Metal Fatigue in Tube Frames?
I am reading about best practices for maintaining old aircraft and it got me to wondering if fatigue is an issue in tube frame construction aircraft. I would think that they would be tougher than aluminum construction if for no other reason than that steel will flex more before failure but that's just a guess. Does anyone have any input on the subject?
Steel doesn't respond to loads the same as aluminum - aluminum will stretch a little bit with repeated loading, steel will not stretch unless the applied load is over the yield strength of the alloy.
(Yea, I know that is a bit oversimplified, but it has been a while since I took materials engineering & I don't feel like remembering a whole lot of theory)
Following up on Mike's post, and keeping it very basic:
Aluminum has a finite fatigue life, and aluminum structures suffer fatigue even under their own weight. Eventually, they will fail.
Steel (at least in theory) doesn't fatigue unless it is overloaded. The main concerns for old steel frames are corrosion and imperfect welds which eventually crack.
Neither steel nor aluminum is going to 'stretch' permanently (yield) unless the stresses are beyond the linear elastic range. Both steel and aluminum exhibit linear elastic behavior below the yield strength and plastic behavior above it, so I don't understand how what you've said is true.
A structure with a static load on it (i.e. an airplane sitting in a hangar) is not going to suddenly suffer a fatigue failure and fall apart. Fatigue failures are related to cyclic loads, not static loads. I've attached a couple of SN curves (stress vs. number of cycles) from MMPDS-01, one for 4130 steel, and another for 2024 aluminum. Both materials will fatigue if the maximum stress is greater than runout stress notated on the curves. There is a relationship between the maximum stress, stress ratio (relating maximum and minimum stresses experienced during a cycle) and cycles. This is probably a bit simplistic, but hopefully you get the idea. The steel frames you speak of do suffer from fatigue, as the corrosion and weld imperfections create stress risers in the structure, which initiate fatigue cracks.
Originally Posted by Kyle Boatright
Someone please correct me if I'm wrong, but what Mike and Kyle posted conflicts with what I was taught about fatigue and materials in engineering school.
What I stated was what I remember being taught in my materials engineering course regarding any kind of repetitive load on steel vs aluminum. I don't have time to dig out my class notes now, but it is the reason drag race engines use aluminum connecting rods while endurance racers use forged steel rods. After only a few runs you can actually measure the elongation of the aluminum rods.
While you can design a structure with aluminum that will never show any evidence of stretching in it's lifetime, you have to oversize the structure so much that you lose any significant weight advantage over steel.
If I get time I will try to dig out the notes on the subject.
As Matt shows, all metals will suffer from fatigue depending on the alloy, heat treat, surface defects, stress and more.
In general, a proper welded steel tube airframe will usually last for decades (60 years+ is not uncommon).
Aluminum tube frames, on the other hand, might not last as long because they are normally bolted or riveted and the holes are stress risers.
I didn't keep my notes from materials science. That class was over 25 years ago. ;-)
Originally Posted by Matt Gonitzke
But look at what the graphs you provided show:
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.
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.
I haven't found my notes (like Kyle I took it about 25 years ago, they are in one of about a dozen boxes my mother sent out when she decided to "clean" my old room and what was in it's proper place on my shelf is now in one of these &(%$^**& boxes) and I haven't done any structural work using anything but steel in 15 years or so, but a quick review of 2 of my texts refreshed my memory somewhat.
When designing with steel, you design to the yield strength of that particular alloy (with an appropriate factor of safety). In most cases steel will not deform permanently until yield strength is surpassed.
When designing using aluminum, because it will deform to some extent below yield strength, (and this is the same for most non-ferrous metals) you design to the published fatigue strength of the alloy, which is usually significantly lower than the published yield strength. Fatigue strength is usually based on a certain (very high) number of stress reversals to failure. Since the yield point of non-ferrous metals can be hard to define, yield strength is defined by the amount of permanent set, usually 0.2 to 0.5 percent of the original gauge length (testing a standard sample of the alloy in a test fixture). If you design an aluminum structure using a design limit in the range between the fatigue strength & yield strength you will, eventually have a failure, it is just a question of how many cycles it will take. Now computer software can to a certain extent predict when this will happen (so you can design it to happen after the warranty runs out and hope the calculations were right). We were taught to design to the fatigue strength unless you were willing to test to failure to determine the life of the part.
A drag race engine's life is measured not in hours or even minutes, but seconds. I think what you are describing is creep, which is related but a different issue. I suspect there may be other factors than fatigue life that are driving the design of the connecting rods in the drag car engine since the lifespan is very short. They could be sized larger for lower stresses, but that might result in them either being too big or too heavy. The steel rods are likely used in the endurance racer engines because they have to last longer. I feel like that's an apples-to-oranges comparison since race car engines don't have a lot in common with aircraft structure, other than perhaps both being made out of some alloy of aluminum.
"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. You keep making it sound like the aluminum structure is going to fall apart due to a static gravity load, regardless of what it is or how it is designed; if it is collapsing under its own weight, it is insufficiently sized, with the yield stress or ultimate stress being exceeded at some location. That would be a static strength failure, not a fatigue failure. That is a design issue and has nothing to do with what material it is made out of. I cannot think of any situation in which the static ground load case is the critical load case for an aircraft.
You could design an aluminum structure with a maximum stress of less than 15-16ksi or so and it would also have a theoretically infinite life, or an equivalent steel structure with a maximum stress less than 40ksi like you stated, and you'd achieve the same thing. The fatigue life of either the steel or the aluminum structure is going to depend on the maximum stress in that structure in accordance with the curves I posted. You could design a structure for a given application made of either steel or aluminum for a certain number of cycles before failure, or for infinite life.
I think you should have kept your materials notes
Can you cite some sort of publicly-available source for this? That is not in any of my materials or structures books, or notes from my materials class or 10+ structures classes in two aerospace engineering degrees. What you are saying doesn't make sense with how allowables are defined. For instance, if an allowable is A-basis, then at least 99% of the population exceeds the statistically calculated property value with a confindence of 95%. (source: MMPDS) If a material is always yielding below its yield stress allowable, then the allowable is bogus. A-and B-basis metallic materials allowables are all defined like this, as far as I know.
Originally Posted by Mike Switzer