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Modeling Loading and Unloading Behavior in Seatbelt Materials

Seatbelt constitutive model, invoked by using *MAT_SEATBELT, in LS-DYNA provides features to model the loading and unloading characteristics from a uni-axial test. Parameter LLCID provides ability to model the loading curve which allows the definition of force as a function of engineering strain. Parameter ULCID, provides ability to model the unloading curve again allowing the […]

Identifying Necking in Metals and Plastics

When characterizing materials such as Metals and Plastics in LS-DYNA, most constitutive models provide a yield criteria that accounts for a 3D state of stress which reduces to a uniaxial yield stress in 1D. This allows us to directly input the true stress-strain curve from a one-dimensional state of stress testing such as in uniaxial […]

Speeding up Simulations for Focused Studies

In several situations, simulation models (new or inherited), are quite often burderend with several expensive features that may have negligible effect on the response that is purely used for comparative numerical studies. When a large portion of any simulation model turnaround time consists of features that are irrelavant to the focued study, it is imperative […]

Initial Penetrations in Contact Interfaces

1. Introduction Contact definitions allow the modeling of interaction between one or more parts in a simulation model and have become a necessity in any small or large deformation problem. The main objective of the contact interfaces is to eliminate any “overlapâ€? or “penetrationâ€? between the interacting surfaces and they accomplish this by first detecting […]

Prescribing Motion to a Rigidbody with Respect to a Local System

LS-DYNA offers several ways to prescribe a motion to a rigidbody using *BOUNDARY_PRESCRIBED_MOTION keyword. To prescribe a motion to a rigidbody along a local coordinate system, LS-DYNA offers two method to accomplish this. To demonstrate the differences in these two methods, we will consider a rigidbody that has initial rotational velocity and a prescribed displacement. […]

Deformable Spotwelds in LS-DYNA

Over the years, spotweld representation for crash and NVH applications have evolved from a crude to a detailed modeling resulting in reduced pre-processing effort while greatly enhancing the accuracy of numerical models. Here is link that gives a brief overview of spotwelding process in LS-DYNA. Please note that the document is over 2 years old […]

Modeling Press-Fit Conditions to Include Initial Stresses

In many designs, parts are often press-fitted as part of an assembly which causes an initial stress condition that is important to consider in simulations. To induce initial stresses that occur in such press-fit conditions, one can either linearly/non-linearly scale the overlap (penetration) or the contact stiffness over a certain interval depending on the following […]

Full-Newton and Quasi-Newton Iterative Schemes

When running problems using Implicit solution sheme in LS-DYNA, the default iterative non-linear solver used is the BFGS method that employs a ‘Quasi-Newton’ method in which the global stiffness matrix is reformed only every ILIMIT steps and in between these a relatively inexpensive update to the stiffness matrix is performed. This default stiffness matrix update […]

Monitoring Incremental Elapsed Time as a Function of Simulation Time

When running explicit simulations in LS-DYNA, it is very important to understand the total CPU clock and the total Elapsed time used by the solver. This information is available at the bottom of every D3HSP file written by LS-DYNA as shown below. The total elapsed time reported in the file is the difference between the […]

Damping for Airbags

When simulating airbag deployment, using the classical uniform-pressure (CONTROL VOLUME) or non-uniform-pressure (ALE, Particle), users frequently encounter oscillations. These oscillations are generally categorized as local and global. To reduce both forms of oscillations, LS-DYNA provides two types of damping that are briefly discussed here. 1. Mass-Weighted-Damping The parameter MWD in *AIRBAG_{OPTION} keyword provides a means […]

General Guidelines for Crash Analysis in LS-DYNA

LS-DYNA is a general purpose finite element software and is designed for use in various applications. Based on the use of the software for a specific application, LS-DYNA offers several parameters that can be changed from their default values to improve the accuracy, robustness, and stability of the simulation. For performing crash analysis using LS-DYNA, […]

