It is observed that repeated loading and unloading weakens objects over
time even when the induced stresses are considerably less than the allowable stress
limits. This phenomenon is known as fatigue. Each cycle of stress fluctuation weakens
the object to some extent. After a number of cycles, the object becomes so weak that it
fails. Fatigue is the prime cause of the failure of many objects, especially those made
of metals. Examples of failure due to fatigue include rotating machinery, bolts,
airplane wings, consumer products, offshore platforms, ships, vehicle axles, bridges,
and bones.
Linear and nonlinear structural studies do not predict failure due to
fatigue. They calculate the response of a design subjected to a specified environment of
restraints and loads. If the analysis assumptions are observed and the calculated
stresses are within the allowable limits, they conclude that the design is safe in this
environment regardless of how many times the load is applied.
Results of static, nonlinear, or time history linear dynamic studies can
be used as the basis for defining a fatigue study. The number of cycles required for
fatigue failure to occur at a location depends on the material and the stress
fluctuations. This information, for a certain material, is provided by a curve called
the SN curve.
SOLIDWORKS Simulation does not support fatigue events that are based on static studies
that include composite shells and static studies with beams only.
Stages of Failure Due to Fatigue
Failure due to fatigue occurs in three stages:
| Stage 1 |
One or more cracks develop in the
material. Cracks can develop anywhere in the material but usually
occur on the boundary faces due to higher stress fluctuations.
Cracks can occur due to many reasons. Imperfections in the
microscopic structure of the materials and surface scratches caused
by tooling or handling are some of them. |
| Stage 2 |
Some or all the cracks grow as a
result of continued loading. |
| Stage 3 |
The ability of the design to
withstand the applied loads continue to deteriorate until failure
occurs. |
Fatigue cracks start on the surface of a material. Strengthening the
surfaces of the model increases the life of the model under fatigue events.