09/24/2018 Exploring Capabilities of XFEM for use in Flaw Evaluations.1
Objectives
Introduce Plan to Investigate Use of xFEM Methods to Investigate PWSCC Growth
– Motivation, Plan, Preliminary Results – Feedback
Discuss International Efforts/Collaboration
2
Motivation
• RES has identified that future PWSCC flaw evaluations may involve 3D Finite Element Models of crack growth with complex stresses in asymmetrical components.
• In order to be able to properly evaluate this kind of model RES plans to continue building upon its computational expertise in this potentially regulatory significant area.
• Developing these capabilities and sharing what we learn we hope to improve and modernize how these issues are evaluated in the future
3
Current FEA Application – Model WRS in complex geometries by simulating weld parameters
1. Perform thermal analysis 2. Impose thermal history in mechanical analysis to generate WRS profile
– Multiple weld histories can be explored without the need for new model or re-meshing
– These Results can then be used to calculate crack growth rates
No Repair
Thermal Model Mechanical Model4
Traditional FEA vs xFEM Traditional FEA • Results in very accurate SIF and
stress states • Requires re-meshing for any
change crack size or geometry • Analysis usually limited to
idealized crack shapes and planar crack growth due to complexity of models
xFEM • Mesh-independent analysis of
• Can model realistic 3D crack growth without re-meshing
Traditional FEA Crack Growth
• Crack growth calculations based on ASME Methodology – 2D approximations – Uses idealized crack shape and
growth model (semi-elliptical) – Assumes planar crack growth,
perpendicular to pipe wall – WRS profile for crack growth is
path dependent and user defined
6
Model Material Behavior – Develop material property relationships using
experimental component geometry (e.g. C(T) Specimen) to benchmark model response.
Industry Relevant Models – Compared xFEM results to similar industry relevant
models evaluated using traditional methods.
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Methodology • ABAQUS can simulate fatigue crack growth using a Paris Law type relationship

=
• ASME Code analysis determines PWSCC crack growth behavior using a similar power law relationship
=
• G and K are interrelated by a linear relationship in LEFM
= 2
′
where E'=E for plane stress and = (1−2)
for plane strain
Benchmark RES
Difference % Error (Handbook
Value of 2.826) 2 2.8537 2.8712 0.61098 1.599 3 2.9643 2.9815 0.57967 5.504 4 3.0027 2.9956 0.23679 6.001 5 2.9696 2.9726 0.10218 5.189
Fatigue Crack Growth (Video)
=
∞
9
Status
• Successfully used ABAQUS simplified fatigue crack growth method, to grow a crack in a static stress field (externally applied or internal) in a 2D model
• Developed preliminary parameter relationships between ABAQUS fatigue model and PWSCC model
• Cracking parameter response (G & K) for 2D models shows good agreement with handbook values
10
International Benchmarking Efforts
• In cooperation with Committee on the Safety of Nuclear Installations (CSNI) the NRC is taking part in a technical round robin project exploring X- FEM Capabilities. – Multinational group (Public and Private, 12 Countries) – Focus is on an preliminary comparison of the X-FEM
capabilities of the different codes used in the nuclear industry
– Goal is to share Methodology and Results in order to evaluate capabilities and develop best practices.
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International Benchmarking Efforts
• Three benchmark problems – Surface Crack in Plate – Embedded Crack in Plate
Subjected to Shear Load – Underclad Crack in Core Shell
of an RPV
loading • Static and transient
Growth – International Collaboration Efforts
Objectives
Motivation
Research Project Plan