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CASE STUDY

CASE STUDY. 100lb. Fixed restraint. cr01.sldprt. CASE STUDY. But welds crack after one day of use (some 50 load cycles) Why?. RPN = R occurrence x R severity x R detection. CASE STUDY. Stress results show no problem FOS=32000/2960=10.8. CASE STUDY.

yoshio-maldonado
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CASE STUDY

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  1. CASE STUDY 100lb Fixed restraint cr01.sldprt

  2. CASE STUDY But welds crack after one day of use (some 50 load cycles) Why? RPN = R occurrencex R severityx R detection

  3. CASE STUDY Stress results show no problem FOS=32000/2960=10.8

  4. CASE STUDY BLF =32 so buckling is not an issue

  5. CASE STUDY Load 100 lb Time 0.07s Electric hoist

  6. CASE STUDY Electric hoist was applying a overload and it was exciting the first mode

  7. CASE STUDY One sided fillet weld 1/8” Post modeled as surface One sided fillet weld 3/16” Weld size calculations

  8. CASE STUDY Weld size calculations

  9. CASE STUDY Hinge 30mmx30mm 30mmx30mm Pin Hinge 2500N case01.sldprt

  10. CASE STUDY x = d2y = 0.043mm σ approx = 3MPa

  11. CASE STUDY case01.sldprt Results of static analysis Displacement results agree with hand calculations

  12. CASE STUDY case01.sldprt Results of static analysis Von Mises stress plot does not show any problem (yield strength 620MPa)

  13. CASE STUDY … but hanger collapses during testing….

  14. CASE STUDY This is the real geometry This is modeled by “stick” model

  15. CASE STUDY Results of static analysis Von Mises still OK

  16. CASE STUDY Results of buckling analysis BLF = 0.87

  17. CASE STUDY 01 FIRST REDESIGN Results of buckling analysis BLF = 1.7

  18. CASE STUDY 01 SECOND REDESIGN Results of buckling analysis BLF = 4.4 much better

  19. CASE STUDY

  20. Verification The process of determining that a model correctly represents the modeler’s conceptual description of the model and the solution of the model Validation The process of determining the degree to which a model is correctly representing reality from the perspective of the intended use of the model.

  21. REALITY Modeling error MATHEMATICAL MODEL Discretization error validation FEA MODEL verification Solution error RESULTS

  22. Verification The process of determining that a model correctly represents the modeler’s conceptual description of the model and the solution of the model Validation The process of determining the degree to which a model is correctly representing reality from the perspective of the intended use of the model. REALITY verification validation

  23. Sinking of Sleipner A platform Failure occurred due to discretization error; model was not verified. http://www.ima.umn.edu/~arnold/disasters/sleipner.html

  24. Hartford Civic Centre Arena roof collapse. Failure occurred due to modeling error; model was not validated. http://www.eng.uab.edu/cee/faculty/ndelatte/case_studies_project/Hartford%20Civic%20Center/hartford.htm#Top

  25. In the case of hanger, mathematical model using 2D trusses was solved correctly, it passes verification. However, it has incorrect geometry and it does not account for the predominant mode of failure: BUCKLING

  26. FEA QUIZ • The finer is the mesh the better are your results • true ? false ? • Geometry should be represented as accurately as possible • true ? false ? • Solids give the best results because they accurately model the geometry • true ? false ? • High accuracy of FEA results from high processing accuracy of the computer • true ? false ? • If your FEA software reports no error, the solution will be correct • true ? false ? • You do not really need any error estimation, the FEA is always accurate enough • true ? false ? • Higher order elements will give you more accurate results • true ? false ? • First order tetrahedral solids are too stiff and should be avoided • true ? false ? • Use a coarse mesh first to find stress concentration, then refine it as needed • true ? false ? • If FEA results correlate well with (e.g.) strain-gauge readings, all FEA results are O.K. • true ? false ?

  27. FEA QUIZ • Always make the finest model possible • true ? false ? • Degenerated elements are O.K. as long as they are far away from stress concentrations • true ? false ? • If you want to know only displacements and do not care about stresses, then you can make a coarse model • true ? false ? • Modal analysis can use coarser mesh that stress analysis • true ? false ? • Finite Element Models offer a deceiving level of detail • true ? false ? • Model geometry is the most readily controlled, loads less so, restrains are the most difficult to control • true ? false ? • Test data always have error and may be inconsistent with FEA assumptions • true ? false ? • Incompetent analysis gives, at best , unreliable results, at worst is positively misleading • true ? false ? • Bad FEA gives you deceiving trust in the design • true ? false ? • FEA makes good engineer better and bad engineer dangerous • true ? false ?

  28. # 1 FUNDAMENTALS OF FEA • State the objective of discretization of continuum • State major assumptions in the design of a finite element • How are FEA equations formulated ? • What is the primary unknown in FEA? • What is the nodal Degree of Freedom ( D.O.F.) ? • What is the relation between total number of D.O.F and total number of nodes in the FEA model ? • What is the price to pay for replacing continuum with a set of finite elements ?

  29. # 2 TYPES OF FINITE ELEMENTS • What is a shape function ? • What is the difference between h-element and p-element ? • Describe major types of finite elements • What are compatible elements ? Non-compatible elements ? • What is a distorted element ? What are types of element distortion ? • What is the difference between 1st and 2nd order element ? • How do elements "communicate" between each other ?

  30. # 3 ACCURACY OF FEA • List major steps in FEA and associated errors • Sort in the order of calculation: stress, strain, displacement • What is the accuracy of FEA results? • What are “artificial” restraints? • What is the objective of convergence analysis? • What are criteria of convergence analysis? • What is an h convergence, why is it called h-convergence? • What is the p-convergence, why is it called p-convergence?

  31. # 4 TYPES OF FEA ANALYSES, MODELING TECHNIQUES • What is calculated in modal analysis • What is calculated in linear buckling analysis • List some types of nonlinear behavior • What is the fundamental difference between linear and non-linear analysis? • Describe some techniques to reduce the size of finite element model • Describe some problems with CAD -FEA interfacing

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