A Few "Words" on Memory Settings in LS-DYNA

Memory in LS-DYNA is specified in “words” at the execution time. The term “word” refers to the amount of data that can be written to or read from a memory in one operation. The following figure will aid in the relationships of bits, the most basic data type, to words on various computers. One bit […]

Modeling Symmetric/Unsymmetric NonLinear Discrete Springs

Discrete springs provide a easy way to model complicated systems by using their responses in the material definitions. This post brings attention to the way LS-DYNA handles the default behavior in tension or compress when the material input does not pass through the origin (0,0) but simply begins from origin. When only one of either […]

Limitations of Penalty Joint Treatment in LS-DYNA

LS-DYNA supports various joint definition types such as spherical, cylindrical, etc (please refer to the LS-DYNA User’s manual for a complete list). Irrespective of the joint definition type and the elements associated, translational constraints are applied to the joint nodes to model appropriate behavior. The constraints are applied using the default penalty formulation whose stiffness […]

Storing Re-Usable Models in a Central Location with *INCLUDE_PATH

Beginning version 971 and later, LS-DYNA allows easy way to store models in a central location for use at run time. This feature can be turned on using *INCLUDE_PATH which takes unlimited number of absolute directory names. When INCLUDE_PATH is used, LS-DYNA first checks the file, specified using *INCLUDE keyword, in the local directory and […]

Best Practices for Modeling Recoverable Low Density Foams – By Example

Attachments: mat57_default.k Modeling recoverable foams poses several challenges in crash worthiness as well as in low-to-medium impact velocity conditions. This is due to its relatively low stiffness when compared with structural materials which has an indirect effect on its contact-impact interactions with other materials. To review the best practices when modeling such components, we can […]

Over the past decade, the ability of simulations driving the design has grown rapidly and today’s confidence in simulations results is a good testament to it. Two significant areas that have contributed to this is the “Numerical Modeling Awareness” and “Design Domain Knowledge” gained over the years by design and analysis community. Numerical modeling awareness […]

Implicit Dynamics – Now with Birth, Death, and Burial Properties

When solving static or quasi-static type problems, the default Implicit Static solver (IMAS=0 in CONTROL_IMPLICIT_DYNAMICS) requires a well-conditioned model, with no rigidbody modes, to get good convergence behavior. It is often difficult to prevent rigidbody modes especially when its dependent on contact-impact conditions. In such cases, use of Implicit Dynamics solver (IMAS=1) can help us […]

Overview of Mass-Scaling in LS-DYNA

Mass-scaling is a term that is used for the process of scaling the element’s mass in explicit simulations to adjust its timestep. The primary motivation is to change (usually increase) the global compute timestep which is limited by the Courant’s stability criteria. LS-DYNA allows two different types of mass-scaling using the DT2MS parameter from *CONTROL_TIMESTEP […]

Example of *DEFINE_CURVE_FUNCTION

Starting in LS-DYNA version 970 and later, we can now use expressions instead of digitized values to define an XY curve. Optionally, the expressions can refer to other curves which could be defined using either digitized points or using expressions themselves. Here is an example of its usage: Polynomial Expressions You can define a generic […]

Modeling Friction in Contact

In contact-impact interactions, friction plays an important role in accurately capturing the sliding behavior. In LS-DYNA, the coulomb treatment of friction is used where a static and a dynamic friction can be defined which are used to determine the shear force while resisting penetration. By default, all contact definitions model a friction-less sliding which is […]

Contact Thickness in Single Surface Contact

Single surface contacts have dramatically eased the way we currently model the contact-impact treatment for complicated simulation models in LS-DYNA. One of the important considerations is the final thickness used by contact algorithms especially for shell elements. By default, LS-DYNA considers the minimum of thickness specified in the *SECTION_SHELL or from *ELEMENT_SHELL_THICKNESS and the 40% […]

