What is fatigue?

Design and Development - Comments Off - Posted on January, 21 at 10:01 pm

Designers normally consider the most important safety consideration to be the overall strength of the component, assembly, or product. To design for this, engineers want to create a design that will stand up to the probable ultimate load, and add a safety factor to that, for insurance.
In operation, however, the design is very unlikely to experience static loads. Much more frequently, it will experience cyclical variation, and undergo multiple applications of such load variation, which may lead to failure over time.
The definition of fatigue, in fact, is: failure under a repeated or otherwise varying load, which never reaches a level sufficient to cause failure in a single application. The symptoms of fatigue are cracks that result from plastic deformation in localized areas. Such deformation usually results from stress concentration sites on the surface of a component, or a pre-existing, virtually undetectable, defect on or just below the surface. While it may be difficult or even impossible to model such defects in FEA, variability in materials is a constant, and small defects are very likely to exist. FEA can predict stress concentration areas, and can help design engineers predict how long their designs are likely to last before experiencing the onset of fatigue.

The mechanism of fatigue can be broken down into three interrelated processes:

1. Crack initiation
2. Crack propagation
3. Fracture
FEA stress analysis can predict crack initiation. A number of other technologies, including dynamic nonlinear finite element analysis, can study the strain issues involved in propagation. Because design engineers principally want to prevent fatigue cracks from ever starting, this paper primarily addresses fatigue from that viewpoint.

Determining the fatigue strength of materials
Two principal factors govern the amount of time it takes for a crack to start and grow sufficiently to cause component failure: the component material and stress field. Methods for determining fatigue testing of materials go back to August Wöhler who, in the 19th century, set up and conducted the first systematic fatigue investigation. Standard laboratory tests apply cyclical loads such as rotating bend, cantilever bend, axial push-pull, and torsion cycles. Scientists and engineers plot the data resulting from such tests to show the relationship of each type of stress to the number of cycles of repetition leading to failure—or S-N curve. Engineers can derive the stress level a material can endure for a specific number of cycles from the S-N curve.
The curve splits into low and high cycle fatigue. Generally, low cycle fatigue occurs at fewer than 10,000 cycles. The shape of the curve depends on the type of material tested. Some materials, such as low-carbon steels, show a flattening off at a particular stress level—referred to as the endurance or fatigue limit. Materials that contain no iron show no endurance limit. In principle, components designed so that the applied stresses do not exceed the known endurance limit shouldn’t fail in service. However, endurance limit calculations don’t account for localized stress concentrations that may lead to initiation of cracks, despite the stress level appearing to be below the normal “safe” limit.

Fatigue load history, as determined by testing with rotating bend tests, provides information about mean and alternating stress. The rate of crack propagation in tests has been shown to be related to the stress ratio of the load cycle, and the load’s mean stress. Cracks only propagate under tensile loads. For that reason, if the load cycle induces compressive stress in the area of the crack, it will not produce more damage. However, if the mean stress shows that the complete stress cycle is tensile, the whole cycle will cause damage.
Many service load histories will have a non-zero mean stress. Three mean stress correction methods have been developed to eliminate the burden of having to carry out fatigue tests at different mean stresses:

Goodman method – generally suitable for brittle materials
Gerber method – generally suitable for ductile materials
Soderberg method – generally the most conservative

Posted in Design and Development | Comments Off

Comments are closed.