FRACTURE MECHANICS IN ANSYS R16 Session will begin @ 10:00 AM (Pacific Day Time) • If you will be connected to audio using your computer’s microphone and speakers (VoIP). A headset is recommended. • Or, you may select Use Telephone after joining the Webinar. Welcome to the Webinar. Please make sure your audio is working Feel free to use computer speakers or telephone Type any questions you have here 1
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FRACTURE MECHANICS IN ANSYS R16Session will begin @ 10:00 AM (Pacific Day Time)
• If you will be connected to audio using your
computer’s microphone and speakers (VoIP).
A headset is recommended.
• Or, you may select Use Telephone after
joining the Webinar.
Welcome to the Webinar. Please make sure your audio is working
• The extended finite element method (XFEM) is a numerical technique for solution of problems which involve singularities and high gradient changes
• In fracture mechanics context, it can be used to model cracks by enriching the DOF in the model
• XFEM eliminates the necessity of remeshingcrack tip regions
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XFEM Method - Features
• Offers a way to model the cracks without explicitly meshing the crack surfaces
• Allows for arbitrary crack growth within the existing mesh. No morphing or remeshing is needed
• Initial cracks must be present in the model and can be modeled as traction free or with cohesive zone behavior
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XFEM Method - Details
• Supported on Ansys Mechanical APDL
• PLANE182 and SOLID185 elements (w/o midside nodes) are supported
• CINT command is used to calculated maximum circumferential stress criterion as crack propagation criterion
• CGROW command is used to define the crack-growth-set, fracture criterion and solution control parameters
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XFEM Method - History
• XFEM was developed in 1999, by Ted Belytschkoof North Western University USA
• XFEM is a numerical method built on the generalized finite element method (GFEM) and the partition of unity method (PUM)
• There are academic and commercial implementations
• Ansys R16 is the first release of XFEM, there were user defined programming implementation with Ansys
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XFEM Method - Overview
• The techniques used in XFEM can be listed as:– Singularity-Based Approach
• Accounts for crack tip singularities as well as the jumps in displacements across the crack surfaces. Cracks may terminate inside a finite element
– Phantom Node Method• Accounts only for jumps in displacements across the crack surfaces. Crack tip singularity
is not taken into account. Cracks terminate at the edge/face of a finite element.
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XFEM Method – Crack-Growth Simulation Process
• An XFEM crack-growth simulation is assumed to be quasi-static (i.e. inertial effects are negligible)
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1. Create a Finite Element Model with an Initial Crack
2. Define the Crack-Growth Criterion3. Define the Decay of Stresses on the Newly
Created Crack Segments4. Specify Cohesive Zone Behavior on Initial Crack5. Perform the Crack-Growth Criterion Evaluation6. Perform the Crack-Growth Calculation
XFEM Step-01: FE model with initial crack
• Define an enrichment region in the model with possible crack propagation
• Multiple initial cracks can be defined in the region• Enrichment region should be limited, as addition of extra internal
nodes in the model requires extra computational time
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XFENRICH, EncrihmentID, CompName, MAT_ID
Command to define enrichment region
Named assigned to identify enrichment region
Component name which includes the elements for enrichment region
Used to define cohesive zone behavior for crack faces
XFEM Step-01: FE model with initial crack
• Crack geometry in an element is defined by specifying two signed distance functions at the nodes of the element
• The two signed distance functions at the nodes represent the position of the nodes from the crack surface and from the crack front
• PHI cannot be zero!• The initial crack cannot cut the element at the nodes of the element
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XFDATA, LSM, ELEMNUM, NODENUM, PHI
Command to define initial crack
Element number
Node number
Signed normal distance of the node from crack
XFEM Step-02: Define crack-growth criterion
• Crack-growth criterion must be specified for newly cracked segments• The crack segments are such that they fully cut the elements ahead of the
crack• The crack propagates at the rate of only one element at a time
• VM287Initial Crack Growth Angle in a Pure Shear Problem with the XFEM Method
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Verification Problem from Ansys Help
• VM287Initial Crack Growth Angle in a Pure Shear Problem with the XFEM Method
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Example Problem from Ansys Help
• help/ans_frac/Hlp_G_FRACXFEM.html#d0e5542
Crack Growth in a Three-Point Bending Specimen
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Example Problem from Ansys Help
• help/ans_frac/Hlp_G_FRACXFEM.html#d0e5542
Crack Growth in a Three-Point Bending Specimen
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Example Problem from Ansys Help
• /device,vector,1
• /pnum,node,1
• /pnum,elem,1
• /number
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XFEM Method – Limitations & Assumptions
• Only linear elastic materials can be used in conjunction with the available fracture criteria
• Multiple initial crack can be defined in the region• The XFEM analysis is assumed to be quasi-static, only!• Crack tip singularity effects are not incorporated into the
analysis (ie. Phantom Node Method is the only supported method)
• In 3D XFEM, hexahedral SOLID185 elements are required where crack is initiated and expected to propagate
• Time stepping needs to be small enough to capture crack propagation
• Element birth/death is not supported• Distributed Ansys is not supported
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Following Week Schedule
• April 9th: 10AM-10:45AM PT Fracture Mechanics with Cohesive Zone and Delamination
• April 16th: 10AM-10:45AM PT Fatigue Crack Propagation