Behind the Pretty Pictures (Presentation With Audio)

8/11/2019 Behind the Pretty Pictures (Presentation With Audio)

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10-Sep-05

How to get the most from your Finite Element Analysis contractors

Behind the Pretty Pictures:

John Davidson WorleyParsons Services Ltd: Pressure Equipment Group


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24/07/20082 FEA presentation.ppt

Introduction

Aims of this presentation:

To give a general awareness of finite element analysis (FEA) and its

common applications

Highlight key issues and potential problems in the methods used

What should be involved in the validation of FEA work?

What should a good FEA report contain?

How do you get the most value out of your FEA contractors?


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Introduction

My FEA background:

Pressure equipment (vessels, piping)

Structural Components (offshore platform

connections, lifting equipment, Bussleton

Underwater Observatory)

Mechanical and thermal, linear and

non-linear, static and transient analyses

Use of ANSYS, Caesar II, FEPipe, SACS, USFOS, ABAQUS On-the-job training with some supplementary external courses.


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Introduction The Finite Element Method

Developed in the early 1940s by Richard Courant and Alexander Hrennikoff to

solve complex elastic structural analysis problems.

They are numerical techniques used for finding approximate solutions of partial

differential equations.

Development progressed in the middle to late 1950s for airframe and structural

analysis through the work of John Argyrisand Ray W. Clough in the 1960s for

use in civil engineering.

By late 1950s, the key concepts of stiffness matrix and element assembly

existed essentially in the form used today, and NASA issued request for

proposals for the development of the first finite element software NASTRAN in1965.



Introduction Common FEA Today

Structural and thermal problems are themost common use of FEA today:

Structural analysis calculates the meshs

node displacementsDisplacement components interpolated

across elements to calculate a

displacement field in the model.

Displacement fields are differentiated to

find strains.

Stresses calculated based on strains and

material elasticity.

Thermal analysis is similar: an interpolated temperature field is differentiated

to find a temperature gradient. Heat flux is calculated based ongradient and

material conductivity.



Introduction Some key issues

Significant increase in software accessibility and hardware power has lead to

a surge in the amount of FEA undertaken.

Increasing costs of materials is raising the importance of design efficiency

FEA offers potential to improve and iterate design for comparatively low cost.

FEA being introduced in university courses is there a danger of being too

software focussed?

Misconception of FEA as an engineering panacea; the new primary designtool.

The pretty pictures are very useful for mollifying upper management

The perform-FEA syndrome often stems from bureaucraticmisunderstanding rather than engineering need for results

Paul Kurowski President ACOM Consulting, USA


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Who should be running your FEA?

FEA is an engineering tool and running the analysis is a specific skill:

Training is required and the opportunity to practice extensively.

Garbage in = garbage out the black box dilemma

Some foundation in the theory behind the method as well as sound engineering

judgment in materials and load conditions is needed as a minimum.

Some software providers are pushing the integration of FEA with CAD

encouraging the use of designers/draftspeopleas FEA operators. This is apotentially dangerous philosophy. Functions such as Automeshing still require an

experienced eye to confirm suitability.

An FE analyst needs to decide which features need modelling, how to apply loadsand boundary conditions, what errors are acceptable, and how theresults are

interpreted against the relevant codes.


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Use of recent graduates as FEA operators:

Computer-savvy, but lack meaningful experience in the fundamentals of good

engineering design.

Understanding the FE method is more important than specific software

commands, which are easily learned.

Unwillingness to ask too manyquestions, graduates may withdraw into

isolated world of simulation. This is of

no benefit to their growth as a good

engineer or to the company employing

them.

A person eager to use newly acquired

skills and lacking a good grasp of FEAis probably the most dangerous user.


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NAFEMS (National Agency for Finite Element Methods and Standards ) has

released a quality supplement to ISO 9001 titled R0013 Finite element

analysis in the design and validation of engineering products

Includes recommendations for the level of experience required tocompletecertain levels of FEA:

Analysis Category Engineer ing

experience

Finite-element

experience after

formal training

Relevant FEA jobs

performed

1. Vital 5 years 6 months

2 X category 1 under

supervisory or 5 X

category 2 properly

assessed

2. Important 2 years 2 months1 X category 1 or 2under supervision or 3

X category 3 properly

assessed

3. Advisory 1 year 1 month Relevant benchmarks


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Codes and Standards Which to Use?

