Development of Reliability Development of Reliability- -Based - - PowerPoint PPT Presentation

Development of Reliability Development of Reliability-

• Based

Based Damage Tolerant Structural Design Damage Tolerant Structural Design Methodology Methodology

Analysis Analysis of Fastener

• f Fastener Disbond

Disbond Arrest Mechanism Arrest Mechanism for Laminated Composite Structures for Laminated Composite Structures for Laminated Composite Structures for Laminated Composite Structures

• Kuen Y. Lin, Chi Ho Cheung, and Phillip Gray
• Department of Aeronautics and Astronautics

University of Washington University of Washington April 21st, 2011

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FAA Sponsored Project Information FAA Sponsored Project Information

• Principal Investigator:
• Dr. Kuen Y. Lin, Aeronautics and Astronautics, UW
• Research Scientist: Dr. Andrey Styuart, UW
• PhD Student: Chi Ho “Eric” Cheung, UW
• Graduate Research Assistant: Phillip Gray, UW
• FAA Technical Monitors: Lynn Pham, Curtis Davies

Oth FAA P l L Il i P t Sh k i h (R t )

• Other FAA Personnel: Larry Ilcewicz, Peter Shyprykevich (Ret.)
• Industry Participants: Marc Piehl, Gerald Mabson, Eric Cregger,

Randy Coggeshall Mostafa Rassaian Cliff Chen Lyle Deobald Randy Coggeshall, Mostafa Rassaian, Cliff Chen, Lyle Deobald, Alan Miller, Steve Precup (All from Boeing)

• Industry Sponsors: Boeing

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Reliability Reliability-

• Based Damage Tolerant Structural

Based Damage Tolerant Structural Design Methodology Design Methodology

• Motivation and Key Issues: Composite materials are being used in

aircraft primary structures such as 787 wings and fuselage In these aircraft primary structures such as 787 wings and fuselage. In these applications, stringent requirements on weight, damage tolerance, reliability and cost must be satisfied. Although currently there are MSG-3 guidelines for general aircraft maintenance an urgent need MSG-3 guidelines for general aircraft maintenance, an urgent need exists to develop a standardized methodology specifically for composite structures to establish an optimal inspection schedule that provides minimum maintenance cost and maximum structural that provides minimum maintenance cost and maximum structural reliability.

• Objective: Develop a probabilistic method for estimating structural

Objective: Develop a probabilistic method for estimating structural component reliabilities suitable for aircraft design, inspection, and regulatory compliance.

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Accomplishments Accomplishments

Work Accomplished: Phase 1

(“Development of Reliability-Based Damage Tolerant Structural Design Methodology”)

• Developed the methodology to determine the reliability and maintenance

p gy y planning of damage tolerant structures.

• Developed a user-friendly software (RELACS) for calculating POF and

inspection intervals.

• Developed software interface (VSTM) with Nastran to facilitate stochastic FEA.
• Implemented stochastic FEA to obtain initial/damaged residual strength

variance.

Current Research

• Develop analytical methods to analyze disbond and delamination arrest mechanisms in

bonded structures under mixed mode loading. g

• Conduct experimental studies to validate analytical methods.
• To apply probabilistic methods to assess reliability of bonded structures with fasteners.
• To apply the developed analysis methods to design and optimization of composite

structure.

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Phase 2: Analysis of Crack Arrest Mechanism Phase 2: Analysis of Crack Arrest Mechanism

• Objectives
• To understand the effectiveness of delamination/disbond arrest

mechanisms mechanisms

• To develop analysis tools for design and optimization
• Tasks

Tasks

• 1. Develop Finite Element models in ABAQUS [completed]
• 2. Develop 1-D (beam) [in progress] and 2D (plate) analytical

biliti [ di ] capabilities [pending]

• 3. Implement reliability analysis capability [in progress]
• 4. Conduct sensitivity studies on fastener effectiveness and stacking
• 4. Conduct sensitivity studies on fastener effectiveness and stacking

sequence effects [in progress]

• 5. Develop and conduct validation experiments [in progress]

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Integrated Composite Structures with Fasteners Integrated Composite Structures with Fasteners

• Model Skin/Stringer as beams

M d l F t i

• Model Fastener as springs

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Analytical Approach Analytical Approach

Th R l i h Rit S l ti i th PMPE

• The Rayleigh-Ritz Solution using the PMPE
• Crack-tip forces resolved from static equilibrium
• Use the VCCT for calculating mixed mode SERR
• Use the VCCT for calculating mixed-mode SERR
• Exponential term to account for stress gradient at the

crack-tip  U W 

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1 2

L A Eq

u U E A dx x         



1 1

 

i i

u a x 

0;  

Total Total

U W   

2 2 1

1 ( ) ; 2

F F

U k u u  

 

u W N N        1

F F

k C 

 

2

t x L i j i j

u a x b e



         



 

A T X L

W N N x



      

1 2

1 1 1 1 2 2 2

a F

t t b C d n t E nt E t E nt E                 

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1 1 2 2 1 3 2 3

2 2 2 d n t E nt E t E nt E    

Analytical Method Flow Chart Analytical Method Flow Chart

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Design Validation Experiment Design Validation Experiment

• The objective is to design a test specimen that will result

in pure Mode II crack propagation Classical “bending type” specimens such as ENF are

• Classical bending type specimens, such as ENF, are

not suitable because

– Relatively thick compared to specimen length; specimen dimensions coupling dimensions coupling – It does not provide enough space for crack propagation

• An “axial type” specimen is proposed

yp p p p

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Design of Mode II Test Specimen V.1 Design of Mode II Test Specimen V.1

