Failure of anchor rod via stripped threads 4

[Ingenuity]

I recently repaired a 60 ton center-hole hydraulic ram (for post-tensioning bar stressing), and to verify that the new seal kit was indeed holding I did a recent load test. I grabbed a 1" dia x 105 ksi F1554 galv anchor rod with heavy hex nuts and washers (I usually use a 150 ksi PT bar but not on this occasion).

Anyway, got to about 3000 psi of gauge pressure (about 36 kip of load) and the threads stripped, pressure dropped back to less than 2,000 psi then I increased it again, and every increment to about 3,000 psi I continued to strip the threads.

So got me thinking that what I thought was 105 ksi materials appears to be only 36 ksi (1" dia tensile capacity is min 35 kips). Opps - and why I usually use 150 ksi PT bar with non-UNC threads.

But I was surprised that the it did not fail at the root of the threads. I have always assumed that the mechanical threads were stronger than the tensile failure of the thread-included section.

The separation you see in the photo is a grinder cut (not section failure) because I had to remove the bar from the test fixture.

Load was applied essentially monotonically, except for the stepping increment after first thread failures.

I learnt something today, besides keeping better account of anchor rod grades.

 

[dik]

Thanks fox... that's what I thought, but didn't have a source or confirmation. With the ductility observed, I was thinking it wasn't such a bad idea to have failure throught the threads and that I'd never considered it. I won't consider it for future.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ingenuity]

Do you want the failure to occur by thread stripping (for ductility) or through the bolt itself (less ductile)?

dik:

What makes you state that thread stripping/shearing will be ductile?

Tensile fracture of the threads-iNcluded section will be precipitated by elongation of the rod and necking of the section (hence ductile), but thread stripping is brittle.

The only thing I will note with thread stripping that I observed in my first post above is that once the threads first strip, the testing load drops off by 25 to 30% of so, then as more load increment is applied then will be further thread stripping, repeating until you decide to stop the test, but in the real-world (not a lab type test with hydraulics) the external load magnitude is presumably always applied, so once thread stripping/shearing commences, it will shear all the remaining threads of the rod projection.

 

[dik]

Shear stripping allows for movement which may have some redistribution, to lessen the stripping. A snapped fasterner may not have the same redistribution.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r6155]

Charpy impact test was stated in the Material Test Report?

Regards

 

[dik]

For the deformation noted, Charpy doesn't mean very much... and a different type of test for what's observed.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r6155]

Was any procedure used to tighten the bolts (torque / elongation, lubrication, qualified personnel, etc...)?

Regards