1 CASE STUDY 100lb cr01.sldprt Fixed restraint

2 But welds crack after one day of use (some 50 load cycles) Why? RPN = R occurrence x R severity x R detection CASE STUDY

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

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

5 Time Load 100 lb 0.07s Electric hoist CASE STUDY

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

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

8 Weld size calculations CASE STUDY

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

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

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

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

13 … but hanger collapses during testing…. CASE STUDY

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

15 Results of static analysis Von Mises still OK CASE STUDY

16 Results of buckling analysis BLF = 0.87 CASE STUDY

17

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

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

20 CASE STUDY

21 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.

22 REALITY verification validation MATHEMATICAL MODEL FEA MODEL RESULTS Discretization error Modeling error Solution error

23 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

24 Sinking of Sleipner A platform Failure occurred due to discretization error; model was not verified.

25 Hartford Civic Centre Arena roof collapse. Failure occurred due to modeling error; model was not validated.

26 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

27 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? FEA QUIZ

28 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? FEA QUIZ

29 # 1FUNDAMENTALS OF FEA 1.State the objective of discretization of continuum 2.State major assumptions in the design of a finite element 3.How are FEA equations formulated ? 4.What is the primary unknown in FEA? 5.What is the nodal Degree of Freedom ( D.O.F.) ? 6.What is the relation between total number of D.O.F and total number of nodes in the FEA model ? 7.What is the price to pay for replacing continuum with a set of finite elements ?

30 1.What is a shape function ? 2.What is the difference between h-element and p-element ? 3.Describe major types of finite elements 4.What are compatible elements ? Non-compatible elements ? 5.What is a distorted element ? What are types of element distortion ? 6.What is the difference between 1 st and 2 nd order element ? 7.How do elements "communicate" between each other ? # 2TYPES OF FINITE ELEMENTS

31 # 3 ACCURACY OF FEA 1.List major steps in FEA and associated errors 2.Sort in the order of calculation: stress, strain, displacement 3.What is the accuracy of FEA results? 4.What are “artificial” restraints? 5. What is the objective of convergence analysis? 6.What are criteria of convergence analysis? 7.What is an h convergence, why is it called h-convergence? 8.What is the p-convergence, why is it called p-convergence?

32 # 4TYPES OF FEA ANALYSES, MODELING TECHNIQUES 1.What is calculated in modal analysis 2.What is calculated in linear buckling analysis 3.List some types of nonlinear behavior 4.What is the fundamental difference between linear and non-linear analysis? 5.Describe some techniques to reduce the size of finite element model 6.Describe some problems with CAD -FEA interfacing