FAA level II Composite Structural Engineering Safety Awareness Course Materials and Process Control Module A Historical Perspective

Transcription

1 FAA level II Composite Structural Engineering Safety Awareness Course Materials and Process Control Module A Historical Perspective 9/14/10 JCHalpin 1

2 Historical Evolution The current allowabls system for nonmetallic materials evolved from the pioneering work for laminated plywood in support of WW2 aeronautical needs. ANC-18 Design of Wood Aircraft Structures Government funded materials characterization material allowable data Until the beginning of the 1990 s the establishment of material allowabls was managed as a government responsibility to provide a standard materials database for both airframe development and continuing airworthiness. MIL HDBK-17 Plastics for Flight Vehicle Structures MIL HDBK-23 Composite construction for Flight Vehicles DOD Acquisition reform initiatives in the 1990 s directed the use of Commercial specification and standards in preference to MIL specification and standards. CMHDBK-17 and /14/10 JCHalpin 2

3 Early Characterization and Material Allowable Efforts: The Fiber Dominated Response: Boron-epoxy Lamina, Effect of Temperature on [0] C Lamina Properties; Boron-Epoxy, RTD (Kaminski 1971) Stable distribution up to ~ F, T G ~ 375 to F 9/14/10 JCHalpin 3

4 The Matrix Dominated Response: Boron-epoxy Lamina, Effect of Temperature on [90] Lamina Properties; Boron-Epoxy, RTD (Kaminski 1971) Stable distribution up to ~ F, T G ~ F 9/14/10 JCHalpin 4

5 Characterization of Temperature Dependence for Statistical Parameters, Weibull Variable estimates for shape parameter, [90] C Analysis suggest that fiber and matrix had distinctive Weibull shape parameters, (f) and (m) Curing Temp. Stable to (T G ~ 50 to 70 0 F) Applied to early USAF development a/c Adopted in to AC20-107A [0] C Property retention curves for translation of (T); (T) f & (T) m Rational: minimize viscoelastic effects in design and certification 9/14/10 JCHalpin 5

6 Material allowabls (Dry) Reflect Temperature Dependence, the Influence of the Glass Transition and the Environmental Compensation Factor, ECF, Early Measurement Of The Glass Transition Temperature 100 RTD ETD E 11 -ε 22 γ 12 ν ε 22 Property Retention Curves Evolved to Become the Basis of the ECLF. % Retention Temperature, 0 F 9/14/10 JCHalpin 6 50 ECF(AMB + T) = RTD/ETD G ε 1 1 NARMCO T300/5208 Historical Data E 22 ε 22 DR Y

7 Why The in-service T <T g 50 ( 0 F). The guidance for the T g - 50 ( 0 F) was first applied to the B-2 Composite structures; Uncertainty in T g measurement Provide a T g margin to avoid time dependent (viscoelastice) behavior Provide a technical basis for RT (dry) substantiation testing and the Environmental Compensation load Factor, ECLF Provide an in-service Tg at 100%RH > F (Local surface heat impingement concern) Process assurance; AS FABRICATED Tg Environmental definition to define End of service moisture and temperatures was recognized as a need; Environmental Sensitivity of Advanced Composites program was initiated in 1976, F C-5324, Grumman PART 2 DISCUSSION Influence of operational usage on temperature and moisture Implications for tailored DESIGN allowable and ECLF 9/14/10 JCHalpin 7

8 Legacy Approach: Property Retention Curves Defined As a Function of Failure Mode Using Location Parameters For Statistical Distributions. (The Retention Curves Where Adjusted For A Lower F Tg Then The Historical Material.) 9/14/10 JCHalpin 8

