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Fatigue & Fracture Mechanics in Finite Element Analysis (FEA)

NAFEMS e-Learning Course

  


Four -Week e-learning Course - (one 2.5 hour session per week)


Course Overview

Fatigue failure occurs when a material is subjected to repeated loading and unloading cycles. The level of stresses present to cause failure may be well below values considered safe for a single static load application. The critical fatigue initiation is usually at a very localized site and may be a result of additional factors such as stress concentration due to component shape, surface finish or corrosion pitting.

Fatigue has been cited as one of the major causes of in-service failure throughout engineering history. The earliest application of rotating machinery with its attendant cyclic nature produced documented fatigue failures. Textile loom machinery, pumping machinery and above all steam railway operations were beset by a mode of failure that was not understood. The early railway axle failures and mining equipment failures prompted fundamental testing and research. The theories on which much of modern fatigue analysis is based on were developed all through the industrial revolution and into the 1920’s.

The advent of more complex structures with more complex loading histories was typified by the introduction of the first jet powered airliners. Sadly new fatigue lessons had to be learned in the period from 1954 as a result of the DH Comet crashes.

The nature and prediction of fatigue is much more understood, and is a requirement for most design products today. However the application of fatigue analysis is not easy and a good background is essential to be able to use the powerful FEA method as a basis for fatigue analysis.

Much of the terminology used in setting up the fatigue problem through a modern GUI is confusing and the choice of options is not always clear.

The objective of this course is to break down the fatigue analysis process into clearly defined steps, give an overview of the physics involved and show how to successfully implement practical solutions using Finite Element Analysis.


Course Process and Details 

In the current climate travel and training budgets are tight. To help you still meet your training needs the following e-learning course has been developed to complement the live class. The e-learning course runs over a three week period with a single two hour session per week.

The course is completely code independent. No software is required.

Each topic in the class is treated as a building block and is presented using an overview of the physics and theory involved. The math is kept simple and the emphasis is on practical examples from real life to illustrate the topic. The mapping to Finite Element analysis techniques is shown with numerous workshops. The tutor will be showing analysis results interactively and involving the students in the process via Q and A periods during each session, follow up emails and a Course Bulletin Board

Students are welcome to send in problems from industry and these will be discussed as time permits.

Full notes are provided for the students, together with personal passwords for e-learning backup material, bulletin board access etc.

Students will join the audio portion of the meetings by utilizing the VoIP (i.e. headset connected to the computer via headphone and microphone jacks) or by calling into a standard toll line. If you are interested in additional pricing to call-in using a toll-free line, please send an email to: e-learning @ nafems.org .


Who Should Attend?

This course is aimed at practicing engineers who wish to learn more about how to apply finite element techniques to fatigue analysis in the most effective manner. Ideally a student should have some experience of FEA analysis, but this is not essential. The material that is presented is independent of any particular software package, making it ideally suited to current and potential users of all commercial finite element software systems. This course is a must for all engineers aiming to use FEA as a reliable predictive tool for fatigue analysis.

E-learning classes are ideal for companies with a group of engineers requiring training. E-learning classes can be provided to suit your needs and timescale. Contact us to discuss your requirements.


Course Program

Note: This is a four-week course. Each session represents one 2-hour session each week. (Note: Sessions may last for 2.5-3 hours, including the Q&A sessions.)

Recordings of each session are made available to course attendees in the event they are unable to participate in one or more of the live meetings, or if they wish to review the material following each session.

The times and dates listed for each session are tentative; we try to schedule these sessions at times convenient for the majority of course attendees.