Identifying Problem Areas for Poorly Converging Implicit Solutions

For non or poorly converging implicit solutions, the parameter D3ITCTL parameter in *CONTROL_IMPLICIT_SOLUTION may come handy to isolate regions of interest. When D3ITCTL is non-zero, LS-DYNA outputs the model information at each Iterative step into a binary file named D3ITER which is in the same format as D3PLOT. The number of steps for which the […]

Mapping of Deformed Nodal Coordinates for SubSystem/Component Analysis

Several analyses sometimes requires the mapping of nodal positions (coordinates) from a previous run for use in current run. For single component this is a rather easy task since it just involves writing a DYNAIN file for part(s) of interest using *INTEFACE_SPRINGBACK_LSDYNA keyword which would consist of final deformed nodal coordinates and element history variables. […]

Number of Shell History Variables in *INTERFACE_SPRINGBACK

*INTERFACE_SPRINGBACK provides an easy way to store a part’s state at the end of a simulation for later use. The part’s state consists of element history variables such as stress and strain tensor (in the form of *INITIAL_STRESS/*INITIAL_STRAIN) and also nodal values such as its final coordinates (*NODE). Optionally, when using the THICKNESS option, it […]

Built-In Filtered Nodal Acceleration Output Using IACCOP in *CONTROL_OUTPUT

Starting in LS-DYNA v971, nodal accelerations can be filtered before being output to the ASCII file “NODOUT”. The type of filter can either be a built-in filter based on low-pass Butterworth frequency filter or can be user-defined. The filtering process is turned on by setting the IACCOP=2 in *CONTROL_OUTPUT keyword. The filtering process works by […]

Contact Force Output to RCFORC

The ASCII file “RCFORC” contains the incremental forces contributed by contact algorithms. Due to the nature of the contact-impact interactions, the raw output tends to be very noisy and is of little value. To eliminate the inherent noise in the contact force output, LS-DYNA averages the force magnitude over the preceing output interval which results […]

Simulation Model Decomposition Using Recursive Coordinate Bisection (RCB) Method

In the area of distributed computing using Massively Parrallel Processing (MPP) LS-DYNA, finite element model decomposition is performed after initial processing of the input deck to “distribute” the model content to compute nodes. There are two primary goals for model decomposition. First goal is of of course to “break-down” the given problem into smaller pieces […]

Contact Modeling in LS-DYNA

This 4 part article, orignally published in the FEAINFORMATION newsletter between August and December 2001, can be downloaded here: Contact Modeling.pdf

Deformability Switching in LS-DYNA

Among several simple yet powerful techniques available in LS-DYNA, switching of bodies that affects their deformability (at run time) is certainly one of them. Switching of bodies with negligible internal strains to a rigidbody at user’s specification can be useful to eliminate the calculation of element-centered variables such as stresses, which in many applications could […]

Airbag Leakage Modeling in LS-DYNA

In the area of numerical simulations involving the use of airbags to absorb impact energy, passively or actively, accurate definitions of airbag leakage parameters play a crucial role in predicting the response of impacting objects (ex. occupants). LSDYNA provides various options for airbag leakage modeling that may appear overwhelming at first but are actually quite […]

Seamless Mesh Coarsening for Shell Elements in LS-DYNA

Element count in finite element simulations are increasing faster than the “Moores law” used in chips. There are several reasons for this increase among which the single-model-multi-loadcase philosophy is certainly one of them. Earlier approach of building load-case-dependent models were both cumbersome, data duplication and big challenges for design change integrations. In today’s simulations, one-model […]

One-Way and Two-Way Contact Definitions

This post will give you a brief summary of one-way and two-way contact definitions. The one-way/two-way refers to the treatment of slave nodes impacting the master segments. One-Way Contacts One-way was one of the first implementations in contact treatment and was done so such that it is computationally efficient. In the one-way contact treatment, the […]