9 Examples Of Failure Mode and Application Correlation Fiber Tension: Laminates Open and filled hole in tension Matrix Tension: Transverse strain, unidrictional Interlaminar Tension Strength, ILTS Open and filled hole in Compression Stringer terminations Shear: Interlaminar Shear Strength, ILSS In-plain Bonded (adhesive) joints (Relatively insensitivity to Fatigue, Temperature and Moisture) (Sensitive to Fatigue, Temperature and Moisture) (Sensitive to Fatigue, Temperature and Moisture) 9/14/10 JCHalpin 9

10 Examples Of Failure Mode Scatter and LEF Implications Fiber Tension: Laminates Open and filled hole in tension (Relatively insensitivity to Fatigue, Temperature and Moisture) Matrix Tension: Transverse strain, unidrictional Open and filled hole in Compression Stringer terminations Interlaminar Tension Strength, ILTS (Sensitive to Fatigue, Temperature and Moisture) Shear: Interlaminar Shear Strength, ILSS In-plain Bonded (adhesive) joints (Sensitive to Fatigue, Temperature and Moisture) α, Weibull Perameter Laminates and Bolted Joints Stringer Terminations 75 Deg. Angles Static Fatigue LEF ~ ILTS ~ Bonded Joints ILSS > /14/10 JCHalpin 10

11 ILT and ILS Fatigue No-growth Designs Typically Limited The Matrix Sensitive ILTS & ILSS Strengths: ILTS ~ < 45% RT static & ILSS ~ < 55% RT static SHOULD ILT AND ILS DRIVE LEF & ECLF? Detail design features include ply drop off rates, local curvature, buckling or no buckling, internally Fraction Static Strength pressurized structure, --, positive process control Log. (LS fatigue) ILT and ILS Fatigue Log. (ILT fatifue) Log N Unidirectional IM7/8552 Tape Data by; A. Makeev, Y. Nikishkov, G. Seon; ICCM /14/10 JCHalpin 11

12 Prepreg Systems Are Synthesized, Both Fiber And Matrix, Through Chemical Process. The USAF took additional actions to address sources of variability beginning: Prepreg Chemical Composition Control to provide a reproducible curing capability and a dry glass transition temperature in the 1960 s Processing Science studies addressing the consolidation and curing of laminated and molded fabrication of structural elements in the 1970 s. Exotherm control Void content Cured ply thickness Residual stress and shape control, ---. Thickness and shape control a reoccurring challenge. Fabrication and assembly implications Manufacturing Review Board (protecting As-Designed margins) New toughened resins have increased process sensitivity (variable viscosity in cure cycle) increasing risk for cured ply thickness variability 9/14/10 JCHalpin 12

13 Two M&P Examples Controlled by Interlaminar Properties Cured ply thickness variations Curved surface Assembly implications (fit-up) for interlaminar damage Spar fabrication Co-bonded Co-cured Assembly implications (fit-up) for interlaminar damage Both examples result in ILTS and ILSS increased potential for in-service delamination 9/14/10 JCHalpin 13

14 Thickness Control A Reoccurring Challenge - New Generation Of Tough Thermosetting Matrix Systems May Have More Complex And Variable Viscosities. Example; Outer Mold Line Tooling For A Curved Panel Performance Requirement: Fabrication of a curved panel with a defined external and inter radius of curvature. Outer mold line tooling selected for aerodynamic shape control. Curved cured skin Tool for curved surface Inter radius of curvature required for fit to internal frame. 9/14/10 JCHalpin 14

15 Assembling The Curved Panel On To A Curved Frame: Will it Fit? Cured Skin Panel inner radius determined by tooling and CURED PLY THICKNESS. Frame curvature defined by outer mold line and an assumed or specified CURED PLY THICKNESS. 9/14/10 JCHalpin 15

16 Assembling The Curved Panel On To A Curved Frame: Will it Fit? Cured Skin Panel inner radius determined by tooling and CURED PLY THICKNESS. The panel does not fit to the frame! WHY? Frame curvature defined by outer mold line and an assumed or specified CURED PLY THICKNESS. IS THE CURED PLY THICKNESS DIFFERENT THAN ASSUMED OR SPECIFIED? 9/14/10 JCHalpin 16

