How is a steel propeller hub tested for cracks?

• A. by eddy current testing
• B. by magnetic particle inspection
• C. by dye penetrant inspection
• D. by radiography

Answer: (B)

Magnetic particle inspection is used because it reliably reveals surface and near-surface cracks in ferromagnetic parts like a steel propeller hub. When the hub is magnetized and fine magnetic particles are applied, any crack disrupts the magnetic field and causes the particles to gather at the defect, creating a visible indication on the surface. This makes it especially effective for a hub with complex geometry where cracks are likely to be near or at the surface and require quick, direct detection. Dye penetrant can show surface openings but needs thorough cleaning and may miss tightly closed flaws; radiography can detect internal flaws but is more costly and less practical for routine hub inspections; eddy current testing is also surface-oriented and sensitive to geometry, making MPI the most reliable choice for this ferromagnetic component.