#SEU12 - 507 an introduction to fea via solid edge and femap - mark sherman

#SEU12

An Introduction to FEA via Solid

Edge and FEMAP

Mark Sherman

Siemens PLM Software

FEMAP

© Siemens AG 2012. All Rights Reserved.

Siemens PLM Software Page 2

Introduction

Mark Sherman

[email protected]

610-458-6502

BS Aerospace and Ocean Engineering –

Virginia Tech

Boeing Helicopter

GE Astro Space

Joined FEMAP (Enterprise Software

Products, Inc. – 1992 )

mailto:[email protected]



FEMAP History

• Founded by George Rudy, 1985

• Mission – PC-based dedicated Pre- and

Post-Processor for Engineering Finite

Element Analysis

• Originally a Pre-Processor Only for

MSC/PAL and MSC/NASTRAN on 64k RAM

IBM PCDOS to Windows Transition 1990-

1992

• Rapid Growth from 1992 on

• Significant strength in Aerospace

accounts

• Laboratory Module of the ISS (Boeing)

• OEM Partnership – MSC/Nastran for

Windows

• Tens of thousands of licenses worldwide



FEMAP

Continuous development with the

same core team! Since 1985 there have been more than 30 releases of FEMAP with

only one major architecture change (DOS to Windows)

FEMAP Development Team is all engineers turned programmers –

FEA By Engineers for Engineers

Product development has been driven by FEA Analyst input



Why use Finite Element Analysis

Understand the behavior of engineered parts and assemblies

Structural Behavior

Static – Stress, Deflection, Load Distribution - Linear

Dynamic

Natural Frequency – Normal Modes

Frequency Response (Sinusoidal) – Accel., Disp., Stresses

Transient Response (General Time-Varying Loads) Accel., Disp. Stresses

Non-Linear

Contact

Geometric Nonlinearity

Material Nonlinearity

• Thermal

Steady-State

Transient




Quality

Material Cost

Weight

Reduce requirement for physical testing

Maximize these benefits - Important to integrate FEA into the Design Process



Customer Success Stories - Duracast

Customer was driven to seek a CAE solution

Rising cost of materials – need to optimize their current designs to save material use and cost (use petroleum based products)

To reduce the level of physical prototyping to save time and cost

Maintain integrity of product – advanced solutions required including nonlinear analysis

A single seat of FEMAP with NX/Nastran saves this company hundreds of thousands of dollars per year in material cost



Space Exploration Technologies

Launch Vehicle Developer

Challenge: Develop rockets that

reduce the cost of space access by a

factor of 10

Approach: Create virtual mockups of

entire rockets using Femap and NX

software

Results: More effective collaboration

between design groups, and a 50%

productivity improvement helped

successfully launch two Falcon 1

rockets

"On the analysis front Femap and

NX Nastran were the clear

winners, not only due to wide

industry acceptance but also from

an ease of use and support

standpoint.”

Chris Thompson, vice president of

Development Operations



A Brief History of FEA and FEM

The concept of a “Finite Element” was introduced by Prof. R.W. Clough of UC

Berkeley in 1960 at an ASCE Conference.

NASTRAN (NASA STRuctural ANalysis) was developed for NASA by a consortium

of several companies for the analysis of the Saturn V rocket.

Siemens PLM Software acquired MSC.Nastran source code in 2003 and has

greatly improved the performance and capabilities of

NX Nastran through the latest release of NX Nastran 8.1

Finite Element Modelers(Pre/Post Processors), the tools used to generate Finite

Element meshes and view results, were first commercialized in the 1970s.

Siemens PLM Software began the first commercial offering of FEM software with

the introduction of SDRC SuperTab in the 1970’s.

Siemens continues to support the analysis community with Femap and NX CAE

pre/post-processors.



The Solution

Consider a single degree of freedom system – a simple spring:

Apply the following conditions to generate a system of simultaneous equations

where displacements are the unknowns:

Equilibrium of forces and moments

Strain- displacement relations

Stress-strain relations

K: spring stiffness P: applied load

u: displacement

K u = P (static analysis)

?



