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2020 | OriginalPaper | Chapter

3. Method to Debug a Model

Author : Raphael Jean Boulbes

Published in: Troubleshooting Finite-Element Modeling with Abaqus

Publisher: Springer International Publishing

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Abstract

The main aim of this chapter is to describe a methodology in order to perform a job diagnostic on a model which did not converge. Following the global mindset in chapter one along with the example of a global overview analysis flowchart methodology in Fig. 1.​1, this chapter will focus on a step-by-step procedure to establish convergence with the model showing different analysis techniques as a function of the modeling phases needed to create the model with Abaqus features. The procedure described here is therefore a deeper understanding of the single cell titled “-Job diagnostics” in Fig. 1.​1, showing users a logical approach toward corrective actions inside a puzzle of numerical difficulties.

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previous chapter Analysis Convergence Guidelines
next chapter General Prerequisites
Footnotes
1
To identify the correct Abaqus version command a simple method is to click right on the shortcut icon to start Abaqus, go the “General” tab and, read the description. For Abaqus version 6.14-5, it shown as “abq6145.bat”.
 
2
Command file, created by the Abaqus execution procedure.
 
3
Printed output file. It is written by the analysis, syntax check, parameter check, and continue options. Abaqus/Explicit and Abaqus/CFD do not write analysis results to this file.
 
4
Log file, which contains start and end times for modules run by the current Abaqus execution procedure.
 
5
Output database. It is written by the analysis and continue options in Abaqus/Standard, Abaqus/Explicit, and Abaqus/CFD. It is read by the Visualization module in Abaqus/CAE (Abaqus/Viewer) and by the convert=odb option. This file is required for restart.
 
6
Model and results file, used by Abaqus/CFD. It is also used by Abaqus/Standard and Abaqus/Explicit when the results format=sim or both option is specified. It is written by the syntax check option. It is read and can be written by the analysis and continue options. This file is required for restart.
 
7
Communications file, used by Abaqus/Standard and Abaqus/Explicit. It is written by the analysis and data check options and is read by the analysis and continue options.
 
8
Results file. This is written by the analysis and continue options in Abaqus/Standard and by the convert=select and convert=all options in Abaqus/Explicit.
 
9
Model file, used by Abaqus/Standard and Abaqus/Explicit. It is written by the data check option. It is read and can be written by the analysis and continue options in Abaqus/Standard, while it is read by the analysis and continue options in Abaqus/Explicit. Multiple model files may exist if the element operations are executed in parallel in an Abaqus/Standard analysis. In such cases, a process identifier is attached to the file name. This file is required for restart.
 
10
Message file. This is written by the analysis, data check, and continue options in Abaqus/Standard and Abaqus/Explicit. Multiple message files may exist if the element operations are executed in parallel in an Abaqus/Standard analysis. In such cases, a process identifier is attached to the file name.
 
11
Part file, used by Abaqus/Standard and Abaqus/Explicit. This file is used to store part and assembly information and is created even if the input file does not contain an assembly definition. The part file is required for restart, import, sequentially coupled thermal stress analysis, symmetric model generation, and underwater shock analysis, even if the model is not defined in terms of an assembly of part instances. This file may also be needed for submodeling analysis.
 
12
Restart file, which contains information necessary to continue a previous analysis and is used by Abaqus/Standard and Abaqus/Explicit. The restart file is written by the analysis, data check, and continue options. It is read by any restarted analysis.
 
13
State file. This is written by the data check option in Abaqus/Standard and Abaqus/Explicit and is read and can be written by the analysis and continue options in Abaqus/Standard. It is read by the analysis and continue options in Abaqus/Explicit. Multiple state files may exist if the element operations are executed in parallel in an Abaqus/Standard analysis. In such cases, a process identifier is attached to the file name. This file is required for restart.
 
14
In case of bolt preload, at least two analyses steps are needed. The first analysis step will only be used to set the applied bolt preload. The second analysis step will be used to apply the external load on the structure but with a propagated load from the previous step regarding the bolt preload in order to fix the bolt preload at a current length in the second step. Otherwise, the bolt preload at the beginning of the second analysis step will not be constrained as a compressive bolt load and consequently convergence difficulties with solver exit will occur.
 
15
See the dynamic load keyword command *DLOAD. This option enables the specification of distributed loads. These include constant pressure loading on element faces and mass loading (load per unit mass) either by gravity forces or by centrifugal forces.
 
