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

3Stress results show no problem

FOS=32000/2960=10.8

CASE STUDY

4BLF =32 so buckling is not an issue

CASE STUDY

5

Time

Load

100 lb

0.07s

Electric hoist

CASE STUDY

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

CASE STUDY

7Weld size calculations

One sided fillet weld 3/16”

One sided fillet weld 1/8”

Post modeled as surface

CASE STUDY

8Weld size calculations

CASE STUDY

9

2500N

30mmx30mm

30mmx30mm

Hinge

Hinge

Pin

case01.sldprt

CASE STUDY

10

x =

d2y = 0.043mm

σ approx = 3MPa

CASE STUDY

11Results of static analysis

Displacement results agree with hand calculations

case01.sldprt

CASE STUDY

12Results 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

14This 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 platformFailure occurred due to discretization error; model was not verified.http://www.ima.umn.edu/~arnold/disasters/sleipner.html

25

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

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

# 1 FUNDAMENTALS 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 1st and 2nd order element ?

7. How do elements "communicate" between each other ?

# 2 TYPES 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

# 4 TYPES 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