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# Automated calculation of Stress Concentrations around Holes using the COM Interface of StressCheck

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The ensuring the structural integrity of ageing aircraft is one of RUAG’s core businesses and is one of the focus points of the structural engineering group. To be able to accurately predict the life of a part the precise calculation of the stress concentrations at potential damage locations is very important. One of the more difficult locations to calculate a stress concentration is around a loaded fastener hole as it depends not only on the through stresses in the part but also on the fastener bearing loads and other factors such as the elastic modulus of the fastener, the thickness of the part or the distance to other fasteners.

The original method to calculate stress concentrations around loaded fastener holes consisted of a number of manual calculations and using the resulting values in different diagrams to come up with the resulting stress concentration at different locations around the hole. This method is very time consuming and error prone even when partially automated using Excel spreadsheets.

The automated solution developed by RUAG makes use of HyperMesh to automatically extract the forces around the hole and transferring them to a local model of the hole surroundings in StressCheck to calculate the stress distribution around the hole.

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### Automated calculation of Stress Concentrations around Holes using the COM Interface of StressCheck

1. 1. Automated calculation of Stress Concentrations around Holes using the COM Interface of StressCheck Luzian Michel, RUAG Aviation EATC 2014, Munich 25.06.2014
2. 2. Motivation  Up to now stress concentrations of loaded fastener holes are being calculated using the methodology of known literature such as Peterson.  The whole process takes a considerable amount of time  loads have to be extracted from the FEM  divided in the unit conditions according to Peterson (manually!)  a clocking analysis has to be performed.  Process is error prone and time consuming as it has to be done by hand  GOAL: Develop a tool which performs automatically a full 3D FE analysis based on a 2D FE model 02.07.2014│RUAG Aviation│2
3. 3. Why StressCheck?  StressCheck is a p-type FEA software  High accuracy through use of higher order polynoms instead of high element count  Low computational time!  Most of StressChecks functions can be used through a COM API  Prerequisite for automated analysis  StressCheck includes  Pre processor  Solver  Post processor  StressCheck is available through the Altair Partner Alliance Program 02.07.2014│RUAG Aviation│3
4. 4. Working principle 02.07.2014│RUAG Aviation│4 HyperMesh • Load model • Extract freebody (geometry, materials, loads, fastener info) Excel • Store freebody information • Run StressCheck macro StressCheck • Build geometry & assign properties • Apply loads & boundary conditions • Perform analysis & extract results Excel • Write Results Sheet
5. 5. Example 02.07.2014│RUAG Aviation│5
6. 6. Step 1: GUI 02.07.2014│RUAG Aviation│6 HyperMesh
7. 7. Step 2: Extract Data from FEM  Read geometry  Node locations  Elements (CQUAD /CTRIA)  Properties  Material  Read fastener information (CBUSH-Elements)  Diameter  Material  Fit  Countersink 02.07.2014│RUAG Aviation│7 HyperMesh
8. 8. Step 3: Build up Excel database  Write all data to a spreadsheet  Transform the forces to a text file for the import to StressCheck 02.07.2014│RUAG Aviation│8 HyperMesh Excel
