Cambodia small group voluntourism trip (sorry the pretty pictures can make it slow to load)
Behind the Pretty Pictures (Presentation With Audio)
-
Upload
yap-yih-shan -
Category
Documents
-
view
220 -
download
0
Transcript of Behind the Pretty Pictures (Presentation With Audio)
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
1/30
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
2/30
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?
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
3/30
24/07/20083 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
4/3024/07/20084 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
5/3024/07/20085 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
6/3024/07/20086 FEA presentation.ppt
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
7/30
24/07/20087 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
8/30
24/07/20088 FEA presentation.ppt
Who should be running your FEA?
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
9/30
24/07/20089 FEA presentation.ppt
Who should be running your FEA?
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
10/30
24/07/200810 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
11/30
24/07/200811 FEA presentation.ppt
Codes and Standards The Hopper Diagram
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
12/30
24/07/200812 FEA presentation.ppt
Codes and Standards Which to Use?
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
13/30
24/07/200813 FEA presentation.ppt
Codes and Standards Which to Use?
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
14/30
24/07/200814 FEA presentation.ppt
Codes and Standards Which to Use?
Determination of stress at FE model
singularities for strength and fatigue
purposes (DNV-RP-C203)
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
15/30
24/07/200815 FEA presentation.ppt
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)
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
16/30
24/07/200816 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
17/30
24/07/200817 FEA presentation.ppt
Modelling Techniques Solids vs Shells
Consider maximum stress locations
Bending stresses at repad edges
Weld geometry, etc
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
18/30
24/07/200818 FEA presentation.ppt
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
19/30
24/07/200819 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
20/30
24/07/200820 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
21/30
24/07/200821 FEA presentation.ppt
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
22/30
24/07/200822 FEA presentation.ppt
Modelling Techniques Element selection
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
shows a maximum Von Misesstress of 32,000 psi.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
23/30
24/07/200823 FEA presentation.ppt
Modelling Techniques Element selection
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)
shows a maximum Von Misesstress of 49,000 psi.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
24/30
24/07/200824 FEA presentation.ppt
Modelling Techniques Element selection
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
25/30
24/07/200825 FEA presentation.ppt
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
26/30
24/07/200826 FEA presentation.ppt
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
27/30
24/07/200827 FEA presentation.ppt
What should a good FEA report contain?
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
28/30
24/07/200828 FEA presentation.ppt
What should a good FEA report contain?
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
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
29/30
24/07/200829 FEA presentation.ppt
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.
-
8/11/2019 Behind the Pretty Pictures (Presentation With Audio)
30/30
24/07/200830 FEA presentation.ppt
Questions?
.. and the obligatory cartoon
Feel free to contact me at [email protected] for further discussion.
mailto:[email protected]:[email protected]