Mastering LS-DYNA input files is essential for anyone working with explicit dynamics simulations. This comprehensive guide walks you through the fundamental keyword commands and syntax rules that form the backbone of every LS-DYNA model. Whether you are just getting started or looking to solidify your understanding, this tutorial provides practical insights into building robust finite element models for crash analysis, impact simulations, and beyond.
1. Keyword Syntax Fundamentals
LS-DYNA operates on a keyword-driven architecture where each command begins with an asterisk (*). Understanding these syntax rules is crucial for creating error-free input files:
- All keywords start with an asterisk (*) and are case-insensitive
- Data lines support both fixed-format and free-format styles
- Comment lines begin with a dollar sign ($)
- Each keyword block terminates at the next keyword or end of file
2. Essential Keywords Every Model Needs
A valid LS-DYNA input file requires these core keywords to define your simulation:
*KEYWORD
Marks the beginning of the input file and must appear on the first line.
*CONTROL_TERMINATION
Specifies when your simulation should stop, typically using end time or cycle count.
*NODE
Defines the coordinates of every node in your finite element mesh, the foundation of your geometry.
*ELEMENT_ (Various Types)
Connects nodes to form elements: *ELEMENT_SOLID for 3D solids, *ELEMENT_SHELL for surfaces, and more.
*MAT_ (Material Models)
Assigns material properties to your components, everything from simple elastic materials to complex composites.
*SECTION_
Defines cross-sectional properties like shell thickness or beam cross-sections.
*PART
The bridge between geometry and physics. It combines material, section, and element type into a single component.
*DATABASE_BINARY_D3PLOT
Controls output of binary result files for post-processing visualization.
*END
Signals the end of the input file.
3. Defining Your Mesh: The *NODE Keyword
The *NODE keyword establishes your computational grid by defining node IDs and their spatial coordinates:
*NODE
NID X Y Z
1 0.0 0.0 0.0
2 1.0 0.0 0.0Pro tip: Node IDs do not need to be sequential, but keeping them organized makes debugging much easier when your model grows complex.
4. Building Components with *PART
Think of *PART as the assembly station where your model comes together. It links the geometric definition (elements) with physical properties (material and section):
*PART
Part Name (descriptive)
PID SID MID
1 1 1Where PID = Part ID, SID = Section ID, and MID = Material ID. This modular approach lets you reuse materials and sections across multiple parts.
5. Element Types and Connectivity
LS-DYNA supports diverse element formulations through the *ELEMENT_ keyword family. Choose the right element type for your physics:
- *ELEMENT_SOLID – 8-node hexahedrons or 4-node tetrahedrons for 3D continuum analysis
- *ELEMENT_SHELL – 4-node quadrilaterals or 3-node triangles for thin-walled structures
- *ELEMENT_BEAM – 2-node beams for structural members and reinforcements
- *ELEMENT_DISCRETE – Spring/damper elements for connections and discrete systems
6. Material Models: *MAT_ Library
LS-DYNA’s extensive material library covers everything from metals to foams to biological tissues. Here are the workhorse materials you will use most often:
- *MAT_ELASTIC (MAT_001) – Linear elastic behavior for metals well below yield
- *MAT_PLASTIC_KINEMATIC (MAT_003) – Classic bilinear plasticity with kinematic hardening, perfect for crash simulations
- *MAT_ELASTIC_PLASTIC_HYDRO (MAT_010) – For materials under high-pressure loading where hydrostatic effects matter
- *MAT_HIGH_EXPLOSIVE_BURN (MAT_008) – JWL equation of state for explosive detonation modeling
- *MAT_NULL (MAT_009) – For fluids, gases, and explosive products where strength is negligible
7. Section Properties: *SECTION_ Keywords
Section definitions provide the geometric properties that complement material behavior:
- *SECTION_SOLID – Defines solid element formulation (constant stress, fully integrated, etc.)
- *SECTION_SHELL – Specifies shell thickness, number of integration points, and hourglass control
- *SECTION_BEAM – Sets beam cross-section geometry and integration rules
- *SECTION_DISCRETE – Defines discrete element properties like spring stiffness and damping
8. Output Control with *DATABASE_BINARY_
Managing your simulation output is critical for efficient post-processing. These keywords control binary result files:
- *DATABASE_BINARY_D3PLOT – The primary visualization file containing geometry, stresses, strains, and more for animation
- *DATABASE_BINARY_D3THDT – Time history data for detailed temporal analysis
- *DATABASE_BINARY_RUNRSF – Restart files for continuing interrupted simulations
- *DATABASE_BINARY_D3DUMP – Core dump files for debugging solver crashes
9. Best Practices for Output Management
Smart output settings save disk space and post-processing time:
- Set appropriate output intervals – Too frequent creates massive files; too sparse misses important physics
- Use *DATABASE_EXTENT_BINARY – Filter which variables get written to reduce file size
- Leverage *DATABASE_HISTORY_ – Extract time histories for specific nodes or elements instead of entire model
- Configure restart dumps – Protect against unexpected crashes by saving intermediate states
10. Fine-Tuning Output with *DATABASE_EXTENT_BINARY
This powerful keyword lets you customize exactly what goes into your D3PLOT files. Key parameters include:
- NEIPH – Number of extra history variables for solid elements
- NEIPS – Number of extra history variables for shell elements
- MAXINT – Number of shell integration points to output
- STRFLG – Flag to include strain tensor output
11. ASCII Output with *DATABASE_ Keywords
For numerical data extraction and plotting, ASCII output files are invaluable:
- *DATABASE_GLSTAT – Global statistics: energy balance, time step, mass scaling info
- *DATABASE_MATSUM – Energy breakdown by material (kinetic, internal, hourglass)
- *DATABASE_NODOUT – Nodal displacements, velocities, accelerations
- *DATABASE_ELOUT – Element stresses, strains, and resultants
- *DATABASE_RCFORC – Contact interface forces
- *DATABASE_SPCFORC – Single-point constraint (boundary condition) reaction forces
Conclusion
Understanding LS-DYNA input file structure and keyword commands is the foundation of successful explicit dynamics simulation. By mastering these essential keywords, from defining your mesh with *NODE to controlling output with *DATABASE_BINARY_, you will be well-equipped to build, debug, and analyze your own finite element models.
Start with simple models and gradually incorporate more complex features. The modular nature of LS-DYNA’s keyword system means you can build sophistication incrementally while maintaining confidence in your results.
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