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CHAPTER-10

DYNAMIC SIMULATION USING LS-DYNA

10.1 Introduction

In the past few decades, the Finite Element Method (FEM) has been developed

into a key indispensable technology in the modeling and simulation of various

engineering systems. In the development of an advanced engineering system,

engineers have to go through a very rigorous process of modeling, simulation,

visualization, analysis, designing, prototyping, testing and finally fabrication. As such,

techniques related to modeling and simulation in a rapid and effective way play an

increasingly important role in building advanced engineering systems, and therefore

the application of the FEM has multiplied rapidly. Commercial softwares, like

PAM-Crash, implement algorithms that include modeling of

contact and are capable of simulating impact conditions.

10.2 Hypermesh Software

Altair Hypermesh is a high-performance finite element pre-and post-processor

tool for major finite element solvers, allowing engineers to analyze design conditions

in a highly interactive and visual environment. Some of the benefits of Hypermesh are:

Reduces time and engineering analysis cost through high-performance finite

element modeling and post-processing

Reduces learning time and improve productivity with an intuitive user-interface

and best-in-class functionality

Reduces redundancy and model development costs through the direct use of

CAD geometry and existing finite element models.

To simplify the modeling process for complex geometry through high-speed,

high-quality auto-meshing.

In this chapter, finite element analysis is carried out using LS-Dyna finite

element code to predict the load-displacement response of sandwich panels and its

component materials, silk-cotton wood and honeycomb core subjected to dynamic

loading. The FE models of these were constructed using Hypermesh pre-processor.

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Processing is done using LS-Dyna and post-prossing is carried out using Hyperworks

software. The dynamic simulation results of these materials are then compared with

their respective dynamic experimental results for degree of convergence.

10.3 General Methodology for Dynamic Simulation

LS-Dyna explicit methods are those in which the information at time step n+1

can be obtained in terms of previous time steps and there is no dependence on the

current time step. Explicit methods are very fast as there is no matrix inversion and the

mass matrix is lumped or diagonal. Linear and highly non linear problems can be

effectively solved. This is the reason that, explicit codes are being used for crash

analysis.

The dynamic FE simulation involves the following steps

a) Part definition

b) FE meshing and material properties

c) Contact analysis

d) Boundary conditions and loading

e) Results

10.4 Dynamic simulation of silk-cotton wood

The compressive stress-strain behavior of wood and crushable foam is

identical [1]. Therefore, crushable foam model has been idealized as wood material

model in the present study. The quasi-static test properties of silk-cotton wood along

the grains are input to the material model. 3-D model geometry was constructed for the

nominal dimensions of 45 x 45 x 45 mm with Hypermesh pre-processor.

10.4.1 FE Meshing and Material properties

Following input data is used for developing FE model for silk-cotton wood.

LS-Dyna material type : 63 (MAT_CRUSHABLE_FOAM)

This material is used for modeling crushable foam with

optional damping. Unloading is fully elastic. The behavior

is treated as perfectly elastic-plastic.

Density : 327 kg/m3

: 7.35 GPa

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Ratio : 0.23

Yield Stress : 350 MPa

Element Size : 2.5 mm

Number of integration points through thickness: 02

Specimen Length : 16 mm (each)

Element Type : 8-Node Hexahedron solid

No. of Elements : 9800

No. of Nodes : 24,080

FE discretization of the wood model was done using two point integration

8-nodal hexahedron elements. Figure (10.1) shows the sketch of an 8-nodal

hexahedron element which is used to mesh wood model, each node of the

element has three degree of freedoms.

Figure 10.1 Eight nodal hexahedron element which is used to mesh the wood model

10.4.2 FE Model Data for Impactor Plate and Base Plates

The velocity input is effected through a defined rigid impactor plate. The

specimen is supported on a rigid base plate. The same material model is used to

construct both the impactor and the base plates. Figure (10.2) shows the meshed model

of silk-cotton wood with the impactor and base plates. The material model details are

as below.

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LS_Dyna material type : 20 (*MAT Rigid)

Parts made of this material are considered to be a rigid

Ratio : 0.3

Element Type : No: 2, Elastic form shell

No. of Elements : 70,000

No. of Nodes : 60,000

Figure 10.2 Meshed model of silk-cotton wood with impactor and base plates

10.4.3 Contact Treatment

Contact occurs between the components or within the component itself when

the components try to come towards each other during the plastic deformation. When

the components touch each other, a force is transmitted across the common interface

between them due to friction. This gives rise to a contact pressure and shear stress. A

high end computational process is required due to the sever discontinuity with respect

to boundary conditions. If there were no contacts defined between the components,

then the components would simply penetrate into each other and this is unphysical.

LS-Dyna software by default has number of defined contact algorithms to detect and

establish the contact between the components automatically. The following line

command enables the automatic contact between the components. The present analysis

uses the, contact type: Contact_Automatic_General. This establishes the physical

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contact automatically between the impactor, wood, within the wood itself and with

base plate.

