Damage Tolerance Testing and Analysis Protocols for Full-Scale Composite Airframe Structures Under Repeated Loading John Tomblin, PhD Executive Director, NIAR and Sam Bloomfield Distinguished Professor of Aerospace Engineering
Waruna Seneviratne Sr. Research Engineer, NIAR
The Joint Advanced Materials and Structures Center of Excellence Motivation & Key Issues
• Produce a guideline FAA document, which demonstrates a “best practice” procedure for full-scale testing protocols for composite airframe structures with examples
• Although the materials, processes, layup, loading modes, failure modes, etc. are significantly different, most of current certification programs use the load-life factors generated for NAVY F/A-18 program. – Guidance to ensure safe reliable approach – Correlate certified “life” to improved LEF (load-life shift)
• With increased use of composite materials in primary structures, there is growing need to investigate extremely improbable high energy impact threats that reduce the residual strength of a composite structure to limit load. – Synthesize damage philosophy into the scatter analysis – Multiple LEF for different stage of test substantiation
The Joint Advanced Materials and Structures Center of Excellence 2 Objectives
Primary Objective • Develop a probabilistic approach to synthesize life factor, load factor and damage in composite structure to determine fatigue life of a damage tolerant aircraft – Demonstrate acceptable means of compliance for fatigue, damage tolerance and static strength substantiation of composite airframe structures – Evaluate existing analysis methods and building-block database needs as applied to practical problems crucial to composite airframe structural substantiation – Investigate realistic service damage scenarios and the inspection & repair procedures suitable for field practice
Secondary Objectives • Extend the current certification approach to explore extremely improbable high energy impact threats, i.e. damages that reduce residual strength of aircraft to limit load capability – Investigate realistic service damage scenarios – Inspection & repair procedures suitable for field practice • Incorporating certain design changes into full-scale substantiation without the burden of additional time-consuming and costly tests
The Joint Advanced Materials and Structures Center of Excellence 3 Approach
The Joint Advanced Materials and Structures Center of Excellence 4 FAA Sponsored Project Information
• Principal Investigators – Dr. John Tomblin and Waruna Seneviratne • FAA Technical Monitor –Curt Davis • Other FAA Personnel Involved – Dr. Larry Ilcewicz – Peter Shyprykevich (consultant ret. FAA)
• Industry Participation
Workshops for Composite Damage Tolerance & Maintenance 2008 CMH-17: PMC Forum & DTTG Meeting 2007 FAA/CACRC/EASA - Amsterdam, Netherlands . 2006 FAA Workshop - Chicago, IL
The Joint Advanced Materials and Structures Center of Excellence 5 Transport Aircraft Applications
We all think about these applications … but …
The Joint Advanced Materials and Structures Center of Excellence 6 Other Applications of Advanced Materials
Cirrus Lancair
Horizon
Adam Aircraft Spectrum Javelin Citation X
Predator Epic Premier I
Honda Bell Helicopter
Global Hawk SpaceshipOne Liberty Toyota Aircraft
The Joint Advanced Materials and Structures Center of Excellence 7 Scatter Analysis
• Background – most test programs reference the Navy/FAA reports by Whitehead, et. al., (1986) and follow that approach – Most test programs have used the conclusions developed in this report regardless of design features, failure modes and/or materials • EADS-CASA study used the same approach (2001) but redefined the shape factors Integrates well into building-block approach based upon design- specific information gained from various coupon and element The Joint Advanced Materials and Structures Center of Excellence 8 Flaw Growth
• Complex failure modes • Compliance change is a • Require extensive NDI function of material, layup, test • Variable B-basis (scatter) at environment, loading mode, different stress levels stress level, etc.
