So, as the title of the post implies, I'm making an FEM code for a composite laminate plate using the Mindlin plate theory. And, as I am trying to get the mode shapes for the plate, I needed to make a mass matrix. Following some other examples and some past code someone gave me, I managed to make one.
But the problem is, when I change the number of elements, like for example, from 4x4 to 5x5 elements, some of the mode shapes, the 4th one specifically, flips upside down. And that is the case when I change to number of elements. Sometimes the 1st and 2nd mode shapes are flipped. Sometimes all of them are. But I still can't find what is wrong.
What could be causing this? Is there some checklist of simple things I should check to correct my mass matrix? If there is an issue you think that might be a problem, please be as specific as possible in coding terms. Thanks in advance!