Automatic Timestep Control for Implicit Simulations

The known advantage with implicit simulations is that the solution is unconditionally stable allowing larger values of timestep. In implicit static simulations (IMASS=0 in *CONTROL_IMPLICIT_DYNAMICS), the simulation time has no real significance but is rather an indication of the applied load magnitude. For example if we have a simulation where a load is applied linearly […]

Use of *SENSOR in LS-DYNA 971

A powerful new feature available in LS-DYNA version 971 is the *SENSOR keyword. To briefly summarize the Keyword Manual description, *SENSOR allows boundary conditions and various model entities to be activated/deactivated based on element, force, and/or node based criteria. Options are available for modeling very elaborate systems, but we will focus on a very simple […]

LS-DYNA Simulation Template

Using the newly implemented feature of embedding FORTRAN type expressions in LS-DYNA using the *PARAMETER_EXPRESSION keyword, it is becoming easier to develop analysis templates that can provide flexible ways of changing analysis settings. A partial input file highlighting the possibilities is shown below, and the full file is available here: main.k. Using this as a […]

Element Formulation Switch for Implicit

Under-integrated elements are widely popular in the explicit field for their robustness and computational efficiency. However, their use in Implicit without appropriate stabilization methods can cause singularities and may result in poor convergence behavior. LS-DYNA offers parameters such as ISHELL, ISOLID, and IBEAM in the keyword *CONTROL_IMLPICIT_EIGENVALUE which can be used to switch to an […]

Strain-rate Effects in Implicit

When migrating input files or using internal switching from Explicit to Implicit solution type, the influence of strain-rate effects, if defined for the materials used, should always be considered. If the problem is intended to be static or quasi-static during the implicit solution, it is recommended that strain-rate effects be ignored. LS-DYNA now offers a […]

Dynamic Solution Type using IMFLAG in *CONTROL_IMPLICIT_GENERAL

There are two classical solution methods available in LS-DYNA to solve a given problem. The widely popular “Explicitâ€? solution scheme, using the central-difference method, is applied to short-duration transient dynamic problems while the “Implicitâ€? solution scheme is used for static problems. Both of these methods have their advantages and disadvantages which depend entirely on the […]

Ensuring Stable, Robust, and Accurate LS-DYNA Models

LS-DYNA simulation models, both large and small, frequently suffer from undetected modeling errors that can cause runs to die prematurely. Sometimes even minor model changes can introduce stability issues indicating a lack of robustness. Runs that die prematurely squander significant resources, something which I generally refer to as “simulation wasteâ€?. A recently developed procedure that […]

Include File Listing

Added support to view the name and type of included files defined in the main input file. Here is a snapshot:

Title Customization

Job Titles can now be easily customized as demonstrated below.

Summary View Customization

Implemented draggable module windows so users can customize their summary view. This feature is demonstrated in the image below.

Model Notes Module

When a model is post-processed, its often very difficult to repeat it when viewed again at a later time since we may have moved on to a different model and would have lost track of the model history. To document a model as soon as we review its results, a “Web Observer” is now made […]

Web Browser to Paper

Implemented feature to print using any standard web browser. Just use the browser’s print option to print any page of the d3hsp tabs. The web view and the print view should be nearly identical. There will be continuing improvements to make the print view consistent with the web view. If you find any major differences, […]

Problem with IE has been fixed

Just wanted to update that the navigational problem when using Internet Explorer has been fixed.

LS-DYNA Warning Messages

Started implementing an interface to view warning messages reported by LS-DYNA. Future upgrades will include more association between warnings and their related entities.

Airbag Definitions

Airbag or Control Volume definitions are now displayed as shown below. Currently, only the general parameters are shown but eventually, all the parameters based on the inflation models will be rendered along with appropriate curve defintions such as mass-flow, temperature, etc…

Export Bill of Materials

Added new feature to support the export of bill of materials (BOM). The current order of the columns are fixed but will later be allowed to be customized. BOM can be exported in either CommaSeperatedValues(CSV)/Excel or a well-formed XML file.