Fatigue, Damage Tolerance, & Airworthiness of Aging Aircraft FAT

Course Length: 3 days

Pre Requisites:

None

Summary:

This course is offered by Fatigue Concepts at Abaris' Reno, NV facility. It is designed for anybody interested in learning about aging aircraft, metal fatigue, corrosion, and many other related issues.

Please visit this link to enroll: https://flipt.wufoo.com/forms/fatcon-enrollment-and-payment-form-4-day-course/

Introduction:

Topics:

Prologue
Cost of Fracture
The Danger of Metal Fatigue
- Early transportation failures from centuries past
  - Bridges
  - Trains
  - Titanic and Liberty ships
  - Wright Flyer, Comet, …
  - F-111 crash
- What is a Failure? USAF-Navy-FAA-NASA Guidelines
- Structural Failure Modes
- Environmentally induced and/or time-dependent failure modes
  - Creep, Fatigue, Corrosion, Wear, Fretting, Hydrogen Embrittlement
- Synergistic processes
  - Corrosion fatigue, fretting fatigue, SCC, high temperature fatigue
- Why High Strength materials are weak in fatigue!
- Crack Initiation and Propagation
- Design Strategies to combat Metal Fatigue
  - Safe Life
    - Safety Factor fantasies
  - Fail Safe
    - 1. Crack Arrest
    - 2. Multiple Load Path (Structural Redundancy)
    - Design for inspectability
Fractography
- What does a fatigue crack look like?
- Macro and Micro Aspects
- Striations and Beach Marks
- Quantitative and qualitative use
  - Ratchet lines
- Marker cycles
- Guides on using Fractography
- Scanning Electron Microscope
- Use of fractography to perform failure analysis
Stress Concentration K_t, Finishes
- Stress Concentrations
  - Load Flow Lines (Streamlines): a useful analogy
- Surface finish
  - Pitfalls in grinding
  - Chrome-plating dangers, zinc and other platings
- Size effects
  - Leonardo da Vinci
  - Similitude or lack thereof
- Scatter, Reliability
- High Temperature Effects
Fatigue Life Prediction
- Cycle parameters
  - Range, amplitude, mean stress, R ratio
- S-N Curves
  - Endurance Limit is largely a myth: Use Fatigue Strength
  - Fatigue Strength
  - Mean stress effects
  - Constant Life Diagrams, Haigh Diagrams
- Low cycle fatigue
  - Neuber’s Notch strain analysis
- Miner’s Myth
  - Extremely limited applicability and lack of defensibility
  - It is not a RULE
- Fracture Mechanics
  - Griffith Criterion
  - George Irwin, Naval Research Lab: Father of Fracture Mechanics
  - Stress intensity factor SIF K_I
  - Beta factors
  - Residual strength
  - Fracture toughness
    - Effect of thickness, orientation, impact, temperature
    - Dynamic fracture toughness and crashworthiness
  - Crack propagation
  - Thresholds
  - Inspection intervals
  - Classroom problem-solving exercises
  - NDE, detectable crack sizes
    - Visual, eddy current, radiographic, ultrasonic, penetrant, magnetic particle
    - Applicability, comparison
    - POD (Probability of Detection)
    - Reliability of Inspections
    - Case Study: MD-88 uncontained engine failure
  - Initial quality, EIFS equivalent initial flaw size
  - Multiaxial stresses, Mixed mode cracking
- Short cracks
  - Can be deadly
- Classroom tutorials in Fracture Mechanics
  - Rigorous, parametric evaluations of various interconnected variables: fracture toughness, initial flaw size, inspection interval, fastener holes, corrosion effects, coldworking, shot-peening, da/dN, material properties C, m, …
- Damage Tolerance
- MIL-A-83444 -> MIL-A-87221 -> JSSG2006
  - 1. Fail-safe structure
    - Arrest of a rapidly running crack
    - Safe partial failure of a load path
      - Skin-stringer partnership
      - Titanium crack stoppers
  - 2. Slow crack growth structure
    - Degree of inspectability
  - Initial flaw assumptions
- Retardation Models
  - Willenborg
  - Wheeler
  - Closure
  - Onera…
- Continuing damage
- ASIP, IATP
Environmentally Assisted Cracking
- Corrosion Fatigue
  - Effect of frequency, waveform, hold-time, …
- Stress Corrosion
  - K_Iscc
  - SCC ratings for aircraft alloys
  - Case study: SCC of a High Strength Steel Frame in a USNavy Fighter
- Fretting Fatigue
  - How surface finish, similar materials in contact, slip amplitude, temperature, and environment affect Fretting Fatigue
- Contact fatigue
  - Gear failures
- Aging aircraft issues
  - Durability vs Damage Tolerance
  - Multi-Site Damage, Widespread Fatigue Damage
  - The Aloha accident
  - Continued airworthiness
  - Supplemental inspection programs
  - Limit of Validity (LOV)
Testing for Substantiation and Data accumulation
- Cycle Counting methods
  - Rain flow method
- In-flight loads monitoring
- Testing
- Coupons, specimen, component, and Full-Scale
  - Standardized Testing
  - Coupon preparation, precracking guidelines
  - E-399 K_Ic
  - E-647 da/dN
  - Service duplication Testing
  - Environmental chambers
  - Time compression issues in corrosion fatigue testing
- Load Spectrum Generation
  - Canned Spectra
    - TWIST, MINITWIST, TURBISTAN, HELIX, FELIX, …
  - Sequence effects
  - Truncation, clipping
- Scatter, Reliability, Risk management
  - Weibull Paper, Example problem
- Material Properties & Useful Data handouts and Sources
  - S-N curves
  - Crack initiation data
  - da/dN crack growth data
  - Fracture Toughness K_cs
  - Data sources and handbooks
  - Journals
Special Topics (Optional)
- Cracking in aircraft engine components
  - ENSIP
  - Sioux City DC-10 crash landing
  - Thermo-mechanical Fatigue
- Creep Fatigue
- Thermal Fatigue
  - Concorde
- Sonic fatigue (Acoustic Fatigue)
- Fatigue of composites
  - Polymers
- ARALL, GLARE and other newer materials to fight fatigue
Do’s and TABOOs
- Methods to improve fatigue resistance
- Clever use of favorable residual stresses
  - Cold-working of holes
  - Shot Peening
  - Residual Stress control measures
- Bolts, Rivets, Springs, Bearings, Welds, …
- Failure Analysis: useful learning tool
  - Care and handling of fractures
- Repair techniques
  - Japan Air Lines 747 crash
- “stop”-drill?
- Helpful treatments and processes
  - plating
- Hi-Tigue and other fatigue rated fasteners
- Guidelines based on experience
Epilogue
No highway

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