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Fatigue rapture
Failure under Fluctuating Stress
Creep rapture
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
