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Cracks produced by the combined action of repeated or fluctuating stress and a corrosive environment, which produces the cracking at lower stress levels or fewer cycles of stress than would be the case if no corrosive environment were present.



Related Terms

EMBRITTLEMENT

An intercrystalline corrosion of boilerplate occurring in highly stressed zones. Cracking may result.

CORROSIVE WEAR

A material deterioration due to the co-joint action of corrosion and mechanical action.

LIQUID EROSION

Removal of films or metal by mechanical action and corrosion of active metal.

SHEAR

An action that causes molecules of a body to be slid past each other. Scissors perform a shearing effect on paper. Shear force is used to punch out holes in a sheet of stainless steel. Shear stress is stress caused by a force that tends to cause particles to slide past each other.

SCC

Stress corrosion cracking.

COUPON

Polished metal strip of specified size and weight used to detect the corrosive action of liquid or gas products or to test the efficiency of corrosion-inhibitor additives. Also known as corrosion coupon.

TEMPERING

Heat treating process that releases internal stress in hardened steel and increases toughness. Tempered steel will not crack under heavy stress or vibration or impact. Should be done as soon after hardening.

CREEP

The time-dependent stretching or strain, heavily influenced by temperature, of a material under stress. Creep is a form of slip which occurs when metal is subjected to a tensile load at high temperature. Creep deformation is plastic and occurs even though the acting stress is 'below the yield stress' of material. At low temperatures the rate of creep is very small but at higher temperatures it becomes increasingly important. For this reason creep is commonly regarded as a high-temperature phenomenon associated with steam plant, gas-turbine technology and turbo-charger blading.

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Plastic Strain from 0 to X which Occurs in Three Stages:

Primary or Transient Creep (0 to P) beginning at a fairly rapid rate which then decreases with time as strain - hardening sets in.

Secondary or Steady-State Creep (P to S) in which the 'rate' of strain is fairly uniform and at its lowest value.

Tertiary (tershari) Creep (S to X) in which the 'rate' of Creep increases rapidly so that fracture occurs at X. This stage coincides with necking of the material.

During Primary and Secondary Creep, plastic deformation takes place due to slip associated with dislocations movements within the grains. This leads to work hardening which, at high temperatures, is balanced by thermal softening. The dislocations eventually move out of the grains and into the grain boundaries. Tertiary Creep coincides with the initiation of micro-cracks at the 'grain boundaries', which leads to necking and the consequent rapid failure of the material. Hence, at the higher temperatures, fine-grained material creep more than course grained material since fine-grained material contains more grain boundaries per unit volume.

SWASH BULKHEAD

Longitudinal or transverse nonwatertight bulkheads fitted in a tank to decrease the swashing action of the liquid contents as a ship rolls and pitches at sea

BOILER SAFETY SHUT DOWN

The action of shutting off all fuel and ignition energy to the burner by means of safety control or controls such that restart cannot be accomplished without operator action.

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