Introducing Residual Stresses in FEA though INISTATE Command Eng. Paulo Rogério Franquetto – IPEN Prof. Dr. Miguel Mattar Netto – IPEN Presentation Topics • IPEN Overview • Problem Description • Methodology • Results • Conclusion • References Nuclear and Energy Research Institute – (IPEN) The Institute was founded in 1956 with the main purpose of doing research and development in the fields of nuclear energy and its applications. It is located at the campus of USP. It has over 1.000 employees. IPEN is recognized as a national leader institution in research and development in the areas of radiopharmaceuticals, industrial applications of radiation, basic nuclear research, nuclear reactor operation, materials science and laser technology. Problem Description Normally, the residual stresses reduce the structure capacity of loads supporting and they contribute to crack propagation. INISTATE command, available in Ansys 15, can be used to take in account the residual stresses in FEA (Finite Element Analysis). The application of INISTATE command to introduce residual stress in FEA will be verified by benchmarked results comparison, considering different types of structures: column, pressure vessel and piping. Methodology – General Approach - A residual stress profile obtained from the literature is introduced in FEM (Finite Element Model) through INISTATE command. - A non-linear buckling analysis is performed in order to verify the influence of the residual stress in the structure behavior. - The FEA results are compared with the reference results (benchmark). CALCULATION INISTATE Residual stress profile obtained from reference FEM FEA results compared with the reference results Methodology – Benchmarking studies Three studies are selected in order to be compared with the obtained results from FEA with INISTATE command. Non linear buckling assessment is performed in all studies. Reference Benchmarking Structure How the residual stress profile was considered in the reference? [1] Column Modifying the material stress x strain curve [2] Ring Stiffened Pressure Vessel Modifying the material stress x strain curve [3] Pipe The residual stress is measured before an experimental colapse test Methodology – General FEA information Based on the studied references, similar FEA are performed using Ansys 15. Hypothesis assumed in all FEA performed - Shell181 finite element is used (6 degrees of freedom Ux, Uy, Uz, Rotx, Roty and Rotz); - Arc-length non-linear solution method is applied; - Geometric non linearities are applied in the model based on senoidal shapes; - Material non linearities are considered (isotropic hardening); and - Residual stress profile is introduced in the integration points of the finite elements using INSTATE, considering the element coordinate system. Column FEA Geometric parameters (mm) Boundary conditions Material Simple support ASTM A-36 steel Column section 0.6 ≤ Length (L) ≤ 6.0 Geometric Imperfection z v( z ) vo sen L Symmetric Column FEA Residual Stress Profile Flange Web Ring Stiffned Pressure Vessel FEA Geometric parameters (mm) Boundary conditions Material Geometric Imperfection HY80 z v( , z ) v1 cos n1 v2 cos M steel Ring Stiffned Pressure Vessel FEA Residual Stress Profile Pipe FEA Geometric parameters (mm) Lp 970 Rm 53.375 t 6.75 Simple support Material Aluminium Boundary conditions Geometric Imperfection z v( , z ) v1 (cos n1 )sin L p Simple support Linear pressure Pipe FEA Longitudinal stress (MPa) Circunferencial stress (MPa) Residual Stress Profile Radius (mm) Radius (mm) Integration points aproximation Experimental data Results - Column The maximum difference of 5% is obtained comparing the results from FEA with INISTATE command with the ones presented in reference [1] . Buckling mode shape Euler FEA w/ INISTATE Results Reference [1] Results – Pressure Vessel The maximum difference of 1.4% is obtained comparing the results from FEA with INISTATE command with the ones presented in reference [2]. Collapse Depth (m) Residual Stress Diference (%) Reference [2] FEA w/ INISTATE Command No 553 561 1.4 Yes 533 537 0.8 Interframes buckling mode GOALS - Piping A difference of 1.0% is obtained comparing the results from FEA with INISTATE command with the experimental collapse results presented in reference [3]. Collapse Depth (m) Residual Stress Reference [3] FEA w/ INISTATE Command No * 3711 * Yes 3660 (Experimental data) 3600 -1.0 Buckling mode shape Diference (%) Conclusion The results provided by FEA with INISTATE are in good agreement with the ones presented by the used references. Based on the results comparison it can be said that the INISTATE command has introduced properly the residual stress profiles in the non linear buckling FEA of the column, of the ring stiffned pressure vessel and of the pipe. References [1] GOMES, C. A. B.. Resistência à Compressão de Perfis H Laminados de Abas Paralelas. Master Teses. UFOP. 2006. [2] GANNON, L.. Prediction of the Effects of the Cold Bending on Submarine Pressure Hull Collapse. Defence R&D Canada – Atlantic. April 2010. [3] LE GROGNECA, Philippe; CASARIB, Pascal and CHOQUEUSEC, Dominique. Influence of residual stresses and geometric imperfections on the elastoplastic collapse of cylindrical tubes under external pressure. Marine Structures. October 2009, Volume 22, Issue 4, Pages 836-854. Legend E – Young modulus v1 – σy – Yield stress v2 – Maximum defect amplitude between stiffiners vo – Maximum column defect – Poisson coefficient Maximum global defect amplitude Rm – Averaged radius n1 – Global buckling mode t - Thickness n2 – Local buckling mode between stiffiners M – Distante between stiffiners θ – Radial coordinate z – axial coordinate L – Column length Lp – Pipe length
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