17 The Curved Panel is Assembled On A Curved Frame: Does Not Fit, What Are The Options? Are Interlaminar Shear and Tension Compromised? Pull down skin to stringer typically using fasteners? Adhesively bonding skin panel to stringer; Leave fasteners in? SHIM to FIT? Discard skin and build another panel? Remove fasteners and fill holes? Damage Tolerance? 9/14/10 JCHalpin 17

18 12 An Example of selected Data: But Not-Uncommon (6 data points per set) Average Cured Thickness Batch Number Spec Thickness (mm) Data indicates to sift in mean and increased variability to the thicker side. 9/14/10 JCHalpin 18

19 Fabrication of C and H section Lay-up of flat prepreg laminate: Automatic Fiber Placement Trim Placement on Mandrel and hot forming Mandrel placement Bagging Heating and vacuum pressure Cooling Un-cured, resin advancement (out-time) Curing C-section option Cure cycle process window T(t) and P(t) Cured part after after cure Shape and thickness as designed? Location specific cure rates differences at s Assembly implications Local bending in radius changes ILTS Force applied to correct shape 9/14/10 JCHalpin 19

20 Fabrication of H Section Lay-up of flat prepreg laminate: Automatic Fiber Placement Trim Placement on Mandrel and hot forming Mandrel placement Bagging Heating and vacuum pressure Cooling Un-cured, resin advancement (out-time) Forming an H-section spar 2 Hot formed sections mated, mandrels Options: Cure H sections for; Assembly of cured parts Co-bonded integrated assembly Un-cured sections for; Co-cured integrated assembly Incorporation into integral box Increasing process discipline requirement. 9/14/10 JCHalpin 20

21 Preproduction Production Verification Pre Production Verification, PPV, was intended to integrate As-designed with As-built. Preproduction Verification, PPV, concepts were discussed and sometimes implemented beginning in the 1980 s to provide a technology support system to assure that the allowabls characterized in the initial design, As Designed structural elements, were essentially the same as the material capabilities in the As Fabricated material elements incorporated into subassemblies and the airframe. These initiatives included: Process simulation for cure cycle selection and management using FEM and finite difference algorithms for curing and consolidation Qualified Process Window Process qualification has typically been with simple structural elements, typically 2 dimensional articles; Rigorous post process inspection before assembly of 2 dimensional items supplemented traditional manufacturing Data to monitor real time pressure and temperature histories were evaluated and sometimes used to provide feedback regarding the integrity of the individual article fabrication processes. 9/14/10 JCHalpin 21

22 Verification of Process Simulation Models, Process Window and Recurring As-Cured Quality Requires Practical Sensors Processing Window For Consolidation and Curing Demonstrated In PPV Manufacturing Test At Critical Locations. Independent Verification Of P(t) And T(t) For Individual Manufactured Articles? 9/14/10 JCHalpin 22

23 Weak Process Verification Post fabrication NDI was introduced and strengthened due to a lack of, inconsistent, commitment to a PPV process qualification. Recent changes in STAT-17 have sought to address the inconsistent commitment to a PPV process qualification through the addition of increased variability in the reduction of allowabls data. In the 1990 s the allowabls system and the PPV concepts were thought to be in balance as the fabrication and assembly concepts involved the curing of individual elements, i.e. skins, spars, --, and then their assembly after an acceptance procedure. Integrated, unitized, co-cured structure poses additional challenges. Question: Does the arrival of integrated, unitized, co-cured structure suggest a reexamination of the need for a process based PPV activity: process window, and some form of real time process data to support coupon based allowabls? 9/14/10 JCHalpin 23

