Aluminum problems

Thank you again guys. As I said, I have done some research (I also went to the forum and asked you) and I passed my recommendations.

Thank you again for the readings.

 

if you are having a fatigue problem than you also have to consider fatigue reistance. the alloy you are using 6063-T5 has very poor fatigue resistance. there are other al alloys and heat treat conditions that have twice the fatigue resistance.

It is difficult to design for fatigue with aluminum because the test data has a very large scatter, and there is no real fatigue limit for most alloys (meaning there is no loading point that will last indefinitely as in steel and other alloys, but even at low cyclic loading will eventually fail).

The problem with aluminum alloys with high fatigue resistance is they can also be suitable to corrosion in a marine environment.

according to an industry publication 5056-0 has double the fatigue resistance over 6063-T5, but also have much higher resistance to corrosion in a marine environment. the 5056 in H18 and H38 have even higher fatigue resistance and still would have good corrosion resistance. 5052-h38 would also be a good choice.

I would look into using 5056 or 5052.

Alclad 7075 also has high resistance to corrosion in a marine environment, and 7075-T6 has very high strength and excellent fatigue resistance. however I have seen how quickly this material gets stress corrosion in a marine environment when not alclad, so if the application could cause the cladding to wear or get nicked or scratched, it would not be a good choice.

you may just be making the parts too light for their use, failure after six months indicates that perhaps it is not as strong as it should be. doubling the fatigue life means it would fail in one year instead of six months, which still not an acceptable useful life.

Good luck.

 

Then i would suggest you're not using the correct FAT curves. These are very well established with consistent data.

I have no idea what you mean by "fatigue resistance"...can you define that please?

5000 and 600 series alloys are pretty much the same when it come to fatigue. The 5000 series is marginally better.

You need to avoid highly strain hardened alloys as much as possible. The higher the strain hardening the more the reduction in its ductility. Looking at 5083 as a typical 5000 series example a H38 temper would be roughly just 30% of the original O temper in terms of it ductility and proof stress. Thus it would not take much of an overload to initiate yielding and thus sites for crack initiation.

One has to be very very careful in selecting "on paper" superior grades in Aluminium. Otherwise you'll make the same mistakes as those on the HSS Stena vessels. The HAZ of the welds corroded badly, all unforeseen. The very small amounts of water present was enough to initiate corrosion, this was not expected.

One needs to be very careful with fatigue and welding with aluminium when the structure is subjected to sea water environments.

 

not really true, there are generally considered "safe" fatigue loads, but look at the typical scatter in the attached table. Also notice there is no lower limit.

it is from a table of relative fatigue life from a publication called "the aluminum data book" by Reynolds metals co. It is an old publication but the data is still relevant. The higher the fatigue resistance the more cycles it can sustain at the same load, or it can take the same number of cycles as another alloy at higher loads. unlike other metals, with aluminum there is no direct correlation with ultimate or working strength, and its fatigue life. Some alloys of AL can take a lot of cycling, but other similar strength alloys can not.

this publication gives different cyclic loading strengths for 5000 and 6000 series alloys, even for the different tempers too.


I am very familiar with both the fatigue and corrosion issues around typical alloys used in aircraft manufacturing. I worked in both the commercial and military aircraft industry for a number of years. So I do have extensive experience with aluminum.

I have very limited experience with aluminum used in marine environments, this is where direct experience is important. I do know that 7075 around salt water is not a good combination from experiance, it was only the corrosion rating in the Reynolds book that listed as suitable for marine environments (which actually surprised me).

 

Ahh, now I understand. You're using incorrect terminology. What you are refining to is the "fatigue limit". It is well known that aluminium has no fatigue limit, unlike steel, which does have a limit (that is to say below a certain value no further damage occurs). Nothing to do with scatter, noted here for ref:



Aluminium like steel has been extensively tested to establish the safe limits of design when designing for fatigue. Here is just one typical FAT curve:



It relates to the detail, the type of weld and the stress direction along with the manufacturing.

You can end up with a series of FAT curves which relate to different joint configurations etc:



Aircraft manufacturing is completely different to shipbuilding. You must not assume the same methodology applies in each, because clearly they do not.

 

Ad Hoc Thank You for those excellent informative posts on aluminum .

 

Thanks VN.

There have also been some good articles in Professional Boatbuilder magazine over the past couple of years too. Worth checking them out.