Solution for Multiple DOFs

Any real structure can be modeled as a collection of elements connected

at nodes

With many elements and nodal dof’s, a matrix approach to the solution is

adopted

All element matrices are assembled into a global stiffness matrix

Kgg =

k11 k12

k21 k22 ka =

Element stiffness matrix ka kb

1 2 3

ka11 ka12

ka21 ka22 + kb22 kb23

kb32 kb33



Modeling of Real Structures

• The behavior of the real structure is obtained by considering the collective behavior of the discrete elements.

• The user is responsible for the subdivision or discretization of real-world structures.

• Element choice has significant influence on the behavior

• A graphic preprocessor such as FEMAP/SE Simulation is the key tool for generating a model that accurately simulates real world structures

Kgg =

ka -ka

-ka ka + kb -kb

-kb kb

• Contributions from all other elements

n x n





Structural Behavior

Static – Stress, Deflection, Load Distribution - Linear

Dynamic


Frequency Response (Sinusoidal)

Transient Response (General Time-Varying Loads)

Non-Linear

Contact



• Thermal

Steady-State

Transient



Linear Static Analysis

90%+ of all FEA projects

100% Linear – if you double the loads, you get double the response

Material stays in the elastic range – return to original shape

Small Deformation Maximum Displacement much smaller than characteristic dimensions of the part being studied, i.e. displacement much less than the thickness of the part

Loads are applied slow and gradually, i.e. not Dynamic or Shock Loading



FEA Building Blocks

Numerous Element Types

This makes it possible to accurately model

the real-world performance of your

engineered structure

Element selection helps balance model size

vs.

Solution accuracy

Hardware resources

Solution time

Results interpretation time

Another consideration for solids is time to

mesh the model if a hex mesh is desired



Choose elements to represent the real structure behavior

Rod Element: Axial and Torsion Stiffness only (pin connected truss)

Beam : Classic Euler-Bernoulli Beam depending on property options used

Shear effects, shear center offset, etc can be included, user must understand the

defaults and options to ensure proper behavior is included

Plate/Shell : Started with Kirchhoff and Mindlin theories but now many different

“tweaks” and modifications included to improve accuracy.

When elements of different types connect, user must be aware of potential

compatibility problems and use special modeling techniques as needed.

Examples: beams connecting normal to plates, plates connecting to solids

Element Basics



Finite Elements – 0D

Scalar Elements

Springs

Node to node axial or torsional

Dampers

Node to node axial or torsional

Mass

Point masses can be used to represent additional mass and inertia in the structure

that is non-structural or where modeling detail is not required

Rigid Elements

Can be used to represent rigid connections within the model

17




Line Elements

Rod

Uniaxial tension compression and

torsion – no bending or shear loads

Used to model pin-ended truss

structures

Bar/Beam

A regular beam that carries axial,

torsion, bending and shear loads

Very versatile – offsets and tapers can

be included

18




Plate and Shell Elements

Used to represent “thin” structures like

sheet metal structures.

Additional features for shear-only,

membrane-only, composite laminate

etc.

Quad4 / Tria3

Isoparametric 3/4 noded

triangular/quadrilateral

Quad8 / Tria6

Higher order isoparametric 6/8 noded

triangular/quadrilateral

19




Solid Elements

Used to fill and model solid volumes

Hexa

Regular hexahedral elements

Penta

Pentrahedral used to mesh transitions

Tetra

Tetrahedral elements can be generated fully

automatically on solids

20



Linear Static Analysis

What can you expect to learn from a linear static Finite Element Analysis

Displacements

Load Paths

Stress*



Linear Static Analysis – Modeling Guidelines

Use the

Displacements

Load Paths

Stress*



Linear Analysis is small displacement, small angle theory

Must use nonlinear analysis if the displacement changes the stiffness or loads

Pressure loads on flat surfaces, have no membrane component unless nonlinear large

displacement solution performed.(load carried by bending stiffness only)

Linear contact is a misnomer, contact condition is iterative solution, but no other nonlinear

effects are considered.

Mesh density required is a function of the desired answers

Must have enough nodes so model can deform smoothly like the real structure.

In general, accurate stresses require more elements than accurate

displacements.

