multiple.

Common Pitfalls:
Assuming isotropic behavior for cast irons; graphite morphology leads to strong tension–compression asymmetry.

Final Answer:
more than