• Axial-type specimen to test crack arrestment behavior

– Allows sufficient length for crack to propagate

S t i 3 b i li i t li f

• Symmetric 3-beam specimen eliminates coupling of

bending and axial deformations that occur in 2-beam

• First proposed specimen is a 3-beam model with load

p p p applied to the center beam

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Mode II Test Specimen V.1 Mode II Test Specimen V.1 -

• Preliminary Findings

Preliminary Findings

• 3-Beam with center loaded results in

mixed mode crack propagation

– Configuration results in opening moment Configuration results in opening moment at crack tip

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Design of Mode II Test Specimen V.2 Design of Mode II Test Specimen V.2

• Reversed Loading elements, i.e. apply tension to the
• uter two beams

Cl i t i t t th k ti

• Closing moment exists at the crack tip
• Results in pure Mode II crack propagation

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Mode II Test Specimen V.2 Mode II Test Specimen V.2 -

• Preliminary Findings

Preliminary Findings

• Outer beam loading

– Pure Mode II crack propagation

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Analytical vs. FEM results Analytical vs. FEM results

• Analytical and FEM results show good correlation

Load vs. Crack Length

(3 x 24-ply quasi-isotropic laminates)

70000 80000

(3 x 24 ply quasi isotropic laminates)

40000 50000 60000

d (lb)

20000 30000 40000

Load

GIIC=7 (Abaqus) GIIC=7 (Analytical) 10000 0 5 1 1 5 2 2 5 3 GIIC=7 (Analytical) GIIC=25 (Abaqus) GIIC=25 (Analytical)

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0.5 1 1.5 2 2.5 3

Crack Length (in)

Effect of Hole Clearance Effect of Hole Clearance

• Crack propagates beyond the fastener for a finite

distance before the fastener begins to load

• 1-D idealization sets a limit on what to expect in testing

and the design of real structures

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Effect of Hole Clearance Effect of Hole Clearance

• A look at how much crack length to expect before crack

arrestment

1 2 1.4

n)

Unarrested Crack Length Past Fastener

0.8 1 1.2

ck Length (in

0.4 0.6

rrested Crac

GIIC = 5.0 lb/in GIIC = 7.5 lb/in 0.2 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045

Unar Fastener Hole Clearance

GIIC = 10.0 lb/in GIIC = 12.5 lb/in 16

Design of the Prototype Specimens Design of the Prototype Specimens

S i f t d b B i

• Specimens are manufactured by Boeing

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Prototype Specimen Details Prototype Specimen Details

• 24-ply quasi-isotropic laminate with 0/90 fabric on top

and bottom S i i t t th i d b di

• Specimen is put together using secondary bonding
• ¼” Titanium fastener installed at half installation toque:

40 in-lb 40 in lb

• Initial cracks are implanted at the secondary bonding

interface with Teflon inserts

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Prototype Specimen Testing Prototype Specimen Testing

S i t t d i t i I t t t hi

• Specimens are tested in tension on Instron test machine
• Crack initiation is followed by ultimate failure

– Filled/Empty-hole tension failure of the outer laminates Filled/Empty hole tension failure of the outer laminates

• Bridging observed, crack jumps from the bondline to a

couple plies into the outer laminates

• Fracture toughness of the secondary bond is too high

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Composite Specimen Composite Specimen Delamination Delamination Inspection Inspection

• C-Scan of tested specimens for fastener vs. no fastener
• Fastener affects the growth of cracks

g

With fastener Without fastener

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Prototype Specimen Summary Prototype Specimen Summary

• Analytical method and FEM were used to design Mode II

specimen C k i iti ti l d i hi h th th lti t f il

• Crack initiation load is higher than the ultimate failure

load of the outer laminates

• The prototype Mode II specimen shows that the fastener

The prototype Mode II specimen shows that the fastener affects crack propagation

• Competing failure modes (filled hole tension) must be

dd d addressed

– Decrease GIIc; co-cured vs. secondary bond – Adjustment to geometry; increase w/a, increase thickness j g y – Change center/outer laminate stiffness

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Work in Progress / Future Work Work in Progress / Future Work

• Compete the current test program
• Generate engineering design curves with 1-D

analytical solution analytical solution

• Develop 2-D plate based analytical model
• Improve and conduct more validation experiments
• Improve and conduct more validation experiments
• Consider multiple fasteners
• Identify key variables for design and optimization
• Identify key variables for design and optimization
• Perform parametric/sensitivity analyses

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A Look Forward A Look Forward

Benefit to Aviation – Provide analysis tools for fastener arrest mechanism Provide analysis tools for fastener arrest mechanism – Provide a fail-safe path to the design of integrated it t t composite structures – Optimization can lead to weight savings while properly addressing safety issue – Integrating with probabilistic analysis method can Integrating with probabilistic analysis method can properly address design uncertainties

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Acknowledgements Acknowledgements

This study was supported by the FAA through the AMTAS (Advanced Material for Transport Aircraft Structures) at the University of Washington, and The Boeing Company. The FAA technical monitors are Ms. Lynn Pham, Mr. Curt Davies and Dr. Larry Ilcewicz. The Boeing technical monitor is Marc Piehl. Other Boeing technical personnel involved are E i C d G ld M b Th i t d l bl Eric Cregger and Gerald Mabson. Their support and valuable discussions are appreciated. Additional thanks to Bill Kuykendall at the University of Washington for his assistance Kuykendall at the University of Washington for his assistance with experimentation.

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Thank You Q i ? Questions? Comments? Suggestions?

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