At 04:49 PM 3/26/2002 -0700, Kappus, John wrote: >Don't forget to check with McMaster Carr for small quantities. >www.mcmaster.com >They have 7075 and an aluminum alloy that I didn't know about that is even >stronger, 7068. > > >Garden variety 6061-T6 Aluminum has a yield strength of around 40ksi. >7075-T6 is around 73ksi >7068-T6 is all the way up at 99ksi!! (Boy is it trouble to bend in a >press!) >This is comparable with unhardened O-1 tool steel and a conservative >estimate for carbon rods! > >Please remember though, that strength is often NOT the important factor when >sizing aluminum wing rods. Deflection (bending) is mostly the issue. All >grades of aluminum have the same modulus of elasticity, so flex the same. >The stronger grades just flex further before they take a permanent set.
I'm confused by what you have written. Metals have three qualities that are important. Stiffness, which is resistance to elastic or recoverable deformation, i.e. modulus or E. (Stiffness in the common sense of the term, is only vaguely related to resistance to bending of a section. The section modulus or functional cross section of the material is more important.) Strength, load where it breaks. Yield, load where it bends permanently or plastic deformation. Above you stated the yield strength for these alloys. You correctly say that the E is the same for all alloys, so they will elastically deform the same. You then say that deflection is the most important issue. Do you mean to say yield? Are you defining "deflection" as permanent bending? If so I agree. Often folks use the term 'deflection' to refer to elastic deformation, i.e. flapping or non permanent deflection. Our main concern is yield strength in an alloy and then only for launch. I suspect we could use a maple wing rod for simply flying around the sky. Launches are killers. If you look at the yield strength of most any low carbon steel, 4340 is a good example, quench it up to a nice hard and tough state, it's yield strength divided by density comes out to higher value than 7075 T6 aluminum also when yield is divided by density. This means that pound for pound, steel has a better specific yield strength than aluminum. Aluminum has a better specific E than steel, thus Boeing buys aluminum by the acre. For wing rods, it might be that a good tool steel, pound for pound is better than aluminum. I haven't run the specific numbers yet. This 7068 you mention might be outstanding for wing rods. I'm not familiar with that alloy. I'd never try to bend this type of alloy in a vise. Take it to any good metal working shop and have someone who knows how to run a break get it to the right angle. At that hardness, I wonder how much you can bend it. Most interesting. So where does someone buy some 7068 T6 round?? Bill -- It comes in pints! Peregrin Took Bill Johns Pullman, WA RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
