Joint failure by bolt bearing

[gyro]

I am currently working on a strut connection where the strut end is a block of aluminium (quite thick)and on each side of this is 4130 steel plates that connect to the fuselage. What I wanted help with is when doing the bolting bearing stress calculations on the hole. I found that the ultimate tensile allowable stress for the tempered plate was higher than that of the AN bolt. Is is possible for the bolt to fail in bearing instead of the hole. In MIL-HDBK 5 (1962) it gives allowable bearing stresses for holes of a given e/D but this is much higher that the tensile allowable stress. I know that these values are to do with other failure modes such as tear out. Does this mean that the hole can be designed for these higher bearing stresses and if so what happens to the bolt. (The bolt is strong enough in shear.)

Any ideas would be appreciated,
Thanks
Daniel.

 

[CoryPad]

I am confused by your question. Perhaps you can restate it using different words. For example, you mention the the allowable ultimate tensile stress for the plate is higher than that for the bolt - what is your point? You still perform the calculations to see if either the steel or aluminium components fail due to bearing stress (or tear out, etc.). I think you are asking if there is a condition when the applied force is less than the ultimate shear force of the bolt, but perhaps high enough to yield/distort/damage the bolt. If the force does not cause a stress that exceeds the bolt ultimate shear stress, then you should be fine. The joint usually is assumed to be tight so that it prevents bolt distortion - you assume the bolt is a pin, and it fails only when its ultimate shear stress is exceeded.

 

[KSshopper]

From what I understand of your problem (sometimes difficult in these forums without a picture), you may want to consider analyizing your connection as a single pin joint. Bruhn has a discussion and design curves for such analysis.

If your plates/blocks are truly thick compared to bold diameter, then you should find the bolt will fail either by shear or pin-bending. Anyway, take a look at Bruhn, it's all discussed.

 

[dooron]

My concern relates to you observing the bolt to have wear damage that you suspect is bearing failure. In static applications I believe the bolt is least likely to fail, and failure is by bending when the span is too large. Very rarely do I see bolts fail by shear at the interface. Often I see bolts that fail by fatigue or wear due to movement. If your bolt shows signs of wear, check out if it is a static application or whether it is rubbing or fretting against something.

 

[gyro]

Thanks for the info, perhaps you will understand better if I say that I wondered that if the, in a pin or truss type joint, the material which bears on the surface of the pin the is harder than the pin in it will cause the pin to fail by deforming the surface. The shear stress of the bolt is not exceeded.
With regard to Bruhn I don't have a copy myself (saving up)but I will see if a friend has a copy.

 

[CoryPad]

gyro,

The pin does not have bearing stress in it. For bearing stress to develop, the surface opposite the force must be supported, which is not the case for a pin. As long as there is no gap in the joint, you only need to look at shear in the pin. If there are gaps, then the pin can bend, which is a different mode than bearing.