# Ellipsoidal Surface to Segment Contact (TYPE15)

TYPE15 interface between surfaces made up of 4-node or 3-node segments and hyper-ellipsoids is a penalty contact interface without damping.

It applies to TYPE14 interface, especially when the mesh is coarser than the ellipsoid size.
Remember that in such a case, TYPE14 interface is able to compute low quality contact forces
even if it fails to find contact.

Interface stiffness is a nonlinear increasing function of penetration, computed in order to avoid penetrations up to half the ellipsoid:

$K={K}_{0}{\left[\frac{Depth}{Depth-Penetration}\right]}^{2}$

Where,

- ${K}_{0}$
- Input stiffness factor

A Kinematic Time Step is computed so that the element does not cross the line Lt within one time step.

A friction coefficient m is input.

Interface takes into account sliding/rolling effects.

Coulomb Friction condition is expressed as:

${F}_{t}\le m{F}_{n}$ for each penetrated element