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LIFE

The LIFE module is used to calculate:

  • Fatigue Life with Strain based approaches

  • Fatigue Life with Stress based approaches (it includes analysis methods for weldings)

  • Extract sequence of stress tensors at selected elements (Virtual Strain Gauge)

  • Assess Multiaxiality at selected FEM elements

When both FEM and internal stresses are imported, a 'surface-stress-resolving' technique is employied in order to handle, during the calculation, the plane stress tensors which occur at the component surface (in case of shell modelled components this is not done as stresses are naturally planar on the top-bottom faces).

In the LIFING database only stresses occurring at the surfaces are stored.

Stresses at the surface, if no pressure is applied, are in fact plane stress by definition.

From a numerical standpoint this is not true, as FEM stresses are calculated at Gauss points (internal) and stresses at the surfaces (element nodal stresses) are extrapolated.

LIFE will ignore stress tensor components which are not the plane stress component ones.

This approach implies that a 'good' mesh refinement is required for a good Fatigue Life estimation.

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Once FEM and stresses are imported, three things are required before running the Fatigue analysis:

  • Materials definition (for the whole FEM or for each sub-part, if the analysed assembly is made of different material components.

  • Spectrum definition: each imported load case (typically 'unit' load cases) is associated to its own time history. At a generic instant of the overall time history, the stress tensors are given by the linear combination of unit load stresses (the istantaneous time history values represent the multiplication factors for the linear combination).

  • Analysis method definition: depending on the type of selected material properties, the analysis method can be selected. If the assemby is defined with multiple materials, each sub-part will be analysed accordingly. The analyst can also assign, to the whole FEM or to the sub-parts, residual stresses (for example deriving from specific surface conditions or treatment).

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After the above steps are done, the Fatigue Analysis can be performed. The analyst has the option to run the analysis at:

  • the entire FEM

  • a sub-part

  • a selected element

In the last case the analyst can dump a set of ASCII files showing all the analysis steps (e.g. how the damage is cumulated cycle-by-cycle, i.e. how the Fatigue Life is obtained).

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On the left, the LIFE workflow is summarized:

1. The FEM (model and related results, i.e. stress stratus, is imported.

2. The fatigue material properties (dependent on the analysis method being adopted) and Spectrum are defined.

3. The analysis is run (on the entire model or on selected sub-parts).

4. The fatigue results are post-processed, ready for reporting.

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The process is very simple and streamlined.

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The following analysis methods are available.

  • Strain based approach

    • Elastic-Plastic Stress calculation with Neuber or E.S.E.D. (Glinka)

    • Multiaxial Elastic-Plastic Stresses calculated with Dowling or Hoffman-Seeger approach by reducing the generic Non-proportional Loading cases to Proportional Loading ones or by using full Multiaxial Non-Proportional Loading cases with Pseudo-Material approach in conjunction with the Mroz-Garud cyclic plasticity model.

    • Critical Plane methods are implemented. Fatigue parameters:

      • Smith-Watson-Topper

      • Morrow's

      • Manson-Halford

      • Brown-Miller

      • Fatemi-Socie

 

  • Stress based appraoch

    • S-N curves defined by points

    • S-N curves as per MIL Standard (MIL-HDBK-5J). When LIFING is installed the MIL-HDBK-5J S-N curves database is installed.

    • Multiaxial analysis with Dang-Van, McDiarmid, generalized Goodman, ...

    • Multiaxial analysis with equivalent stresses

    • Multiaxial analysis with uniaxial reduction (critical plane search)

    • Mean stress accounting with:

      • Goodman

      • Gerber

      • Soderberg

      • Walker

      • Morrow

      • Smith-Watson-Topper

      • Haigh diagram

      • Rupp

 

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Fatigue based on PSD is handled: some load channels can be 'fed' with PSD signals and equivalent time histories are calculated with the Dirlik, Narrow Band or Stainberg methods. 

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LIFE can also be used for 'straight analyses':

  • The user can import a sequence file, instead of a FEM model: in this case the sequence is recognized as a stress sequence and the user can perform a uniaxial fatigue analysis with any of all implemented methods.

  • The user can import a stress tensor file, instead of a FEM model: in this case LIFE creates a single element database where internal stresses are those imported from the file and fatigue can be calculated at this element with any of all implemented methods.

  • The user can import a set of sequence files coming from a real strain gauge (uniaxial or 0-45-90 or 0-60-120 or 0-120-240): as for the above, the fatigue analysis can be therefore carried out with any of all implemented methods.

Other than just analysing, the LIFE module can be used for stress tensor time history extraction at selected locations with Virtual Strain Gauge and related Multiaxial Assessments.

The analyst can put on any FEM location (on the surface), a strain gauge and can orient it.

For the selected element

  • the sequence of stress tensors can be visualized and dumped in ASCII files

  • Mohr Circles at defined instant can be visualized

  • Multiaxial Assessment can be performed (scatter plots for showing Maximum Principal direction variation over the time as well as Biaxiality ratio variation over the time.

Time histories can be exported and/or handled as following:
This module allows to perform the following tasks.

  • Filter sequence. The following methods are included:

  • Non-turning points filtering (always active)

  • Racetrack filtering

  • Modified. The following options are available:

      Scaling (entire sequence or a portion)    

      Offsetting (entire sequence or a portion)

      Negative values scaling

      Clipping

  • Cycle counted. The Range-Pair method is implemented as per the ASTM STP1006 standard. The output cycles sequence is dumped in a ASCII file. The option to dump the counted sequence in AFGROW format is available.

  • Exceedence plots and Range-Mean Hystograms can be visualized

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​​​​​​​​​​​​​​​​​​​​​​​​​​Element stress tensor sequences can be also filtered with the Multiaxial Racetrack Filter.

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​The LIFE module development Roadmap includes the following enhancements:

  • Termo-structural Fatigue calculation

  • Additional Multiaxial Fatigue analysis methods

  • Interaction with the GROWTH module in order to calculate crack initiation at a hole which has been drilled inside the GROWTH module.
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