Strange behavior in normal mode analysis of shells with laminates.

[W_A_R_W_]

Hi,

When performing normal modes analysis on a shell with a laminate attached to it (e.g a aluminum/aluminum sandwich panel), Nastran seems to be unable to find the correct normal modes when picking a low, non-zero value for the Young's modulus of the core. A value of zero or a value of about 1000 MPa or higher seems to give correct answers but lower ranges show incorrect normal modes.

I was wondering if somebody else has experienced this or knows what causes it.

Any insight would be appreciated. 

[John_Holtz]

Hi @W_A_R_W_ 

Numerical solutions can lose accuracy when the ratio of (maximum stiffness)/(minimum stiffness) becomes "too large". The stiffness is related to the material properties (mainly the modulus of elasticity) and the mesh (the size and distortion of the elements). It is not surprising that the results change when the material properties change.

The question is how the various stiffness contributes to the real answer. For example,

• a stiff core in conjunction with stiff face sheets causes a frequency of X.
• When the core is very weak, the frequency is controlled by the stiffness of the face sheets and causes a frequency of Y. 
• As the stiffness of the core is increased, the frequency may change very little if the stiffness is controlled by the face sheets. Once the core stiffness reaches some value, it begins to have an effect on the overall stiffness, and the frequency begins to change noticeably.

So if 1000 MPa is the lowest core stiffness that gives reasonable answers, try running the model with higher and higher a core stiffness. The graph of the results will show whether a stiffness lower than 1000 MPa will really make any difference or not, and if so, you may be able to extrapolate out to the desired stiffness.

I'm curious about your statement "unable to find the correct normal modes". Does this mean the analysis fails? Or does it calculate the modes but they are wrong? (And if wrong, how obvious is it that they are wrong?)

John