FAQs on CONTACT – ANALYSIS
Terminology: STS – Surface to Surface Contact. NTS - Nodes to Surface Contact .NTN - Node to Node Contact. FKN or Kn - Normal Contact Stiffness. E - Young’s Modulus. ICONT - Initial Contact value. PMIN & PMAX - Minimum and Maximum initial Penetration range. PINB - Pinball region Radius. FTOLN - Penetration tolerance. Mu - Friction Coefficient
FAQ# 1. On what factor can a contact analysis be judged as "OKAY"?
Answer: Penetration In physical reality, penetration between 2 contacting bodies never occurs. This is a mathematical creation so as to activate a contact stiffness between two bodies. Keeping penetration to a minimum is a best way to simulate a contact analysis. For achieving this, the contact Stiffness may be specified as high as possible as long as a converged FE solution is possible.
FAQ# 2. What affects the accuracy and convergence of a contact analysis?
(FKN) Stiffness ud for the Contact is a major factor affecting Convergence of Contact problems. Using higher values of Stiffness may diverge the solution. Its advid to start with a small enough value and get a converged solution. Re-solve with incread value of stiffness so long as the problem stops converging. Trial and Error + Experience will help you predict the Contact Stiffness needed to solve a Contact Analysis. Another recommended practice is to run the analysis with an initial small contact stiffness and then slowly increa the stiffness over a ries of load steps. This will ramp the contact stiffness value from the initially considered Stiffness, thus improving convergence behavior. The results for the last load-step would have been analyzed with more stiffness and hence would be more accurate than previous results.
FAQ# 3. What is Hertz Stiffness? How is it calculated?
Hertz Stiffness provides an approximate value for the Penalty Stiffness value. Kh = L*E where, Kh --> Hertz Stiffness. L --> Size/edge length of the element at the contact surface
E --> Young’s Mod of Contact Surface. For 2D models, the thickness of element is considered for L.
FAQ# 4. Simulating contacts between 2 Rubber material bodies. How can I evaluate E for the rubber so that I can roughly estimate the contact Stiffness?
Consider E for the rubber material portion as: E=6(a+b) where ‘a’ and ‘b’are the first Two Mooney-Rivlin constants.
FAQ# 5. While using NTN contact elements between two (3D) surfaces, is there any recommended practice for meshing at tho two surfaces?
Meshing between the two sides were varying along X. Surface 1 Y X finer coarr Surface 2 $ sign indicates the Node-To-Node Contact Element between the Nodes of Surface 1 and 2 respectively. If you obrve the mesh variation along X direction, it has varied from Finer to coarr. In such situations, u of a uniform Kn value for all the N-T-N contact elements could lead to un-even contact pressures. It is advisable to specify diff
erent values of Kn. If this step was not taken care of, then the finely meshed portion will experience more stiffness compared to the coarly meshed portion. To avoid the issues, its recommended practice to u the same edge length between the two surfaces 1 & 2.
FAQ# 6 what are the guidelines to estimate FKN for a contact Analysis.
FKN = 0.1 to 10 ---> Factor of E For contact between two slender bodies (thin / bending dominated structures) FKN = 0.01 to 1 ---> Factor of E Its always advid to start with a lower value of FKN and proceed later with higher values. The default tting in Ansys is1, irrespective of what kind of bodies is participating are getting in Contact.
FAQ# 7. When two different materials (say Steel and Aluminum) are in Contact, which material's E should be considered for evaluating FKN?
E of the softer material be always considered for estimating FKN. In this ca prefer E of Aluminum.
FAQ# 8. How to specify Sticking and sliding conditions for Contact between two bodies?
'Mu' - Friction coefficient and TAUMAX are the two inputs ur needs to specify .If Ft >= Mu*Fn where Fn --> Normal Force & Ft --> Tangential
Force, the bodies slide relative to each other and the two contacting bodies experience a Shear force of TAUMAX.Sticking happens when Ft < Mu*Fn.
FAQ# 9. How to estimate TAUMAX?
TAUMAX = (Von Mis Yield Stress/1.732) this is an empirical formula.
FAQ# 10. I have higher order elements (with mid-side nodes) on the two contacting surfaces & I want to analyze the problem using NTN Contact. Can I create NTN Contact between the Mid-side Nodes?
Kinematically consistent reaction forces at the nodes of midside-noded elements, NTN co
ntact should NOT be applied to mid-side nodes. In effect, usage of NTN implies that only lower order elements be ud for underlying mesh of contacting parts.
FAQ# 11. While editing the real constants of a Contact element, like FKN can -ve values be entered?
Answer: In the Real Constant menu, a negative value indicates an absolute value; and a positive is considered as a factor. So if FKN was specified as 0.1, its considered as 0.1*E El, if FKN was specified as -2e10 then the value of FKN for the analysis is -2e10.