24 Summary Current guidance based in historical experience and focused R&D investments. Both regulator and OEM staff s need to understand historical data base and experience. Tailored Design Allowable concept is technically feasible BUT significant effort required by OEM and PRIME s. Increased Attention to interlaminar data and design criteria needed. Chicken fasteners and NDI are NOT a substitute for control of matrix sensitive failure modes. Damage arrestment options. Preproduction Verification activities and In-process Health monitoring need strengthening. Process health monitoring real time needed Better option than Structural health monitoring 9/14/10 JCHalpin 24

25 Tailoring Allowabls: Design Usage Influence 85% RH is a MATERIALS Characterization condition Operational defined end-of-life moisture content can be defined for worst Case operations: Hot and humid locations Operational usage Environmental definition to define End of Service moisture and temperatures was recognized as a need in the 1970 s; Environmental Sensitivity of Advanced Composites program was initiated in 1976, F C- 5324, Grumman Runway storage environments based on world wide data. Basis for preliminary analysis. Coordination of hub locations and operational usage Material data base interpolation PART 2 DISCUSSION Influence of operational usage on temperature and moisture Implications for tailored DESIGN allowable and ECLF 9/14/10 JCHalpin 25

26 Some other examples of historical data demonstrating consistent material failure mode and statistics. 9/14/10 JCHalpin 26

27 Fiber Controlled Lamina to Laminate, Boron-epoxy, RTD (Kaminski 1971) 9/14/10 JCHalpin 27

28 Stress Concentrations Shift the Location of the Distribution but the Variance (Shape) Is Unaltered. QI Boron-Epoxy Laminate, RTD (Waddoups and Halpin 1974) Similar results for QI Graphite - Epoxy & Glass-Epoxy 9/14/10 JCHalpin 28

29 The Physics Based Argument for the Assumption for a Specific Statistical Description, alpha, Was the Assumption That Fiber Bundle Dispersion Determined Fiber Dominated Static Strength (Tensile). Stress, KSI Historically fiber tensile strength and variability have translated into tensile and flexural strength; Example; Narmco 5208/T300, O 0 RTD test (McKague, Bullock and Head, J Comp. Mat 1973) 9/14/10 JCHalpin 29

30 Matrix Controlled Lamina to Laminate Transition, Boron-epoxy, RTD (Kaminski 1971) 9/14/10 JCHalpin 30

31 FAA level II Composite Structural Engineering Safety Awareness Course Part 2 Recommendation For Application-specific Environmental Design Criteria For Design Allowable And ECLF Definition 9/14/10 JCHalpin 1

32 Application-specific Environmental Design Criteria For Design Allowable And ECLF Definition AC20-107B guidance and Static Strength Substantiation of Composite Airplane Structure, Policy Statement PS-ACE as implemented through AC20-107B : 4.0 Environmental Exposure and Repeated Load 4.1 Long-Term Time Related Degradation 4.2 Solar/Thermal Design Criteria 4.3 Peak Temperature Analysis 4.4 Peak Moisture Content 4.5 Analyses and Tests 4.6 Full Scale Test Considerations 4.7 Other Exposures Section 4.4 Peak Moisture Content contains a default requirement for a moisture absorption in equilibrium with an ambient 85% relative humidity, RH. Tailored, end of life, moisture content is acceptable for thick locations when supported by a proper analysis. 9/14/10 JCHalpin 2

33 Historical Origin of Design Moisture Content Section 2.02 Moisture Content, page 8. Wood is a hygroscopic material, continually giving off and taking up moisture in accordance with the relative humidity and temperature to which it is exposed. Thus, while the strength of a piece of wood may be increased to a relatively high value by drying to a low moisture content, some of that increase may be lost, if, in use, it is exposed to atmospheric conditions which tend to increase the moisture content. ---it is desirable, therefore, to design a structure on the basis of the strength corresponding to the conditions of use. ---It is for this reason that the design values for wood used in aircraft are based on a moisture content of 15 percent which has been found to be about the highest of the average moisture contents prevailing in the various regions of the United States. 9/14/10 JCHalpin 3

