After you perform a fatigue analysis, you can plot life, damage, and
factor of safety plots for the model.
Suppose you run a static analysis and define a fatigue study with the
static analysis being the only event that occurs N times. Consider the S-N curve for the
material as shown:
The program, based on the loading ratio, determines a corrected
alternating stress from the static study's stress value for each node.
In the graph above, the green dot represents the alternating stress (Y
coordinate) and number of cycles for which the event occurs (X coordinate) for one node.
One out of three possible outcomes can occur for each node:
- The dot lies above the curve. Fatigue failure is predicted at
that location.
- The dot lies below the curve. Fatigue failure does not occur at
that location.
- The dot lies outside the range of S-N curve. The highest
corrected alternating stress in the model must be within the stress range of the
S-N curve. Also, the number of cycles (N) should be within the cycle range of
the S-N curve. Otherwise, instead of an intersection point, the program uses the
S-N curve's end point.
To simplify the discussion, we did not consider the scenarios where
multiple S-N curves for a material and several events are defined.
This table describes Life, Damage, and Factor of Safety.
| Fatigue Plots |
Description |
Relation |
Notes |
| Life |
The number of cycles that
causes failure at a model location. In the graph it is the intersection
between the horizontal line for σ and the S-N curve. |
Life = N - ΔN |
- Life plot is available only when you define a
single event.
- Life value depends only on the stress value at a
location and the S-N curve. It does not depend on the number of
fatigue cycles.
- If the corrected alternating stress in the model
falls below the stress level of the last point of the S-N curve,
the program takes the number of cycles defined for the last
point of S-N curve, unless you define and select the Infinite Life option in the
fatigue study's Properties, in which case the Infinite Life will be taken to
calculate the Damage.
- If the event is Variable Amplitude of Constant Amplitude), then the
Life results given by the fatigue analysis are in terms of
blocks instead of cycles, where blocks are defined as the total
load history (including the number of repeats of the event in
the curve) specified by the user.
|
| Damage |
The percentage cumulative
damage factor or the percentage life consumed. |
Damage = N/(N -
ΔN) |
- A value of 100 indicates that the defined
fatigue events consume 100% of the model life at that
location.
|
| Factor of Safety |
The ratio of the stress
value that causes fatigue failure over the stress σ. |
FOS = (σ-Δσ)/σ |
- The FOS plot is available only when a single
event is defined in the fatigue study.
- A factor of safety of 2.0 at a location predicts
failure at that location when applied loads are multiplied by
2.0.
|