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Exercise 15 Introduction to Contact Contact 33, 34, 36, 46

Problem description Outline

Creation of a pen and a plane model, and different contact types are used for presenting their function

Analysis type(s):

Explicit, 3D analysis

Element type(s):

Solid, Shell

Materials law(s):

Elastic_Plastic_Solid; Elastic_Plastic_Shell

Model options:

Boundary conditions, Nodal loads, Hourglass control

Key results:

Displacements, Shear Force, Normal Force

Prepared by:

Wang Kunpeng, Pierre Culiere

Date:

February 2008

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Background information Contact-impact phenomena occur in many fields of applied mechanics. For reasons of computational economy or accuracy, however, distinctions are often made in the numerical treatments. Contact interfaces work in two phases: -

Contact search: detecting the objects that should not interpenetrate.

-

Contact treatment: applying forces (penalty methods) or monitoring node positions/velocities (kinematic methods).

The computer time of contact-intensive models (e.g. crash simulation) may be dominated by the contact algorithm used. It is therefore of prime importance to achieve computational efficiency, and much effort has been devoted to this subject: specific types of interfaces have been developed for some problems, and parallel efficiency is regularly increased. In this tutorial, the four basic types of contacts used in crash simulation will be introduced. Problem data and description Units: kN, mm, kg, ms, Kelvin Description: The pen has an initial velocity, and moves towards the plane’s center, which is also the center of an element. This model demonstrates the basic contact interface between master segments and slave nodes. This document does not describe how to create the pen model, but you can get the model for free. The following image shows the configuration of the pen and plane model.

Pen Model

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Plane Model

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Pen’s imposed velocity curve

Plane’s boundary condition

Material: Plane: Elastic_Plastic_Shell (type 103, parameters are shown in right picture)

Pen: Elastic_Plastic_Solid (type 1, parameters are shown in right picture)

Remark: Assign the created material to Pen and Plane through PART card. Supplied datasets It is recommended that you use the following names for the PAM-CRASH input and results files: -

For surface-to-surface contact (type 33): Pen_Model_33.pc.

-

For node-to-surface contact (type 34): Pen_Model_34.pc.

-

For the self contact (type 36): Pen_Model_36.pc.

-

For edge-to-edge contact (type 46): Pen_Model_46.pc.

In each case, completed PAM-CRASH datasets are available.

Part 1: Background Information of Contact Terminology Contact Interface Types

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Contact Surface: Set of segments that contact themselves or each other.

Contact Segment: Three or four node polygons composing the contact surface. In general the contacts are eventually reduced to a node to segment treatment.

Contact pair: One slave node and one Master segment that are potentially in contact.

Master Segment and Slave node: ”Slave” usually indicates the impacting node, and “Master” indicates the impacted segment.

You can check the contacting surfaces through right-hand image.

Penetration: A slave node is within a distance less than the contact thickness of the master segment. Perforation: (also inter penetration) A slave node has penetrated beyond the mid surface of the master segment. Intersections will then occur as well. Introduction to Contact type 34, 33, 36 and 46 Nodes-to-Surface (Contact 34) A contact interface of type 34 is defined as a set of (slave) nodes contacting a (master) surface. The slave nodes should not penetrate the master segments. This treatment is not symmetric. Using this contact you must choose the slave and master parts wisely, otherwise it will not effectively prevent penetration. Surface to Surface (Contact 33) A contact interface of type 33 is defined as a (slave) surface contacting another (master) surface. However, here,

master

and

slave

have

absolutely symmetric roles and can be exchanged: a contact type 33 actually works as two contacts of type 34, using the nodes of each surface as slave of the other surface.. It will be use more computations than type 34, but it can prevent more potential penetrations and perforations.

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Self Impacting (contact 36) In a self contact interface of type 36, each slave node sees non-connected segments of the same surface as masters. Edge to Edge (Contact 46) The close search of contact 46 finds the closest point between two finite lines, which can prevent penetration between edges. This kind of contact can be used in the following cases: •

Model contact in bar/beam structures;

Improve contact for Shell/Solid structures with sharp edges or corners (to be used in addition to a node-surface type)

Improve contact in structures where sharp edges can be formed, especially membrane structures.

