Ann, you mention end of life phases when talking about the life cycle cost analysis. You mention recycling and disposal. Steel and aluminum of this type will certainly be recycled. It has been known for some time that aluminum recycling is extremely effecient. I think it uses about 5% of the energy to recycle aluminum as it does to refine it from bauxite. Steel is also effecient, although I am not sure of the ratio. Steel mini-mills are the most effecient steel mills becuase they use scrap. So, I assume in this case the wheels of both types will be recycled.
It is good, though to see such comprehensive analysis. If you are looking at full lifecycle costs, then you really have to look at everything.
Lou, you're certainly right about the recyclability of aluminum, and steel too to a somewhat lesser extent. But it's also true that LCA has to look at everything. In fact, the latest concept of the life cycle is "cradle-to-cradle", not "cradle-to-grave." CtoC includes that last link in the chain that closes the loop (to mix metaphors) of recycled material going back into the product.
@Ann: Although steel is stronger than aluminum, it's not really too surprising that forged aluminum wheels are stronger than steel wheels made from sheet metal -- the designs and manufacturing processes are completely different. A forged steel wheel would be much stronger than a forged aluminum wheel, but would also weigh a lot more. (Forged steel wheels make sense for railroad cars, but definitely not for on-road applications).
The lifecycle analysis is very interesting. It is very comprehensive, and all of the assumptions seem to be reasonable. It makes a convincing case.
@Charles Murray: The elastic modulus of aluminum is about 1/3 that of steel (10,000 ksi vs. 29,000 ksi), and the tensile strength of a typical alloy used for aluminum wheels is about 1/2 that of a typical high-strength steel sheet used for wheels (45 ksi vs. 90 ksi).
Based on that, it would be easy to conclude that steel wheels should be stronger than aluminum wheels. But this would be a mistake.
If steel is twice as strong as aluminum, then why are aluminum wheels stronger than steel wheels?
Aluminum wheels are stronger due to their design, not their mechanical properties. Compare the section thickness of a forged aluminum wheel to a typical steel wheel. The steel wheel is made from relatively thin sheet metal. The aluminum wheel is much thicker.
If you made them both the same thickness, the steel would be stronger, obviously. But if you made a steel wheel the same thickness as a forged aluminum wheel, it would weigh about 3x as much (7.8 g/cm² vs. 2.7 g/cm²). Nobody would want to put this heavy of a wheel on a truck.
Ann, are you able to foreseen any significant advantage in replacing the steel wheel by Aluminum wheels. I don't think any advantages because we cannot reduce the number of wheels in any heavy weight carriers or trailers. The number of wheels is proportional to the curb weight and length of base platform. Moreover, some road transportation laws restricts any change in length of these base platform and height of the vehicle.
Agreed that the design of the Aluminum wheel is a major factor in creating a wheel that is stronger (and lighter) than steel. But... I would like to see the FEA on the aluminum wheel under stress. Steel Wheels are flexible, and therfore are forgiving under load. Will the aluminum wheel be as flexible? At all temperatures?
Some years ago there was a move toward a "super tire" that was mounted on a different wheel, which was aluminum (Alcoa, I believe). One super tire combination would replace two wheels/tires on a trailer with the one (per side per axel). I have not seem many of them on the road (I'm in NM), but they were touted to be improvements over the (then) current technology. One drawback acknowledged at the time was that they would cost $1200 per wheel; also, the normal truck tire repair guy could not service them routinely.
Hope there is some real motion toward improved technologies. Reduced weight should be safer as well, as any reduction in un-sprung weight is a good thing (though it's probably a reach to expect much improvements in handling on a truck/trailer combination). PSI loading on the pavement may not benefit from a reduced footprint on the super tire, either. Anyway, it's a good idea, and the benefits to the environment are always a plus.
Thanks for your comments, Dave, that's a good point about the difference between forging versus sheet metal production. That also made me wonder--in your failure analysis work, to what extent do you encounter LCA issues, facts, details, studies, etc.? Is any of that data relevant to failures?
@Ann: Environmental issues rarely come up in failure analysis per se, but they do come up in material selection.
On the other hand, I think very few companies would actually base major decisions on a lifecycle analysis. For most companies, "corporate social responsibility" is more about public relations than anything else. Acidification, ozone depletion, eutrophication, climate change, etc. may have significant costs to the economy as a whole, but they don't directly impact the company's bottom line.
The only potential benefit is increased sales, if you can convince customers that your product is "greener" than another product. This was clearly Alcoa's intent in funding this study.
Large companies may be doing lifecycle analysis (for instance, Pepsico recently completed a fairly comprehensive analysis of its packaging, using tools developed at Columbia University), but it's not clear that they actually using it to make decisions yet.
Personally, I think elimination of waste, conservation of natural resources, and minimization of environmental harm, are important goals. Unfortunately, I think few companies are really paying much more than lip service to these goals.
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