Interesting Material Failure 7

[dik]

I have also posted this in the Welding forum.

We are doing some remedial work to a structure that is approx 40 years old. The steel used is specified as A36, but we have not confirmed this.

The structural Tees are welded to a bulkhead (all around)and cracks have developed in the flange on either side of the web. They appear to be intermittent and in some areas extend approximately 4' from the end. The span of the ST is approximately 10'. The cracks also appear to have initiated at the bulkhead. Loading is relatively high and the hydraulic load is applied to the thin plate at the bottom shown in the Detail Elevation. There is also a slight vibration that may have initiated a fatigue type of failure as well as the steel being subjected to zero degree C /32F temperatures. Welding appears to be sound and has a good appearance with no apparent 'splattering' or inclusions.

We are in the process of obtaining additional testing information to determine the full extent of the cracking as well as the depth; we don't know if the crack is the full depth of the flange, yet.

We are thinking that the cause of the cracking is from a triaxial stress condition caused by welding the ST all around to the bulkhead.

Any comments or suggestions?

 

[dik]

Trueblood...

The top of the tee is in tension as you have noted. The end weld to the bulkhead/frame does constrain it a bit, but shear stresses (in a plane parallel to the web) are so low as to preclude cracking. In addition, it has been determined that the cracks do not go through the flange; they are approximately 1/4" deep.

I'm just formulating a rough draft of some 'head scratchings'...

Thanks, Dik

 

[JAE]

Termites with diamond capped teeth?

Dik,
Do let us know what you find out eventually. Interesting problem.

 

[btrueblood]

Oh, so the cracks only propagate to the midline of the top web? Makes one suspect that the cracks are propagating due to bending stresses. Again, the vertical leg of the tee is carrying the shear/bending load, but the top web is transferring all of the load into the endplate. Local bending of the web, adjacent to that restraint, is likely, i.e. the top of the tee near the welds would tend to want to bend to form a downwards-facing cup? I just did a quickie little FEA analysis, and it does show some stress instensification in that area with only the top cap constrained as described.

 

[dik]

JAE:

I will... still having a problem wrappin' my ears around this. I'll be meeting with a NDT tekkie this afternoon to go out and locate a few sample areas to take coupons. Hopefully looking at the crack may give some information. If it's not too badly corroded, then possible britle fracture, fatigue and shear failure.

I've forwarded some ramblings... not a formal report outlining some of the possible means of failure (not sure it's a classic failure, since the cracking may not have an impact on the behaviour of the structure). I've also indicated that the cause has not been determined, yet, since the cracking is 'odd'. I've learned that the tees have been subjected to a fairly intensive vibratory load with 'modelled' pressures varying from 23' of head to about 18' of head over a sustained period... a month or so each year for 40 years. I'll see if bug spray really works <G>.

btrueblood:

At one area tested with mag particle, the cracking is intermittent along the full 11' of the structural tee.

Dik

 

[dik]

Just to cap this...

We have received additional information about the cracks; they are very shallow, about 0.2 mm deep. We are awaiting some sections to examine the crack surface as well as info on the material and chemical properties. Cracking of the welds was not observed at any location.

There is a good chance that my 5am ramblings may be the cause and that cracking was present from day 1.

Dik