Part 2: Contact Card Definition Open model Pen_Model.pc, and generate 4 models, each one with a different type of contact interfaces. 1. Definition of Contact 33 Right click on the tabs Crash>Contacts and choose the Type as 33-SYMMETRIC NODE-TO-SEGMENT WITH EDGE TREATMENT like the following image. (1) We input some important parameters as shown (highlighted in green frame). (2) We choose the Slave Selection as Seg and select the pen’s triangular head as shown (yellow highlight). After selection click the Update Selection button to confirm. (3) Choose the Master Selection as Seg again, and choose Plane’s all segment (highlighted in yellow lines). After selection click Update Selection button to confirm. (4) Click the Apply button to finish creating the contact card. Remarks: You can check the selection part by the following image. You can also select the object as Part but choosing the exact selection can save CPU time. Export model as Pen_Model_33.pc 2. Definition of Contact 34 Contact 34 has the same definition steps as contact 33. You can define this contact card using the anterior four steps except for the contact type choice; just replace 33-SYMMETRIC

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NODE-TO-SEGMENT

WITH

EDGE

TREATMENT

into

34-Non-Symmetric

Node-to-Segment Contact with Edge Treatment. Export model as Pen_Model_34.pc

3. Definition of Contact 36 (1) Right click on the tabs Crash>Contacts and choose the Type as 36- Self-impacting Node-to-Segment Contact with Edge Treatment (2) The key parameters is the same as Contact 33 (hcont, FRICT, IREMOV). (3) Choose the Slave Selection as Ele. and select the pen’s triangular head and Plane’s all elements. After selection click the Update Selection button to confirm. (4) Click the Apply button to finish creating the contact card. Remarks: In this contact you need not to choose the master parts, every part are treated as slave elements. Export model as Pen_Model_36.pc 4. Definition of Contact 46 (1) Right click on the tabs Crash>Contacts and choose the Type as 46-Edge-to-Edge Self-impacting Contact (2) The left steps are the same as contact 36. Remarks: In this contact you has better choose Slave Selection as Edge, but this selection is kind of time-consuming. There is no IREMOV and FRICT parameter in this card. Export model as Pen_Model_46.pc

Part 3: Evaluating the results (Visual Viewer) 1. Results of Contact 33 Click Results > Contour and under Entity types activate Node and select Displacement_Z to show the contour distribution of nodal displacement in Z axis. Through the result we can see that there is no penetration.

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The following images are respective the contact force time histories and contact pressures on mesh plots of contact 33.

Contact Force Time Histories Contact Pressure 2. Results of Contact 34 To show the contour distribution of nodal displacement in Z axis. Through the result we can see that there is no penetration.

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If in VCP you have chosen the pen as master and the plane as slave, then the simulation will show an apparent perforation. The contact 34 is not symmetrical. If you are sure of your master and slave parts selection, you can make good use of this contact type. It is computationally cheap. In some cases, you do not have a choice and need to use it. For example, if you work with SPH elements, these elements can only interact with FE as slave nodes in a node-to-surface contact. If you are not sure which part should be slave or master, you should choose a contact of type 33, which uses more CPU but is safer. The following image shows the result of wrong selection of contact 34.

3. Results of Contact 36 To show the contour distribution of nodal displacement in Z axis. Through the result we can see that there is no penetration. This contact is the easiest to define, as there is no need to worry about master and slave definitions. You can just select all parts of your model. However, it may be more expensive and requires extra steps if you need to know the contact force: you need to define a section force of type “contact�.

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The following images are respective the section force and contact pressures on mesh plots of contact 36.

Section Force

Contact Pressure

4. Results of Contact 46 With a type 46 only, we have an apparent perforation. Although there is no edge-to-edge perforation here, the head node of the pen has gone through the center of one element. This can be prevented by the addition of a node-to-segment interface: add an interface of type 33, 34, or 36.

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The following images are respective the contact force time histories and contact pressures on mesh plots of contact 46.

Contact Force Time Histories

Contact Pressure

Another Example for Edge-to-Edge Contact In order to show the function of contact 46 in details, here we introduce into another simple model: a plane to plane impact. 1. Define two planes, one (part 1) is perpendicular to another (part 2). 2. Part 1 moves towards part 2 with an initial velocity and added mass. 3. In order to demonstrate the interest of contact 46, we will set another model with contact 33 to compare their behaviors. 4. Display the result of contact 33 and 46: you can see that only contact 46 prevents penetration.

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Comparison between contact 33 and contact 46 We can see that part 1 has crossed part 2 without any resistance, which means the contact 33 does not prevent the edge to edge impact. When contact 46 is defined, the edges of plane 1 cannot be penetrated by the edges of plane 2. A penalty force is applied to both planes, which leads to the bending of plane 2.

The following images are respective the contact force time histories and contact pressures on mesh plots of the plane to plane impact.

Contact Force Time Histories

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Contact Pressure

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Contact Study