Behind the Pretty Pictures: How to get the most from your Finite - PowerPoint PPT Presentation

Behind the Pretty Pictures: How to get the most from your Finite Element Analysis contractors John Davidson – WorleyParsons Services Ltd: Pressure Equipment Group 10-Sep-05

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? 2 24/07/2008 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. 3 24/07/2008 FEA presentation.ppt

Introduction – The Finite Element Method Developed in the early 1940’s 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 Argyris and 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 in 1965. 4 24/07/2008 FEA presentation.ppt

Introduction – Common FEA Today Structural and thermal problems are the most common use of FEA today: • Structural analysis calculates the mesh’s node displacements •Displacement 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 on gradient and material conductivity. 5 24/07/2008 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 design tool. • The pretty pictures are very useful for mollifying upper management ‘ The “perform-FEA” syndrome often stems from bureaucratic misunderstanding rather than engineering need for results ’ Paul Kurowski – President ACOM Consulting, USA 6 24/07/2008 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/draftspeople as FEA operators. This is a potentially 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 loads and boundary conditions, what errors are acceptable, and how the results are interpreted against the relevant codes. 7 24/07/2008 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. • “Unwillingness to ask too many questions, 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 FEA is probably the most dangerous user . Understanding the FE method is more important than specific software commands, which are easily learned. 8 24/07/2008 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 to complete certain levels of FEA: Analysis Category Engineering Finite-element Relevant FEA jobs experience experience after performed formal training 2 X category 1 under supervisory or 5 X 1. Vital 5 years 6 months category 2 properly assessed 1 X category 1 or 2 under supervision or 3 2. Important 2 years 2 months X category 3 properly assessed 3. Advisory 1 year 1 month Relevant benchmarks 9 24/07/2008 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 requires experience and good knowledge of differences between primary and secondary, general and local stresses. • Stress intensity (Max shear stress / Tresca stress theory) compared against multiples of the material design strength. Max shear stress theory usually more conservative and simpler to calculate. 10 24/07/2008 FEA presentation.ppt

Codes and Standards – The ‘Hopper’ Diagram 11 24/07/2008 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 Mises stress 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. 12 24/07/2008 FEA presentation.ppt

Codes and Standards – Which to Use? Structural Components Far fewer codes available that give guidance on FEA use. Some European standards (eg BS 7608, DNV-RP-C203) specify methods for extracting suitable stresses from FEA for fatigue assessment. Some analysts use AS3990 (or similar) based on comparing average stress through sections against a proportion of material yield strength. Clients and analysts must consider what form the loads are given in: - Working Stress design - Limit State design 13 24/07/2008 FEA presentation.ppt

Codes and Standards – Which to Use? Determination of stress at FE model singularities for strength and fatigue purposes (DNV-RP-C203) 14 24/07/2008 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 from UTS/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) 15 24/07/2008 FEA presentation.ppt

Material Design Strength “Sps , is computed as the larger of the quantities 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” is required! For steady state conditions (say pressure + external loads at operating temperature) it is more correct to determine the design strength at the operating temperature. 16 24/07/2008 FEA presentation.ppt

Modelling Techniques – Solids vs Shells • Consider maximum stress locations • Bending stresses at repad edges • Weld geometry, etc… 17 24/07/2008 FEA presentation.ppt

Modelling Techniques – Solids vs Shells Nozzle thickness …corresponds with… Shell + repad Shell thickness thickness Neutral Axis No bending stress at repad edge accounted for! t 2t t Bending stiffness ⍺ (2t) ³ Bending stiffness ⍺ 2(t ³ ) Shell + repad? Somewhere in between… 18 24/07/2008 FEA presentation.ppt