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