Environmental Compensation Factor Influence on Composite Design and Certification

Transcription

1 Environmental Compensation Factor Influence on Composite Design and Certification 2012 Technical Review Waruna Seneviratne & John Tomblin Wichita State University/NIAR

2 Environmental Compensation Factor Influence on Composite Design and Certification Motivation and Key Issues Moisture absorption characteristics of composites, which follow Fick s second law, can be coupled with realistic environmental data to design structurally efficient and economic composite components. This research will provide guidance to establish practical levels of moisture content and corresponding environmental compensation factors for composite structures. Objective Develop guidelines for the development of environmental enhancement factors for static strength loading 2

3 Approach Develop guidelines for the development of environmental enhancement factors for static strength loading Use data developed at lamina, laminate, element and subcomponent to demonstrate application Incorporate a probabilistic model, which accounts for the environmental factors affecting composite design Address any additional research & development needs with environmental factors as budget allows, i.e. effects of non-fickian processes such as capillary action along fiber/matrix interface and through cracks and voids, effects of surface cracking in the resin at free edges due to swelling stresses resulting from moisture desorption on subsequent moisture absorption, environmental factors for adhesive joints and sandwich construction, etc. 3

4 Environmental Compensation Factor Influence on Composite Design and Certification Principal Investigators & Researchers John Tomblin, PhD, and Waruna Seneviratne, PhD Upul Palliyaguru, Shawn Denning, Janith Senaratne FAA Technical Monitor Curtis Davies, Daivd Westlund Other FAA Personnel Involved Larry Ilcewicz, PhD, and Peter Shyprykevich (ret.) Industry Participation Cessna, Bombardier, Hawker Beechcraft, and Spirit Aerosystems 4

5 Environmental load factor to satisfy FAA certification requirements for composite structures, FARs require compliance with , , and (can apply also to Part 25 aircraft). General guidelines for a composite structure should be considered which are over what is normally done for metallic certifications (i.e., account for the difference between composite and metallic structures in certification) an approach which may be used, when combined with analytical modeling, is to apply these overloads within the model to demonstrate compliance after a successful static structural test (may also be applied during the test) and demonstrating positive margins of safety throughout the structure 5

6 Static Load Factor SLF = Static Load Factor» represents the difference in load factor between a composite and metallic structure SLF = C composite var iability F metals variablity C F composite temperature metals temperature C F composite moisture metals moisture Will depend on material system, layup (lamina or laminate), failure mode, damage.. Based upon some existing data, this could be as high as 1.32 Room for improvement in this based upon failure mode in temperature (based upon FEM M.S. model) and amount of moisture actually expected in structure during lifetime Based on percentage of strength at 180 F, approximately

7 Design & Certification Strain DRY WET Ambient external flight condition loading Δ due to thermal loading Δ due to moisture ε ATM Maximum Applied Strain Minimum Margin of Safety Applied Loading Temperature Ambient external flight-load strain unaffected by addition of thermal moisture strains Ambient + Thermal + Moisture Ambient + Thermal Ambient Strain Strain ε A ε AT ε ATM 7

8 Analysis Assumptions Fickian Diffusion Assumptions Moisture concentration behaves according to: c t = D z 2 c 2 z Diffusivity Constant ( in 2 /day ) T700/#2510 (12 ply) 85% RH 8-ply 12-ply hd D ( ) = z T π 4 M D=Ax B m 2 M 2 M t2 t Diffusion behavior constant through thickness Cloth vs. Uni differences are negligible Steady state only Two sided diffusion Through the thickness diffusion dominates End effects neglected Temperature ( F) 8

9 Saturation Levels Ø G(T,t) is the ratio of moisture level at a given time to the saturated moisture content (M m ) 100% 75% Decreasing T and Dz G(T,t) 50% 25% T700/#2510: 12 plies, 85% RH G(80 F,t) G(120 F,t) G(160 F,t) G(180 F,t) 0% Time, t (years) G( T, t) D = 1 exp 7.3 z ( T) t h

10 Moisture Absorption 2.5% Moisture Content ( % ) T700/#2510 PW: 12 ply laminate, 85% RH 80 F 120 F 160 F 180 F % Moisture Concentration 2.0% 1.5% 1.0% T700/#2510 (48 ply) 160 F / 85% RH 1 Hour 1 Week 1 Month 60 Days 100 Days 1 Year 3 Years 6.7 Years h Time (days 0.5 ) 0.5% y 0.0% Distance (z/h) z 10

11 Moisture Absorption for Full Scale Articles What is realistic? Years for 99% Saturation Laminate Thickness (in) F/85%RH F/85%RH Time (years) for 99% Moisture Content T700/# ply 90 F/85%RH 145 F/85%RH ply 12-ply Laminate Thickness (h ) 11

12 Environmental Data 12

13 Service life Assumptions Assume 10 years continuously in worst case conditions (conservative!). This does not indicate a 10 year service life. Merely the worst case that would happen in 10 years with no time at altitude or other drier locations. Max. saturation levels and conditioning criterion Condition a representative article such that it reaches the same saturation level expected for the full scale aircraft in a worst case environment 100% 100% G(T,t) 75% 50% 25% 0% Increasing Laminate Thickness 90 F / 85% RH 12 plies 48 plies 354 plies G(T,t) 75% 50% 25% 0% Increasing Laminate Thickness 180 F / 85% RH 12 plies 48 plies 354 plies Time, t (years) Time, t (years) 13

