1 / 55

IBEX 2002 - Session 602 Finite Element Analysis

IBEX 2002 - Session 602 Finite Element Analysis. Paul H. Miller, D. Engr. Assistant Professor of Naval Architecture United States Naval Academy. Presentation Overview. What is FEA and what will it do for us What FEA will not do for us Limitations of FEA

silas
Download Presentation

IBEX 2002 - Session 602 Finite Element Analysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. IBEX 2002 - Session 602Finite Element Analysis Paul H. Miller, D. Engr. Assistant Professor of Naval Architecture United States Naval Academy IBEX 2002 - SESSION 602

  2. Presentation Overview • What is FEA and what will it do for us • What FEA will not do for us • Limitations of FEA • Working with Finite Element Analysts • Case Studies IBEX 2002 - SESSION 602

  3. Getting started with a simple exampleA new mast step for an old wooden sailboat • Designer: L. Francis Herreshoff, 1955 • Built: 1962 Lunenberg, N.S. • Original Mast Step was Red Oak (not designed that way) • It broke at a bad moment! t=0.5” IBEX 2002 - SESSION 602

  4. The grain is longitudinal The Mast StepAs it’s thickness is about the same dimension as its width, we must use solid elements. • Loads – 7000 lb down • Geometry – 24”x4”x4” • Material – Black Locust • Boundry Conditions – supported by 3 oak floors IBEX 2002 - SESSION 602

  5. Black Locust mast step • The actual boundary conditions with the three floors. Floor grain is vertical Forward White Oak floors t=0.5” IBEX 2002 - SESSION 602

  6. Deformation (300x) IBEX 2002 - SESSION 602

  7. Displacement Maximum displacement is 0.0084” IBEX 2002 - SESSION 602

  8. Stress with vertical grain floors Max Stress = -1889 psi IBEX 2002 - SESSION 602

  9. Stress with transverse grain floors Max Stress = -2706 psi 43% higher! But floor loads are more even IBEX 2002 - SESSION 602

  10. Rolling Shear Stress Maximum shear stress is 559 psi IBEX 2002 - SESSION 602

  11. Mast Step Analysis Results • The analysis took 5 hours • The predicted weight was 7.5 pounds • The minimum factor of safety for bending was 10.2 • The minimum factor of safety for shear was 4.4 • The recommended minimum FOS is 4 • Therefore LFH over-designed it by 3/8”! • I built it to LFH’s drawing… IBEX 2002 - SESSION 602

  12. Deform – strain (in/in) If the strains are always proportional to the load it is “linear deformation” If not, then “non-linear” Have internal stress (psi) Are made of materials Which could be linear or non-linear themselves Discrete Forces Pressures Vibrations (or fatigue) Accelerations Gravity Dynamics Temperature Moisture What is Finite Element Analysis?In the real world of structural response… Loads include: Objects with loads on them: IBEX 2002 - SESSION 602

  13. Stiffness matrix Loads Displacements and Rotations (DOFs) In the world of mathematics… • FEA divides the object up into multiple small parts (up to 100K+!) • Each part is represented by stiffness constants (like springs, f=k·x • All the parts are combined mathematically (by matrix algebra) into a global structure • The solution is found from equilibrium (ΣF=0, ΣM=0) IBEX 2002 - SESSION 602

  14. Solving the basic equation for the unknown degrees of freedom… • Finding the final displacement gives us the elongation • Elongation gives us the strain • Strain and area gives us the stress • Stress and failure criteria give us the Factors of Safety! OK, Ready for the quiz?! IBEX 2002 - SESSION 602

  15. Physical modeling of structures • An FEA model is made of simple structural “elements” connected at “nodes” • The basic building blocks (elements) are: • Beams (1 primary dimension) • Plates/shells (2 primary dimensions) • Solids (3 primary dimensions) “Primary” means “much bigger than the other dimensions” IBEX 2002 - SESSION 602

  16. Just To Avoid Confusion! An element with 2 Primary Dimensions, a shell element, has a length and a width, but is thin compared to the other two dimensions. It can be either used in either 2-D analysis (x and y axes) or in 3-D analysis (x, y and z axes). IBEX 2002 - SESSION 602

  17. Common Structural Element Types • Mass • Gap • Immersed pipe • Buoy • Magnetic • Fluid/heat • Solid • Shell • Beam • Cable • Truss • Radiation IBEX 2002 - SESSION 602

  18. FEA can handle almost any structure • It’s greatest power (and cost) is with complex structures. • The structure needs to be envisioned in terms of element types which are available, and suitable. • The structure is then represented with many (often thousands) of these elements. IBEX 2002 - SESSION 602

  19. Example of Beam/Cable/Truss Elements:What they are 2 nodes, Each node has up to 6 degrees of freedom, giving 12 per element • • IBEX 2002 - SESSION 602

  20. Example of Beam Elements: A Mast tube is shells, spreaders are beams, rigging is cables IBEX 2002 - SESSION 602

  21. Example of Shell Elements: What they are 4 nodes, Each node has up to 6 degrees of freedom, giving 24 DOF per element IBEX 2002 - SESSION 602

  22. Example of Shell Elements: A 77-foot Hull Note the beam elements IBEX 2002 - SESSION 602

  23. Example of Solid Elements: What they are 8 nodes, Each node has up to 3 degrees of freedom (translation only), giving 24 DOF IBEX 2002 - SESSION 602

  24. Example of Solid Elements:The Mast Step (again) Solids are sometimes called “brick elements” IBEX 2002 - SESSION 602

  25. What FEA does beautifully! • Handles complex geometry. (Indeterminate structures) • Isotropic materials (materials with consistent properties in all directions) • Static and simple dynamic problems • Examples • A steel keel, a bronze rudder shaft • Metal hulls (tanker fatigue) • Accuracy is within 0-5%! IBEX 2002 - SESSION 602

