Design for fatigue strength:

The failure of fatigue strength accounts for about eighty percent of the failures in the field of engineering. This failure occurs due to subjection in fluctuation loads. Most of the times, fatigue fractures occur due to a crack that starts with a discontinuity in the material or at a location where other stress gets concentrated. This fracture or crack grows gradually under the effect of the repeated application of the load. As this crack increases in size, the stress on the cross section bearing the load increases till it reaches a level that is high enough to cause a catastrophic fracture of that part of the machine.

Fracture surfaces exhibit smooth areas corresponding to the gradual crack growth stage and rough areas that correspond to the catastrophic fracture stage. The smooth parts of the fracture surface generally show beach marks that occur due to changes in the magnitude of the fluctuating fatigue load.

In most of the cases, the fatigue strength or limit of endurance of the material is based on a test of samples that are carefully prepared under laboratory conditions. Such values as these cannot be used directly for the purpose of design because the behavior of a structure or component under the effect of fatigue depends not only on the endurance limit or fatigue of the material but also on many others factors. These factors are:

• Size and shape of the structure or component
• Type of loading and the state of the stress
• Stress concentration
• Operating temperature
• Service environment
• Method of fabrication.

A damage tolerant design is one that has an extension of fail-safe criteria and assumes that there are existing flaws in the component before they are used. Fracture mechanics techniques are used to find out if a crack will grow large enough to result in failure before being detected.

Questions:

• How do fatigue failures occur?
• How do you check if a crack is dangerous or not?