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Q_Goest

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There's a common belief that for any given bolted joint, the higher the preload, the longer the fatigue life. I’d like to challenge that preconception and suggest that increasing preload above that needed to keep the joint together and acting as a pair is generally a bad thing and results in shorter fatigue life for the bolt. Here's why.

Bolted joints are generally modeled as two linear springs acting against one another – the

This change in force therefore produces a given change in strain which produces a given change in stress. From this, we can find the limits of stress that the joint undergoes. The limits are:

- The stress produced under the preloaded condition.

- The stress produced under the loaded condition. For the sake of simplicity, let's assume the loaded condition only acts in one direction, and that force is parallel to, and along the centerline of, the axis of the bolt.

The median of these two stress levels is called the mean stress. The mean stress is equal distance from these two stress extremes, and that distance is called the alternating stress.

The question then is, "If we have a given alternating stress and are only able to change the mean stress, then what is the best mean stress to have in a bolted joint to maximize fatigue life given that the alternating stress remains the same regardless of mean stress?" Figure 8.16 (attached) shows various types of stress combinations, all of which correspond to an equal fatigue life. From left to right, what it shows is that as the mean stress increases, alternating stress must decrease in order to maintain the same fatigue life. This can be seen also on a Goodman diagram in which we plot the fatigue life against alternating and mean stress as shown in Figure 8.21 (attached). On this Goodman diagram, the parallel lines running horizontally and then converge on the Uts (150 ksi) represent the fatigue limit of the material for given cycles. The operating point is then placed on this chart to determine if it is inside or outside any of these lines. If the operating point is outside the line, fatigue failure is predicted. The operating point is simply a point on this graph which is determined from the mean stress and alternating stress. That point is plotted where x is the mean stress and y is the alternating stress. Since the y value of alternating stress stays constant, it is obviously best to minimize the x location (mean stress) because all the lines drop off to the right, and the farther you go out to the right (positive x direction) the less alternating stress it takes to exceed the fatigue limit of the material.

Unfortunately, calculating the fatigue life of a part is never easy given all the various factors involved, but fortunately, those factors don't change depending on stress level. The factors such as gradient factor, load factor, surface factor and stress concentration factor, all remain the same regardless of the mean and alternating stress. The conclusion I've come to therefore is that to maximize the strength of a bolted joint in pure tension, I want to have the minimum mean stress possible that allows the bolted joint to act as a pair under all conditions which means

Thoughts?

Bolted joints are generally modeled as two linear springs acting against one another – the

*and the*__bolt__*. As long as the bolt doesn't come away from the joint, the spring rate of the assembly is constant. Because the spring rate is constant, the stretch (or change in length of the bolt) that occurs when the bolted joint undergoes a given change in load is independent on the initial and final load. The stretch of the bolted joint due to a given change in load dF, is given by dx = dF/k where dF is the change in load and k is the spring constant of the bolted joint. So the strain in the bolted joint is linearly dependant on the change in force imposed on the joint as long as the bolt and joint stay together and act as a pair. This proved to be the case when we modeled a joint using FEA and also modeled it using the canned formulas in texts on the subject.*__joint__This change in force therefore produces a given change in strain which produces a given change in stress. From this, we can find the limits of stress that the joint undergoes. The limits are:

- The stress produced under the preloaded condition.

- The stress produced under the loaded condition. For the sake of simplicity, let's assume the loaded condition only acts in one direction, and that force is parallel to, and along the centerline of, the axis of the bolt.

The median of these two stress levels is called the mean stress. The mean stress is equal distance from these two stress extremes, and that distance is called the alternating stress.

The question then is, "If we have a given alternating stress and are only able to change the mean stress, then what is the best mean stress to have in a bolted joint to maximize fatigue life given that the alternating stress remains the same regardless of mean stress?" Figure 8.16 (attached) shows various types of stress combinations, all of which correspond to an equal fatigue life. From left to right, what it shows is that as the mean stress increases, alternating stress must decrease in order to maintain the same fatigue life. This can be seen also on a Goodman diagram in which we plot the fatigue life against alternating and mean stress as shown in Figure 8.21 (attached). On this Goodman diagram, the parallel lines running horizontally and then converge on the Uts (150 ksi) represent the fatigue limit of the material for given cycles. The operating point is then placed on this chart to determine if it is inside or outside any of these lines. If the operating point is outside the line, fatigue failure is predicted. The operating point is simply a point on this graph which is determined from the mean stress and alternating stress. That point is plotted where x is the mean stress and y is the alternating stress. Since the y value of alternating stress stays constant, it is obviously best to minimize the x location (mean stress) because all the lines drop off to the right, and the farther you go out to the right (positive x direction) the less alternating stress it takes to exceed the fatigue limit of the material.

Unfortunately, calculating the fatigue life of a part is never easy given all the various factors involved, but fortunately, those factors don't change depending on stress level. The factors such as gradient factor, load factor, surface factor and stress concentration factor, all remain the same regardless of the mean and alternating stress. The conclusion I've come to therefore is that to maximize the strength of a bolted joint in pure tension, I want to have the minimum mean stress possible that allows the bolted joint to act as a pair under all conditions which means

__the best bolted joint will have the lowest possible preload (torque) on the joint that is sufficient to keep the bolted joint together and acting as a pair during all foreseeable operations.__Thoughts?