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 of 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 21 st , 2011 1

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 Other FAA Personnel: Larry Ilcewicz, Peter Shyprykevich (Ret.) FAA P l L Il i P t Sh k i h (R t ) • 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 2

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. 3

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. 4

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 capabilities [pending] biliti [ di ] 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] 5

Integrated Composite Structures with Fasteners Integrated Composite Structures with Fasteners • Model Skin/Stringer as beams • Model Fastener as springs M d l F t i 6

Analytical Approach Analytical Approach • The Rayleigh-Ritz Solution using the PMPE Th R l i h Rit S l ti i th 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          0; 0 U U W W  2   Total Total 1 u  L      U E A dx    A Eq   i x u a x 2 0 i 1 1 1 1      2 t U k ( u u ) ; k    x L  F F 2 1 F     2 2 C i j u a x b e   F i j      u             W W N N N N  A T   x  X L    a   t t b 1 1 1 1        1 2  C F         2 2 d d n t E n t E nt E nt E 2 2 t E t E 2 2 nt E nt E 1 1 2 2 1 3 2 3 7

8 Analytical Method Flow Chart Analytical Method Flow Chart

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 9

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 • Symmetric 3-beam specimen eliminates coupling of S t i 3 b i li i t li f 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 10

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 11

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 outer two beams • Closing moment exists at the crack tip Cl i t i t t th k ti • Results in pure Mode II crack propagation 12

Mode II Test Specimen V.2 Mode II Test Specimen V.2 - - Preliminary Findings Preliminary Findings • Outer beam loading – Pure Mode II crack propagation 13

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) (3 x 24 ply quasi isotropic laminates) 80000 70000 60000 50000 d (lb) 40000 40000 Load 30000 GIIC=7 (Abaqus) 20000 GIIC=7 (Analytical) GIIC=7 (Analytical) GIIC=25 (Abaqus) 10000 GIIC=25 (Analytical) 0 0 0 0 5 0.5 1 1 1 5 1.5 2 2 2 5 2.5 3 3 Crack Length (in) 14

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 15

Effect of Hole Clearance Effect of Hole Clearance • A look at how much crack length to expect before crack arrestment Unarrested Crack Length Past Fastener 1.4 n) 1.2 1 2 ck Length (in 1 0.8 rrested Crac 0.6 GIIC = 5.0 lb/in 0.4 GIIC = 7.5 lb/in Unar 0.2 GIIC = 10.0 lb/in GIIC = 12.5 lb/in 0 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 Fastener Hole Clearance 16

Design of the Prototype Specimens Design of the Prototype Specimens • Specimens are manufactured by Boeing S i f t d b B i 17

Prototype Specimen Details Prototype Specimen Details • 24-ply quasi-isotropic laminate with 0/90 fabric on top and bottom • Specimen is put together using secondary bonding S i i t t th i d b di • 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 18

Prototype Specimen Testing Prototype Specimen Testing • Specimens are tested in tension on Instron test machine S i t t d i t i I t t t hi • 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 19

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 20