mechanicalFEA
Mechanical
Hello,
When calculating the number of cycles to failure during a random vibration test fatigue calculation based on minors cumulative damage, should you use the first resonant frequency, the frequency which has the larges mass participation or calculate the number of RMS zero crossing points?
I have seen many examples of this most using the first resonant frequency, presumable because its this frequency which contribute significantly to the overall damage and the one the part is most likely to resonate at.
But surley during random varbration, when the frequency and amplitude is constantly changing the part will not simply resonate at a single frequency so why should we assume one when estimating number of cycles?? I would have thought it would be far more accuate to define an average/ estimate number of cycles for the given time period based on the number of crossings if the frequency band width is known?
My issue with this calculation is that by using the first resonant frequency you are assuming the frequency remains constant, as such if a part has a low first resonant frequency the damage calculation will simply assume that it will not experience as many cycles as a part with a high first resonant frequency so the estimation of total number of cycles for a given time period is highly variable and as such the damage contribution also.
Any thoughts or views on this topic?
When calculating the number of cycles to failure during a random vibration test fatigue calculation based on minors cumulative damage, should you use the first resonant frequency, the frequency which has the larges mass participation or calculate the number of RMS zero crossing points?
I have seen many examples of this most using the first resonant frequency, presumable because its this frequency which contribute significantly to the overall damage and the one the part is most likely to resonate at.
But surley during random varbration, when the frequency and amplitude is constantly changing the part will not simply resonate at a single frequency so why should we assume one when estimating number of cycles?? I would have thought it would be far more accuate to define an average/ estimate number of cycles for the given time period based on the number of crossings if the frequency band width is known?
My issue with this calculation is that by using the first resonant frequency you are assuming the frequency remains constant, as such if a part has a low first resonant frequency the damage calculation will simply assume that it will not experience as many cycles as a part with a high first resonant frequency so the estimation of total number of cycles for a given time period is highly variable and as such the damage contribution also.
Any thoughts or views on this topic?
