Introducing Residual Stresses in FEA though INISTATE

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