34 Equilibrium Moisture Content: EMC Airframe White Spruce: Historical Moisture Requirement ANC-18 15% Moisture Wood is hygroscopic, it responds to changes in relative humidity, RH, gaining or loosing moisture as the working environment changes. For a given RH level a balance is reached in which the wood is no longer gaining or loosing moisture. This moisture content is the EMC for wood (or composites) 9/14/10 JCHalpin 4

35 Environmental Exposure PS-ACE , Static on-ground exposure joint USAF & NASA effort 4.2 Solar/Thermal Design Criteria 4.3 Peak Temperature Analysis Environmental definition to define End of Service moisture and temperatures was recognized as a need in the 1970 s; Environmental Sensitivity of Advanced Composites program was initiated in 1976, F C-5324, Grumman AFFDL-TR Vol. 1 Runway storage environments based on world wide data. NASA, in-house simulations Operational defined end-of-life moisture content can be defined for worst Case operations: Hot and humid locations Operational usage 9/14/10 JCHalpin 5

36 The USAF And NASA ACE Programs Provided The Background For Policy Statement PS- ACE Static Strength Substantiation Of Composite Airplane Structure. Illustration Of An Early Hot Structural Requirement. Maximum Surfaces Temperatures Due to Solar Radiation, Lockheed L 1011 (1981) Composite Airframe Study (Temperatures approach max after 20 minutes, 60 minutes soak shown) Ambient Still Air Temperature = F (44 0 C) Inboard Aileron F (98 0 C) Vertical Fin F (89 0 C) Upper Fuselage F (106 0 C) Dark Painted Surfaces; Solar Absorptivity = 0.5 Emissivity = 0.2 Wing Box: no Fuel F (96 0 C) & w Fuel F (79 0 C) Lower Fuselage, F (74 0 C) Runway Temperature ~ F (58 0 C) G. W. Davis and I.F. Sakata, Design Considerations for Composite Fuselage Structure of Commercial Transport Aircraft NASA CR , March /14/10 JCHalpin 6

37 Transient Convective Thermal and Moisture Simulation Status Transient analysis was a 4-min taxi run with a constant relative wind velocity of 20 kn followed by constant takeoff, acceleration, and climb to 190 kn in 1.2 min. Point selected as the earliest possible time that the aircraft could be subjected to significant maneuver or gust loads and would occur at 1.2 min after brake release. At the end of 1.2 min of flight, a stabilizer stringer would be at 82 0 C (180 0 F). [~ 71 0 C (160 0 F) recent analysis] The minimum temperature of C (-75 0 F) was based on the lowest ambient temperature experienced in flight modified by the effect of aerodynamic heating. DEFAULT Moisture Guidance; absorbed moisture in equilibrium with 85% RH. Independent of thickness Independent of utilization and operations PRELIMINARY SIMULATIONS in 1975 to 1980 time period indicated medium and long haul airframes would see about 75 to 60% of this value. LOCAL THICK SECTION structural could be between 45 to 50% of the 85% guidance. WEIGHT driven by Hot Wet allowable and Environmental Compensation Test Factor, ECLF. Initial Development used operational based simulation to establish realistic Hot Temperature design requirement Operational based simulation will be used for establishing realistic moisture requirement 20 May 2010 Jchalpin 7

38 Sample Ground Environment AFFDL-TR Vol. 1: HSTAB At Charleston SC 9/14/10 JCHalpin 8

39 Southeastern US CIVIL Hubs Are Similar Annual Temperature And Humidity To Other Worst Case MIL Short Haul Locations 9/14/10 JCHalpin 9

