• Poorly shaped elements—Elements that have sharp angles, excessive curvature, excessive spanned angles, or high aspect ratios—or that are excessively slender or thin—may cause your model to be poorly constrained, may cause delayed convergence, or may yield poor results. Reshape the existing elements or create additional elements in the region. This is particularly true for shell elements.

• Sharp corners, reentrant corners, and/or notches—These features can exist along the load flow path, on the surface of the structure, or on the boundary between elements with different material types. You should replace them with filleted joints or surround them with excluded elements.

• Excessively small numerical dimensions—The overall dimensions of your model should not be excessively small numerically. For example, it is best to create a model with all dimensions in millimeters or micrometers, rather than with all dimensions in small fractions of meters.

• Springs without mass—In a modal analysis, each degree of freedom at the endpoint of a spring must be connected to a beam, shell, solid element that has mass, or an element that is connected to a concentrated mass element. Otherwise, the endpoint must be constrained. For example, the rotational degrees of freedom of a spring attached to a solid element should either be constrained or have a concentrated mass with rotational inertia.