Learning Forum

[Md_Salem]

l am doing modal analysis of a cantilever using ANSYS 19.R2, in the result tables (effective mass & participation factor) there is a column called (cumulative mass fraction) , what is cumulative mass fraction and how to be calculated ? .Also when i tried to find the ratio between effective mass to total mass (by dividing effective mass/total mass) it didn't match the mentioned ratio in the last column in the case of ROTATION directions (about x,y&z axis), what is wrong with that ?

[Erik Kostson]

the cumulative mass fraction for mode ÔÇ£nÔÇØ is the the addition/sum of individual Effective Mass Fractions for modes 1 through to the number of modes we requested and got.

So say in the Y-direction for the first mode the effective mass is 48 kg and the total effective mass in Y is ~ 68 kg then the Cumulative Effective Mass Fraction is 0.7 as shown in the image below.
As we add more and more modes the effective mass fractions from modes are added together.
So in the example below for the 2nd mode it is small and we can neglect say here (so I do not have to write this out), for the 3rd mode it is 14kg and the Effective Mass Fraction for that mode is then ~ 0.22 (14.8/67.9), so add that (0.22) to 0.7 from the first mode (we accumulate) we get 0.92 as shown below (3rd row), and so on so forth.

[Md_Salem]

well , I am appreciated for your answer which makes sense, but if you look at attached figure, for the 3rd mode in Y direction , the cumulative mass fraction in 3rd row should equal to the ratio of the effective mass to the total mass since the upper two rows equal zero !!

[Erik Kostson]

So this is wrong: "the cumulative mass fraction in 3rd row should equal to the ratio of the effective mass to the total mass since the upper two rows equal zero!!". What you mention here is closer to the ratio of effective mass to total mass for a mode (so last column in the solve.out table)


Please read through my example above it is not the total mass we divided with it is the total effective mass (sum) which is 0.130237 in your example in the Y-dir, and it was 67.9 kg in my example (the total mass in that example is somewhere around 75 kg).

So the cumulative mass fraction in you case up to mode 3 is for the 3 modes: 0/0.130237 + 0/0.130237 + 0.992688E-1/0.130237 = 0.762216 which is what we get.
So the value in the solve.out from the solver is of course correct.

[Md_Salem]

I got your point , appreciated so much.

[Erik Kostson]

More than welcome all the best

Erik