Pressure Vessels

Previously ASME VIII, Division 2 and AS1210-Supplement 1:1990.

Almost identical in their guidance for numerical analysis Biased towards linear elastic analysis (written before the FEA

software boom) the hopper diagram

Although simple to analyse, interpretation of stresses requiresexperience and good knowledge of differences between primary and

secondary, general and local stresses.

Stress intensity (Max shear stress / Trescastress theory) comparedagainst multiples of the material design strength. Max shear stress

theory usually more conservative and simpler to calculate.


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Codes and Standards The Hopper Diagram


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Pressure Vessels

ASME VIII Div 2 rewritten in 2007

Very prescriptive section on design-by-analysis (Ch 5)

Focussed on protection against:

- Plastic collapse - Buckling

- Local failure - Failure under cyclical loading

Specifies methodology for linear (elastic) and non-linear (plastic) analyses now uses Von Misesstress rather than stress intensity.

AS1210 to be revised later in 2008 (?)

Some improved guidance on FEA New ASME VIII Div 2 methods may be included in later amendments

Pressure Piping

AS4041 currently directs users to AS1210 for complex geometries.


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Structural Components

Far fewer codes available that give guidance on FEA use.

Some European standards (egBS 7608, DNV-RP-C203) specify methods forextracting suitable stresses from FEA for fatigue assessment.

Some analysts use AS3990 (or similar) based on comparing averagestress

through sections against a proportion of material yield strength.

Clients and analysts must consider what form the loads are givenin:

- Working Stress design

- Limit State design


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Determination of stress at FE model

singularities for strength and fatigue

purposes (DNV-RP-C203)


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Material Design Strength

Determination of material design strength is one of the key issues in

correctly interpreting FEA results (particularly for pressure

equipment).

Some variation in design strength between AS1210, ASME VIII and ASME

B31.3 etc remember the fundamental intent behind them!

AS1210 f is typically lesser of Yield/1.5 and UTS/3.5 (amended fromUTS/4 except for flanges).

Local membrane stresses limited to 1.5*f (Max = 1* yield strength)

Local primary + secondary stresses limited to 3*f(Max = 2* yield strength).

Care must be taken with standards using different calculations for f,

eg: AS4041 Class 2P (f =0.72*Yield), AS2885 (f= 0.8*Yield)


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Material Design Strength

Sps , is computed as the larger of thequantities shown below.

1) Three times the average of the S values

for the material at the highest and lowest

temperatures during the operational cycle.

2) Two times the average of the Sy values

for the material at the highest and lowest

temperatures during the operational

cycle ASME VIII Div 2

CAUTION Careful consideration

of the definition of load cycle isrequired!

For steady state conditions (say pressure + external loads at operating

temperature) it is more correct to determine the design strength at the

operating temperature.


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Modelling Techniques Solids vs Shells

Consider maximum stress locations

Bending stresses at repad edges

Weld geometry, etc


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Modelling Techniques Solids vs Shells

Nozzle thickness

Shell thickness

Shell + repad

thickness

Neutral Axi

corresponds

with

No bending stress at repad edge

accounted for!

Bending stiffness (2t) Bending stiffness 2(t)

Shell + repad?Somewhere in between

2t

t

t


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Modelling Techniques Hexahedra vs Tetrahedra

A second-order hexahedral elementA volume built from first-order tetrahedral elements

Three basic approaches to reducing % element error in FEA:

- h method: the element order (p) is kept constant, but the mesh is

refined infinitely by making the element size (h) smaller.- p method: the element size (h) is kept constant and the element

order (p) is increased.

- h-p method: the h is made smaller as the p is increased to create

higher order h elements.


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Modelling Techniques Hexahedra vs Tetrahedra

510MPa348MPa

An 8-noded hexahedron formedfrom five tetrahedrons has greater

discretisationerror than a single 8-

noded brick because the fivetetrahedrons cannot assume all the

displacement fields handled by the

8-noded element. (1st

ordertetrahedra elements also have

constant strain behaviour compared

to linear strain behaviour across a

1st order hexahedral element).