 

Session 1

Finite Element Analysis Overview

Introduction to Fatigue analysis

High Cycle Fatigue method

  • S-N Curveo Definition and Usage
    • Endurance Limit
    •  Data Sources
  • Mean Stress Effects
  • Fatigue Correction Factors
  • Loading Environment

Low Cycle Fatigue methods – overview

  • Strain Life
  • True Stress and Strain
  • Cyclic Stress Strain history

FEA application of Fatigue Analysis

Workshops and homework

Session 2: 

Homework Review

Notch Effects in High Cycle Fatigue

Low cycle fatigue

  • Notch Effects
  • Neuber Method
  • Peterson Method
  • Stress Gradient Method
  • Worked example
  • True stress strain definitions
  • Mean Stress Effects

Loading History definition

  • Cycle Counting Methods

More FEA implications

  • Surface Stresses
  • Stress Concentration Idealization

Workshops and homework

Session 3: 

Homework Review

Multiaxial Fatigue

  • Proportional Loading
  • Non-Proportional Loading
  • Solution Methods
  • Checking Methods

Vibration fatigue

  • Review of Random Vibration Analysis
  • Apparent Frequency and RMS values
  • Stress components
  • Von Mises results – caution
  • Damage calculation methods

Homework


Session 4: 

Homework Review

Introduction to Fracture Mechanics methods

  • Fracture mechanics
  • Crack Loading Modes
  • Stress Intensity Factor
  • FEA Implementation Methods
  • Virtual Crack Closure Method
  • Crack growth and re-meshing

Fatigue in Composites

  • Overview of Fatigue in Composites
  • Micro-mechanical behaviour
  • Fatigue Prediction
  • Practical Applications

Workshops

 

PSE

The PSE Competencies from the Fatigue technical area that are addressed by this training course are listed below.

