Metal Fatigue in Tube Frames?

[Mike Switzer]

Matt

There is no question that aluminum WILL deform at values below the published yield strength, why else would fatigue strength be the published design standard for non-ferrous metals while yield strength is used for steel? Everything in my last post was paraphrased from 2 of my engineering texts, I don't think materials science has changed in ~25 years.

[Mike Switzer]

Can you cite some sort of publicly-available source for this?

Mechanical Engineering Design, Shigley & Mitchell 4th edition

[Kyle Boatright]

Kyle-

"The fatigue curves for steel appear to be headed towards a limiting value of about 40ksi. So under a 40ksi load, steel will never fail, regardless of the number of cycles." True, but if the structure had a stress higher than 40ksi, this would no longer be true. Both curves I posted have the same trend, but obviously different numbers because they are different materials.

"The fatigue curves for aluminum are still pretty steep at 10KSI. If you had the entire curve, it would limit out at zero, meaning that aluminum will eventually fail after only one fatigue cycle, with that cycle coming from the structure's own weight." You are misinterpreting the chart. The bottom edge of the vertical axis is 10ksi...there is no data down there, as the lowest run-out is at about 17ksi. One cycle is off the left end of the chart; notice it starts at 10^3 cycles. You have the entire curve...it does not 'limit out at zero', the numbers 'run out' below a certain stress level, meaning that the test specimen did not fail before the conclusion of the test.

The point is that you can design a steel structure which will never fail. As the 4130 graph shows, if you keep the stress under 40ksi on 4130, it will never fail, regardless of cycles. An extrapolation of the aluminum graph will show that an aluminum structure will eventually fail, regardless of the load applied. And no, the test samples didn't necessarily fail, but they didn't run an infinitely long test, either - it isn't practical. It is certainly possible to design an aluminum structure with a lengthy and predictable service life, and that is often the best compromise in aviation. But that well designed structure will eventually fail.

[Mike Switzer]

If a material is always yielding below its yield stress allowable, then the allowable is bogus.

This is why published yield strength is not allowable (as a design value) for aluminum structures - fatigue strength is, as the cycles required for failure at that level will probably never be achieved.

[Eric Marsh]

Thanks for all the great info. My understanding is that aluminum rods are used in some drag racing engines because they absorb some of the shock of detonation which on some motors, especially those running fuel, can make the difference between bearing life and death. My airplane has 53 years on it's tube frame so I guess that means I've got another seven years left. :-) I was really curious about this because at first I thought that aluminum construction was the only way to go but after buying an airplane with tube construction I'm changing my mind. However, since this is kind of new to me I didn't know if fatigue would be an issue. What I'm hearing is that unless there is a real rust problem then cracked welds are the biggest concern.

[Matt Gonitzke]

Well, I stand corrected...I found endurance limit explanation for non-ferrous metals in Bruhn. Not sure why it isn't in any of my newer books, but it wouldn't be the first time I've found an older book more useful than a new one Oh well, at least I learned something tonight.

"This is why published yield strength is not allowable (as a design value) for aluminum structures - fatigue strength is, as the cycles required for failure at that level will probably never be achieved." I agree...no airplane is designed for one cycle. I think what I've been trying to say all along is that a steel structure may also have a limited, finite fatigue life if the stress level is high enough, just like an aluminum structure. I wish I'd have thought of that a few hours ago, as it would have saved us all an evening...

[Mike Switzer]

Matt, I'm curious, I graduated from Rose Hulman in 88, when did you graduate? I'm wondering if they are teaching fatigue life somewhat differently now - I haven't been involved in the automotive field for maybe 15 years, but even then I was seeing some trends in design that I & some of the other older engineers didn't agree with, designing components to values significantly over fatigue strength for both cost & weight savings, and the beancounters & six-sigma idiots didn't care as long as it made it past warranty & was not what the lawyers would consider a "life threatening" failure mode.

Like I said, I'm wondering if this area of design is being taught differently now? I never designed anything for consumer use that operated in a range over fatigue strength, but there were some things for military applications & for that big 500 mile race in Indiana that were on the ragged edge of yield strength & we hoped they made it....

[Mike Switzer]

I was really curious about this because at first I thought that aluminum construction was the only way to go but after buying an airplane with tube construction I'm changing my mind. However, since this is kind of new to me I didn't know if fatigue would be an issue. What I'm hearing is that unless there is a real rust problem then cracked welds are the biggest concern.

Eric - the design I am working on will be welded steel tube - I chose that because long term maintenance will be easier & I am more comfortable doing the calculations for that type of structure.

[Matt Gonitzke]

Mike-

I graduated this spring. Is your degree mechanical engineering or aerospace? I suspect an ME degree would go a bit more in depth into fatigue, as we had to have as many or more aerodynamics classes to be more well-rounded. Essentially, we had problems where we would determine the fatigue life of some notched-specimen shaped thing, given its material properties, geometry, and loads. Rather academic and not much of a real-life scenario, for sure.

Most of the focus was on static strength analysis; positive margin at ultimate load for no failure, and no permanent deformation at yield for static loads. Obviously, the sizing of many aluminum parts of an aircraft structure will be driven by fatigue life and not static strength, but we used the static strength and those allowables for initial sizing. There isn't enough time in a semester-long design class to do both the detail design and analysis, and the analysis probably would have been of no interest to those people in the class that didn't want to become stress engineers. Hopefully by the time I need to do the stress analysis on the aircraft I am designing I will have had enough training/exposure to fatigue analysis methods through my day job to comfortably do so.

[Mike Switzer]

Matt - I am a ME. I will openly admit my math background wasn't good enough for the higher level aero classes.
A couple of my friends were ME/Aero, and both the required 300/400 level classes & the electives were different. I'm going off memory here, but I believe we were on the same track thru Statics & the first Dynamics class (Sophmore year) but we went separate ways from junior year on. The basic aero course I took was a 300 level course, as were the ME track courses I took that got into advanced design & fatigue, etc.

We had quite a bit of real world stuff in class, as our professors had quite a bit of experience - the Aero prof had been a Boeing test engineer, the materials guy was one of the designers of the Pershing missile, the engine prof had been an engineer on the Dodge Ramcharger racing team, and at the time at least a third of the ME department had worked at some point at the Langley research center.