Element selection can be a matter of preference but there are some key

issues to be aware of.


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Modelling Techniques Element selection

Model 1 - 1st order tetrahedraconstant stress across element

one element through thickness cannot represent bending stress

elements will be highly distorted

shows a maximum Von Misesstress of 18,000 psi.

From When GoodEngineers Deliver

Bad FEA - by Paul

Kurowski ACOM

consulting

The mesh and results from the finite-element analysis of a bracket


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Model 2 -

2nd

order tetrahedra linear stress across element one element through thickness mesh still too coarse to model stress

concentrations

some elements will be highly distorted



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Model 3 -

2nd

order tetrahedra, enough elements to model stress distribution reasonablyaccurately (good starting point)

analyst needs to successively refine mesh to check that % error in stress is

within permissible limits (stress will increase with each refinement)



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Model 4 -

Adaptive-order p elements software automatically iterates element order ateach location until a user-specified accuracy is achieved (accuracy based on

local strain energy, local displacements or global RMS stress etc)

Shows a maximum Von Misesstress of 62,000 psi.

Not all software handles adaptive-order elements.


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Validating third party results

Due to general incompatibility between software packages, it is often difficult to get

models validated.

Not necessarily cost-effective to re-build and re-analyse a model from scratch. Some gains are being made in FE softwares ability to export CAD-type geometry

files (.IGES, .SAT) which may be imported into clients CAD packages to allow

geometry checking (and vice-versa for model development.)

Often have to wait until receipt of final reports to highlight any possible problems

with the analysis and assumptions by then is it too late?

Can be totally reliant on contractors internal checking procedures


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Validating third party results

As part of the contractors checking process, the following should be done:

Confirmation of use of correct element type (shell vssolid, 1st or 2nd order)

Confirmation of use of correct analysis type (linear vsnon-linear, small vs large deflection)

Check of material properties, loads, boundary conditions and sum of reaction forces Hand calculation of results away from geometric discontinuities(PD/2t, Roarks Formulae)

Check that stress variation across elements is within acceptable limits (mesh density)

Element Order % Stress Variat ion across element

0 10%

1 20%

2 30%

>2 40%

Check that units are consistent Check contact (if applicable)

Check model for free edges

Check for solution convergence


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Software package and version

List of assumptions:

Codes/Standards applied

Design strengths applied Description of failure modes considered

Material Properties

Longitudinal and

circumferential

restraint at free

end

Nozzle loads

applied at centre of

flange face and

transferred to face

via rigid constraint

equations

Symmetry

constraints

applied on Y-Z

plane

14MPa pressureapplied to all

internal surfaces

0.3MPa pressure

applied to bottom

surface of upper

support plate and

top surface of

lower support plate

Plot of model geometry (including list of

thicknesses if shell elements used)

Plot(s) of FE mesh at critical areasType of elements used (shape and

order)

Plot of applied loads and boundary

conditions (including notation for clarity)

Plot c/o ContractDesign and

Management

Services Pty Ltd


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Plots of the final deflected shape withcolour contouring

Key results plots

Stress / Strain / Temperature etc Clearly indicating type of stress (Von

Mises, Tresca) or strain (true,

engineering)

Other helpful information:

Summary of reaction forces at

boundaries. Additional hand calculations to verify

results away from discontinuities


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Getting the most value from your contractor

Avoid the Perform FEA syndrome..

Stay involved with the FE contractor during the process

Develop and agree on a design basis with the key assumptions addressed

(loads, materials, failure criteria, critical regions in the component/assembly)

Review preliminary results to ensure that the output is as expected.

Understand that, what may seem like a minor design change at your end,

can be significantly more complex to implement in an existing model.

Review your contractors internal auditing:

Do they have other analysts with enough experience to independently check the

analysis?

Do they have an analysis verification checklist?

Documentation should contain enough information that a third party can replicate

the analysis long after the original author is gone.


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Questions?

.. and the obligatory cartoon

Feel free to contact me at [email protected] for further discussion.
mailto:[email protected]:[email protected]

Behind the Pretty Pictures (Presentation With Audio)

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