Elements LSDYNA

8/17/2019 Elements LSDYNA

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M.S. Ramaiah School of Advanced Studies, Bengaluru

Elements in LS-DYNA

Session delivered by:Session delivered by:

Mr.Suman M.L.J.Mr.Suman M.L.J.



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Element Library available in LS-Dyna

Element formulation

Hourglassing

Negative volume

Mass Scaling

Session Topics

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ELEMENTS AVAILABLE IN DYNA• Different solid elements

• 8-node thick shells• Different 3- and 4-node shells

• Beams

• Welds

• Trusses and cables

• Nodal masses

• Lumped inertias• Arbitrary Lagrangian/Eulerian

elements

• Eulerian elements

• Element Free Galerkin

formulations

• SPH elements• Elements for 2D-analysis

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•Accuracy requirements

• speed requirements

• type of material to model

• type of simulation

The choice of Element Formulations

depends on

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Description of Elements

Purpose: To define section properties for solid continuum and fluid

elements

• 8-node solid element by default uses one point integration plus

viscous hourglass control.

• Fully integrated brick elements are also available. They perform

better where element distortions are large (like soft materials, such as

foam). but are about four times more costly.

•When full integration is used no hourglass control is needed, as

there are no zero-energy modes.

•Wedges and tetrahedral are simply degenerate bricks (i.e. some of

the nodes are repeated). They cause problems in some situations sothese type of solid elements are avoided

1. Solids

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Solid Element formulation options

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2. Shells

Purpose: Define section properties for shell elements

Belytschko Tsay element (B-T):

•Default shell element is the Belytschko Tsay (B-T) element.It uses

reduced one-point integration

•Not recommended when element experiences excessive warping.

• Hughes Liu:Hughes Liu (HL) element available in reducedintegration and fully integrated formulations. Substantially slower than

B-T formulation

S/R Co-rotational Hughes-Liu: This type of formulation uses fully

integrated element, so hourglass deformations does not occur (but

much more costly).

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Shell Element formulation options

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HOURGLASSING Hourglassing is a zero energy mode of deformation that

oscillates at a frequency much higher than the structure’s globalresponse.

Hourglassing typically have no stiffness and give a zig zagdeformation appearance to a mesh.

Undesirable phenomenon that occurs due to reduced integration(single point).

The expression “full

integration” refers to the

number of Gauss points

required to integrate the

polynomial terms in an

element's stiffness matrix

exactly when the element has

a regular shape.

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Integration

The stiffness and mass of an element are calculated

numerically at sampling points called “integration points”

within the element.

The numerical algorithm used to integrate these

variables influences how an element behaves.

Dyna includes elements with both “full” and “reduced”

integration.

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Affects brick, quadrilateral shell and 2-D elements.

It Can be eliminated through full integration

Can be identified through the hourglass energy reported in

the d3hsp file and other output files.

Should normally be less than 5% of deformation energy

Hourglass control brings additional stiffness or viscous

damping to prevent such modes.

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Element with hour glassing mode

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MINIMIZING HOURGLASSING1. Avoid Single point loads

- Single point loads are known to excite hourglass modes.

Since one excited element transfers the mode to its

neighbors, point loads should not be applied.

2. Use fully integrated elements

- Fully integrated elements do not experience Hourglassingmodes. Hourglass control implemented through the use of

the keyword *HOURGLASS section

3. Globally adjust the models bulk viscosity

- Hourglass deformations are resisted by a structures bulk viscosity. It is possible to increase the bulk viscosity of a

model by using various Hourglass viscosity type which is as

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4. Globally adding elastic stiffness

- Hourglassing can be eliminated by adding elastic stiffness.

This can be done for the entire model by increasing theHourglassing coefficient

5. Can normally be minimized through good modeling

practices

6. Use of a uniform mesh (i.e, Mesh refinement in general)

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NEGATIVE VOLUMES

•In materials that undergo extremely large deformations, such as

soft foams, an element may become so distorted that the volume

of the element is calculated as negative.

•This may occur without the material reaching a failure criterion.

There is an inherent limit to how much deformation aLagrangian mesh can accommodate without some sort of mesh

smoothing or remeshing taking place.

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•A negative volume calculation in LS-DYNA will cause the

calculation to terminate unless ERODE in *CONTROL_TIMESTEP

is set to 1 and

•DTMIN in *CONTROL_TERMINATION is set to any nonzero

value in which case the offending element is deleted and the

calculation continues (in most cases).

•Even with ERODE and DTMIN set as described, a negative volume

may cause an error termination.

How to control Negative volume

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Some approaches that can help to overcome negative

volumes include the following

1. In many cases, the problem lies in stress strain curve

2. Simply stiffen up the material stress-strain curve at large

strains. This approach can be quite effective.

