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One end of the tube has the thrust nut so the tube can accept a clevis; the other end is being fixed to a thick steel counterface plate.
The collar is fitted to the stud and the stud passed thru the plate and nut and washer attached. A small amount of preload makes the collar grip the counterface and now it will not spin when the tube is fitted over it.
The clevis end thread and taper is lubricated with graphite grease and torqued up to 86 lbf ft, the plate end thread and taper is dry and torqued to 100lbf ft.
Far right - the tube section in place on the load test rig.
At 3 tonf tensile load in the tube the 4mm thick square section steel beam is already heavily embedded and deformed
This is the applied pressure from the test rig, the red zone being max safe pressure - 3500 psi (15 tonf ram load).
The steady state gauge reading here – left for over an hour - is 1400 psi.
Failure in the tube occurred around 1600 psi.
Calculating moments around the beam the tensile load in the tube at failure comes out at 4.46 tonf
This is the plate end of the tube, showing failure due to hoop stress in excess of tensile.
The fastener did not pull out, in fact it exerted increasing radial force as the tensile load increased until the tube burst.
The copper alloy band quite naturally fractured too.
Cracking of the tube, as expected, has propagated from the change of collar section adjacent notch and slot at the open end.
This end of the tube was torqued up to 100lbf ft and it is quite possible that it failed first because the tube was overstrained during preload.
Whereas at the clevis end, the tube has fractured at the same place but the restraining band is still holding.
Note the ‘clam-like’ mode of expansion of the collar under heavy load.
As anticipated, the mild steel XPC collar has fractured along the notch line. The true mode of expansion is clam-like, and fracture has no effect on holding strength. In the other had, fracture facilitates disassembly, since the parts drop out when the load is released.
The M16 capscrew used for making these rough prototypes is completely intact and only exhibits some embedding and internal deformation at the socket end, as expected .
Production XPC are made from solid or forgings with surface treatment such as Nitrotec which is ideal for the stud, and this obviate the need for any additional lubrication or corrosion protection.
Nitrotec will actually increase the radial force exerted by the collar, by reducing the friction in the tapers compared with bare metal.
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