It's more than just one thing. Why the steps fail.
Hi All,
I've been lurking around the forum for a while and have been reading a lot about this particular issue. Some very well informed people have made some interesting observations and comments. It's my opinion that the issue may not be any one culprit, but a cumulation of several issues with the materials and construction, geometry, and use. Here are my thoughts:
1. The steel connection has two things working against it; residual tensile stresses, and thermal hardening from the welding process. Welding creates stresses in the material as a result of the heating and cooling process. These residual stresses result in a zone of internal compression and tension in the welded element. The residual tension stresses occur at the weld - this is an important factor in compression fatigue failures. The thermal hardening causes the material to gain strength but lose ductility. The result is failure is brittle and occurs more suddenly.
2. Geometry is an issue as the location of the failure is in a notch, formed by the mounting plate and step strut; this is a stress riser. The welds interface angle to the tube also can create a notch stress riser here exasperating the problem.
3. The steps hang down so you can put your weight on them while getting in or out. And hanging down in the prop wash means they are subject to those forces. Both these action would cause oscillation of the step. The former creates a high load - low frequency movement, the latter a low load - high frequency one. I believe this is why some fail before startup, and others after hundreds of hours of service. Since people weigh different amounts, and step (or jump) on the steps differently, the high load scenario can have forces that vary significantly resulting in different failure rates. Both scenarios oscillate the step and lead to fatigue.
This is a fatigue failure. Compression fatigue cracks like these breed in residual tension zones, with high compressive stresses (amplified by notches), and oscillating motions. The brittle (non-ductile) material makes things worst by not yielding, increasing the crack propagation rate (the material strength is higher, but does not exceed the stresses).
There are three ways to fix any structural problem: change the material, the geometry, or the loads. Since the steps are steps and carry people loads on them, and steel is a good material choice, the geometry is the best thing to change. We see this in the Van's demo plane, with the addition of a bracket to support the connection (it also eliminates the notch). This needs to extend down the step strut far enough that the forces induced under load are less than the failure stress.
Well that's my 2-cents on this issue. I am surprised Vans has not come out with an official fix or improved part. They appear to have employed one on their demo plane - why not spread the love? I want a RV-7 anyways, so no need for steps - they just slow you down!
Cheers,
CanadianAce
(Lurker now - Builder later)