Fatigue Analysis
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.
Stages of Failure Due to Fatigue
Failure due to fatigue occurs in three stages:
Fatigue cracks start on the surface of a material. Strengthening the
surfaces of the model increases the life of the model under fatigue events.
Related Topics
Fatigue
Check Plot
Performing
Fatigue Analysis
Rainflow
Cycle Counting Method
SN
Curve
Theory
of Accumulative Damage