Processes, examples, and how initiation is prevented or minimised

Description: the combined action of cyclic (repeated) stress and a corrosive environment. Even though the peak stresses are well below the fatigue limit of the material in air, corrosion initiates or accelerates cracks: corrosion pits or local attack act as stress raisers and weaken the material, and the corrosive medium (sea water, moist air) attacks the freshly-created crack surfaces, so a crack grows at stresses below what would otherwise be the endurance limit, leading to sudden fracture.

Example in marine engineering: propeller shafts, crankshafts and the stern tube/shaft/coupling region exposed to sea water; hull plating and welds near the waterline/fouled areas.

Prevention/minimisation: remove/reduce the corrosion (protective coatings, cathodic protection - ICCP/sacrificial anodes), specify a material with a high fatigue/endurance limit and corrosion resistance, eliminate stress raisers and sharp corners, apply surface peening/compressive residual stress, keep surfaces clean (de-biofouling) and control the environment; design so cyclic stresses are low and avoid resonance.

Description: the growth of cracks in a metal under the combined action of a tensile stress (often well below the yield) and a specific corrosive environment, e.g. chloride ions for austenitic stainless steels in sea water (chloride stress-corrosion cracking), or caustic/ammoniacal cracking. The environment attacks grain boundaries or sites, and the tensile stress opens the crack so corrosion proceeds at the tip.

Example in marine engineering: stainless steel exhaust piping/bellows, mono/damped fixtures and pressure parts in chloride-bearing sea water/steam or where fluid contains ammonia; brass/copper fittings.

Prevention/minimisation: choose materials immune to the specific environment (e.g. avoid susceptible stainless grades for marine/chloride service), reduce tensile residual stresses by stress-relief annealing, use compressive surface treatments (peening), control water chemistry (avoid chlorides/ammonia), and avoid stress-raising notches; maintain regular inspection and protect surfaces.

Description: a crack that slowly extends (creeps) through the material under repeated below-fracture loading or under sustained strain (time-dependent crack growth), so that the part fails at a low nominal stress after many cycles / long loading. It is a slow, progressive crack growth (as opposed to sudden overload fracture). It can also represent creep (high-temperature deformation) cracking.

Example in marine engineering: fatigue cracks at welds, at plate edges, in crankshaft fillets, through bolts/studs and in exhaust/manifold components where cyclic loading continues after a crack has started.

Prevention/minimisation: design to keep cyclic stresses below the endurance limit with adequate factor of safety, eliminate stress raisers/weld toes, inspect with NDT (magnetic particle, ultrasonic) and repair small cracks before they propagate, use fatigue-resistant materials, shot-peen for compressive stress, and reduce stress amplitude by balancing/alignment.

Description: cracks that form in a metal casting during solidification/cooling - either hot tears (cracks at elevated temperature while the metal is weak, at the weld/liquidus) or cold cracks (cracks from internal stress on cooling when the metal is brittle and contraction creates tensile stress) or shrinkage cracks. They are usually internal or at the casting corners/feed sections.

Example in marine engineering: propeller blades, gear blanks, cylinder heads/blocks and other large castings if cooling is too fast or the design has poor section transitions.

Prevention/minimisation: correct foundry practice - controlled cooling in the mould, suitable mould design (fillets, uniform sections, generous corners), correct pouring temperature and composition, use of a designed runner/gating, adequate risers to allow feeding, stress-relief annealing/normalising after casting, and inspection (ultrasonic/magnetic particle) to detect and reject cracked castings; redesign sections to avoid weak, sharp transitions.