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Fundamentals of Fatigue Analysis

Fatigue is a structural failure mode that must be recognized and understood to develop products that meet life cycle durability requirements. In the age of lightweighting, fatigue strength is an important vehicle design requirement as engineers struggle to meet stringent weight constraints without adversely impacting durability. This technical concept course introduces the fatigue failure mode and analysis methods. It explains the physics of material fatigue, including damage accumulation that may progress to product failure over time, and it provides the needed foundation to develop effective fatigue prediction capabilities.

Participants will investigate the assumptions and applicability of fatigue analysis methods, such as stress-life (SN), strain-life (EN), and crack growth. They’ll solve example problems by hand calculation, emphasizing and reinforcing the practical aspects of key concepts and methods in preparation for putting the knowledge into action.

Student computers are NOT required but may be useful for note taking.

What Will You Learn

By attending this seminar, you will be able to:
  • Describe fatigue as a failure mode
  • Compare fatigue analysis methods, such as stress-life (SN), strain-life (EN) and crack growth
  • Define procedures and assumptions for applying these methods to strain gage- and CAE-based fatigue calculations
  • Articulate what occurs inside fatigue analyses
  • Apply key concepts to reduce fatigue and improve product durability

Is This Course For You

This course has been developed for engineers involved in design and development of structural parts and structural durability, including fatigue life prediction, improvement, or validation testing. It’s popular with both experienced analysis engineers and with those transitioning into a durability or analysis role.

The content is conceptual in nature, so it’s suitable for the broader engineering community. Engineering analysts will learn more about the durability-related tasks for which they are responsible. Product engineers and managers will learn key points about the durability concepts and analyses they need to understand to meet product durability requirements.

Materials Provided

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Course Requirements

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Topics

DAY ONE
  • Fatigue as a structural failure mode
    • The definition and background of fatigue
  • Cyclic loading and material response
    • Structural failure mechanisms
    • Micro- and macro-structural material response
  • Common fatigue analysis methods
    • Crack initiation and crack growth
    • Common methods of assessing durability
      • Hands-on: paper clip fatigue
  • Stress-Life approach – SN
    • Concepts of high cycle fatigue and endurance limits
      • Hands-on: Design stress
    • Material fatigue properties and their measurement
      • Hands-on: SN analysis with mean stress
    • Palmgren-Miner’s Rule for damage accumulation
      • Hands-on: Fatigue of an industrial press
  • Strain-Life approach – EN
    • Concepts of high and low cycle fatigue
    • Material fatigue properties and their measurement
    • Localized yield and Neuber elastic-plastic corrections
    • Stress concentrations
    • Rainflow cycle counting
      • Hands-on: rainflow cycle counting
DAY TWO
  • Fatigue and multiaxial stresses
    • Fatigue damage accumulation under complex stress states
    • Methods for predicting fatigue under multiaxial stresses
  • Fracture mechanics and crack growth
    • Linear elastic fracture mechanics (LEFM)
    • Assessing stressed cracked structures
    • Fast fracture and crack growth analysis
      • Hands-on: Fatigue and fracture of an aluminum wheel
  • Fatigue of welds
    • Weld vs. parent metal performance
    • Methods for predicted weld fatigue life
      • Hands-on: Fatigue of a welded oil riser
  • The role of temperature in fatigue
    • Changes to material behavior at high temperature
    • Methods for assessing durability under cyclic and thermal loading
  • Fatigue of composites and anisotropic materials
    • Characterizing strength and stiffness of short- and long-fiber materials
    • Methods for predicted fatigue life of composite materials
  • Practical applicability and summary
    • Fatigue with measured strain gauge data
    • Fatigue with FEA stresses
    • Method comparison – SN vs. EN vs. crack growth
    • Probability of failure: understanding scatter and its implications
    • Discussion of fatigue modeling and the 5-box trick