Goal is for a small stress gradient across any individual element

Normal modes should always be run before any dynamic solution

Confirm model behavior, stiffness and mass properties are correct

Important Guidelines



Finite Element Analysis is an Approximate Solution to a Complicated

Problem :

Therefore, Sound Engineering Judgment is Required

Our Answers are only as good as the assumptions we make

Common sources for analysis uncertainty:

Numerical round off (usually small)

FEM : mesh density, element formulation, element connections

Boundary conditions

Loads and environments seen by the structure

Important Guidelines



Linear Statics - Stresses

To accurately recover stresses in shell and solid elements, the mesh must

be very dense in areas of high stress gradients

Stress Changing Too

Fast Across One Element



Stresses from the Web




To accurately recover stresses in shell and solid elements, the mesh must

be very dense in areas of high stress gradients

Stress Changing Less Across

an Element – More Accurate




Keeping Model Size “Reasonable”

Increase the Mesh Density where you need it, decrease it where you don’t






Guidelines for Good Stress Interpretation -

Singularities



Guidelines for Linear Static Analysis - Stresses

• Remember the limitations of “Linear” analysis

• Increase Mesh Density in High Stress Regions

• Ignore Stress Answers at Singularities

• Zero Radius Fillets

• Inside Corners

• Loaded and Constrained Nodes



When you need FEMAP

Monaco Coach

Solid Mesh Impractical

Shells alone, not enough

Beam – Lumped Mass required

Transient Dynamic Analysis a

requirement

Courtesy of Predictive Engineering



Real FEA - Examples

Bechtel River Project

Solid Mesh Impractical

Shells alone, not enough

Beam – Rigid, and Lumped Mass required

Transient Dynamic Analysis a requirement

Post-processing Data Processing Required



Real FEA - Examples

Boeing - International Space Station

Laboratory Module



Real FEA - Examples

Why can’t we just tetra-mesh this?

Just this little section is 565,405 Nodes – 275,558 Elements and there’s

only one element through the thickness

Full Model would be billions



SE Simulation FEMAP

Model Size Limitations – when one has to idealize a structure beyond

the Solid, Shell and Beam Elements available in SE Simulation

Modeling Limitations

• Composite Laminates

• Nonlinear Geometry - Large

Displacements

• Nonlinear Materials - outside

the elastic range, or non-elastic

materials (rubber).

• Time or Temperature Dependent Loading

• Specialty Elements like CBUSH where you can have displacement

dependent stiffness and damping

• CWELD - CFAST



SE Simulation Continues to Dive Deeper into FEA

Functionality

Without Pre-Load/Contact – First Mode 1.472608 Hz




Functionality

With Pre-Load/Contact – First Mode 7.586033 Hz




Functionality

First Mode 415% Higher!!!





Structural Behavior

Static – Stress, Deflection, Load Distribution – Linear

Linear Contact

Dynamic


Frequency Response (Sinusoidal)

Transient Response (General Time-Varying Loads)

Non-Linear

Contact



• Thermal

Steady-State

Transient



Advanced Dynamics Examples

Sample Model

Tubular Structure

Supports Offset Payload/Mass

Subject to Lateral +/- X Forces




Idealized Model – beam elements for tubes and lugs, shell mesh at base




Always do a modal run first and make sure everything makes sense

Evaluate basic modes/natural frequencies




Frequency Response – Check Response at the end of the support arm to

frequencies between 0 and 30 Hz.

Request responses

between 0.0 and 30.0,

every 0.2 Hz



Advanced Dynamics Example

Frequency Response – Limit Output, in this sweep, we are asking for 150

sets of Output, recover responses where they matter most, in this case, out

at the end of the arm.




Use Stick and Plate model for preliminary design/sanity check

Update design as necessary

Create detailed Shell or Solid model of final design, run Frequency

Response at actual operating frequencies for final validation




Stick and Plate Model – 2104

Nodes

All Shell Model – 18,115

Nodes



Frequency Response Results



Overview - Discussion

To get good results, accurately model your structure

• Material Properties, linear, non-linear

• Linear or nonlinear overall behavior

• Boundary Conditions

• Loads

• Structure – Does your mesh accurately reflect the structure

• Most Critical Stress condition may not be covered by linear static analysis



Contact Information

Mark A. Sherman

[email protected]

610-458-6502

mailto:[email protected]

#SEU12

Thank You! Questions?

Technology

#SEU12 - 507 an introduction to fea via solid edge and femap - mark sherman