Fracture Mechanics
Capabilities in Marc
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Presenter Information
• Zhong Qin @ Storrs, Connecticut
• Lead Technical Representative
(12 years with MSC)
• Primarily support Marc/Mentat, Nastran and Patran
• Also familiar with Dytran, MSC.Fatigue, Laminate
Modeler,...
• Education background
• B.S., Mechanical Engineering
• M.S., Mechanical Engineering
• Ph.D., Computational Mechanics
• Joe Satkunananthan @ Newport Beach, CA
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Seminar Agenda
• Overview of fracture mechanics capabilities• J-Integral – Lorenzi option
• Virtual Crack Closure Technique – VCCT option
• Delamination – crack initiation based on stress criterion
• Crack propagation – direct and fatigue
• Simulation of crack propagation• Crack initiation
• Crack propagation types
• Crack propagation methods
• Crack propagation direction
• Crack propagation criteria
• Demo
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Fracture Mechanics - Overview
• J-Integral
• A path-independent contour integral
for the analysis of cracks
• Equivalent to the energy release
rate for linear elastic material model
• Stress intensity factors KI, KII & KIII
by mode separation
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Fracture Mechanics - Overview
• J-Integral (cont.)
• Integration domain is defined
by rigid regions
• Rigid region may be defined
• Topology search (connectivity)
• Geometry search (radius)
• Manual
• Shift vector can be defined
manually or automatically
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Fracture Mechanics - Overview
• VCCT
• Virtual Crack Closure Technique
• A simpler but more general way for
obtaining energy release rate G
• G = change in potential energy by
a change in crack length
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Fracture Mechanics - Overview
• VCCT crack propagation
• Direct and fatigue crack propagation
types are supported
• Crack grows when G is greater than
material fracture toughness Gc
• Fatigue crack growth increment
calculated by Paris’ Law
• Crack can grow by remeshing
or splitting elements
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Fracture Mechanics - Overview
• Crack initiators
• Modeling existing flaws
• Splitting mesh
• Cutting mesh along a curve
• Remeshing with faceted surface
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Fracture Mechanics - Overview
• Delamination
• Crack initiation due to service loading
• Based on stress criteria
• Split mesh upon failure
• Insert VCCT definition
• Insert cohesive interface elements
• Terminate loadcase when crack is initiated
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VCCT Crack Propagation
• Crack propagation analysis types
• Low cycle fatigue
• High cycle fatigue
• Direct
• Both fatigue and direct
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VCCT Crack Propagation
• Crack growth methods
• Remeshing
• Release constraints
• Split elements
• Cut through elements
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VCCT Crack Propagation
• Crack growth direction
• Maximum hoop stress
• Along pure mode
• Along mode I
• User defined
• Mode I and normal
• User subroutine
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VCCT Crack Propagation
• Fatigue crack propagation properties
• Fixed growth increment size
• Fatigue time period
• Crack growth increment
• Cracks grow with constant increment at the end
of fatigue time period
• Useful for crack growth path
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VCCT Crack Propagation
• Fatigue crack propagation properties
• Growth increment calculated by Paris’ law
• In terms of energy release rate
• In terms of stress intensity
• Perform every fatigue cycle
• Useful for low cycle fatigue
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VCCT Crack Propagation
• Fatigue crack propagation properties
• Fixed with scaling
• Specified growth increment is scaled
• Fatigue law scaling
• Exponential scaling
• High cycle fatigue count
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VCCT Crack Propagation
• Direct crack propagation properties
• Total energy release rate (fracture toughness)
• Individual modes
• Power law mixed mode
• Reeder mixed mode
• Total using Lorenzi
Geometry based rigid region
preferable for 3D problems
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VCCT Crack Propagation
• Available results
• Energy release rate
• Stress intensity factor
• Crack growth direction
• Crack local system
• VCCT failure index
• Accumulated crack growth
• Number of fatigue cyclesGlobal variable
Summary
• Marc supports J-integral and VCCT for fracture mechanics
• For J-integral evaluation, rigid region can be defined by
automatic topology search or geometry search
• Crack initiator may be used to generate initial crack for VCCT
crack propagation
• Stress-based delamination option may work alone or
generate initial crack for VCCT crack propagation
• In fatigue crack propagation, a load sequence is repeated
and crack grows after each sequence
• Paris’ law is used to calculate high cycle fatigue count
• In direct crack propagation, crack grows if a given crack
growth criterion is fulfilled
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DemoFatigue crack propagation in a lug
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Demo (cont.)
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Demo (cont.)
Thank You
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