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Hello

I am lately working with 3D-solid mechanics-Eigen Frequncies. I have problems with simulation when the aspect ratio of the 3D beam is really high. 3D beam is composed of 3 layers gold(0.04 m thickness), chromium(0.01 m thickness) and silicon nitride(0.1 m thickness). So the height(sum of three layers) of the 3D beam is 0.15 m, width is 400 m and depth is 0.3 m.

With this structure, I get the first eigen frequency as pure imaginary number it was around 6000i Hz. There is no other first mode() The structure have fixed constraint for 6 faces(for each end of the three layers). There seems to be no problem in the memory of the computer. Also there seems to be no problem in meshing. I used swept mode sho the distribution was at least 15 for height. Also triangular method is used for meshing and size was customary, which the maximum value was 0.1 m.

Also I am suffering from strange mode shape of eigen frequency. I expected transverse mode oscillating with y or z direction, but the result was like combination of two (I can't understand this because it isn't even symmetric structure). This strange mode shapes seems to vanish as it becomes higher frequency modes. Also this strange mode shape vanishes as the width become smaller.

As a result, there must be some problem when the width of the beam become larger ( when aspect ratio becomes higher). How can I solve this problem? How can I interpret with this pure imaginary eigenfrequency?