The stresses from modal analysis are not the actual stresses. They doesn't have meaning but can be used to compared some areas of the modal to other areas so that we know which area has higher or lower stress compared to other area. Since random vibration is based on modal analysis, does that mean that the stress RMS output from RVA is also meaningless? Is it only used in comparative sense as well?
The Von Mises RMS stress is not an actual stress on several levels.
RMS values are an indication of "likely" stress values as the actual stresses vary w/time. RMS is a specific averaging method and the values can also be used as an indication of peak stresses (e.g. a factor of 3 applied to the RMS value is often used to indicate the maximum likely value that might occur over time from a probibalistic point of view.). Similarly, the Von Mises stress is an indication of stress level severity at a point and its closeness to failure levels.
Modal analysis is simply a convenient way of decomposing output levels by determining the contributions of each natural mode for a linear system.
Stresses from modal analysis are actual but you need to determine how much each mode contribute to response. Remember that response to random excitation is also random. RMS is a type of averaging of all stresses, hence it is relevant. The idea is anything about it is monetarily is covered satisfactorily by the satisfactorily margin.
Compare RMS with the arithmetic mean. RMS value is slightly above it.
The stresses from RVA are not just used to compare, they are real (provided you've given the right data). The value of these stresses can be calculated differently; for example by calculating the RMS stress from the stress corresponding to each mode, or by averaging the stresses corresponding to each mode ....
It's all a matter of what you think is the adequate measurement of stress in that point.
Most software give results in RMS by default though.
The Von Mises RMS stress is not an actual stress on several levels.
RMS values are an indication of "likely" stress values as the actual stresses vary w/time. RMS is a specific averaging method and the values can also be used as an indication of peak stresses (e.g. a factor of 3 applied to the RMS value is often used to indicate the maximum likely value that might occur over time from a probibalistic point of view.). Similarly, the Von Mises stress is an indication of stress level severity at a point and its closeness to failure levels.
Modal analysis is simply a convenient way of decomposing output levels by determining the contributions of each natural mode for a linear system.
Please keep in mind that any stress is coming from a constitutive model of the material, applied to the strain tensor to evaluate stresses. This means also that you need to know about which tensor component you are speaking: sigma_ij, or first principal stress, or... Von Mises, as well as Tresca, suggested a criterion on which make an equivalence for failure modalities. And sure, as above suggested, RMS is not actual quantity, for its intrinsic integral definition.
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