The Joint Advanced Materials and Structures Center of Excellence 9 Load-Life Combined Approach
Increase applied loads in fatigue tests so that the same level of reliability can be achieved with a shorter test duration LEF
N Structure is tested for additional fatigue life to achieve the desired level of reliability
The Joint Advanced Materials and Structures Center of Excellence 10 Material Databases
– FAA-LEF »AS4/E7K8PW » T700/#2510 PW » 7781/#2510 8HS – FAA-Laminate » T700/#2510 UNI » T700/#2510 PW » T700/E765 UNI » T300/E765 PW » AS4C/MTM45 UNI » AS4C/MTM45 8HS – FAA-Adhesive Fatigue » Loctite Paste » PTM&W paste (two bondline thicknesses) » EA 9696 film – FAA-Adhesive Effects of Defects » T700/#2510 PW & EA9394
The Joint Advanced Materials and Structures Center of Excellence 11 Shared Databases
Laminate Statistical Allowable Generation for Fiber-Reinforced 40/20/40 Single Shear Bearing-Tension -- (RTD) 45.4085 Double Shear Bearing-Tension -- (RTD) 49.696 Composite Materials: Lamina Variability Method Bearing-Bypass 50%-Tension 43.4258 Bearing-Bypass 50%-Compression 42.102 Bearing-Bypass 50%-Tension [t/D=0.475] 40.0401 [DOT/FAA/AR-06/53] Bearing-Bypass 50%-Tension [t/D=0.570] 42.5935 ReliaSoft's Weibull++ 6.0 - www.Weibull.com Bearing-Bypass 50%-Tension [t/D=0.949] 38.1976 Reliability vs Time Open Hole-Tension [w/D=6] 27.2021 1.00 Weibull Filled Hole-Tension 20.2959 Static No Hole-Tension 29.8203 W2 RRX - SRM MED No Hole-Compression 20.5843 F=48 / S=0 Open Hole-Compression 30.4534 Critical Hole-Tension -- (CTD) 29.9075 0.80 Critical Hole-Tension -- (ETD) 25.1923 V-Notched Rail Shear 59.2079 Open Hole-Tension [w/D=3] 20.594 Open Hole-Tension [w/D=4] 27.0538 0.60 Open Hole-Tension [w/D=8] 25.4413
25/50/25 Double Shear Bearing-Tension -- (CTD) 25.7206 Double Shear Bearing-Tension -- (RTD) 43.8267 Double Shear Bearing-Tension -- (ETW) 34.7752 0.40 Reliability, R(t)=1-F(t) Single Shear Bearing-Tension -- (CTD) 28.956 Single Shear Bearing-Tension -- (RTD) 18.1315 Single Shear Bearing-Tension -- (ETW) 33.8501 Bearing-Bypass 50%-Tension 44.2636 Bearing-Bypass 50%-Compression 48.0284 0.20 Open Hole-Tension [w/D=6] -- (CTD) 35.8156 Open Hole-Tension [w/D=6] -- (RTD) 34.0488 Open Hole-Tension [w/D=6] -- (ETW) 25.2227 Waruna Senev iratne No Hole-Tension -- (CTD) 51.1531 NIAR 0 2/6/2007 22:52 No Hole-Tension -- (RTD) 40.1864 0 14.00 28.00 42.00 56.00 70.00 No Hole-Tension -- (ETW) 38.383 Time, (t ) 0 40.00 80.00 No Hole-Compression -- (CTD) 31.498 No Hole-Compression -- (RTD) 27.0743 β=2.9412, η=39.8364, ρ=0.9955 No Hole-Compression -- (ETW) 23.6762 T700SC-12K-50C/#2510 -Plain Weave Fabric Open Hole-Compression -- (CTD) 34.4747 Open Hole-Compression -- (RTD) 46.9989 863 specimens Open Hole-Compression -- (ETW) 33.3186 α 2.941 V-Notched Rail Shear 16.4582 β 39.836
10/80/10 Bearing-Bypass 50%-Tension 65.4454 Bearing-Bypass 50%-Compression 74.3601 Open Hole-Tension [w/D=6] 51.7133 MODAL (EXTREAM) No Hole-Tension 58.0843 αR 34.587 No Hole-Compression 36.0558 Open Hole-Compression 50.909 V-Notched Rail Shear -- (CTD) 9.9634 V-Notched Rail Shear -- (RTD) 17.2784 12 V-Notched Rail Shear -- The(ETW) Joint Advanced13.1027 Materials and Structures Center of Excellence Fatigue Scatter Analysis Techniques
• Individual Weibull 50.0 12.0 45.0 Stength (Static) Stength (Pooled) 10.0 40.0 CV (Static) • Joint Weibull CV (Pooled) 35.0 8.0 • Sedeckyj Equivalent Strength Model 30.0 25.0 6.0
20.0 Strength (psi) Data Pooling Techniques 4.0 15.0 Coeff. Variation (%)
10.0 2.0 5.0 Sendeckyj (w / static) 7.0 0.0 0.0 Sendeckyj (w /o static) (R=5) (R=5) 6.0 Individual Weibull (w /o static) (R=0) OH (R=-1) OH (R=-1) OH OH (R=-1) OH CAI- BVID CAI- OHT (R=0) OHT TAI - BVID - TAI Flex (R=0) OHC (R=5) OHC CAI - BVID DNC (R=-1)DNC OH (R=-0.2)OH DNC (R=-0.2)DNC TAI - VID (R=0) CAI - VID (R=5) CAI - VID (R=5) CAI - VID (R=5)
