Interesting Material Failure 1

[dik]

I have also posted this in the Structural 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?

 

[MintJulep]

It would help to know the directions of the loading. The hydraulic loading you mention, that is upward from the bottom of the plate?

The crack is right where the tensile stress in the flange will be the highest.

The fillet weld at the bulkhead built-in cracks when the think was fabricated, and they have propagated over the years as the flange saw changes in tension. Remember that cracks propagate perpendicular to the direction of stress.

The welds should have been full penetration.

 

[metengr]

Agree with MintJulep that this looks to be a design problem and not a materials problem.

 

[dik]

MintJulep

The loading is upward on the thin plate at the bottom; the thin plate acts as a bit of a flange for the ST and is the loaded flange. The crack is running parallel to the direction of maximum stress.

Cracking is present in many of the ST flanges (about 60% of the members of that ilk) and runs parallel to the web.

thanks for the speedy reply... Dik

 

[MintJulep]

You may want to consider NDT on the flanges that do not exhibit visible cracks.

Mag particle of ultrasonic I would think, but NDT is not really my thing.

 

[dik]

We'll be checking all of them... using mag particle, initially. If the cracks do not go through the flange, then we'll go ultrasonic to try to establish a depth.

Dik