LIFE (CRACK INITIATION)

LIFE

The LIFE module is used to calculate fatigue Life in a FEM (acting as a 'Digital Twin' of a real structure) with a broad palette of Strain based and Stress based approaches.

The GUI is very intuitive and the analysis process is straightforward.

When both FEM and internal stresses are imported, a 'surface-stress-resolving' technique is employed 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.

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

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.

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

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