F 5.0 10/80/10 0/100/0 HRH 10 25/50/25 40/20/40 α Sandwich 4.0
3.0
Life Parameter, 2.0
1.0 NADC Fatigue Scatter Analysis 0.0
αI > αJ > αS OH (R = -1) = (R OH -1) = (R OH OH (R = -1) = (R OH OHT (R = 0) = (R OHT OHT (R = 0) = (R OHT OHC 5) (R = DNC (R = -1)DNC (R = OH (R = -0.2) = (R OH DNC (R = -0.2) DNC (R = TAI - VID (R = 0) = (R - VID TAI CAI - VID (R = 5) (R = -CAI VID 5) (R = -CAI VID t =0.010 (R 0.1) = t =0.060 (R 0.1) = CAI - BVID (R = 5) (R = -CAI BVID TAI - (RBVID 0) = CAI - VID [40-ply] (R = 5) = (R [40-ply] - VID CAI CAI- BVID [20-ply] (R = 5) = (R [20-ply] BVID CAI- 10/80/10 0/100/0 Sandw ich Bonded 25/50/25 40/20/40 Joints
The Joint Advanced Materials and Structures Center of Excellence 13 Adhesive Scatter Analysis
Shape Parameters for Fatigue Data [2, 5, and 10 -Hz combined] w/ Static Data 6000
CTD RTD RTW # of Specs. 5000 Loctite 0.805 0.662 0.682 95 Kassapoglou 4000 EA9696 0.847 0.403 0.379 103 Sendeckyj PTM&W (0.06") 0.870 1.051 0.681 101 3000
PTM&W (0.16") 0.363 0.856 0.671 91 Stress (psi)
2000 Experiment Residual Strength Total 390 Equivalent Static Sterngth 1000 Sendeckyj α 3.8538 Kassapoglou
β 0.7641 0 1 10 100 1,000 10,000 100,000 1,000,000 Modal Value 0.707 Cycles (N) w/o Static Data (Sendeckyj) w/o Static Data (Individual Weibull) CTD RTD RTW # of Specs. CTD RTD RTW Loctite 0.821 1.624 0.644 86 Loctite 1.069 1.226 1.109 EA9696 4.119 1.389 2.189 88 EA9696 2.372 2.077 1.110 PTM&W (0.06") 1.376 1.483 1.169 92 PTM&W (0.06") 1.541 1.179 1.417 PTM&W (0.16") 0.669 4.296 1.618 82 PTM&W (0.16") 1.165 2.170 1.061
Total 348 α 1.6789 α 3.3394 β 2.016 β 1.6255 Modal Value 1.176 Modal Value 1.461
The Joint Advanced Materials and Structures Center of Excellence 14 Life Scatter Summary
3.0
2.5
2.0
1.5
1.0 Fatigue Factor Scatter 0.5
0.0 adhesive adhesive adhesive adhesive adhesive adhesive w/ static & static w/ w/ static & static w/ w/ static & static w/ w/ static & static w/ w/ static & static w/ w/ static & static w/ w/o adheive w/o adhesive w/ w/o adheive w/o adhesive w/ w/o adheive w/o adhesive w/ w/o static & static w/o & static w/o w/o static & static w/o & static w/o w/o static & static w/o & static w/o FAA-LEFAS4/E7K8 & FAA-Adhesive PW FAA-LEFT700/#2510 & FAA-Adhesive PW FAA-LEF7781/#2510 & FAA-Adhesive 8HS
The Joint Advanced Materials and Structures Center of Excellence 15 Methodology Automation
350
300
250
200 Equivalent Static 150 Strength Data Stress (ksi) Stress
100
50
0 1 10 100 1000 10000 100000 1000000 10000000 Cycles, N The Joint Advanced Materials and Structures Center of Excellence 16 Scatter Analysis Guidelines
N
0 R 0 L NF 0.5 1.0 1.5 3.0 AS4 ~ Individual (C) 32.193 4.057 2.070 1.196 1.096 1.041 0.954 AS4 ~ Individual (C+A) 32.193 2.083 4.418 1.151 1.101 1.072 1.025 1.30 AS4 ~ Sendeckyj (C) 32.193 1.438 9.312 1.140 1.105 1.085 1.052 AS4 ~ Sendeckyj (C+A) 32.193 0.772 88.705 1.132 1.114 1.103 1.085 NAVY AS4 ~ Sendeckyj (C) +Individual (A) 32.193 1.421 9.589 1.139 1.105 1.085 1.053 CASA T700 ~ Sendeckyj (C) +Individual (A) 32.845 1.576 7.516 1.139 1.102 1.080 1.045 AS4/E7K8 - C 7781 ~ Sendeckyj (C) +Individual (A) 36.416 1.660 6.715 1.126 1.091 1.071 1.037 AS4/E7K8 - C+A NAVY 20.000 1.250 13.558 1.229 1.177 1.148 1.099 CASA 19.630 2.740 3.019 1.285 1.167 1.103 1.001 T700/#2510 - C T700/#2510 - C+A 7781/#2510 - C 1.20 7781/#2510 - C+A LEF Reduced test matrix Factors affecting LEF 1.10 Shared database Choice of analysis techniques
1.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Test Duration, N The Joint Advanced Materials and Structures Center of Excellence 17 Case Studies
1.30
NAVY CASA • Shared database Scenario 1 Scenario 2 Scenario 3 Scenario 4 • Multiple scatter analysis 1.20 techniques LEF
1.30 1.10