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3. Avoid fully-integrated solids (formulations 2 and 3)

which tend to be less stable in situations involving large

deformation4. Use the default element formulation (1 point solid) with

type 4 or 5 hourglass control (will stiffen response).

Preferred hourglass formulations for foams are:

- type 6 with coeficient = 1.0 if low velocity impact- type 2 or 3 if high velocity impact

5. Model the foam with tetrahedral elements using solid

element formulation 10 although this approach may

give an overly stiff response.

6. Increase the DAMP parameter (foam model 57) to the

maximum recommended value of 0.5.

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MASS SCALING

• When FE model contains a few small or stiff elements, theefficiency of explicit time integration method is compromised

severely, since the time step of the entire mesh is set by these

very stiff elements. Several techniques are available for

overcoming this difficulty.

• The masses of stiffer elements are increased so that the time

step is not decreased.

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Activated primarily through the use of the DT2MS parameter

available with the keyword CONTROL_TIMESTEP

Positive DT2MS values for quasi-static analyses or time

history analyses with negligible inertial effects

Negative DT2MS values imply mass scaling will be

implemented if time step values fall to lower than TSSFAC*DT2MS

When the dynamic effect is big, such as in crash forming

simulation. In this case, less mass scaling and low punch

velocity should be used.

How to control Mass scaling

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RIGID BODY DEFINITION

RRZZRRYY

RRXX

ddzz ddyy

ddxx

)f(R )f(dd CGCGn

A rigid body cannot deform.

Rigid body has 6 degrees of freedom, 3 transnationaland 3 rotational.

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RULES FOR RIGID BODIES:

• Two rigid bodies cannot share the same node

• Constraints must be applied to part or all nodes.

nn

M1M1

F1F1

CG1CG1

M2M2

F2F2

CG2CG2

MM

CGCG

FaFa

FbFbn1n1

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RIGID BODY DEFINITION

FEATURES FOR RIGID BODIES:

• Extra nodes can be assigned to rigid bodies.

• Rigid bodies can be merged, i.e. slaved to each other.

• Rigid bodies can be connected by joints.

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•Avoid small elements whenever possible as they will

significantly reduce the time step size.If small elements are

required,use mass scaling.

• Minimize the use of triangular/tetrahedron/prism elements.

Although these elements are supported,they are highly not

recommended.

• Avoid acute angled elements and warped shells, as they will

degrade the accuracy of the results.

• Fully integrated elements can be defined in regions of amodel where hourglass control is needed.

GENERAL ELEMENT

GUIDELINES

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Material library of LS-Dyna

Session delivered by:Session delivered by:

Mr.Suman M.L.J.Mr.Suman M.L.J.



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Overview of material models in LS-Dyna

Brief description of various material models

Session Topics

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MATERIAL MODELS AVAILABLE IN DYNA

1. Provide Constitutive equations for more than 120 material

models

2. Default parameters from best practices

3. Material Models

• Elastic

• Elastic-Plastic

• Viscoelastic

• Rubber

• Foams

• Composites and many more….

4. SECTIONS

•Solids

•Shells

•Bars

•thick shells

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MATERIAL LIBRARY AVAILABLE IN

LS-DYNALinear Elastic Models

•Isotropic (MAT1)

•Orthotropic (MAT2)

•Anisotropic (MAT2)

Nonlinear Elastic Models

•Blatz-Ko Rubber (MAT7)

•Mooney-Rivlin Rubber (MAT27)

•Viscoelastic (MAT6)

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Plasticity Models

•Bilinear Isotropic (MAT3)

•Temperature Dependent Bilinear Isotropic (MAT4)

•Bilinear Kinematic (MAT3)

•Plastic Kinematic (MAT3)

•Powerlaw Plasticity (MAT18)

•Rate Sensitive Powerlaw Plasticity (MAT64)

•Strain Rate Dependent Plasticity (MAT19)

•Piecewise Linear Plasticity (MAT24)

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Foam Models

•Low Density Foam (MAT57)

•Viscous Foam (MAT62)

•Mooney-Rivlin Rubber (MAT27)

•Viscoelastic (MAT6)

Spring Damper Models•Linear Elastic Spring (MAT18)

•Linear Viscous Damper

•Nonlinear Elastic Spring

•Nonlinear Viscous Damper

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•Elasto-plastic spring

•General Nonlinear Spring

Composite Models

•Composite Damage (MAT22)

•Enhance Composite Damage(MAT54-55)

•Laminated composite Fabric (MAT58)

Others

•Rigid (MAT20)

•Cable (MAT71)

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1.Linear Elastic Material Models

There are three different material models available in the linear

elastic family:

• Isotropic: Material properties are same in all directions.

