Why a material may fracture below its yield point

A material may fracture at a stress well below its yield (proportional/elastic) limit because of stress concentration and/or sub-critical crack growth, even though the nominal overall stress is low. At local discontinuities the actual stress is multiplied far above the nominal value; when this local stress exceeds the local fracture strength a crack starts. Causes:

• Stress raisers: notches, sharp corners, keyways, bolt holes, surface scratches, corrosion pits, weld toes, weld porosity, inclusions and laminations concentrate stress by a factor possibly of 2-3.
• Internal flaws/inclusions and lamination acting as micro-cracks.
• Corrosion, pitting and stress-corrosion creating crack initiation sites; fatigue from cyclic loading growing a crack by striations until the remaining ligament fails (fatigue failure and corrosion fatigue happen below the yield/static strength).
• Brittle material behaviour at low temperature or in a notched condition, or hydrogen embrittlement.
• Residual tensile stresses from welding or cold work increasing the effective stress.

Examples of components that may fracture in this way without precautions

• Propeller shafts at the keyway/cone or at corrosion pitting (fatigue cracks).
• Crankshafts at fillet radii or oil holes.
• Connecting rods, bolts, studs and keyways in propeller boss.
• Weld seams in hull plating at the weld toe.
• Tie rods, gear teeth, turbine/blower blades, piping at weld defects.
• Stern tubes, gear couplings, rudder stocks at stress raisers.

How such fractures can be avoided

By material choice:

• Select ductile, tough, fatigue-resistant alloys with a good endurance limit; avoid materials prone to stress-corrosion/hydrogen embrittlement; use clean, inclusion-free steel.

By careful design:

• Avoid sharp corners and abrupt changes of section; provide generous fillets and generous radii at fillets, smooth tapers, and cold-rolled/peened surfaces to give compressive residual stress.
• Design so nominal stress is low with a suitable factor of safety, keep stress raisers away from high-stress regions and at the free surface.
• Ensure smooth surface finish, adequate radii, correct keyway/keyless design, and apply shot peening to induce beneficial compressive surface stress.

By good workmanship:

• Accurate machining without tool marks, correct heat treatment (no overheating/damage), controlled welding with complete fusion, peening/grinding to remove weld toes, eliminate inclusions and laminations, and correct bolting/assembling.
• Regular non-destructive testing (magnetic particle, ultrasonic, dye penetrant) and inspection of critical components (shafts, crankshafts, welds) for cracks before they propagate; polished surfaces where needed.
• Surface protection/coating and cathodic protection to prevent corrosion pits initiating cracks.