Mechanical Behavior and Testing
This domain tests interpretation of standard mechanical tests - tensile, hardness, impact, fatigue, creep and fracture toughness - and the ability to convert raw readings into the property the design code actually needs.
Core rules
- Engineering stress uses the original area; true stress uses the instantaneous area. Necking begins at the ultimate tensile strength, where dσ/dε = σ (Considère criterion).
- Yield strength is taken at 0.2% offset for materials without a distinct yield point. Ductility from % elongation depends on gauge length; % reduction in area does not.
- Hardness is an indentation response, not a fundamental property. Conversion tables (HB/HRC/HV) are valid only within the ferrous family for which they were established.
- Charpy V-notch impact energy is comparative and transition-temperature based (FATT); it is not a design stress parameter.
- Fatigue: S–N curves show a plateau endurance limit in many steels but not in aluminium alloys. Striations mark one cycle each; beach marks are macroscopic arrest lines.
- Creep has primary, secondary (steady-state) and tertiary stages; Larson–Miller or Sherby–Dorn parameters extrapolate rupture life.
Worked example
A 10 mm diameter tensile specimen carries 50 kN at yield. Area = π × 5² = 78.54 mm², so σ = 50,000/78.54 = 637 MPa. If the same specimen shows 50% reduction in area at fracture, true fracture strain = ln(1/(1 − 0.5)) = ln 2 = 0.693.
Traps that cost marks
Designing to UTS rather than yield with a safety factor. Quoting a hardness conversion across material classes. Comparing Charpy energies from different specimen geometries or temperatures. Treating fracture toughness KIC as interchangeable with impact energy.