Landing Struts - what makes them overstress?

[The_Rocketeer]

I have a very technical reason for asking a fairly simple question, but I'm hoping for as much input as I can get in the hopes of understanding what is happening.

In your experience, what causes, or under what circumstances, or how do landing struts fail due to overstress? Very technical answers are welcome, but equally purely anecdotal answers may provide the key to solving my quandry.

Why I'm asking:

I am building an engine for a stock propellor plane with a stock bearing. Normally I use Juno engines to blow a turbine to power the propellor, but lately I've been experimenting with landing struts for the same purpose. The basical principle runs thus: the struts are always extended, and as the blades of the turbine move into the collision ray for the strut's "foot", the strut instantly compresses and then extends again, so applying torque to the turbine. I got the idea when I ran over a discarded stack separator and saw it fly off, and realised this was a potential source of reciprocating power. Anyway... when the craft is attached to the ground with stability enhancers, the engine works well, and I can get a steady rate of rotation without any issues (obviously the bearing part isn't attached to the ground, but the part with the struts on it is). However, when I release the stability enhancers, or launch without them (standing on wheels), it's very common for the struts to quickly overstress and disappear in a cloud of smoke.

So far I've not been able to work out what's going on. I would probably be better equipped to avoid the problem if I understood 'the rules' for landing struts a little better. So, here we are.

[W. Kerman]

Landing a tad to hard, 15-30 m/s smashs them.

[SpaceFanatic10]

Either sometimes your ship is to heavy,or your landing to hard and fast

[Mastikator]

Force = mass x acceleration. Exceeding the force limits causes them to break. I don't know what the exact force limits is and how faithful KSP is to real world engineering, but I suspect it's somewhere between "passable" and "pretty good".

They also have an impact tolerance which breaks them even if the mass is very low.

[Wcmille]

If the landing strut is compressed too much, it will fail.

stress is created in two ways:

the speed at which the strut is compressing times the damping coefficient.

the square of the distance that the strut is compressed times the spring coefficient.

[katateochi]

This is a bit anecdotal but with the smallest landing struts I've noticed that if they are mounted parallel to the ground (shhh I have my reasons) they can sometimes overstress much sooner than expected for the same weight.  I think it's because as they extend they immediately contact the ground and register as being fully compressed. 

[The_Rocketeer]

Thanks for chiming in folks, I should be able to try at least a few new ideas on the back of your remarks.

[Fearless Son]

Something to keep in mind for your design is that landing legs do take pressure into account.  Several landing legs divide the weight of an object between them.  A single landing leg will take all the pressure itself.  I think this will have an effect on overall leg compression rates.  Several legs will compress less on the same amount of weight than fewer legs will.  That might be something to keep in mind with your design.