Choice of material for application in harsh environment - toughness vs. yield strength vs. hardness.

[bojans]

I'm designing an in-roadway LED light enclosure for traffic safety applications installed at pedestrian crosswalks. The product is buried two inches into the surface of the road with the top sticking out of the road by 1/2". It must be able to survive regular abuse from vehicle tires and our main enemy - the street sweeper. The occasional visit by the street sweeper has proven to be deadly for our current generation of products made of 6061-T6 Aluminum. It gauges the enclosure enough to cause failures of the other parts in the enclosure such as the LED lens/window and o-ring seal. The street sweeper's vacuum box rides low to the ground on ski-like steel framework. This framework damages our current Aluminum enclosures on impact and they eventually fail after about a year or two. I'd like the next generation product to be made of something tougher and longer lasting. It also has to be corrosion free. The best answer that I could come up with is 17-4 PH steel. I am not sure if I should put more emphasis on hardness, yield strength, or toughness. Any advice would be much appreciated.

Thanks,
Bojan

 

[dgallup]

17-4 PH will increase all those material properties. Generally hardness gives you the most improvement in wear resistance but if your device is failing due to impacts then toughness will be more important. While making it out of a tougher, stronger & harder material has got to help, I would also want to give the unit a gradual slope so the street sweeper (& snow plows!) can ride up it rather than impact into it.

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[EdStainless]

What is failing? Is the top part actually breaking, or at the fasteners breaking?
You need to be careful if you go with Al base and SS top, as you will risk serious galvanic corrosion issues.
If it is an impact issue then perhaps some 17-4PH rails that fit over the top and provide a ramp for deflecting impact energy. You are still going to need toughness so don't go for max strength, maybe the H1050 condition would be good.
Have you looked at a higher strength Al alloy? Along with some mechanical design to minimize impact this might be enough.
There are some fairly high strength SS grades that don't heat treat, especially if you have a part that is being cold formed. You could use 219 (or some other 2xx grade) and stamp them, this would give them higher strength and they would still have good toughness. Perhaps a cap to fit over the existing design (if you don't want to alter it).

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

You might take a look at Ni-resist cast iron alloys. They are tough and wear well, and they are lower cost than 17-4. They have fairly good corrosion resistance, but not as good as 17-4 cres.

 

[btrueblood]

I'm wondering if the opposite direction might make more sense - i.e. encapsulate your electronics into a hollow "bubble" of wear-resistant, yet flexible, polyurethane rubber or pvc. Opening said container for repair might be a problem, but it sounds like your current design is already somewhat a disposable item - what do you do when the roadway gets re-paved?