Unrestricted © Siemens AG 2014

FEMAP SYMPOSIUM 2014

Discover New Insights Femap Symposium 2014

May 14-16, Atlanta, GA, USA

Femap and LS-DYNA: Rocket

Science Made Easy

George Laird, Ph.D., P.E., Principal Mechanical Engineer

Adrian Jensen, EIT, Sr. Staff Mechanical Engineer

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Agenda LS-DYNA: Unified Numerical Architecture for FEA

• Focus of Presentation

(Abstract)

• Why Explicit

• Implicit versus Explicit

• Element Quality is Paramount

• Building LS-DYNA Models

• Best Practices

• LS-DYNA Unified Solver

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Femap + LS-DYNA:

Rocket Science Made Easy

ABSTRACT

LS-DYNA is well-known in the world of transient, dynamic nonlinear

analyses but is often perceived as a difficult code to use on a daily basis

for general implicit-type work that more standard implicit codes have

dominated for years. To address this perception gap, this paper

demonstrates how easily Femap can be used to drive LS-DYNA in the

solution of simple linear static analyses all the way to the most complex

simulations involving aviation seats and satellites. Along the way, the

capabilities of Femap and LS-DYNA will be showcased to show how

one analysis package can solve problems from rigid-body dynamics to

normal modes analysis (PSD) to complete system level models. Case

studies will be used to highlight the effectiveness of this combination.

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Why Explicit?

LS-DYNA: Unified Numerical Architecture

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Explicit Solves Where Implicit Can’t

Femap FEA Model FEA + LS-DYNA Model Validation is Gold

16g Seat Crash Test

TSO-C127a / SAE AS8049A / 14 CFR Part 25.562

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Explicit Solves Where Implicit Can’t

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Explicit Solves Where Implicit Can’t

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Explicit Solves Where Implicit Can’t

Cargo Net Development

Impact Analysis

Impact of Plastic Foams

Plastic Thread Design

Modal Analysis

Digger Tooth Failure Simulation

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Explicit Solves Where Implicit Can’t

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Implicit versus Explicit?

LS-DYNA: Unified Numerical Architecture

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Implicit versus Explicit

Explicit only works when there is acceleration (dynamic) whereas an implicit approach can solve the dynamic

and the static problem. For dynamic problems, this means that we are solving the following equation:

𝑚𝑎𝑛 + 𝑐𝑣𝑛 + 𝑘𝑑𝑛 = 𝑓𝑛

where n=time step. A common terminology is to call the 𝑘𝑑𝑛 part the internal force in the structure. The basic

problem is to determine the displacement at some future time or 𝑑𝑛+1, at time 𝑡𝑛+1.

In conceptual terms, the difference between Explicit and Implicit dynamic solutions can be written as:

𝐸𝑥𝑝𝑙𝑖𝑐𝑖𝑡: 𝑑𝑛+1 = 𝑓 𝑑𝑛, 𝑣𝑛, 𝑎𝑛, 𝑑𝑛−1, 𝑣𝑛−1, …

All these terms are known at time state “n” and thus can be solved directly .

For implicit, the solution depends on nodal velocities and accelerations at state n+1, quantities which are

unknown:

𝐼𝑚𝑝𝑙𝑖𝑐𝑖𝑡: 𝑑𝑛+1 = 𝑓 𝑣𝑛+1, 𝑎𝑛+1, 𝑑𝑛, 𝑣𝑛, … .

Given these unknowns, an iterative solution at each time step is required.

Explicit is fast since it is a direct linear algebra calculation to arrive at all quantities for the future time. It also

means that you can’t jump very far ahead. Implicit, you have to calculate future entities and requires a

decomposition of the stiffness and mass matrices – pain in the ass and slow. But one can use large steps.

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Implicit versus Explicit

Explicit: Direct – No Iteration Implicit: Matrix Algebra & Iteration

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Implicit versus Explicit: Explicit Time Step

Explict: F = ma + cv + ku and solve for 𝑢𝑖+1 = 𝑓 𝑢𝑖 , 𝑣𝑖 , 𝑎𝑖 , 𝑢𝑖−1, 𝑣𝑖−1, …

(no F = Ku solution with 𝑢 = 𝑘 𝑓 since there is no mass)

𝐶𝐴𝑐𝑐𝑜𝑢𝑠𝑡𝑖𝑐𝑊𝑎𝑣𝑒𝑆𝑝𝑒𝑒𝑑 =𝐸𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙

𝜌𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙

∆𝐸𝑥𝑝𝑙𝑖𝑐𝑖𝑡𝑇𝑖𝑚𝑒𝑠𝑡𝑒𝑝 =𝐿𝑒𝑛𝑔𝑡ℎ𝐸𝑙𝑒𝑚𝑒𝑛𝑡

𝐶𝑊𝑎𝑣𝑒𝑠𝑝𝑒𝑒𝑑=

1"

11,400 𝑀𝑃𝐻= 5.0𝑥10−6second

CAluminum =

10e6(1 − 2)

2.75x10−4= 11,400 mph

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Implicit versus Explicit

Thoughtful Application Helps a Lot

Standard Mesh: Solution Controlled

by Misc. Small Elements Better Hex Mesh & 3x Faster

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Implicit versus Explicit

Mass Scaling

“Mesh Controls” but there are tricks: Mass Scaling & CPU Scaling

Anytime you add nonphysical mass to increase the

timestep in a dynamic analysis, you affect the results

(think of F=ma).

