CREEP -The slow and progressive deformation of a material with time at constant stress is called creep.

At elevated temperature, the strength of a material becomes very dependent on both strain rate (rate of deformation) and time of exposure. A number of materials under these conditions behave in many respect like viscoelastic materials. A material subjected to a constant tensile load at an elevated temperature will creep and undergo a time - dependent deformation is viscous flow. After creep sets in, it continues until sufficient strain has occurred so that a necking down and a reduction of cross - sectional area occurs. After this and until rupture, the rate of deformation increases because there is less area to support the load.

The phenomenon of creep is observable in metals, ionic and covalent crystals and amorphous materials such as glasses and polymers. Metals, generally exhibit creep at high temperatures, whereas plastics, rubbers and similar amorphous materials are very temperature - sensitive to creep.

TYPES OF CREEP -

Logarithmic Creep - At low temperature, the creep rate usually decreases with time and a logarithmic creep curve is obtained. The deformation produced is proportional to time.

Recovery Creep - At higher temperatures, in the range of 0.5 to 0.7 of the melting point measured on the absolute scale in O°K, The influence of work - hardening is weakened and there is a possibility of mechanical recovery. As a result, the creep rate does not decrease rapidly as at low temperature.

Diffusion Creep or plastic Creep - At very high temperature, i.e. beyond 0.7 times melting point, the creep is primarily influenced by diffusion and the stress applied has little effect.

CREEP CURVE - The variation of extension of a metal with time for a constant nominal stress is shown.The first stage of creep called primary creep, represents a region of decreasing creep rate. Primary creep is a period of predominantly transient creep in which the creep resistance of the material increases by virtue of its own deformation. For low temperatures and stresses, as in the creep of lead at room temperature, primary creep is the predominant creep process.

The second stage of creep, known as secondary creep, is a period of nearly constant creep rate which results from a balance between the work hardening process and recovery. The average value of creep - rate during secondary creep is called the minimum creep rate.

The third stage of creep or tertiary creep mainly occurs at an accelerated rate. It actually represents a process of progressive damage resulting in an imminent fracture of the material through intercrystalline or other causes.

MECHANISM OF CREEP - The mechanism of creep is related to the movements of dislocations. At low temperatures, strain is restricted because the dislocation movements are stopped by grain boundaries or by impurity atoms. However, at high temperatures atomic movements permit the dislocations to climb, jog out of its plane of slip or even be annihilated. As atoms move to and from the dislocation, the dislocation climbs out of the initial slip plane, thus permitting the continuation of incremental stress or creep. The climb is ascribed to the thermal agitation of surrounding atoms. Caught in this and failure is common in aluminium alloys, low carbon steels and brasses. Noticeable in pipelines, heat exchanges etc.

MECHANISM - When a material is under the effect of tensile stress, it affects the bonding between the atoms of the metal. In a perfect crystalline structure; the stress is absorbed uniformly between all the bonds concerned. But when a defect in the structure is present, the stress distribution cannot be uniform. Some bonds will therefore be under greater stress than others. Thus application of a tensile stress to a crystal lattice, which is otherwise in equilibrium* results in a rising of the thermodynamic energy of some of the atomic bonds. If the effect is localized at the surface, anodes will be formed, even though the material is being stressed well below the elastic limit.

The crystal structure suffers bond breaking and reforming so that its shape is permanently altered, and dislocation occurs. The movement of dislocation will be halted when they reach either the surface of the metal or a grain boundary. The pile-up of dislocations at grain boundaries results in the anodic polarization of these regions because of the increased irregularities in the crystal structure. At the surface a local blemish occurs, on an otherwise 'smooth' surface, this is known as a slip step and it is at this site that the material is most vulnerable to initial corrosive attack.

Alloys which rely on thin film of oxide or other material for their corrosion protection are especially vulnerable because the slip step uncovers a microscopic quantity of bare metal which is highly anodic compared to the surrounding surfaces.

If the metal is able to repassivate quickly, then little danger ensues, but if the passivation time is long enough to allow corrosion of the exposed area to occur and a pit to form, then stress corrosion cracking initiates. Even in metals which are not passivated, the formation of slip steps on the surface represents a corrosion problem for the discontinuity of crystal structure causing local anodes.

HYDROGEN EMBRITTLEMENT

It is a condition of low activity in metal resulting from the absorption of hydrogen produced by corrosion. If a metal is under a high tensile stress, brittle fracture can occur.

Caustic embrittlement is a special case of hydrogen embrittlement. It occurs when steel is highly stressed and exposed to hot solution containing alkalies. Hydrogen and caustic embrittlement is therefore, a stress corrosion phenomenon occurring on mild steel exposed to alkaline solution at high temperature and stresses.

 

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