# Mechanic problems

**I am looking for a FEA software in order to get the eigenfrequency of a steel piece. Can I use something from your modules?**

Unfortunately, there is no such a feature in our package.

**I am trying to figure out the directions of the principal stresses shown on an axisymmetric model field picture results? The principal stresses are 1, 2 and theta. Which is vertical, horizontal, etc?**

You can view principal stress vectors direction if you switch on Tensors in the Field view dialog. The tension vectors are blue. The compression vectors are red.

**We are working with the stress analysis module. We want to simulate a wheel-rail contact. With no load, the wheel and the rail are in contact at one point (line). How can we manage the geometry, so that the boundaries of wheel and rail won't overlap when we put a load on the wheel?**

To correctly represent the contact problem you need special algorithms and elements, able to deal with geometric non-linearity. Contact area should depend upon the load, and this is far beyond linear stress model QuickField uses.

**What are what kinds of gradients are G1 and G2?**

G = grad (E), where E is a vector, so G is 4-component tensor G = { Gxx, Gxy, Gyx, Gyy }. It can be rewritten in canonical form that only Gxx and Gyy remain nonzero. Geometrically it means rotation the tensor. If we do that, the G1 is a maximum of (Gxx, Gyy), and G2 = min (Gxx, Gyy). Another term for them is "main values of the tensor".

**Underneath the boxes for entering the thermal expansion coefficients, there is a box for "difference of temperature" What is that about?**

The "Difference of temperature" box is used when you work with separate stress problem, without link to thermal problem. In this case if you want to perform a thermal stress you should specify the difference of temperatures between strained and strain less states. In linked problem the difference is calculated automatically.