Improvements to MFSP 1.20 1.00
0.0 2.0 4.0 NF 6.0 8.0 10.0 Test Duration, N • Improved LEF LEF
1.10 NAVY CASA • Improved N Scenario 1 F Scenario 2 Scenario 3 • Improved “life” Scenario 4 – Load-life shift 1.00 0.1 1.0 Further NF 10.0 improvements to Test Duration, N MSSP
The Joint Advanced Materials and Structures Center of Excellence 18 Full-Scale Specimens
Beechcraft Starship Forward Wings (BSFW) Approx. average of 1000 flight hours (assume minimal aging effect), NDE examination
The Joint Advanced Materials and Structures Center of Excellence 19 Damage Scenarios
Categories of Damage & Defect Considerations for Primary Composite Aircraft Structures Category Examples Safety Considerations (Substantiation, Management) Category 1: Damage that may BVID, minor environmental Demonstrate reliable service life go undetected by field inspection degradation, scratches, gouges, Retain Ultimate Load capability methods (or allowable defects) allowable mfg. defects Design-driven safety Category 2: Damage detected VID (ranging small to large), Demonstrate reliable inspection by field inspection methods @ mfg. defects/mistakes, major Retain Limit Load capability specified intervals (repair scenario) environmental degradation Design, maintenance, mfg. Category 3: Obvious damage Damage obvious to operations in Demonstrate quick detection detected within a few flights by a “walk-around” inspection or Retain Limit Load capability operations focal (repair scenario) due to loss of form/fit/function Design, maintenance, operations Category 4: Discrete source Damage in flight from events Defined discrete-source events damage known by pilot to limit that are obvious to pilot (rotor Retain “Get Home” capability CAT1 flight maneuvers (repair scenario) burst, bird-strike, lightning) Design, operations, maintenance Category 5: Severe damage Damage occurring due to rare Requires new substantiation Ultimate CAT2 Design created by anomalous ground or service events or to an extent Requires operations awareness 1.5 Factor Load of Safety flight events (repair scenario) beyond that considered in design for safety (immediate reporting) CAT3 Level Limit ~ Maximum load per lifetime AMTAS Spring 2006 Meeting Federal Aviation 1 1 Administration Continued April 11, 2006 safe flight
REFERRENCE: Ilcewicz, L., “Composite Damage Tolerance and Maintenance Safety Issues,” Allowable Critical Damage Damage Limit Threshold FAA Damage Tolerance and Maintenance Workshop, Rosemont, IL, July, 2006. (ADL) (CDT) Increasing Damage Severity The Joint Advanced Materials and Structures Center of Excellence 20 Load-Life-Damage (LLD) Hybrid Approach
LOAD FACTOR • Provide an opportunity to further investigate large VID without additional test articles
CAT1 LIFE FACTOR
CAT2 DAMAGE CAT3 • Incorporate the effects of damage to scatter analysis • Minimum risk of premature failure of full-scale article • Application to hybrid structure • post certification – Additional load cases that were not included in the original certification, but found to be significant or worth investigating can be studied
The Joint Advanced Materials and Structures Center of Excellence 21 Damage Tolerance Investigation
CAT1 Introduce Defect VID Help define ADL Durability Damage Tolerance Load DLT & LEF DLT & LEF Limit Limit Load Limit Load Load in in critical in critical critical condition condition condition No-growth Threshold and load Damage Category requirements up to large VID 1 2 3
UL
LL
Help define inspection intervals Help define CDT
1 with LEF NF with no LEF (Typically LEF is applied to reduce test duration) The Joint Advanced Materials and Structures Center of Excellence 22 Design Change Substantiation
1.20
Tested LEF NEW LEF LEF
1.10 LEFT
LEFR %∆Load
1.00
1.0 NR NT NF NRT 10.0