10.4.4 Boundary conditions

The base plate is constrained in all the directions

Enabled the automatic contact generation

The velocity input of 3.8 m/s is given through the command in the load

collector

Figure 10.3 shows the simulated model of silk-cotton wood specimen.

Figure 10.3 Simulated model of silk-cotton wood specimen

10.5 Dynamic Simulation of Aluminum Honeycomb

The model geometry of honeycomb was constructed for (100 x 100 x 50) mm

nominal size with Hyper mesh pre-processor. Figure (10.4) shows the 3-D geometric

model of aluminum honeycomb. LS-Dyna defined honeycomb material model 24 is

selected for the present study to accommodate the elastic-plastic behavior of the

honeycomb material. This model represents the piecewise linear plasticity to consider

the strain hardening modulus. The quasi-static test properties of aluminum honeycomb

are input to the material model.

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Figure 10.4 Geometric model of aluminum honeycomb specimen

10.5.1 FE Meshing and Material properties

The FE material model input data are as below.

LS-Dyna material type : 24 (*MAT_PIECEWISE LINEAR_PLASTICITY) This model is for Elasto-plastic material with an

arbitrary stress-strain curve and arbitrary strain rate dependency.

Density : 2700 kg/m3

: 69 GPa

Ratio : 0.3

Yield Stress : 165 MPa

Element Type : Co_rotational, Belystchko Lin Tsay Shell

Element Size : 2 mm

Number of integration points through thickness: 02

Thickness of the Shell : 0.068mm.

Specimen Length : 50 mm

FE discretization of the honeycomb model was done using two point integration

4- nodal shell elements. Figure (10.5) shows the sketch of a 4-nodal

shell element which is used to mesh honeycomb model. Each node of the element has

6 degree of freedom, three translational and three rotational.

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Figure 10.5 4-nodal shell element used for meshing of honeycomb model.

10.5.2 FE Model of Honeycomb

Figures (10.6) shows the meshed model of aluminum honeycomb along with

the impactor and base plates

Figure 10.6 Meshed model of aluminum honeycomb with impactor and base plates

10.5.3 Boundary Conditions

1. The specimen was supported on a rigid surface (base plate).

2. Base plate was fixed in all directions

3. The impactor plate is constrained to move only in Z-direction

4. Enabled the automatic contact generation

5. The velocity input of 4.7 m/s is given through the command in the load

collector

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Figure 10.7 Simulated aluminum honeycomb specimen

10.6 Dynamic Simulation of Sandwich Panel

10.6.1 Part Definition

The model geometry was constructed with Hyper mesh pre-processor. The

FE model of the sandwich panel consists of a base plate, two silk-cotton wood skins,

honeycomb core and an impactor plate.

A 3-D geometric model was constructed for the similar dimensions to that of

experimental specimens of sandwich panels. Figure (10.8) shows the geometric model

of the sandwich panel.

Figure 10.8 Geometric model of the sandwich panel.

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10.6.2 FE Meshing and Material Properties

The model is then meshed with Hypermesh pre-processor. The procedure was

repeated for FE meshing of the components of the sandwich panel.

10.6.3 FE Model for Sandwich Panel

Figure (10.9) Shows meshed model of sandwich panel with base and impactor

plates and Figure (10.10) shows simulated model of the crushed specimen.

Figure 10.9 FE Model of sandwich panel with base plate and impactor plates

10.6.4 Boundary Conditions and Loading

1. The specimen was supported on a rigid surface (base plate).

2. Base plate was constrained in all directions

3. The impactor plate is constrained to move only in Z-direction.

4. For each run, one face of the top wood skin was constrained for all degrees of

freedom.

5. Enabled the automatic contact generation

6. The impactor plate was given with a velocity of 6.7 m/s.

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Figure 10.10 Simulated model of sandwich panel specimen

10.7 Results and Discussion

Figures [10.11 to 10.13] shows the comparative load-displacement responses

obtained through the dynamic simulation of silk-cotton wood, aluminum honeycomb

and Type-I sandwich panels respectively. The simulated behavior of these materials

closely resembles when compared with the behavior of these materials tested under

dynamic experiments. Table 10.1 shows the comparison of results obtained by

FE dynamic simulation and dynamic experiment. The results of dynamic simulation

shows a decreasing trend when compared with the experimental results. For the case of

silk-cotton wood, aluminum honeycomb, and Type-I sandwich panel the decrease in

the energy absorption capacity with respective to the experiment is about 6.5%, 4%

and 4.4% respectively.

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Figure 10.11 Comparative load-displacement responses of wood using simulation and experiment.

Figure 10.12 Comparative load-displacement responses of aluminum honeycomb using simulation and experiment.

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Figure 10.13 Comparative load-displacement responses of Type-I Panel using simulation and experiment.

Table 10.1 Comparison of simulation and the dynamic experimental results

Cellular Material/Structure Energy Absorbed (J)

Dynamic Experiment

Dynamic Simulation

Silk-cotton wood (along the grain) 472 441

Honeycomb (out-of-plane) 457 439

Sandwich panel (Type-I) 976 933