ABSTRACT
Whether or not a fastener is safe from fatigue failure can be determined through the use of a fatigue diagram. Here, the mean stress (fastener preload) is plotted against the dynamic or alternating stress. Next is is necessary to define the failure criteria. There are several failure theories. The most commonly used is the Goodman criterion. Figure 4 is a fatigue diagram including the Goodman line. Only points below that line are safe. The Goodman line extends from the endurance limit at zero mean stress to the tensile strength at zero alternating stress. This indicates that, just as in Section 7 for combined tensile and torsional stresses, a decrease or increase in the alternating stress must accompany an increase or decrease, respectively, in the mean stress. The likelihood of failure is evaluated by plotting the mean and alternating stress conditions for the fastener. The mean stress is the in-service fastener preload. Determination of this value is described in Chapters 18 and 36. Fastener alternating stress is not the full alternating load applied to the joint; it is the stress due to that portion of the alternating applied load that the fastener experiences. This value is determined by using joint diagrams as described in Chapter 36. The Goodman line can also be expressed using an interaction equation. Expressing the alternating and mean stresses as ratios of their respective allowable values, the interaction equation incorporating a factor of safety would be
s (16)
C>a
where
See Chapter 37 for a further discussion of fatigue.
