5456-H116 Plate ST Cracking

[boo1]

5456 H116 - 2” plate is fabricated in to a hinge that is part of a welded structure. The system has operated in the marine environment for 15 years. There has been several cases this cracking issue. There is machinery close to the structure that vibrates at 21 and 84 Hz.

The hinges are cracking (multiple) in the Short Transverse (ST) plane. The cracks are perpendicular to primary load axis and are propagating on planes parallel to the rolled surfaces of the material which implies environmentally enhanced cracking due to ST loads. Fracture surface appears to exhibit a combination of crystallographic chemical attack, fatigue, and ductile mechanical overload. ASTM-G67 Mass Loss is low (<15mg/cm2) indicating the material is not sensitized.

Fractography photos are attached

Does anyone have information on 5456 ST fatigue or recommendations of how to procede with analysis the cracks? Would tensile testing in the ST direction provide useful data for modeling the structure?

 

[metengr]

boo1;
Nice pictures regarding the fracture surfaces. However,has there been a proper failure analysis performed by a competent metallurgical lab? If not, this is your first step in determining a root cause.

 

[boo1]

a metallurgical lab did the G67 and fractography

They said, "Fracture surface appears to exhibit a combination of crystallographic chemical attack, fatigue, and ductile mechanical overload" and the plate is non-Isotropic.

Since the loading in the plane of the cracking is low (the hinge should be free to rotate) I am trying to understand where to go next in the metallurgical analysis and FEA model. Looking at the 5456 in saltwater 10**8 cycles the allowable is ~3 ksi. Should I have them do the tensile testing in the ST direction?

 

[metengr]

Ok, very good. However, there is no mention of metallographic examination Did the metallurgical lab evaluate the microstructure (metallographic examination) of the failed 5456 alloy? My limited experience with this alloy is that because of the higher magnesium content, this alloy if exposed to temperatures in excess of 120 deg F will sensitize over time and result in susceptibility to intergranular cracking.

I suspect sensitization occurred along the grain boundaries, and an ASTM G67 test would prove it. Sensitization of the magnesium-rich phase would also render a preferred path for fatigue crack propagation even under low stress conditions.

 

[EdStainless]

corrosion fatigue, 15 years at 84Hz
A tiny bit of corrosion, not enough to detect in G67, and then enough fatigue to open a slight crack, then a little more corrosion.....
I would look at the the non-homogeneous properties, especially the ST.


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Plymouth Tube

 

[boo1]

what tests should we run?

 

[metengr]

I just noticed that you reported the G67 results so my statement regarding sensitization would not appear to be applicable in your case.

I am not sure of what use tensile testing will provide in your analysis. If the hinge failed after 15 years under low stress or strain amplitude service conditions, and it appears to be environmental-assisted cracking (which is a catch-all phrase for SCC or fatigue or corrosion fatigue), you can either change the material to a more corrosion resistant grade of Al, or modify the hinge geometry to avoid any resonance vibration.

 

[stanweld]

Is the location of cracking adjacent to the weld?

 

[boo1]

The cracks are propagating on planes parallel to the rolled surfaces of the material and are not in the HAZ zone.

 

[CoryPad]

The images you provided appear to be of a corroded surface and are not suitable for determining fatigue or overload failure modes.

A sketch of the part and the loading vectors would be helpful.