14 Diffusivity Constant Chamber Temperature ( F) Initial Dry Conditioning (180 F / 85% RH) Specimen Configuration Fully Exposed Sides Covered Fully Covered Initial Wet Conditioning (180 F / 0% RH) Specimen Configuration Fully Exposed Sides Covered Fully Covered - 60 F F F F F F F Total Specimens Required Moisture Absorption of Carbon/Epoxy [+45/0/-45/90]4s 32-Ply Laminates for All Temperatures (All Configurations) - 60 F - 40 F - 20 F 40 F Weight Gain [%] F F F Time [days] 14

15 Diffusivity Constant - Absorption Diffusion Constant for Carbon/Epoxy [+45/0/-45/90]4s 32-Ply Laminates as a function of Temperature Diffusivity Constant [in 2 /day] All Sides Exposed Two sides Covered with Aluminum All sides Covered with Aluminum Tape Temperature ( F) 15

16 Diffusivity Constant - Desorption Diffusion Constant for Carbon/Epoxy [+45/0/-45/90]4s 32-Ply Laminates as a function of Temperature Diffusivity Constant [in 2 /day] All Sides Exposed Two sides Covered with Aluminum All sides Covered with Aluminum Tape Temperature ( F) 16

17 Diffusivity Constant - Summary Diffusion Constant for Carbon/Epoxy [+45/0/-45/90]4s 32-Ply Laminates as a function of Temperature Diffusivity Constant [in 2 /day] Absorption Desorption Temperature ( F) 17

18 Aircraft (Fleet) Environmental Exposure Cruise Climb Descent Ground/Taxi Ground/Taxi T VC RH VC P VC Determination of end-of-life moisture content (conservative approach) 18

19 Cyclic Moisture Distribution Moisture Content [%] Through Thickness [in] Moisture Absorption [%] ~ 52 Days Moisture Desorption [%] ~ 2 Days Residual Moisture [%] = 0.58% 19

20 Effects Moisture Distribution TTT Ultimate Flexural Strength [ksi] Ultimate Compression Strength [ksi] RTD Baseline EMD UMD Moisture Distribution 0 RTD Baseline EMD UMD Moisture Distribution 16 Ply Ultimate Compression Strength [ksi] 32 Ply Ultimate Compression Strength [ksi] 8 16 Ply Ultimate Flexural Strength [ksi] 32 Ply Ultimate Flexural Strength [ksi] 10 Flexural Modulus [Msi] Compression Modulus [Msi] RTD Baseline EMD UMD Moisture Distribution 0 RTD Baseline EMD UMD Moisture Distribution 16 Ply Flexural Modulus [Msi] 32 Ply Flexural Modulus [Msi] 16 Ply Compression Modulus [Msi] 32 Ply Compression Modulus [Msi] 20

21 FLUID-INGRESSED SANDWICH STRUCTURE 21

22 Fluid-Ingressed Sandwich Mode I Testing Material Core Type Core Thickness (in) Facesheet (per F/C) Cell Size (in) Core Density Baseline Static Fluid Ingressed Baseline Fatigue Fluid Ingressed HRH-10 Hexagonal ply [0/45]S 1/ / / ply [0/45]4S 1/ / / OX-Core ply 3/ ply 3/ Sub Totals Total Specimens 288 GIC [kj/m2] da/dn [mm/cycle] 1.E E E E E E- 07 Crack Length a [mm] NIS11- SDT- 41- AD- BL- MODE1- SL1-1 NIS11- SDT- 41- AD- BL- MODE1- SL1-2 NIS11- SDT- 41- AD- BL- MODE1- SL1-3 NIS11- SDT- 41- AD- BL- MODE1- SL1-4 NIS11- SDT- 41- AD- BL- MODE1- SL1-5 NIS11- SDT- 41- AD- BL- MODE1- SL GIC [J/m 2 ] 22

23 SCB Mode I RTD Summary 23

24 SCB Mode I RTD Summary 24

25 SCB Mode I (RTD) Fatigue Summary 25

26 Failure Mode(s) 26

27 Skydrol Conditioning of SCB Specimens P 27

28 Skydrol Conditioning Study Condi&oning Timeframes Con&nuous Chamber &ll Acidity Saturate and Then Room Temp Temperature [ F] Skydrol [%] Water [%] Weeks After 5 Weeks 70 F 120 F 160 F 28

29 Skydrol Conditioning 29

30 Acidity Level Monitoring Samples were conditioned continuously at prescribed temperature 30

31 Acidity Level Monitoring Samples were conditioned at prescribed temperature and kept at room temperature after reaching targeted acidity level 31

32 Skydrol Conditioning Procedure Mix the needed amount of 50% Skydrol and 50% water solution in the air tight container. Place the container inside the conditioning camber at 160 F for 14 days, mixing thoroughly once a day. Remove the container from the conditioning camber and let set at room temperature until cooled. The solution should now be at 3-4 ph and will remain so for at least 90 days, if stored at room temperature. 32

33 Summary Fickian Diffusion is effected by temperature, moisture concentration, and pressure Environmental history on ground condition is important in tracking moisture content through the thickness of composite parts Guidelines for design and certification of composite structures related to environmental knockdown based on practical levels of moisture content and operational usage is in progress SCB Testing Fluid ingression phenomenon and the progressive damage growth due to entrapped fluids in sandwich structures 33

34 Looking Forward Benefit to Aviation Systematic approach for developing environmental knockdown factors based on structural details Possibility of extending the methodology for life extension strategies Guidelines for substantiating sandwich structures Fluid ingression phenomenon GAG effects on damage growth Effects of geometry and sandwich parameters on fracture toughness and damage growth rates Future needs Test articles representing modern day composite structures Environmental history data 34

35 End of Presentation. Thank you. 35