9. 9. Step 4: StressCheck Model  Create 2D geometry in StressCheck  based on Elements from DFEM  Extrude 2D geometry to the required thickness  Subtract fastener holes  Countersink is not being modeled  Automesh  Pentamesh for flat geometries  Tetramesh for anything else 02.07.2014│RUAG Aviation│9 HyperMesh Excel StressCheck
10. 10. Step 4: StressCheck Model 02.07.2014│RUAG Aviation│10 HyperMesh Excel StressCheck
11. 11. Step 5: Loads  Import nodal forces and moments  Apply forces and moments to the borders of the plate 02.07.2014│RUAG Aviation│11 Source: StressCheck 9.2 Master Guide HyperMesh Excel StressCheck
12. 12. Step 5: Loads  Import fastener loads  Apply the loads on holes in structure 02.07.2014│RUAG Aviation│12 Source: StressCheck 9.2 Master Guide Forces Moments HyperMesh Excel StressCheck
13. 13. Step 6: Solve & Evaluate Stresses  Solve StressCheck model.  Read tangential stress around hole  Top and Bottom Surface  Midthickness  Determine StressCheck solver error  Create report sheet for each analysis 02.07.2014│RUAG Aviation│13 HyperMesh Excel StressCheck
14. 14. Step 7: Report  Create report sheet for each analysis 02.07.2014│RUAG Aviation│14 HyperMesh Excel StressCheck Excel
15. 15. Evaluation 02.07.2014│RUAG Aviation│15
16. 16. Open-Hole Cases  Test Scenario: Open Hole with thru stress  2 different thicknesses tested  d/H = 0.25  d/l = 0.25  From literature: 02.07.2014│RUAG Aviation│16 Source: Peterson 2nd Ed., Chart 4.33 𝜎 𝑚𝑎𝑥 𝜎 = 3.13
17. 17. Open-Hole Cases  Impact study performed to assess:  Edge effect  Shadowing factors  Thickness effects  Example model: 02.07.2014│RUAG Aviation│17
18. 18. Open-Hole Cases 02.07.2014│RUAG Aviation│18 B1 B2 B3 B4 B5
19. 19. Open-Hole Case - Results  Results:  Conclusions  Mean-values a bit below, peak values a bit too high  The larger the area the better is the result  Thickness effect is considered in the analysis 02.07.2014│RUAG Aviation│19 B1 B2 B3 B4 B5 Ref. value Mean ktσ t=0.2 in. 3.37 3.29 3.19 3.19 3.19 3.25 t=0.0625 in. 3.53 3.33 3.16 3.16 3.17 3.16 Peak ktσ t=0.2 in. 3.45 3.38 3.29 3.29 3.30 3.25 t=0.0625 in. 3.58 3.38 3.20 3.19 3.20 3.16 Thickness factors incl.
20. 20. Off-Axis loading  Lug is modelled in StressCheck  Analysis performed for  2D  3D  As reference a second model was created in Hypermesh  2D  Sin distributed loads  Variation of the load angle θ 02.07.2014│RUAG Aviation│20
21. 21. Off-Axis loading  Comparison StressCheck 2D ↔ 3D  Very good comparison for small load angles. Variations exist for other load angles. 02.07.2014│RUAG Aviation│21 -2,00E+00 -1,00E+00 0,00E+00 1,00E+00 2,00E+00 3,00E+00 4,00E+00 5,00E+00 0 20 40 60 80 100 120 140 160 180 kt(phi) theta [°] phi=0° phi=10° phi=20° phi=30° phi=40° phi=50° phi=60° phi=70° -2,00E+00 -1,00E+00 0,00E+00 1,00E+00 2,00E+00 3,00E+00 4,00E+00 5,00E+00 0 20 40 60 80 100 120 140 160 180 kt(phi) theta [°] phi=0° phi=10° phi=20° phi=30° phi=40° phi=50° phi=60° phi=70° 2D 3D
22. 22. Off-Axis loading  Comparison StressCheck ↔ Hypermesh (2D, sinusoidal load)  Good match between StressCheck und HyperMesh. Some variation exists for higher load angles. 02.07.2014│RUAG Aviation│22 -2,00E+00 -1,00E+00 0,00E+00 1,00E+00 2,00E+00 3,00E+00 4,00E+00 5,00E+00 0 20 40 60 80 100 120 140 160 180 kt(phi) theta [°] phi=0° phi=10° phi=20° phi=30° phi=40° phi=50° phi=60° phi=70° -2,00E+00 -1,00E+00 0,00E+00 1,00E+00 2,00E+00 3,00E+00 4,00E+00 5,00E+00 0 20 40 60 80 100 120 140 160 180 kt(phi) theta [°] phi=0° phi=10° phi=20° phi=30° phi=40° phi=50° phi=60° phi=70° StressCheck HyperMesh
23. 23. Off-Axis loading  Conclusions:  2D models show higher Kt values than 3D models  Good match between the FE programs StressCheck and Hypermesh  Overall good match to literature data 02.07.2014│RUAG Aviation│23
24. 24. Outlook  Include pin-bending effect  Countersink modeling 02.07.2014│RUAG Aviation│24
25. 25. Thank you! Any Questions? 02.07.2014│RUAG Aviation│25