ANSYS接触分析精华

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 used for the Contact is a major factor affecting Convergence of Contact problems. Using higher values of Stiffness may diverge the solution. Its advised to start with a small enough value and get a converged solution. Re-solve with increased 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 increase the stiffness over a series 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 those two surfaces?

Meshing between the two sides were varying along X. Surface 1 Y X finer coarser Surface 2 $ sign indicates the Node-To-Node Contact Element between the Nodes of Surface 1 and 2 respectively. If you observe the mesh variation along X direction, it has varied from Finer to coarser. In such situations, use 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 different values of Kn. If this step was not taken care of, then the finely meshed portion will experience more stiffness compared to the coarsely meshed portion. To avoid these issues, its recommended practice to use 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 advised to start with a lower value of FKN and proceed later with higher values. The default setting 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 case 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 user 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 Mises Yield Stress/1.732) this is an empirical formula.