Evaluating results

After running the analysis, the results can be evaluated using one of 4 post-processors:

  1. Graph plot (single node results)
  2. Contour plot (full model color plot)
  3. Vector plot (principal stress directions)
  4. Hot-spot analysis (single node or extrapolated results)
  5. Path plot (stress evolution along path)

The appropriate post-processor is selected from the tabs in the Results panel.

In general, the Matlab 3D plotting engine is not too fast. It is therefore highly recommended to keep the model size (number of elements/nodes) down.

For necessarily large models, it is recommended to divide these into smaller parts/bodies. These can then be selected on the main figure under Show parts, and Fatlab will then only show these.

Results can also be exported to an Excel spreadsheet using the Export results button. The file will be saved in the working directory, overwriting any existing results file.

Please refer to the Theory reference for technical details.


 

1. Graph plots

A single node is selected either by entering its node number in the Selected node textbox or graphically using the Select node toolbar button The selected node will be highlighted with a red dot. The Probe nodes toolbar button does not select a node as such; it is used for probing the results of many different nodes and updating the selected graph results online.

The following graph results can be selected from the Graph popup menu:

Graph Description
Damage This bar plot shows the damage accumulated in the selected node for each load case along with the total damage sum.
Stress This graph shows the stress time history for the selected load case and stress component (not necessarily the one used for fatigue analysis).
Dominating load This bar plot shows an estimate of the damage contribution of each load component in the selected load case. The estimate is determined by calculating the damage for the node by one load component at the time (i.e. setting all others to zero). The interaction between load components (one helping another) is thus lost in this estimation and. However in general, it is sufficient to determine which load component dominates the damage generation in the selected node. This information can be used e.g. to select in which load case stress extrapolation should be carried out for welded joints.
Markov matrix The Markov matrix is a visualization of the distribution of range and mean values of the stress cycles acting the selected node. It cannot be generated for single-cycle loads.
Stress spectrum The stress spectrum is calculated for the selected fatigue stress and shows the stress ranges ordered from large to small over the cumulative number of cycles. The plot also shows the currently selected SN curve together with two damage equivalent constant amplitude spectra.
Range/mean distribution This bar plot is similar to the Markov matrix, but showing only the distribution of ranges or mean values.
FE stress components This shows the load-stress relationship for the selected load and stress component.
Critical plane damage This plot shows the calculated damage in each search plane. Simultaneously, the critical plane  can be shown in the model view.
Mohr’s circle This plot shows the Mohr’s circle for discrete time instances for the selected load case. The time instance is selected using the slider appearing below the plot. The principal stress directions are visualized in the model view for the same time instance.
Multiaxiality analysis This plot shows the numerically largest principal stress magnitude as a function of its direction.

Example result plots:

 


 

2. Contour plots

The contour plot tab provides several options for visualizing values using a color-scale in the model view. The Results popup menu provides the following contour plot options:

Result Description
Model only Clears any result plots from the model view.
Damage The damage accumulated in the selected load case.
Utilization Utilization ratio from the sum of damage from all load cases.
Max stress Maximum stress value occurring during the selected load case.
Min stress Minimum stress value occurring during the selected load case.
Max stress range Maximum stress range occurring during the selected load case.

Most results refer to the selected load case, except Utilization, which refer to all load cases.

A limit on the color-scale can be imposed by changing the max value of the selected result set.

Example result plots: Left: contour plot of the damage. Middle: critical plane in selected node. Right: principal stress vectors in selected node, at selected time instance.

2.1 Stress time series

This function visualizes the selected stress component over time in the selected load case. The time instance to plot is selected using the Time slider. Press the Calculate button first.



 

3. Vector plots

The vector plot tab provides features for visualizing directional information (currently just principal stress directions).

Initially, the user must press the Calculate button to calculate the necessary information. Then the Time slider can be used to animate the principal stress directions over time.

The principal stress directions can be plotted either for the single selected node or for all nodes depending on the selection in the Scope popup menu.

Option Description
Scale by magnitude Scales the direction vectors according to the size of the associated principal stress.
Show P1
Show P2
Show P3
Select which of the principal stress direction vectors to show.
Show Pnmax Show the direction of the numerically largest of P1 and P3.

Example:

 


 

4. Hot-spot analysis

The Hot-spot tab is mean for detailed analysis of locations of special interest, e.g. a weld or a notch. Hot-spots can be added/deleted using the add/remove row buttons.

The hot-spot table is useful e.g. for comparing different stress components or different SN curves, as shown below. It is possible to define additional SN curves for hot-spot analysis without running the entire model again.

The table takes input in the form of node numbers that can be entered manually or selected graphically from the model view by first placing the cursor in the relevant cell and then selecting the nodes.

Subsequently, the user must specify a method, a stress and an SN curve if non-default values are required.

Method Description
Single node Use nodal stress  directly (converged stresses)
3p extrapolation Weld: Linear extrapolation from node 3 through node 2 to node 1.
0.4t 1.0t
0.5t 1.5t
Weld, type A hot spot: Linear extrapolation
0.4t 0.9t 1.5t
0.5t 1.5t 2.5t
Weld, type A hot spot: quadratic extrapolation
5mm 15mm Weld, type B hot spot: Linear extrapolation
4mm 8mm 12mm Weld, type B hot spot: Quadratic extrapolation

Pressing the Calculate button then determines the damage (D), utilization ratio (UR) and dominating load component ( DLC) for each hotspot.

hot_spot_table

Hot-spot analysis

Each hot spot will be marked in the model view using a magenta square (Single node) or a magenta arrow (extrapolation) as shown below.

weld


 

 

5. Path plot

The Path plot feature works much like the one in ANSYS, i.e. a stress component can be plotted along a geometric path. In Fatlab, the path is defined by picking a series of nodes, e.g. radiating away from a notch or along some other line of interest. This feature is particularly useful for evaluating the correctness of hotspot stress extrapolation for welded joints.

 

 

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