IDCompetence
FATkn1 List the conditions necessary for fatigue failure.
FATkn2 List the possible sources of cyclic loading in your company products.
FATkn3 List potential sites for fatigue in your company products.
FATkn4 Sketch a sinusoidal stress variation and show the maximum stress, minimum stress, mean stress, alternating stress (or stress amplitude), stress range and stress ratio.
FATkn5 List a common source of harmful tensile residual stress in your company products.
FATkn6 List ways of inducing beneficial compressive stresses in your company products.
FATkn7 Sketch a fatigue diagram, showing the Modified Goodman, Gerber, Soderberg and Langer/Yield lines.
FATkn8 Sketch typical welds, highlighting features detrimental to fatigue performance.
FATkn11 Define the terms Nominal stress, Notch stress, Equivalent stress and Weld-Throat stress.
FATco1 Discuss the initiation, propagation and fast fracture stages of Fatigue in metallic materials.
FATco2 Describe how the data used to construct an S-N curve are obtained.
FATco3 Discuss the term high cycle fatigue, highlighting a common source in your company products.
FATco4 Discuss the statistical nature of fatigue and explain how this is handled in relevant design standards and codes of practice.
FATco5 Discuss the salient features of an S-N diagram for steels and explain the terms endurance limit, infinite life and low cycle fatigue.
FATco6 Discuss the typical appearance of a fatigue failure surface in a metallic component and explain how the source of the fatigue failure is commonly identified.
FATco7 Discuss the observed relationship between endurance limit and static tensile strength for steels and explain why this relationship does not hold for welded steels.
FATco8 Discuss the philosophy of Safe Life Design.
FATco9 Explain the term Damage Tolerant Design.
FATco10 Contrast the Stress-Life and Strain Life / Manson-Coffin approaches to fatigue assessment.
FATco11 Explain the use of Endurance Limit Modifying Factors in Stress-Life based fatigue assessment.
FATco13 Discuss the effects of corrosion on fatigue life and highlight how this is typically handled in relevant standards and codes of practice.
FATco14 Discuss the term Fatigue Strength Reduction Factor  in relation to stress concentrations and explain how this has traditionally been handled in relevant design standards and codes of practice.
FATco15 Discuss the concept of cumulative damage and explain how this is commonly handled.
FATco16 Explain why a multiaxial stress field can complicate an analysis and discuss approaches to handling this.
FATco17 Discuss the significance of the choice of equivalent stress used in the fatigue assessment of welded joints
FATco18 Outline a conservative approach to situations where the directions of principal stresses vary during a stress cycle.
FATco21 Discuss why weld toe grinding can be beneficial and explain how a design standards and codes of practice will typically allow for this improvement.
FATco25 Reflect on why fatigue is such a long-standing and persistent cause of failure.
FATco26 Discuss the nature of and typical locations for, stress singularities in a finite element model and explain how they would typically be handled in a fatigue analysis.
FATco27 Describe the approximations inherent in a plate/ shell idealisation of welded joints and how these could influence fatigue assessment.
FATco28 Discuss the term Effective Notch Stress and Nominal Stress.
FATco29 Explain how a Cyclic Stress-Strain Curve is constructed and used.
FATco30 Explain Neuber's Rule and its limitations and why corrections to the elastic strain range from an elastic analysis may be necessary.
FATco31 Discuss the term Local Plastic Strain Amplification Coefficient and Elastic Follow-Up.
FATco32 Discuss the possible effects of radiation on fatigue life
FATco33 Explain why corrections for mean strain are often unnecessary for low cycle fatigue.
FATco34 Explain the term ultra high cycle or gigacycle fatigue  and discuss findings in relation to the concept of endurance limit.
FATco35 Discuss the term endurance limit for many non-ferrous metals, steels in a corrosive environment and the possible effects of load sequencing.
FATco37 Reflect on how variable amplitude load sequencing can affect the prediction of damage accumulation and fatigue life.
FATap1 Employ a fatigue diagram, consisting of Modified Goodman and Langer lines, to assess fatigue performance of components.
FATap2 Carry out elastic fatigue assessment using design standards and code guidelines for components and structures including any special procedures for ancillary components such as bolts,
FATap5 Use Reservoir Counting / Rainflow Method or similar to specify the necessary stress ranges, number of cycles and loading history for any component to be analysed.
FATap6 Employ a finite element analysis system for the fatigue analysis of a component or structure.
FATap7 Use hot spot stress techniques (extrapolation and/ or linearization) to determine structural stresses for fatigue assessment.
FATsy1 Prepare a fatigue analysis specification, highlighting any assumptions relating to geometry, loads, boundary conditions and material properties.
FATsy2 Plan a fatigue analysis, specifying necessary resources and timescale.
FATsy3 Prepare quality assurance procedures for fatigue analysis activities within an organisation.
FATsy4 Specify whether a Fracture Mechanics approach is more appropriate.
FATev1 Assess the significance of neglecting any feature or detail in any idealisation being used for fatigue assessment.
FATev2 Assess the fatigue significance of simplifying geometry, material models, loads or boundary conditions.

*Note: While we will make every attempt to follow the course outline, the schedule may be shifted at some point. However, ample notice will be given prior to the start of the course date with regards to the course schedule.


Special Note(s):

Telephony surcharges may apply for attendees who are located outside of North America, South America and Europe. These surcharges are related to individuals who join the audio portion of the web-meeting by calling in to the provided toll/toll-free teleconferencing lines. We have made a VoIP option available so anyone attending the class can join using a headset (headphones w/ microphone) connected to the computer. There is no associated surcharge to utilize the VoIP option, and is actually encouraged to ensure NAFEMS is able to keep the e-Learning course fees as low as possible. Please send an email to the e-Learning coordinator (e-learning @ nafems.org ) to determine if these surcharges may apply to your specific case. 

Just as with a live face-to-face training course, each registration only covers one person. If you plan to register a large group (5+), please send an email to e-learning @ nafems.org in advance for group discounts.  

For more information, please email e-learning @ nafems.org .





Upcoming Session:


View the NAFEMS Professional Simulation Engineer competence statements addressed by this training course.


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Course Tutor:
Tony Abbey 

Tony Abbey - NAFEMS Tutor







Read Tony Abbey's bio on the NAFEMS tutors page