Fatigue and creep rapture

Fatigue rapture

Failure under Fluctuating Stress

Creep rapture

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The failure of metal under alternating stresses is known as Fatigue.

Under fluctuating / cyclic stresses, failure can occur at lower loads than under a static load.

90% of all failures of metallic structures (bridges, aircraft, machine components, etc.)

Fatigue failure is brittle-like –

even in normally ductile materials. Thus sudden and catastrophic!

FatigueFailure under fluctuating stress

Fatigue: Cyclic StressesCharacterized by maximum, minimum and mean Range of stress, stress amplitude, and stress ratio

Mean stress m = (max + min) / 2

Range of stress r = (max - min)

Stress amplitude a = r/2 = (max - min) / 2

Stress ratio R = min / max

Convention: tensile stresses positive

compressive stresses negative

Fatigue: S—N curves (I)

Rotating-bending test S-N curves

S (stress) vs. N (number of cycles to failure)

Low cycle fatigue: small # of cycles

high loads, plastic and elastic deformation

High cycle fatigue: large # of cycles

low loads, elastic deformation (N > 105)

Fatigue: S—N curves (II)

Fatigue limit (some Fe and Ti alloys)

S—N curve becomes horizontal at large N

Stress amplitude below which the material never fails, no matter how large the number of cycles is

Fatigue: S—N curves (III)

Most alloys: S decreases with N.

Fatigue strength: Stress at which fracture occurs after specified number of cycles (e.g. 107)

Fatigue life: Number of cycles to fail at specified stress level

Fatigue: Crack initiation+ propagation (I)Three stages:

1. crack initiation in the areas of stress concentration (near stress raisers)

2. incremental crack propagation3. rapid crack propagation after crack reaches critical size

The total number of cycles to failure is the sum of cycles at the first and the second stages:

Nf = Ni + Np

Nf : Number of cycles to failure

Ni : Number of cycles for crack initiation

Np : Number of cycles for crack propagation

High cycle fatigue (low loads): Ni is relatively high. With increasing

stress level, Ni decreases and Np dominates

Fatigue: Crack initiation and propagation (II) Crack initiation: Quality of surface and sites of stress concentration

(microcracks, scratches, indents, interior corners, dislocation slip steps, etc.).

Crack propagation

I: Slow propagation along crystal planes with high resolved shear stress. Involves a few grains.

Flat fracture surface

II: Fast propagation perpendicular to applied stress.

Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.

Crack eventually reaches critical dimension and propagates very rapidly

Factors that affect fatigue life Magnitude of stress

Quality of the surface

Solutions: Polish surface Introduce compressive stresses (compensate for applied tensile

stresses) into surface layer.

Shot Peening -- fire small shot into surface

High-tech - ion implantation, laser peening. Case Hardening: Steel - create C- or N- rich outer layer by atomic

diffusion from surface

Harder outer layer introduces compressive stresses

Optimize geometry

Avoid internal corners, notches etc.

Factors affecting fatigue lifeEnvironmental effects

Thermal Fatigue. Thermal cycling causes expansion and contraction, hence thermal stress.

Solutions:

change design!

use materials with low thermal expansion coefficients

Corrosion fatigue. Chemical reactions induce pits which act as stress raisers. Corrosion also enhances crack propagation.

Solutions:

decrease corrosiveness of medium

add protective surface coating

add residual compressive stresses

The Macroscopic Character of Fatigue Failure

Because of the manner in which the fracture develops, the surfaces of a fatigue fracture are divided into two areas with distinctly different appearances.

In most cases, the surface will have a polished or burnished appearance in the region where the crack grew slowly.

In the last stage, the surfaces developed are rough and irregular.

Fractograph of fatigue failure in SAE 1050 pin, induction hardened to a depth of 5 mm ( 3/16 in.) and surface hardness of 55 HRC. Core hardness: 21 HRC. Fatigue initiated inside the grease hole at the metallurgical notch created by the very sharp case-core hardness gradient.

Schematic representation of fatigue fracture surface marks produced on smooth and notched components with circular cross sections under various loading conditions.

Creep

Creep testing

Furnace

Time-dependent deformation due to constant load at high temperature

(> 0.4 Tm)

Examples: turbine blades, steam generators.

Creep test:

Stages of creep

1. Instantaneous deformation, mainly elastic.

2. Primary/transient creep. Slope of strain vs. time decreases with time: work-hardening

3. Secondary/steady-state creep. Rate of straining constant: work-hardening and recovery.

4. Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary separation, necking, etc.

Stages of creep

Parameters of creep behavior Secondary/steady-state creep:

Longest duration

Long-life applications

(creep rate)

Time to rupture ( rupture lifetime, tr):

Important for short-life creep

t/s

tr

/t

Creep: stress and temperature effects

With increasing stress or temperature:The instantaneous strain increasesThe steady-state creep rate increasesThe time to rupture decreases

Creep: stress and temperature effectsStress/temperature dependence of the steady-state creep rate can be illustrated by

Mechanisms of Creep

Different mechanisms act in different materials and under different loading and temperature conditions:

Dislocation Glide Dislocation Creep Diffusion Creep Grain boundary sliding

Different mechanisms different n, Qc.

Grain boundary diffusion Dislocation glide and climb

Dislocation glide- Involves dislocations moving along slip planes and overcoming barriers by thermal activation. This mechanism occurs at high stress levels.

Dislocation creep- Involves the movement of dislocations which overcome barriers by thermally assisted mechanisms involving the diffusion of vacancies or interstitials.

Mechanisms of Creep

Diffusion creep- Involves the flow of vacancies and interstitials through a crystal under the influence of applied stress. This mechanism occurs at high temperatures and low stress levels.

Grain boundary sliding- Involves the sliding of grains past each other.

Mechanisms of Creep