I have a wind blade. I want to perform a fatigue test on it. Some standards state that to accelerate the fatigue, one can use excitation with frequency very near to the natural frequency of the blade.
I wonder if this resonant excitation will damage the blade or it can survive a certain number of cycles of excitation?
Fatigue test at resonance frequency is a common practice for a large structure such as the wind turbine blade. Yes, it will produce fatigue crack which will eventually lead to failure. It won't fail right away since the blade has large damping capacity so the amplitude at resonance will stay limited.
Dear Jin,
You are right in case of a blade totally made from composite but my blade is manufactured by putting steel chassis covered with composite laminae.
I think damping capacity will not that high.
The damping capacity would depend on the volume ratio between the composite part (high damping) and the steel part (no damping). But I still cannot imagine the structure will break in a short time. If worried, you also can reduce the response amplitude by getting the excitation frequency a little off from the resonance frequency.
The idea of the resonant method is to minimize the amount of energy required to obtain large strains in large structures by taking advantage of the structure dynamic properties. Therefore it should be possible to tune your resonator (often a simple eccentric mass in rotation) so that it provides any level of excitation you want. You might be able to use the frequency, mass, time in action or oscillator position to tune the blade behavior. May be also use more than one oscillator with different properties can be useful.
As to wondering if the loading will damage the blade, well... Fatigue testing is usually considered to be a destructive test so you are aiming at two diverging goals if you want to do non-destructive fatigue tests.
As I see it, either you want to proof test your blade OR gain information on the damage process and fatigue lifetime. In the first case, you use a pre-determined loading and target life and test to see if your design reach the target design brief. In this case, you fix objectives of acceptable damages and inspect after the desired test duration to see if the part is fit for service.
If you want to gain information on the fatigue damage evolution and fatigue life, I think you need to have some damage occur.
Fatigue in the resonant frequency to minimize the amount of energy needed to carry them out and reduce the duration of these tests.
You will need to select the appropriate parameters of the experiment. Particularly important is the power of the excitation signal. If it is too large the can achieve load causing plastic deformation in a very short period of time.
Fatigue Test at Resonant Frequency will certainly damage the material as stated by other researchers. You can test using Electrodynamic shaker where you can control the frequency precisely and the frequency can be tracked by standard vibration software. You can track the shift in natural frequency and once the shift is more than 3% you can stop the test as the stiffness is changed and thus life can be calculated. If you do the damage will not be major.
If servo hydraulic machine then you are advised to carry out the fatigue testing in wherein you can test in strain/displacement controlled mode and you can stop the test as soon as the displacement increases and check for crack using NDT methods but still it is also a destructive testing but can save your component from total failure.
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