CREEP

Muhammed Labeeb

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CREEP

▪ Creep is the tendency of a material to permanently deform under constant load, even well below the yield stress

▪ Creep is more severe at elevated temperatures

▪ Creep test and rupture test are used to evaluate elevated temperature strength of materials

▪ Creep test is used to measure the dimensional changes which occur from elevated- temperature exposure.

▪ Rupture test measures the effect of temperature on the long-time load-bearing characteristics

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CREEP CURVE

▪ Engineering creep curve is determined by applying a constant load at a constant temperature, and strain of the specimen is determined as a function of time

▪ It is specified by ASTM E139-70 standard

▪ Slope of creep curve is referred to as creep rate

▪ Following rapid elongation of specimen, creep rate decreases with time, then reaches steady state and finally the creep rate increases rapidly with time until fracture occurs

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CREEP CURVE

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STAGES OF CREEP

▪ Creep has three stages; Primary creep, secondary creep and tertiary creep

▪ Primary creep: Represents a region of decreasing creep rate. In this stage creep resistance of material increases by its own deformation. For low temperatures and stresses, this is the predominant form

▪ Secondary creep: Nearly a constant creep rate which results from a balance between the competing process of strain hardening and recovery. It is usually referred to as steady state creep.The average value of creep rate during secondary creep is called as minimum creep rate

▪ Tertiary creep rate: It occurs when there is a effective reduction of area either because of necking or internal void formation. It is associated with metallurgical changes like recrystallization, coarsening etc…

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STRUCTURAL CHANGES DURING CREEP

▪ Variation of creep rate with time is due to the changes in the internal structure of material with creep strain and time

▪ The principal deformation processes at elevated temperatures are Slip, Subgrain formation and Grain-boundary sliding

▪ Deformation by slip: New slip systems become operative when metal are deformed at elevated temperature. Slip under high-temperature creep conditions occurs on many slip planes for small slip distance.

▪ Subgrain formation: Near lattice boundaries creep deformation leads to lattice bending. This results in formation of excess dislocations of one sign, and in elevated temperatures dislocations arrange themselves into a low-angle grain boundary. This is mostly found in metals with high stacking-fault energy

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STRUCTURAL CHANGES DURING CREEP

▪ Grain-Boundary Sliding: At elevated temperatures the grains in polycrystalline metals are able to move relative to each other. Grain boundary sliding is a shear process which occurs in the direction of the grain boundary.The main importance of grain-boundary sliding is that it initiates grain boundary fracture

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MECHANISMS OF CREEP DEFORMATION

Chief creep formation grouped as:

▪ Dislocation glide: Dislocation moving along slip planes and over coming thermal barriers by thermal activation.Stress range –σ/G >10-2.

▪ Dislocation creep:Overcome barriors by thermally assisted mechanisms involving the diffusion of vacancies. Stress range 10-4< σ/G <10-2.

▪ Diffusion creep:The flow of vacancies and interstitials through a crystal under the influence of applied stress.This mechanism includes Nabarro-Herring and Coble creep.Stressrange σ/G < 10-4.

▪ Grain boundary sliding :Sliding of grains past each other.

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DEFORMATION MECHANISM MAPS

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▪ Creep predominates at temperature above 0.5Tm.

▪ Arrhenius rate equation

Assumption-creep is a singly activated process.

εs =Ae-Q/RT

ACTIVATION ENERGY FOR STEADY STATE CREEP

FRACTURE AT ELEVATED TEMPERATURE

Metals undergo a transition from transgranular to intergranular fracture as the temperature increases.

EQUICOHESIVE TEMPERATURE-Temperature at which the grains and grain boundaries have equal strength.

▪ Decreasing the strain rate lowers the ECT which increase tendency for intergranular fracture

▪ The amount of grain boundary area decreases with increasing grain size ,so a material with large grain size will have higher strength above the ECT

▪ Fracture mechanism map for Nickel

▪ Cavities form continuously through out the creep process.

▪ In third stage of creep the nucleation and growth of grain boundary voids and cracks occurs at an accelerated rate.This process is called creep cavitation.

▪ Nucleated cavity must be large enough to grow in the face of capillarity forces which tend to make it shrink.

▪ Cavity growth will occur only if the grainboundary sliding displacement exceeds a critical value

δ≥1/a2(2γs/σ)3)

TYPES OF INTERGRANULAR CRACKS IN CREEP

▪ Wedge-shaped cracks-mostly at grain boundaries which are aligned for maximum shear.

▪ Round or elliptical cavities-in the grain boundaries that are aligned normal to the tensile stress.

CREEP UNDER COMBINED STRESSES

ASSUMPTIONS: Material is incompressible.

Principal shear strain rates are proportional to principal shear stresses.

ε1=2/3C[σ1-1/2σ2+σ3)]

Similarly ε2 and ε3 .

REFERENCES

▪ George E. Dieter, Mechanical Metallurgy, McGraw-Hill Book Co.,1988.

▪ wikipedia.org/wiki/Creep_(deformation).