Nonlinear Analysis:  changing contact or interference between parts of an assembly  [Requires: contact/target elements on surfaces which may change contact, plus settings and properties for the contact surface]

CONTACT ELEMENT CHOICES

• "Contact Pairs,"  ANSYS: Contact Wizard for elements: TARGE169, 170 and CONTA171, 172, 173, 174
• (older technology) point-to-point contact, ANSYS CONTAC12, 52

ANSYS contact models (element options, real constants, material properties):

• formulation: Augmented LaGrange method (default) or penalty method
• load increment control: predicting contact changes
• behavior: sliding with friction, bonded, hook, initial interference, and more
• Status: open = no stiffness, closed = stiff spring (stiffness may default or may require user specification), and near/far determination

Guidelines for Target/Contact Surface Selection:

• if a convex face will contact a flat or concave face, the flat or concave face should be the target surface.
• if one surface has a coarser mesh, it should be the target surface.
• if one surface is stiffer than the other, it should be the target surface.
• if one surface is higher-order and the other is lower-order, the lower-order surface should be the target surface.
• if one surface is larger, it should be the target.

ALWAYS use large deformations 

WHAT TO DO WHEN THE SOLUTION WON'T CONVERGE

• check the solution output and error log files for messages
• apply the load in more, smaller substeps
• allow more equilibrium iterations
• try alternative material models (bilinear vs. multilinear, different hardening rules, etc.)
• inspect the mesh in high stress regions for poor element shapes
• re-evaluate you choice of loads (get rid of point loads, overconstrained edges)
• look at the last good solution for any irregularities
• re-check EVERYTHING ! [element options, and material properties, and real constants, and load values, and dimensions]
• experiment with convergence enhancement tools in the code
• deactivate one or more of the nonlinear conditions to find the problem
• (rarely) loosen the convergence criteria - to find the problem - then tighten it up again

READING ASSIGNMENT:

ANSYS Help System

Contact Technology Guide, Chapters 1 - 6, 8 - 11