Which Fatigue Methods/Features Should FatPy Implement?

[Vybornak2]

Hi everyone,

As FatPy develops, we want to know fatigue calculation methods and features that you would like to see implemented.

Beyond the fundamentals, what specific capabilities would make FatPy most useful for your work or research?

Some potential areas could include for example:

• Specific life models (Strain-Life E-N, Crack Growth - Paris Law?)
• Advanced Mean Stress Corrections (Morrow, SWT?)
• Cycle Counting Algorithms (Rainflow?)
• Multiaxial Fatigue Criteria (Findley, Dang Van?)
• Weld Fatigue specific methods?
• Load history processing or other utilities?

What specific features, algorithms, or calculation methods would you most like to see added to FatPy?

Please share your ideas and briefly mention why they'd be useful. Your input will help guide our development efforts!

Thanks!

[JanHPapuga]

Hi all,

Well, according to me, we should first focus on methods, which are needed for the items proposed by Jan.

1. Stress/strain tensors evaluation: von Mises, principal stresses & principal directions for general 3D stress state, stress state projection into a specific plane, splitting the tensor to deviatoric and hydrostatic part, converting the stress tensor into the 5D Ilyushin's deviatoric space.
2. Shear stress path evaluation: minimum circumscribed circle and ellipse in 2D and 5D (plus methods to simplify/quicken the calculation, in addition, there are multiple minimum circumscribed ellipse definitions), maximum prismatic hull, moment of inertia (Meggiolaro & Castro), ...
3. Definition of normal line to the surface for mesh plus isolating the surface nodes.
4. Enlarging the domain of interest: Searching for elements neighbouring given nodes and nodes included within an element. Thus allowing to get from a single hotspot to the mesh volume at which the stress exceeds a given threshold.
5. Hot-spot evaluation: In addition to the previous item, looking for the closest node on the opposite surface to get the local thickness.
6. Rain-flow: converting the output to rain-flow matrices (automatic ranges vs user-set limits). RF in 1D, Wang's & Brown's RF for multiaxial cases.
7. S-N curve description: Alternative S-N curve formulations as the material/part input. Regression analysis over provided S-N data.
8. Stress gradient: Calculating the relative stress gradient at a given node - perpendicularly to the surface, but also in other directions to reach its maximum and minimum values.
9. Stress and strain tensors within the mesh: Interpolation of stresses and strains across the elements, not only at nodes and integration points.
10. Maximum damage optimization: Methods to quicken the search for the plane with maximum damage

Well, there will be many more, that's just for the start :-)

[gokberkozcicek]

Thanks to @JanHPapuga for clear explanation👍

[Vybornak2]

Hello, here is a full list of methods we would like to implement.

Core

Decompositions

• Rain-flow in 1D stress or strain
• Rain-flow on a specific stress/strain component or equivalent stress while monitoring other stress tensor component features
• Shear stress path analysis in 2D (precise combination algorithms vs optimization algorithms)
• Minimum circumscribed circle
• Minimum circumscribed ellipse
• Maximum prismatic hull
• Moment of inertia method
• Load path analysis in Ilyushin's 5D space
• Minimum circumscribed hyperball
• Minimum circumscribed hyperellipsoid
• Maximum prismatic hull
• Moment of inertia method
• Filtering techniques to isolate only local peaks
• Wang-Brown multiaxial rain-flow
• Load path reduction (minimum convex hull – to simplify its analysis)
• Frequency domain e.g. Dirlik

Plane-based methods

Damage Cumulation

• Miner-Hailbach
• Miner-Elementary

Stress life

Base Methods

Correction Methods

• Stress gradient involvement – various fatigue factors definition
• Retrieval of an equivalent stress at the critical distance
• Fatigue limit correction, S-N curve slope correction
• Overlapping of individual corrections
• Size effect
• Roughness effect
• Notch effect
• Technology effect
• Mean stress effect
• Statistical effect

Damage Parameters

• Multiaxial fatigue strength criteria
  • Dang Van
  • Findley
  • Papuga QCP
  • Crossland
  • McDiarmid
  • Liu&Zenner
  • Böhme&Papuga
  • Etc.

Strain life

Base Methods

Correction Methods

• Elastic to elastic-plastic states conversion
  • Neuber
  • Glinka's ESED
  • Hoffman-Seeger
  • Pseudo-stress & pseudo-strain solutions
  • Application both to cyclic stress-strain curve and to hysteresis loop branch – memory effect involvement
  • Mean stress effect involvement
  • Landgraf
  • Morrow
  • Bergmann
  • Feltner
  • Heitmann
  • SWT
  • Walker

Decompositions

Damage Parameters

• PSWT
• PRAM
• PRAJ
  (parameters acc. to FKM-NonLinear)

Material laws

• Ramberg-Osgood
• SeegerBeste
• Wohler curve representations – Power law, Kohout-Věchet curve
• S-N data regression
• Hooke's Law
  • 1d
  • 2d
  • 3d

Structural mechanics

Equivalent stresses

• Principals
  • All principals
  • Max principal
  • Min principal
  • Abs principal
• Von mises stress
  • Signed Von Mises
• Von mises strain
  • Signed Von Mises
• Tresca
  • Signed Tresca
• Mean stress
• Stress amplitude

Tensor transformations

• Stress tensor projection on a plane
• Stress tensor transformation to Ilyushin's deviatoric space
• Morh circle (plane stress)

Utils

• Signal
• Pre-Processing
• Post-Processing

Mesh

• Hot-spot Detection
• Gradient
• Interpolation
• Surface detection
• Defining a normal line to the surface
• Critical volume calculation
• ecs -> gcs transform

Examples

• Stress invariant method

Norms / Standards

• To be added

[Vybornak2]

Full list of methods can be found in Component List to FABER members.