16
All nominal strain components.
 
17
Maximum principal nominal strains.
 
18
Minimum principal nominal strains.
 
19
Regarding contact penetration, in some cases Eulerian material may penetrate through the Lagrangian contact surface near corners. This penetration should be limited to an area equal to the local Eulerian element size. Penetration can be minimized by refining the Eulerian mesh or adding a fillet to the Lagrangian mesh with a radius equal to the local Eulerian element size.
 
20
A normal Lagrange contact chattering is an issue which often occurs with the normal Lagrange method. If no penetration is permitted, then the contact status is either open or closed (a step function). This can sometimes make convergence more difficult because contact points may oscillate between open/closed status and is called “chattering”. If some slight penetration is allowed, it can make it easier to converge since contact is no longer a step change.
 
21
It is strongly recommended, especially with a high coefficient of friction value set between both surfaces of, greater than 0.2.
 
22
The CARTESIAN connection does not impose kinematic constraints. It defines three local directions at node “a” and measures the change in the position of node “b” along these local coordinate directions. The local directions at node “a” follow its rotation.
 
23
Connection type CARDAN provides a rotational connection between two nodes where the relative rotation between the nodes is parameterized by Cardan (or Bryant) angles. A Cardan-angle parameterization of finite rotations is also called a 123 or yaw–pitch–roll parameterization. Connection type CARDAN cannot be used in two-dimensional or axisymmetric analysis. When connection type CARDAN is used with connector behavior, the relative rotation axis with the highest resistance to rotational motion should be assigned to the second component of relative rotation (component number 5) in order to avoid gimbal lock, a singularity in the rotation parameterization for relative rotation angles. The CARDAN connection does not impose kinematic constraints. A CARDAN connection is a finite rotation connection where the local directions at node “b” are parameterized in terms of Cardan (or Bryant) angles relative to the local directions at node “a”. Local directions are positioned relative to by three successive finite rotations.
 
24
Connection type TRANSLATOR imposes kinematic constraints and uses local orientation definitions equivalent to combining connection types SLOT and ALIGN. The connector constraint forces and moments reported as the connector output depend strongly on the order and location of the nodes in the connector. Since the kinematic constraints are enforced at node “b” (the second node of the connector element), the reported forces and moments are the constraint forces and moments applied at node “b” to enforce the TRANSLATOR constraint. Thus, in most cases, the connector output associated with a TRANSLATOR connection is best interpreted when node “b” is located at the center of the device enforcing the constraint. This choice is essential when moment-based friction is modeled in the connector since the contact forces are derived from the connector forces and moments, as illustrated below. Proper enforcement of the kinematic constraints is independent of the order or location of the nodes.
 
25
It is important to be careful with vocabulary here as, this energy is an artificial energy caused by the hourglassing effect. An energy analysis will have to minimize this effect as much as possible with meshing techniques. When reduced integration is used in the first-order elements (the 4-node quadrilateral and the 8-node brick), hourglassing can often make the elements unusable unless it is controlled. In Abaqus the artificial stiffness method and the artificial damping method given in Flanagan and Belytschko [1] are used to control the hourglass modes in these elements.
 
26
SIM is a high-performance software architecture available in Abaqus that can be used to perform modal superposition dynamic analyses. The SIM architecture is much more efficient than the traditional architecture for large-scale linear dynamic analyses (both model size and number of modes) with minimal output requests. SIM is used with the eigensolver to calculate modal frequencies using Lanczos or AMS techniques but is not applicable for subspace iteration techniques.
 
27
As defined in Abaqus Analysis User’s Guide v6.14 Sect. 4.2.1 Abaqus/Standard output variable identifiers paragraph Total energy output quantities.
 
28
As defined in Abaqus Analysis User’s Guide v6.14 Sect. 4.2.2 Abaqus/Explicit output variable identifiers paragraph Total energy output.
 
Literature
1.
Flanagan DP, Belytschko T (1981) A uniform strain hexahedron and quadrilateral with orthogonal hourglass control. Int J Numer Methods Eng 17:679–706CrossRef
2.
Belytschko T (1976) Survey of numerical methods and computer programs for dynamic structural analysis. Nucl Eng Des 37:23–34CrossRef
3.
Hibbitt HD, Karlsson BI (1979) Analysis of pipe whip, EPRI, Report NP-1208
Metadata
Title
Method to Debug a Model
Author
Raphael Jean Boulbes
Copyright Year
2020
Book
Troubleshooting Finite-Element Modeling with Abaqus

Print ISBN: 978-3-030-26739-1

Electronic ISBN: 978-3-030-26740-7

Copyright Year: 2020

DOI
https://doi.org/10.1007/978-3-030-26740-7_3

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