Test Duration, N The Joint Advanced Materials and Structures Center of Excellence 23 Impactor Setup
• 1-inch Wedge (pry bar) • 3-inch Wedge (wood-splitter)
The Joint Advanced Materials and Structures Center of Excellence 24 Damage Infliction - BSFW Front Spar -
• Primary load path • Scaling CAT3 damages into full-scale article • Thick part - unitape • Wound - no layers Energy Window for damage infliction for CAT3 After 1000 ft-lb damage at FWS 66.5 (top skin – front spar) Thin Parts Limit Load Large VID Starship Forward Wing Ultimate Load Front Spar BVID
Thick Parts
Extremely Realistic Energy Energy Improbably 25 The Joint Advanced Materials and Structures Center of Excellence Energy Full-Scale Validation
Static Damage Tolerance
Element ST001, ST002, ST003 Static Tests (CAT1) NDI NLFEM
Full-scale Impact Tests
BDLL Æ NRLL CAT2 & CAT3
ST004(R) ST001(R) ST004 Fatigue Test Static Test (CAT2) Fatigue Test (CAT2) (CAT3Æ1)
ST005 ST006 Static Test (CAT3) Fatigue Test (CAT3) Repair Durability
Load-Life LLD Hybrid Shift Approach
The Joint Advanced Materials and Structures Center of Excellence 26 Full-Scale Substantiation - LLD Hybrid Approach -
CAT2 Repair Defect VID
Damage Tolerance Repair Durability
DLT & LEF DLT & LEF (TBD) (TBD) Ultimate Limit Load Limit Load Load in in critical in critical critical condition condition condition
• Demonstrate LLD hybrid CAT3 approach Defect • Application of load-life shift Damage Tolerance
DLT (until failure) • Determine fail safety LEF = 1 Min/Max Fail spectrum Safety • Fatigue capability of CAT3 load
The Joint Advanced Materials and Structures Center of Excellence 27 Benefits to Aviation
• FAA guideline document, which demonstrates a “best practice” procedure for full-scale testing protocols for composite airframe structures with examples – Cost effective and reliable certification approach(s) – Integrate design specific details gained from coupon and subelement tests into the LEF approach • Layup, loading modes/R ratios, Environments, .. • Bonded joints, interlaminar shear, sandwich, .. – Address evolution/maturity of material systems, manufacturing processes, test techniques, etc. • Reduced test matrix • Shared database concept 1.30 – Realistic analysis approach for scatter NAVY CASA Scenario 1 Scenario 2 Scenario 3 • Appropriate analysis techniques for diverse design details Scenario 4 1.20 • User-friendly automated procedures and analysis guidelines
LEF • Notch effects on scatter for damage tolerance testing
1.10
Improvements to MFSP 1.00
0.0 2.0 4.0 NF 6.0 8.0 10.0 Test Duration, N The Joint Advanced Materials and Structures Center of Excellence 28 Benefits to Aviation
• Incorporation of damage into scatter analysis: Load-Life-Damage – Investigate large VID damage – Further studies
• Load-Life Shift – Investigate different categories of damages/repairs in the same full-scale test article damage – Design changes, i.e. gross weight increase
– LEF during certification vs. improved LEF LOAD FACTOR • Reliability of designed life
Category 1 Damage LIFE FACTOR E G A Category 2 Damage M A D Category 3 Damage
The Joint Advanced Materials and Structures Center of Excellence 29 Future Needs
200 0.00 Life •Training 50% -NRLL 0 0.25 Life
0.50 Life -200 0.75 Life
-400 1.00 Life
1.25 Life Axial Strain (microstrain) (microstrain) Strain Axial -600 50% +NRLL • Reliable NDI and health monitoring techniques1 50 Life for damage characterization during full-scale testing and service ~ & Training • Further studies on extremely improbable high energy impact threats and their impact on the residual strength of the composite structure and inspection intervals
The Joint Advanced Materials and Structures Center of Excellence 30 Questions
The Joint Advanced Materials and Structures Center of Excellence 31