c. Working stress and factor of safety
The working stress __, also called the allowable stress, is the maximum safe axial stress used in design. In most designs, the working stress should be limited to values not exceeding the proportional limit so that the stresses remain in the elastic range (the straight-line portion of the stress-strain diagram). However, because the proportional limit is difficult to determine accurately, it is customary to base the working stress on either the yield stress __ or the ultimate stress __, divided by a suitable number N, called the factor of safety. Thus,