Fatigue data of counterbored bolts in shear - steel

[trainguy]

All,

Does anyone have a reference for the fatigue behaviour of bearing-type bolted joints in a counterbored hole? Main loading is shear alternating between zero and a design value. I imagine there is quite the stress raiser at the edge of the bore. This is for a steel freight car.

Thanks!
tg

 

[SwinnyGG]

Think about the stress distribution- if the part is truly loaded in shear, what stresses are present in the material near the bolts and above the floor of the counterbore?

Not a lot.

 

[trainguy]

jgKRI - that sounds promising.

Our other option is switching to counter-sunk fasteners. Assuming this remains a bearing-type connection, does anyone have opinions/references for:

1) Static strength (bearing strength) design equations

2) Fatigue strength of such a connection

My guess is that any fatigue success will be achieved through a friction type (slip-critical) connection instead of bearing type. In this case, is there any guidance on clamp loads for bolts with counter-sunk heads?

tg

 

[Screwman1]

If those are recessed drive counter-sunk fasteners there is a good chance that they are head critical and will break through the recess at a load less than the theoretical full strength of the threaded section. One key is to keep the center line of the countersinks perfectly centered on the axis of the hole to prevent bending. In general, because of the angled bearing surface, countersunk style parts are used more on shear applications than tensile applications. Check out MIL hdbk 5 (MMPDS) for some ideas.

 

[drawoh]

trainguy,

Is there an approved, tracked, possibly high strength flat head fastener out there? I have specified phillips socket screws for aircraft hardware.

If you clamp the bolts down hard, there should be minimal fatigue of the bolts. If your bolts are not tightened down hard, your joint is doing all sorts of weird things, all of which you must take into account.

If your flat head screws are driven into tapped holes, you have no allowance for positional tolerances on your holes. Your screws are being bent to line up, which must affect stresses and fatigue. If you have a nut and a large enough clearance hole, this is not an issue.

--
JHG