• Orthotropic: properties have 3 mutually orthogonal planes of

symmetry

• Anisotropic: properties are independent of position at a point

within a material

Linear elastic materials do not undergo any plastic deformations and

are fully defined by generalized Hooke’s law

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1.1 MAT_ ELASTIC

This is Material Type1.This is an isotropic elastic material and is

available for beam,shell and solid elements in LS-DYNA. This typeof material is also used for modeling of fluids.

Card Format used:

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1.2 MAT_ OPTION TROPIC_ELASTIC

This is Material Type 2. This material is valid for modeling the elastic-orthotropic behavior of solids,shells and thick shells.Anisotropic

option is available for solid elements.

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2. Nonlinear Elastic Material Models

There are three different material models available in the non-

linear elastic family:

•Blatz-Ko Rubber: Used for compressible foam-type

materials such as polyurethane rubbers.

•Mooney-Rivlin Rubber: Used to define behavior of

incompressible rubber materials

•Viscoelastic: Defines the behavior of glass and glass-like

materials.

Non-linear elastic materials can undergo large recoverable elastic

deformations

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• MAT_BLATZ-KO_RUBBER

- This is material Type 7.This material allows the modeling ofnearly incompressible continuum rubber

- Here the Poisson's ratio is fixed to 0.463

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• MAT_VISCOELASTIC

This is material Type 6. This model allows the modeling ofviscoelastic behavior for beams (Hughes-Liu),shells,and solids.

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• MAT_PLASTIC_KINEMATIC

- This is Material Type 3. This model is suited to model isotropicand kinematic hardening plasticity with the option of including

rate effects.

- It is a very cost effective model and is available for

beam(Hughes-Liu),shell and solid elements.

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3. Plasticity Material Models

• There are different plasticity models available in LS-DYNA

• The selection of a specific model depends on the type of material

being analyzed and the availability of material constants.

• It is very important to select the correct category for the material

being analyzed. It is less important to select the specific model withina category, which is usually controlled by the material data available.

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• MAT_PIECEWISE_LINEAR_PLASTICITY

- This is Material Type 24. It is an elasto-plastic material with an

arbitrary stress verses strain curve and arbitrary strain rate

dependency is defined.

- Here failure based on a plastic strain or a minimum time step

size can be defined

p

200 0.0

220 0.0002

235 0.0008

245 0.002

250 0.005

252 0.010

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4.Foam Material models

•There are different foam models available in the LS-DYNA

program.

•The selection of a specific model depends on the type of

material being analyzed.

•All of the foam models in LS-DYNA are primarily used inautomotive impact applications.

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• MAT_LOW_DENSITY_VISCOUS_FOAM

- This is Material Type 73.It is mainly for Modeling Low Density

Urethane Foam with high compressibility and with rate sensitivity.

- Its main applications are for seat cushions,padding on the side impact

Dummies (SID),bumpers and interior foams.

- Optionally, a tension cut-off failure can be defined

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• MAT_VISCOUS_FOAM

- This is Material Type 62. This type of Material represents the Con-

Foam on the ribs of EuroSID side impact dummy.

- It is only valid for solid elements,mainly under compressive loading

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• MAT_HONEYCOMB

-This is Material Type 26. The major use of this material model is

for honeycomb and foam materials with real anisotropic behavior.

- A nonlinear elastoplastic material behavior can be defined

separately for all normal and shear stresses.

- This type of material model is developed for the front end material

of a side impact bumper and for aerospace structures.

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• MAT_COMPOSITE_DAMAGE

-This is Material Type 22. This model is developed for failure of

Composite materials which is used for energy absorption.

- An orthotropic material with optional brittle failure for composites

can be defined.

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• MAT_RIGID

- This is Material Type 20.Parts made from this material are considered

to belong to a rigid body.- The coupling of a rigid body with MADYMO can be defined via this

material.

- Here global and local constraints on the mass center can be optionally

defined.

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• MAT_FABRIC

-This is Material Type 34.This material is especially developed

for airbag materials.

-This model is more suited when the fabrics experiences large

deformation.

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• MAT_CABLE_DISCRETE_BEAM

-This is Material Type 71.This model permits elastic cables

to be realistically modeled.

- In this model during compression no forces are developed

MID Material Identification

RO Mass density

E GT 0.0: Young's modulus

LCID Load curve ID

F0 Initial tensile force69

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M S R i h S h l f Ad d St di B l

Material Models - Guidelines

• Not all material models are available for every element

type.Check the Elements Manual to see which models can be used.

• For each material model, not all constants and options are required

for input.For example,failure strains can be incorporated into a

material that does not have strain rate effects by setting the Cowper-

Symonda constants to zero.

• Make sure to use consistent units when defining your material

properties.Incorrect units will not only effect the material

response,but will also effect the contact stiffness.

•Don’t underestimate the importance of having accurate material

data. Spend the extra time and money to obtain accurate materialdata.

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Elements LSDYNA

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