Fatigue of a rotor

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1. Nov 23, 2016

strive

I have a fast rotating rotor spinning over 2 axis (over its main axis and over an external axis - the two axis are perpendicular to each other). Easiest to imagine this is like a windturbine yawing as it follows the wind direction.

Each of the two rotations obviously causes a distinct centripetal force which causes stress on the rotor.

1. Do I correctly presume these loads can be understood as static (like a preload) as long as the rotations have constant velocities?

Additionally as the rotor is rotating with its axis perpendicular to the gravity field, gravity is causing a cyclic load on rotor parts. Plus i have a small aerodynamic force which is variable. This adds up to peaks and dips in the stress experienced by the rotor - on top of the static loads described under the point 1. above.

2. How do i predict the max number of cycles the rotor can endure?

I presume i need data for fatigue with applied preload, but i am unable to find any for metals. I presume i could somehow calculate the life expectancy using equivalent strain, but i am not sure how to go about it.

3. Is it possible to calculate the life expectancy using equivalent strain? If yes, can you please point me into the right direction, thank you.

At the moment my “static” stresses amount to 400 MPa and my variable cyclic stresses amount to 20 - 140 MPa.

20 MPa are the high frequency (sinus) cycles and 120 MPa are low frequency (step).

Since the high frequency (sinus) cycles are only 5% of the main load, i presume they may be considered as vibrations? But then again it depends on the yield strength, correct?

2. Nov 23, 2016

Nidum

Please start again from the beginning . Draw the mechanism , describe what it does and show some basic loads , speeds and dimensions .

Last edited: Nov 23, 2016
3. Nov 23, 2016

strive

It is a small impeller/turbine.
Imagine it powering an acrobatic plane - in turns it can experience up to 6 g's, and surges in the airflow.

Roots of the blades - where the blades are molded together with the hub - experience very high stresses. I need to determine how long these blades will endure at specific flight conditions/patterns.

I can do physical tests but at this point it would require several iterations which would be costly. So i'm looking to estimate the life expectancy first.
Due to the complex blade geometry i am using fea to calculate stess distribution in the blades. But i can't use it to estimate the life expectancy.

For what it's worth, in one of the flight patterns the stresses/time would roughly look like on the attached picture.
(the graph is non-normalized - sorry i quickly made it just for my own refference yesterday)

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