Low Cycle Fatigue (LCF)

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Low Cycle Fatigue (LCF) High Cycle Fatigue (HCF)
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What is Fatigue?   The ASTM definition..... The
process of progressive localized permanent
structural change occurring in material subjected
to conditions which produce fluctuating stresses
and strains at some point or points and which may
culminate in crack or complete fracture after a
sufficient number of fluctuations.    
Translation
Cyclic damage leading to local cracking or
fracture.
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Requirements have evolved for Gas Turbine
Engines.... Emphasis today is on Cyclic
Properties...

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Emphasis today is on Cyclic Properties...
High Cycle Fatigue 8 Allowable vibratory
stresses   Low Cycle Fatigue 8 Crack initiation
life 8 1/1000 to small crack
8 Component
retirement   Crack Growth 8 Remaining life from
crack
8 Safety inspection interval
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Inspection size requirement
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Turbine Disk Design Requirements
Most Severe Structural Challenge High structural
loads, fatigue, creep

• Environmentally friendly
• Fatigue cracking resistance
• initiation
• propagation
• Creep resistant
• Strong
• Lightweight
• Predictable/Inspectable
• Affordable
• Environmentally stable

Nickel Superalloy Balances All Requirements
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Combustor, Turbine Components Present a Severe
Thermal Fatigue Cracking Challenge

• Mechanical fatigue, caused by cyclic thermal
  strains

• High temperature accelerates fatigue damage

• Exacerbated by crack tip oxidation

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Fatigue is a Major Challenge for Many Engine
Components, Including Fan Blades
fatigue crack initiation site
Compressor blade tested in a vibratory fatigue
test rig
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Cyclic vs. Monotonic Curves Behavior can be
significantly different ...
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Crack Size How big is big? ...
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PWA Stress Control HCF Test Apparatus
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Fully Reversed Stress/Strain Cycle
Specimen
S/N Plot
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Basic Cycle
Terms to Remember
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Cyclic Deformation Parameters Fatigue loop
illustration ...
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PWA Strain Control LCF/TMF Test Apparatus
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Cyclic Fatigue Testing Parameters of Interest
...
Strain Range - De   Stress
Range - Ds P/A smax - smin   Max. Tensile
Stress - sT   Mean Stress - sm 0.5(smax
smin)   Inelastic Strain - ei, ep  
Temperature
-
T
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Cyclic Loading Key Relationships ...
Elastic Modulus,
(monotonic) or (cyclic)
Stress Ratio,
where
Max. Stress,
Min. Stress,
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Total Strain Elastic Strain Range Plastic
Strain Range  
Where and
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Cyclic Stress-Strain Behavior Derived from loci
of cyclic endpoints ...
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Rate dependent test data and model correlation
ANSYS analysis of constitutive specimen
Model parameter temperature dependencies
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Understanding Metallurgical Aspects of Fatigue
Metallurgical Aspects...     Relevant
Topics 8      Crystal Structure 8     
Deformation Mechanisms 8      Crack Initiation ..
Sequence of Events 8      Visual Aspects -
Fractography
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Deformation for crystal structures can be
visualized like a sliding row of bricks...
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Metals have a highly ordered crystal structure...
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Two predominant deformation mechanisms in
metals...
Dislocation occurs at all temperatures,
but is predominant at lower temperatures.
Diffusion important at higher
temperatures, especially above
one half the melting temperature
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Can you find the Illustrated Dislocation Defect?
Edge dislocation. (a) Bubble-raft model of an
imperfection in a crystal structure. Note the
extra row of atoms. (b) Schematic illustration
of a dislocation. Bragg and Nye, Proc. Roy.
Soc. (London), A190, 474, 1947.
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Pure metals are easily deformed. Several methods
are used to inhibit deformation...
8      Dispersion strengthening
8      Solid solution strengthening 8     
Precipitation hardening 8      Microstructure
control (grain size and morphology, precipitate
control, etc.)
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Solid Solution Strengthening Perturbations to
crystal lattice retard dislocation motion...
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Precipitation Hardening Local areas of
compositional and/or structural differences
retard dislocation motion...
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Grain Boundary Strengthening Crystallographic
and/or compositional boundary. Strengthens at low
temperature but weak link at high temperature...
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Grain Boundary Resistance Will resist
dislocation motion at the boundary...
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Grain Boundaries Illustrated Notice the
vacancies and excess atoms at boundaries...
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Grain Boundary Mechanics   Crystallographic
and/or compositional boundary. Strengthens at
low temperature weak link at high temperature...
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Persistent Slip Band Formation A product of
cyclic deformation important to fatigue
initiation for ductile metals ...
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Diffusion A high temperature deformation
mechanism ...
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Diffusion Usually considered at temperatures
above half the melting point (?K) ...
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Grain Boundary Sliding A diffusion controlled
deformation process ...
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Grain Boundary Sliding Can provide large
deformation at boundary with relatively small
intergranular deformation ...
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Fatigue Crack Initiation Occurs when enough
local deformation damage accumulates to produce a
crack ...  
8      from dislocations - as in slip   8     
from diffusion - as in grain boundary
sliding   8      or from both
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Fracture Stages Steps of an Idealized Fatigue
Process ...  
Stage I Crystallographic
Fracture, along a few planes, brittle appearance,
at angle to principal loading direction.     Stag
e II Usually transgranular, but
numerous fracture planes normal to principal
loading direction. Striations often seen at
high magnification for more ductile
alloys.     Stage III Final fracture brittle,
ductile or both.
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Fracture Stages Fatigue origin often at a
Mechanical or Metallurgical Artifact ...
Schematic of stages I and II transcrystalline
microscopic fatigue crack growth.
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Typical Fatigue Fractures Several Common
Features ...     1. Distinct crack initiation
site or sites.   2. Beach marks indicative of
crack growth arrest.   3. Distinct final
fracture region.
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Fatigue Features Initiation sites . . .
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Fatigue Features Beach marks ...
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Fatigue Features Final Fracture ...
Final Fracture
Fatigue Area
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Ramberg-Osgood Relationship Describes cyclic
inelastic behavior ...
IN100, (Tests Conducted in Air at 650C,
Frequency, 0.33 Hz)
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Typical Failure Modes General Characteristics
...
  Failure Mode
Some General Characteristics Overstress
Rapid fracture, may be
ductile or brittle, large
deformation, often transgranular, often the final
stage of some other
fracture mode.   Creep/Stress Rupture
Usually long term event, large deformation,
intergranular, elevated
temperature   High Cycle Fatigue
Often short term event, small deformation,
transgranular   Low Cycle Fatigue
Moderate time event, moderate
deformation, fracture
dependent on time/temp.   Thermomechanical
Fatigue Moderate time event, subset of LCF with
deformation due largely to
thermally induced stresses, fracture
usually shows heavy oxidation/alloy
depletion
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Cyclic Behavior Must be Modeled After Tensile
yield, there are two models which describe
compressive behavior ...
Isotropic - assumes symmetrical behavior in
tension and compression.  
Kinematic - assumes yield stress,
following inelastic deformation, is degraded
...  
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Hardening Models Defines the Bauschinger effect
...
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Cyclic Effects on Stress-Strain Behavior
Progressive changes occur during cyclic loading
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Material Copper in 3 Conditions
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Summary
8      Cyclic properties are important to our
product.   8      Principal deformation
mechanisms are slip at low temperature and
diffusion at high temperature.   8      Cracking
can be crystallographic, transgranular, or
intergranular.   8      Simple deformation models
can be used to consolidate data and predict local
stresses and strains.