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FAILURE MODES

OBJECTIVE: IDENTIFY AND PREVENT STRUCTURAL BREACH

WARNING: BRITTLE FRACTURE OCCURS WITHOUT WARNING. NO PLASTIC DEFORMATION. NO TIME TO EVACUATE.

CASE A: DUCTILE SHEAR

MECHANISM

Material yields before breaking. Neck-down visible at stress concentration point. Energy absorbed via plastic deformation.

SIGNS
  • Cup-and-cone fracture surface
  • Visible elongation prior to separation
  • Rough, fibrous appearance
ROOT CAUSE

Overtorque beyond 110% of proof load. Friction coefficient miscalibration.

MITIGATION

Strict adherence to 3-pass star-pattern protocol.

CASE B: BRITTLE FRACTURE

MECHANISM

Instantaneous cleavage along crystal planes. Zero warning. Catastrophic release of stored elastic energy.

SIGNS
  • Flat, crystalline fracture surface
  • Chevron patterns pointing to origin
  • No neck-down
ROOT CAUSE

Hydrogen embrittlement during plating. Improper tempering. Sub-zero thermal shock.

MITIGATION

Bake-out at 195°C for 24h post-plating. Avoid Grade 8 in cryogenic service.

Crystalline fracture surface showing brittle failure pattern

FIG 2.0: BRITTLE FRACTURE SIGNATURE. PIXABAY LICENSE.

CASE C: THREAD STRIPPING

MECHANISM

Shear failure of internal nut threads due to excessive preload. Bolt remains intact; nut spins freely.

SIGNS
  • Sudden torque drop during tightening
  • Loose fit post-event
  • Debris in thread valley
ROOT CAUSE

Insufficient engaged thread length (<2 diameters). Misalignment creating edge loading.

MITIGATION

Use locknuts. Verify alignment before Pass 1.