Describtion of Abaqus terminology

Before starting the description of the software developed to interact with Abaqus input files, it is better to describe how Abaqus creates these files and how it is possible to modify them and to use them for the purpose of defining proper boundary conditions.

The input file of Abaqus

The input file is an ASCII file which describes the model to be solved by Abaqus. This file contains the description of the mesh (nodes and elements), the material properties, the boundary conditions (constraints and loads), equations, sets, fields to be calculated during te solution. etc.

thanks to a proper syntax.

Considering the ASCII format of the file, it can be easily modified (or created) by a user developed software before sending it to the solver. An example of a very simple input file is the following:

Example of Abaqus input file

*Heading

** Job created as example

*Preprint, echo=NO, model=NO, history=NO, contact=NO

*Node

1, 0., 0., 20.

2, -14., 0., 20.

3, -28., 0., 20.

4, -42., 0., 20.

5, 0., 0., 10.

6, -14., 0., 10.

7, -28., 0., 10.

8, -42., 0., 10.

9, 0., 0., 0.

10, -14., 0., 0.

11, -28., 0., 0.

12, -42., 0., 0.

*Element, type=S4R 1, 1, 2, 6, 5 2, 2, 3, 7, 6 3, 3, 4, 8, 7 4, 5, 6, 10, 9 5, 6, 7, 11, 10 6, 7, 8, 12, 11

*Nset, nset=AllNodes, generate 1, 12, 1

*Elset, elset=AllElements, generate 1, 6, 1

*Shell Section, elset=AllElements, material=Steel 10., 5

*System

5.5. IMPLEMENTATION IN ABAQUS/STANDARD

*Nset, nset=BlockedNodes, generate 4, 12, 4

*Elset, elset=BlockedElements 3, 6

*Nset, nset=LoadedNode 1,

*Material, name=Steel

*Elastic 206000., 0.3

*Step, name=Step-1

*Static

1., 1., 1e-05, 1.

*Boundary

BlockedElements, ENCASTRE

*Cload

LoadedNode, 1, 1.

*Output, field, variable=PRESELECT

*Output, history, variable=PRESELECT

*End Step

At the beginning *Heading defines just a title for the analysis and *Preprint selects the printout that will be obtained from the analysis input file processor. Then the file continues with the definition of nodes by *Node and the elements by *Element. *Nset and *Elset define sets of nodes or elements respectively. Other important parts are the material definition, using the key word *Material, and the boundary conditions definition, thanks to *Boundary and *Cload.

This file can be easily managed by user developed software. In particular it is important to analyze the nodes and apply the correct boundary conditions to the model.

Nodes sets

In Abaqus the definition of nodes sets is done by the use of the special word *Nset followed by:

• nset= plus the name of the set (e.g. nset=BlockedNodes)

• unsorted. If this parameter is included, the nodes in this node set will be assigned to the set in the order in which they are given. If this parameter is omitted, the nodes in the set are sorted into ascending order of their node numbers, with duplicates eliminated.

• generate can be used in order to generate a nodes set by the definition of one or more rules to select which nodes must be included. The following lines should give a first node n1, a last node n2 and the increment in node numbers between these nodes, i. Then, all nodes going from n1to n2in steps of i will be added to the set. i must be an integer such that ⁿ²⁻ⁿ_i ¹ is a whole number.

5.5. IMPLEMENTATION IN ABAQUS/STANDARD

After this line a series of lines are needed to assign the nodes number to add to the set (apart from when generate is used). Each number is separated from another by a comma (,) and the maximum number of nodes in a line is 16. Also a label set is allowed, but only if it has been set before.

An example of a definition of a node set of 20 nodes is:

Example of nodes set definition

*Nset, nset=Set-Example, unsorted

187, 570, 114, 804, 113, 236, 170, 168, 269, 271, 475, 477, 935, 937, 434, 432 802, 930, 548, 823

Equations in Abaqus

The equations are used to define linear multi-point constraints in the form of an equation. In Abaqus the definition of equations is made by the use of the key word *Equation followed by:

• a line that contains the number of the terms in the equation

• the equation as sum of coefficients multiplied by a DOF of a certain node or nodes set The equation finally can be written as follow

�N i=1

Ai· u^rjiⁱ= 0 (5.32)

where i is the term’s index, j stands for the j^thdegree of freedom of the node (or nodes set label) pointed by r. Each term is thus composed of three parts:

• node number or node set’s label, rⁱ

• degree of freedom involved in the equation, jⁱ

• coefficient of the term, Aⁱ

This has to be repeated as many times as needed to define all terms of the equation, but more than four terms can not be defined in a line. The first term can be defined either as a node number or a nodes set label. However if the first term is defined as a node, then the others have to be a single node (or a set made by a single node).

An example of a definition of such an equation is:

Example of equation definition

*Equation 2

Set-up, 3, 1.0, Set-down, 2, -2.0

5.5. IMPLEMENTATION IN ABAQUS/STANDARD

that has to be seen as

1.0 · u^Set-up3 − 2.0 · u^Set-down2 = 0

The way the equations link to the nodes inside node sets depends on the position of each node on the set. So a simple equation like the afore reported links the first node of set Set-up with the first node of set Set-down, the second node of Set-up with the second node of set Set-down and so on.

Boundary conditions in Abaqus

In Abaqus the definition of boundary conditions is done by the use of the special word

*Boundary followed by:

• node number or nodes set’s label

• first degree of freedom constrained

• last degree of freedom constrained (this can be left blank if only one degree of freedom is being constrained)

• magnitude of the variable (displacement, velocity, etc.). If this magnitude is a rotation has to be given in radians

All parameters has to be separated by comma (,). An example of a definition of a equation is:

Example of boundary condition definition

*Boundary

Set-Point-A, 3, 3, -0.01

that has to be seen as

uSet-Point-A

3 = −0.01

in the unit of measurement of the model, that is defined by the user.