Theory of Critical Distances for Structural Analysis of Welds on a Small Wind Turbine Nacelle using ANSYS Mechanical

Transcription

1 Titelmasterformat A Comparison of the Effective Notch Presentation title durch Stress Method Klicken and the bearbeiten Theory of Critical Distances for Structural Analysis of Welds on a Small Wind Turbine Nacelle using ANSYS Mechanical Fiachra Ó Brádaigh, R&D Engineer, Airsynergy David Smyth, R&D Engineer, Airsynergy CADFEM

2 Airsynergy founded 2008 Dublin/Granard Product Partners development and licensing in Ireland/UK/USA/Germany Products: Offgrid Passive 6 Microturbine/Solar Hybrid (Lidl/JJ Rhatigan/Illinois IT) Exhaust Louvre kw HAWT in development CADFEM

3 Problem: Nacelle frame design & analysis IEC Structural Static Idealised Longitudinal Machined Joints: SWT certification & fatigue analysis cases for thrust/yawing/braking etc. steel construction (static elastic) C-channels with lateral L-channels for support reference plane for proper alignment non penetrating fillet welds & bolted fasteners CADFEM

4 Submodelling: Cut Boundaries Find displacements in overall coarse model Find notch stresses in finely meshed submodels Nine submodels using breaks in geometry and reasonable element counts Cut boundaries avoid stress gradients Local loads applied as needed Different load cases may require different cut boundaries CADFEM

5 Submodelling: element count & solver resources Model Element Solve time Ram GB count hh:mm Base model 1,388, :44 Sub 01 6,553, :47 Sub 02 5,297, :13 Sub 03 2,510, :39 Sub 04 2,637, :02 Sub 05 2,441, :03 Sub 06 5,083, :28 Sub 07 8,431, :17 Sub 08 5,139, :15 Sub 09 5,117, : GB peak 59:44 total CADFEM

6 Effective notch method: Geometry and meshing Modify Fillet Fine Stress Fatigue geometry to allow analytical solution of stress field (Radaj, Neuber). welds modelled as chamfers with 1 mm fillet radii at toe and root. mesh for normal and tangential stress gradients (quadratic elements 0.25 mm) measured on surface of weld toe notch average max principal stress on fillet surfaces strength assessed e.g. IIW FAT225 S-N curve. ρ ρ s ρ f = ρ + s ρ = actual notch radius = factor for multiaxiality = microstructural length It is a type of distance method CADFEM

7 TCD method (Theory of Critical Distances) Stress Length For Point Line Stress/distance field fully resolved in FEA - Quadratic elements scale L = 1 π K 2 c 1 or σ u π mild steels, L = 0.43 mm K th σ o 2 Method (PM): Critical distance = 0.5*L Method (LM): Average over 2*L curves plotted perpendicular to local max principal stress CADFEM

8 TCD with Mesh Control Fixed Defeatured Linear Error No Coarser mesh size = 2.87*L in vicinity of notch length scale is the mesh size mesh with hard sizing tetrahedral elements intentional error records hot-spot stress equal to Point Stress path information required read toe hot spots mesh possible than with other methods ACT extension in development by CADFEM Use Automate look up table for material data mesh sizing process CADFEM

9 Comparison max principal stresses (MPa) at max retarder torque Stresses TCD But very low ok for fatigue methods compared to notch method: errors mostly <20% some outliers (>20%) to be investigated - further work but TCD is promising Fillet Weld ID TCD PM TCD LM TCD Mesh Control ENM % err (PM - ENM)* % err (TCD Mesh Control - ENM)* * Minus indicates underprediction CADFEM

10 Benefits of TCD with Mesh Control Understandable in terms of mechanical properties K c, σ u etc. Preprocessing: System No Mesh detailed geometry modification easier for geometry iterations settings defined for all submodels resources: Effective Notch Method TCD LM/PM TCD mesh sizing #Elements 7,384,589 (TET10) 5,717,622 (TET10) 2,810,411 (TET4) Postprocessing: RAM 54.7 GB 15.2 GB 7.5 GB No additional coordinate systems/result paths CADFEM

11 Future work with TCD analysis Multiaxial Load Unsteady/variable Validate Certification problems shaft/rotor hub histories for fatigue assessment (LDD etc.) amplitude/out-of-phase loads with experimental testing (rotor hub) body requirements? CADFEM

12 References Taylor, D., Barrett, N. and Lucano, G., Some new methods for predicting fatigue in welded joints. International Journal of Fatigue, 24(5), pp Vargiu, F., Sweeney, D., Firrao, D., Matteis, P. and Taylor, D., Implementation of the Theory of Critical Distances Using Mesh Control. Theoretical and Applied Fracture Mechanics. Fricke, W., IIW recommendations for the fatigue assessment by notch stress analysis for welded structures. Paris: International Institute of Welding CADFEM

13 Q&A CADFEM