Design for static strength:

Designs have been determined for cyclic loading of working stress from the ultimate or yield point values of the material divided by a factor of safety. This gives reliable and safe results for static loading due to static strength. There are numerous machine parts that are subject to a non steady and vary continuous stress. The failure in such parts of machines is often caused by repeated loading and at stress below the yield point. When the part is below the endurance limit or fatigue value, it lasts indefinitely. The failures in fatigue that result form bending are very common and occur across the crystal.

The fatigue failure is a result of separation or fracture that does not have a visible distortion or yielding. A stress that is purely reversing or a cyclic stress is one that is seen when the stress is alternating between equal negative and positive peak stresses at each cycle of he operation. The cyclic stress over a time period can be represented graphically by a sinusoidal curve. In pure cyclic stress, the average stress is generally zero. When a machine part is subject to cyclic stress, also called range stress, the failure of the part has been observed to occur after numerous stress reversals even when the magnitude of the range stress goes below the yield strength of the material. On a more general front, the higher the range stress is, the less are the number of reversals required for failure of that part of the machine.

As per the maximum normal stress theory, the failure that occurs in a machine part is when the maximum normal stress in that part exceeds the shear strength of the material. This is determined by unaxial testing. The maximum normal stress theory deals only with brittle materials.

Questions:

• What is a cyclic stress?
• What does the maximum normal stress theory postulate?