Aluminium Casting Failure

[Batswings]

I was sailing on a boat this weekend. The wind was about 20 knots; therefore, the potential loads are high but not extreme. We only had the jib up (front sail), did not use the main sail; therefore, the loads should not have been that high. I am sure the boat has seen higher loads in the past.

On top of the mast there is an aluminium casting were some the mast supports attach (forestay and backstay = front and back support guy wires). The boat is 20 years old.

The back side of the casting were the backstay is attached fractured, lost the mast support, the mast bent, and came down. Nobody was hurt.

The color of the casting were the fracture occured is dark and homogenous; however, on one spot (on the edge) there is an area that has a different color (more like white instead of dark grey).

I am wondering if the loads were too high or there was an existing crack at this location, due to to a void in the casting, fatigue (20 years) or cycling (due to waves). Will a catastrophic crack in Aluminium be homogenous or would it have a different texture at the high stress point were the crack starts? Or is the differnt color/texture due to corrosion of an exising crack?

Thanks,

Walt

 

[redpicker]

The answer to all your questions is Yes.

Without being able to see the fracture, any response would be a guess. If I was to guess, I'd suspect a fatigue crack growing from a casting imperfection. However, Stress Corrosion Cracking is another possibility. Where is the boat stored? (storage conditions can often have a more significant impact than service conditions) Many other questions come to mind, but without looking at the fracture, they may or may not be pertinent.

rp

 

[1gibson]

Are support wires steel? Is there potential for cathodic reaction in a wet (but not submerged) environment? That's a real question, I only know enough to mention it.

 

[Batswings]

I will try to post some pictures.

I am wondering the different colors at the fracture.

Galvanic corrosion is a problem in boats: Aluminium structural members with stainless steel fasteners. However, in this case the fracture was not near a stainless fitting.

Walt

 

[CoryPad]

The different colors suggest corrosion was involved. The base metal would be light gray, while aluminium oxide starts white and turns dark when hydrated.

 

[blacksmith37]

And to muddy the water: I expect the original specification and NDE were pretty loose. So, the question may be, "How did it last so long ?", given its initial condition.

 

[Batswings]

Attached are the pictures.

On the main casting on the right, under the blue and green ropes, the fracture is grey just like on the left. But below the ropes you can see that the fracture is white.

Walt

 

[Batswings]

Here is the other side

 

[swall]

There is a lot of corrosion on all surfaces. Particularily note the angle iron piece, which appears to be galvanized and the corrosion has highlighted the grain or spangle. I am curious about the gray rope with the blue chevrons--it appears to have some sort of metal fitting where it attaches to the casting? There appears to be a "white rust" band at the attachment, which suggests localized galvanic corrosion of the aluminum where the fitting (if I am interpreting things correctly) attaches. I was also wondering about the light colored region at the corner of the fracture. Is it really white in color, or is it shiny and faceted?

 

[metengr]

Just based on the two pictures, it looks to me that the casting failure was caused by a one time, tensile overload event.

 

[Batswings]

Swall:
The light colored region at the lower right corner of the fracture is definitely very white, while the rest of the fracture is a very dark grey. And you are right, a galvanized angle is bolted on the casting, I need to check if the white around the bolt is corrosion or some type of sealant.

Metengr:
It could have been tensile overload, but the loads were not that high, I am pretty sure there has been higher loads in the past. Trying to find with the sailors if by not having the second sail I overstressed the casting.

To All:

There are galvanic corrosion problems in boats. As you can see the mast is Aluminium and the castings are also Aluminium since they are lighter. However, all the fasteners are Stainless Steel 316, 18-8, or Monel (better due to salt water environment) as you can see in the pictures. High Tech boats have Carbon Fiber masts with Stainless Steel fittings; therefore, there are galvanic corrosion problems here too.

This saiboat is from a friend of mine. It is American made and it is not a Cadillac. I have an European made sailboat, and the manufacturer has put more time into the details. For example: most contacts between disimilar metals have an isolator (nylon washer, paint, gaskets), but what can you do when you need to put a Stainless Steel rivet in an Aluminum Part (Aluminum rivet might not be strong enough).

Walt

 

[metengr]

Batswings;
Wind loads can be quite unpredictable because I have seen failures on wind turbine components caused by variable wind loads.

Second point, if you evaluate the geometry of the mast casting, the section of the casting that failed had a net section thickness about half of the main casting section thickness.

Third point, the failure of the casting exhibits no noticeable ductility. This implies that the casting has little tolerance for dynamic loading events and internal defects. All it takes is a sudden gust of wind, applied along a certain direction, and with an inherent stress riser (thin to thick net section thickness change) you have failure.

 

[Batswings]

Metengr:

You are right on the above points.

This sailboat is a monohull. When sailing with the wind on the side (on a reach as was the point of sail when failure occured), as the forces increase (a gust for example) the boat leans (the mast is not anchored like a wind turbine). As the boat leans, the projected area is less; therefore, less force, counter force is the weight of the keel.

The boat is suppose to be able to take twice the load that was applied since we had only the front sail up. The boat was not leaning, which means there was not excess projected area for the wind load. However, the boat was not balanced since the force was only applied to the front (the boat tried to turn away from wind). The back sail (main) balances the forces (turn boat into the wind).

If the wind is too high the sail area needs to be reduced in order to keep the boat leaning not more than 15-20°.

W

 

[swall]

One more potential factor for the overload hypthesis: The several high magnesium aluminum casting alloys, which offer excellent corrosion, rate the poorest when it comes to castability, i.e. they will be more prone to casting defects. Since we don't know what the alloy is question was, this point is somewhat hypthetical, but a casting with inherent flaws would be more likely to fail in this manner.