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, the attached document provides a list of best practices that can be used to improve simulations. Please note that the recommendations in the document are based on experience and apply to the software version that existed at its original publication date. It must be noted that the guidelines, if implemented in simulation models, do not always guarantee stability, accuracy, and robustness as they depend on a number other factors as well. The document was prepared by my dear colleague, Mr. Jim Day, and myself. Crash_Guidelines.pdf ...

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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 is a basic unit of data which can be transmitted and can be either 0 or 1. A sequence of 4 bits is called as a Nibble and a sequence of 8 bits is called as one 'Byte'. 16-bits or 2 bytes is equal to one word on a 16-bit computer. 32-bit or 4 bytes is equal to one word on a 32-bit computer. 64-bit or 8 bytes is equal to one word. Memory Settings for SMP LS-DYNA In Shared Memory Parallel (SMP), multiple CPUs solve a given problem in parallel using shared memory as shown below. To allocate sufficient memory, we can specify "memory=XXX" in the execution line where XXX refers to the size of the ...

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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 compression or tension data is provided, LS-DYNA extrapolates the slope, obtained from the first two pair of points from the defined region, and uses it to compute the force for any deformation in the undefined region. The default cross-region extrapolation may be inaccurate when the behavior of the discrete spring in the undefined region is supposed to represent either a symmetric or unsymmetric non-linear behavior. For representing both types of data, symmetric or unsymmetric , in tensile and compressive regions, the user must always provide a force-displ ...

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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 depends on the maximum frequency of all joints in the model, timestep and the timestep-scale-factor (TSSFAC). The default penalty formulation computes the necessary forces in an attempt to model the joint behavior based on the joint type but in some instances the forces generated by the penalty treatment may be insufficient to maintain the constraint. As a good practice, it is always recommended to monitor the relative displacements between the joint nodes to see if there is any separation which would ind ...

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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 if its not available, it will scan all the directories listed using INCLUDE_PATH in the order of definition. This feature eliminates the need to specify absolute file names. A sample use of this feature is shown below. (Click image to enlarge) ...

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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 consider a simple example of a rigid steel ball (solid elements) impacting a block of foam (solid elements). To model the interaction between the rigid block and the foam, a two-way contact such as *AUTOMATIC_SURFACE_TO_SURFACE is included. The foam is constrained using SPC definitions on the bottom face. The complete model set up is as shown in Figure 1. The recoverable low density foam is modeled using *MAT_LOW_DENSITY material model whose inputs include density, elastic-modulus, and a load c ...

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Simulation Based Product Design Using LS-DYNA – Single Code & Single Model Benefits

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 is an expertise that is developed over time and goes beyond the ability to learning specific software. It is an awareness in which an analyst, and in essence the corporation, gains expertise in a single or multiple numerical tool set which is used and calibrated against established prototype results to gain more confidence in the modeling procedures and the numerical intricacies involved in a complex software such as LS-DYNA. Design domain knowledge is again an expertise which is gained by a deep understanding of the product and ...

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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 to achieve better convergence since rigidbody modes has negligible effect on the convergence. In versions prior to 971 (revision 3), when IMAS=1, LS-DYNA included dynamic terms for the entire duration of the problem. Consequently, when using the implicit dynamics solver for solving static or quasi-static problems, the loading rate would have be reduced substantially to reduce the dynamic effects. Starting from 971 (revision 3), LS-DYNA now allows birth, death and burial times for the implicit solver ...

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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 with the default set to no mass-scaling. When DT2MS is less than zero, LS-DYNA adds mass of each element whose timestep is below abs(DT2MS) such that the element's updated DT is equal to abs(DT2MS). When DT2MS is greater than zero, LS-DYNA adds mass to elements whose DT is below abs(DT2MS) and "removes" mass from elements whose DT is greater than zero. DT2MS>0 is seldom used while DTM2<0 is frequently used for overcoming the smallest computed timestep. Care must be taken when using DT2MS<0 to ensure that the added mass d ...

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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 polynomial of any order upto a maximum of 30 using the general expression label "POLY" (Please note that the current ls-dyna manual shows this as POLYL which is incorrect) and its argument list is given by: POLY(abscissa, abscissa0, coeff0, coeff1, coeff2,..., coeff30) or POLY(x, x0, a1, a2,..., a30) The built-in expression used is: P(abscissa) = coeff0+coeff1*(abscissa-abscissa0)+coeff2*(abscissa-abscissa0)*power(2)+... or P(x) = a0+a1(x-x0)+a2*(x-x0)*power(2)+...+an*(x-x0)*power(n) Below is an example that prescribes a displacement to a node us ...

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