  26. Complex materials Composites Wood Non-linear static deformation (x5) Buckling of isotropic materials (x2) Increased uncertainty From 1-5% potential error To 3-30% error HIGHER MIN FOS! Increased manhours required to prepare model What FEA does “OK”… This means: Examples: IBEX 2002 - SESSION 602

  27. Model took 127 manhours to build Predicted deformations within 4% for static loads Static strains within 6% Example: A Composite Sailboat IBEX 2002 - SESSION 602

  28. Composite Sailboat • Fatigue-influenced dynamic strains were predicted within 14% when compared to strain gages and coupons. IBEX 2002 - SESSION 602

  29. Non-linear deformationHigh Aspect Ratio Rudder • 8 foot span/16 lb • 20” of tip deflection • High membrane stresses reduce predicted deflection and stress • 5% error in deflection IBEX 2002 - SESSION 602

  30. Tsai- Wu Factors of Safety IBEX 2002 - SESSION 602

  31. Non-linear Mast Deformation • Small dinghy mast • Used to size spreaders, wire and pretension • Input was gust spectrum • 8% error in deformation IBEX 2002 - SESSION 602

  32. What FEA does not do well • Dynamic impact (slamming loads) • Joints ( composites or metal ) • Buckling of “real world” composites. • Misc details unaccounted for in element formulations. • Error can be 30-300%! IBEX 2002 - SESSION 602

  33. Dynamic Analysis • FEA has great strengths in dynamic analysis for certain types of problems. • Standard FEA doesn’t handle slamming impacts well. • One of the major difficulties are in the definition of the loads. • The other is in the speed of the transient nature of the load. IBEX 2002 - SESSION 602

  34. Joint Analysis with FEA • FEA is good for extracting loads at joints. • FEA is weak in micro analyzing joint designs • This is primarily due to difficulty with material properties and failure mechanisms. IBEX 2002 - SESSION 602

  35. Joint Design with FEA(some variation with programs) • Normal FEA solution assumes joint is perfect • Either a) list nodal forces • b) use nodal stresses and area • Determine stress concentration factors for specific joint geometry • Calculate joint loads by spreadsheet (isotropic or wood) or • Use laminate analysis program and spreadsheet (for composites) IBEX 2002 - SESSION 602

  36. Not all aspects of structres can be accounted for in FEA models IBEX 2002 - SESSION 602

  37. Failure mode prediction is only as good as it’s modeling. This means realistic material testing to support the FEA. “Special” failure mode analysis (post-processing) using spreadsheets or macros IBEX 2002 - SESSION 602

  38. Uncertain loads Slamming Impact Transient Unanalyzed loads! IACC cockpit example Uncertain materials Testing QA/QC from builder Model Errors Mesh density Linear or non-linear analysis Wrong elements Boundary conditions Results analysis Limitations of FEA= High Error Possibility! IBEX 2002 - SESSION 602

  39. A Multiple Issue Problem! • Loads, materials and boundary conditions • FEA assumes “continuum mechanics” Eventually we got the deflections to match within 10%,But the strength was under predicted by 110%. IBEX 2002 - SESSION 602

  40. Working with a Consultant- and getting good value from it -an overview • Choosing a consultant • What you’ll be asked to supply • Getting what you expect • Several projects outlined IBEX 2002 - SESSION 602

  41. Choosing a ConsultantQuestions to Ask • Analyst versus engineer or designer • Their experience/education • Your relationship (micromanagement?) • Experience with similar projects • Loads • Materials (isotropic or orthotropic?) • Track record of success and failure • Rates and availability (current range is $25-275/hour) IBEX 2002 - SESSION 602

  42. Information You’ll Be Asked To Supply: • Geometry • Loads (are they biased or real?) SES • Material types and properties (guess or test?) • Goals • Deliverables • Any guesses increase the error and may make using FEA uneconomical! IBEX 2002 - SESSION 602

  43. Deliverables: • Do you expect guidence in making decisions? • Do you want a specifc question answered or do you want design work? • Do you expect formal documentation? • If so, then in what form? • Report, e-mail, spreadsheet, tables, drawings (dxf, dwg, igs, etc) • Do you want nice color stress plots? IBEX 2002 - SESSION 602

  44. Extra hints to make your life easier • Be specific on the design criteria • Stiffness, deflection, strength, FOS • Be flexible on the geometry • Don’t be stuck on a particular design (just because it worked in the past doesn’t mean it is the best) • Keep the design simple! • Communicate! IBEX 2002 - SESSION 602

  45. Another Case Study • A 77-foot performance cruiser • Designed by Carl Schumacher • Under Construction in Seattle IBEX 2002 - SESSION 602

  46. Project Overview • Began in January 2000 • Structures to meet ABS and realistic loads if not specified • Multiple materials intended • Goal is “ULDB” cruiser • Light but strong with a deep bulb keel IBEX 2002 - SESSION 602

  47. FEA work • Designer subcontracted out structural FEA design • Designer provided dxf files for all geometries (hull, appendages) • FEA consultants optimized and specified construction • Designer did hull structure drawings • Consultants did keel structure drawings and interfaced with keel and hull manufacturer to ease construction IBEX 2002 - SESSION 602

  48. Design Limit Load Cases • Upwind in heavy air, wave height equal to freeboard, wave length equal to boat length • Slamming • Grounding • Lifting Each load case drove the design of different parts of the boat. IBEX 2002 - SESSION 602

  49. Upwind in 30 knots on port tack Rig loads supplied by mast maker IBEX 2002 - SESSION 602

  50. Displacements (25x) Maximum displacement = 3.32” Max rotation 0.5 degrees IBEX 2002 - SESSION 602

More Related