Low Cycle Fatigue (LCF) High Cycle Fatigue (HCF)

“Cyclic damage leading to local cracking or fracture.” 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.”

Requirements have evolved for Gas Turbine Engines....   Requirements have evolved for Gas Turbine Engines.... Emphasis today is on Cyclic Properties...   Time Design Requirements Material Properties Historical Basic Engineering Strength, Creep 1960’s - 1970’s Add ... Fatigue HCF, LCF, TMF Late 1970’s Add ... Damage Tolerance Crack Growth

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 8 Inspection size requirement

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

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

Fatigue is a Major Challenge for Many Engine Components, Including Fan Blades Caused by Load Cycling Occurs at cyclic loads well below the Ultimate Strength High Cycle Fatigue (HCF) Caused by vibration/flutter Low Cycle Fatigue (LCF) Caused by engine cycling fatigue crack initiation site Compressor blade tested in a vibratory fatigue test rig

Cyclic vs. Monotonic Curves: Behavior can be significantly different ... From Metal Fatigue in Engineering, H.O. Fuchs and R.I. Stephens, John Wiley & Sons, NY, 1980

Crack Size: How big is big? ...

P&WA Stress Control HCF Test Apparatus

Fully Reversed Stress/Strain Cycle Specimen Fully Reversed Stress/Strain Cycle S/N Plot

Basic Cycle Terms to Remember

Cyclic Deformation Parameters: Fatigue loop illustration ...

P&WA Strain Control LCF/TMF Test Apparatus

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

Cyclic Loading: Key Relationships ... Elastic Modulus, (monotonic) or (cyclic) Stress Ratio, where Max. Stress, Min. Stress,

Total Strain = Elastic Strain Range + Plastic Strain Range   Where and

Cyclic Stress-Strain Behavior: Derived from loci of cyclic endpoints ...

Constitutive Modeling Approach Here, we show the ability of the model to correlate data in the rate dependent regime. Note the execellent agreement for a test in which considerable plasticity, creep and creep-relaxation are present. To address the TMF case, an alloy is tested over a range of temperatures, from room to the solution temperature of the alloy. The model constants are then plotted as a function of the temperature (center figure), and a function fitted to the data. The resulting model, and model material constants are integrated into the structural analysis (FEM) code, enabling viscoplastic analyses to be conducted. Rate dependent test data and model correlation Model parameter temperature dependencies ANSYS analysis of constitutive specimen

Constitutive Modeling Approach strain controlled testing procedures are used. standard profiles with unique sets of strain end levels, strain rates and dwells are designed for low (rate independent), intermmediate, and elevated (rate-dependent) temperature levels. Approximately 30 specimens are required to characterize an alloy. Approximately 20 are used for model calibration, over a temperature range from room to alloy solution temperature. The additional specimens are used to develop evaluation and verification data with which to check the model. All profiles are designed such that test creation, evaluation, machine setup and calibration, test execution and data reduction can be done in a standard 8 hour work day. Ultimate goal is of course, introduction of the new model/alloy parameters in a structural analysis code. specimen correlation specimen prediction component analysis

Understanding Metallurgical Aspects of Fatigue   Relevant Topics: 8      Crystal Structure 8      Deformation Mechanisms 8      Crack Initiation .. Sequence of Events 8      Visual Aspects - Fractography

Deformation for crystal structures can be visualized like a sliding row of bricks...

Metals have a highly ordered crystal structure...   Cubic Arrangement Hexagonal Close-Packed Structure Zn, Mg, Be, a-Ti, etc.

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

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.]

Pure metals are easily deformed 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.)

Solid Solution Strengthening: Perturbations to crystal lattice retard dislocation motion...

Precipitation Hardening: Local areas of compositional and/or structural differences retard dislocation motion...

Grain Boundary Strengthening: Crystallographic and/or compositional boundary. Strengthens at low temperature; but weak link at high temperature...

Grain Boundary Resistance: Will resist dislocation motion at the boundary...

Grain Boundaries Illustrated: Notice the vacancies and excess atoms at boundaries...

Grain Boundary Mechanics:   Crystallographic and/or compositional boundary. Strengthens at low temperature; weak link at high temperature...

Persistent Slip Band Formation: A product of cyclic deformation important to fatigue initiation for ductile metals ... From Metal Fatigue in Engineering, H.O. Fuchs and R.I. Stephens, John Wiley & Sons, NY, 1980

Diffusion: A high temperature deformation mechanism ...

Diffusion: Usually considered at temperatures above half the melting point (K) ...   Melting Point (F) 1/2 Melting Point (F) Aluminum 1220 379 Titanium 3035 1288 Nickel 2647 1094 Iron 2798 1170 Cobalt 2723 1132 Ice 32 -213

Grain Boundary Sliding: A diffusion controlled deformation process ...

Grain Boundary Sliding: Can provide large deformation at boundary with relatively small intergranular deformation ...

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

Fracture Stages: Steps of an Idealized Fatigue Process ...   Stage I Crystallographic Fracture, along a few planes, brittle appearance, at angle to principal loading direction.   Stage 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.

Fracture Stages: Fatigue origin often at a Mechanical or Metallurgical Artifact ... Schematic of stages I and II transcrystalline microscopic fatigue crack growth.

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.

Fatigue Features: Initiation sites . . .

Fatigue Features: Beach marks ...

Fatigue Features: Final Fracture ... Fatigue Area

IN100, (Tests Conducted in Air at 650°C, Frequency, = 0.33 Hz) Ramberg-Osgood Relationship: Describes cyclic inelastic behavior ... IN100, (Tests Conducted in Air at 650°C, Frequency, = 0.33 Hz)

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

- assumes symmetrical behavior in tension and compression. 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 ...  

Hardening Models: Defines the Bauschinger effect ...

Cyclic Effects on Stress-Strain Behavior: Progressive changes occur during cyclic loading ... Material: Copper in 3 Conditions From Metal Fatigue in Engineering, H.O. Fuchs and R.I. Stephens, John Wiley & Sons, NY, 1980

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.