The design and working stresses
All criteria of strength are modified in some way in order to obtain a
design criterion. In its simplest form,the design criterion is a design
stress, which may also be called a safe stress or allowable stress.
The stress used in design must be a safe stress if failure is not to
occur, and such a stress is said to be allowable.
The design stress is the stress value which is used in the mathematical
determination of the required size of the machine member. It may be
considered to be the stress that the designer hopes will not be
exceeded under operating conditions. When the properties of the
material are definitely known and when the stress can be accurately
determined, this stress may be as high as 80 per cent of the yield
stress, but 50per cent is the usual value for nonshock and nonfatigue
conditions.
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