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Implicit versus Explicit

Don’t Fear the Explicit……

Standard Linear Implicit Explicit / Implicit Explicit

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The Importance of Mesh

LS-DYNA: Unified Numerical Architecture

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The Importance of Mesh Quality

Classic Explicit

Clean, Mapped Mesh

Classic Implicit

Random Element Shapes

• Explicit propagates at the speed of sound

o Mapped mesh provides greater confidence

o Typically worth the effort to spend time on mastering the quad and hex meshing techniques

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The Importance of Mesh:

Looks Ain’t Everything

Explicit Elements Use 1-Point Guass Integration (default)

• Explicit can be very sensitive to

mesh density due to element

formulation and nonlinear effects

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The Importance of Mesh:

A Couple of Bad Elements Can Kill You

Bad Elements Can Generate Interesting results

• Solution stability can be affected by bad elements

• Femap provides Explicit Time Step checking

• Time spent up-front typically pays dividends

• Explicit contains more physics and it helps to

understand them in debugging

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Building LS-DYNA Models in

Femap

LS-DYNA: Unified Numerical Architecture

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Building LS-DYNA Models

Render Unto Caesar…….

Airplane Seat 16g Sled Test with ATD

• Build model in Femap

o Check Explicit Time Step

o Add Contacts

o Material Laws

o Perform FEA checks (component masses)

• Add anthropomorphic test device

o Leverage LS-PrePost Dummy Positioning

o Create *Include file and add to Femap

• Post Process in LSPP

o Fast

o Extensive support of results files and cross-plotting

o Tracking ability for objects that move

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Building LS-DYNA Models

Glue the Un-Hexable and Tet the Rest….

Femap Fully Supports Axisymmetric

(super sweet for seal analysis)

Viscoelastic Plastics and Seal Design

• Hex mesh is more robust

o Gluing between hex regions

o Easy within Femap

• Four-Node Tetrahedrals

o Tet Growth Ratio = 1.0

o ELFORM=13

• Full Contact / Full Nonlinear

o Material Stress/Strain Curves

o Femap base material support

o Robust model setup for quick runs

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Building LS-DYNA Models

Be Economical…..and Play Chess Hex

Blade-Out Containment

• Turbine Blade Hexed

o Complex Hexing in Femap

• Model Organization

o LS-DYNA likes Properties

o Contact based on Properties

• Implicit Spin Up

o Femap verification - Static

• Implicit / Explicit Switching

o Femap base material support

o Robust model setup for quick runs

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Building LS-DYNA Models

Leverage the Complete Toolbox

Bird-Strike (FEA + SPH)

• Build in Femap

o RBE2 to CNRB

o Composite to *PART_COMPOSITE

o Model Checkout

• LSPP for SPH

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Best Practices

LS-DYNA: Unified Numerical Architecture

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Best Practices

No Substitute for Organic Learning – Get On-Line and Ramp Up

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Best Practices

No Substitute for Organic Learning

Our LS-DYNA Class Notes Are Open to Femap Clients

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Best Practices

No Substitute for Organic Learning

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Best Practices

Attend Classes and Conferences – Networking is Awesome

Visit

Predictive Engineering

at Booth 300

We will be presenting papers on

DEM and SPH Methods

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LS-DYNA Implicit and Multi-Physics

LS-DYNA: Unified Numerical Architecture

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LS-DYNA:

Unified Numerical Architecture for Everything

LS-DYNA’s Objective is MPP for Every Discipline

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LS-DYNA: One Code Strategy

Unified Numerical Architecture for Everything

“Combine the Multi-Physics Capabilities into One Scalable Code for Solving

Highly Nonlinear Transient Problems to Enable the Solution of Coupled

Multi-Physics and Multi-Stage Problems”

Implicit Capabilities (partial list) Static and Dynamic Linear and Nonlinear Analysis

Normal Modes Analysis and Flexible Body Analysis

Frequency Domain Analysis, Response Spectrum, PSD, NVH, Fatigue

Acoustic Analysis (BEM Method and FEM)

Explicit Capabilities (partial list) o Dynamic Nonlinear Analysis and Rigid Body Dynamics

o FSI with ALE or CFD (compressible / incompressible solvers)

o DEM, SPH, EFG, Electromagnetism, Optimization, Topology

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LS-DYNA: One Code Strategy

Unified Numerical Architecture for Everything

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LS-DYNA: Unified Solver Capabilities

LS-DYNA’s Objective is MPP for Every Discipline: Implicit Mechanics

CPU Scaling Normal Modes / PSD

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LS-DYNA: Unified Solver Capabilities

Strong Coupling Between DEM and FEA

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LS-DYNA: Unified Solver Capabilities

FSI with ALE (short duration / explicit) and CFD (long duration / implicit)

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Predictive Engineering, Inc.

George Laird, Ph.D., P.E.

Principal Mechanical Engineer

Adrian Jensen, BSME, EIT

Sr. Mechanical Engineer

Predictive Engineering, Inc.

2505 SE 11th, Suite 310

Portland, OR 97202-1063

Phone: +01 (503) 206-5571

Mobile: +01 (503) 201-2688

E-mail:

FEA@PredictiveEngineering.com

Located in Portland, OR