40 Example of a Worst Case Hub location: Extra Long Haul Operations 9/14/10 JCHalpin 10

41 Example of a Worst Case Hub location: Medium Haul Operations Charleston SC 9/14/10 JCHalpin 11

42 Example of Worst Case Hub locations and Operations Example: medium and extra long haul usage: Long haul: Southeast US to Honolulu US. Flight operations 24 hours Mix of 8 and 12 hour flights ~ 2 to 3 hour ground environment between flights Mix of ground and flight operations: ~ 3 to 5 hours ground day time 19 to 21 hours in-flight per operational day night and day Medium haul: Southeastern US locations selected based on Temperature, Humidity and typical flight lengths. No flight operations between 2400 and 600 hours Mix of 2 and 3 hour flights 1 hour ground environment between flights Mix of ground and flight operations: 13 hours ground 11 hours in-flight per operational day 30 year historical temperature, humidity, solar radiation, rain, --, by averaged by hour for 12 month annual cycle 9/14/10 JCHalpin 12

43 HSTAB Skin Simulation: AS/ Like Moisture Properties Assumed 9/14/10 JCHalpin 13

44 9/14/10 JCHalpin 14

45 Alenia Material Resin system a blended epoxy-thermoplastic matrix; UD IMS-977/2 Moisture 85% RH like simulation ~ 1.1% moisture matrix Preliminary ~ 0.76% Expected result 0.69 reduction in moisture levels for the like resin ~ 0.50 to 0.54% moisture weight gain at end of life simulation waiting for data: equilibrium Moisture RH Diffusion data (preliminary available at 1 temperature) Tg moisture reduction (preliminary in material specification) 9/14/10 JCHalpin 15

46 Polynomial Equations Are Often Used For Interpolation Of Materials Characterization Allowable Data To Other Conditions. CMHBK-17 guidance identifies a characterization plan for material allowable data: temperature, humidity, loading, open and filled holes, laminate stacking, --- Design applications often implement structural location specific conditions intermediate between identified material characterization conditions. Interpolation between characterization points provides a basis to tailor material allowable data to application specific design locations. (Temp., Moisture, Stacking, --) MATERIAL ALLOWABLE to DESIGN ALLOWABLE 9/14/10 JCHalpin 16

47 Comparison Of OHC Wet And Dry Property Retention. (Moisture Reduces The Tg Shifting The Retention Curve To Lower Temperatures.) 9/14/10 JCHalpin 17

48 Environmental Compensation Load Factor The ECLF is a load factor that reflects the local temperature and moisture effects for critical structural locations when tested at ambient structural test facility conditions and as an additional margin when assessing the margin of safety by analysis. Margin of safety for bonded repair Derived from the property retention curves Fiber control Matrix tension control Matrix shear control ECLF Reference temperature is RTDry not RTAmb (see chart 21) 9/14/10 JCHalpin 18

49 Equilibrium Moisture Wt. Gain & Glass Transition Transition, T G 0 C ( type matrix) Laminate Weight in Laminate Resin Illustration Of Historical Relationship For Basic Resin Systems: ~25% Weight Fraction in Composite Epoxy Resin Tg in Laminate Toughened Matrix Are Resins Blends, Some Resin Components Do Not Absorb Moisture BUT The Epoxy Phase Will Have The Same Tg Moisture Relationship

50 Static Strength Assessment: Tailored Environmental Compensation Factor, ECF, Require Interpolation Procedures To Adjust Ambient Temperature And Humidity Tests For Effects Of Hot Wet Operational Conditions. X = X ref [a (T T R )]*[exp-(t/t G ) b ] Matrix - Wet Fiber -Dry Property Matrix -Dry Ambient Hot Temperature, T (K 0 ) 9/14/10 JCHalpin 20

51 Comparison Of Laminate To Internal Resin Moisture Weight Gain, Resultant Tg Depression And For a Generic Modified Toughened Matrix (Reduced Epoxy Content) 9/26/10 JCHalpin 21

52 Static Strength Assessment: Test and Analysis for Multiple Structural Locations and Loading conditions (Non-linear load-displacement response) ECF Margin By Analysis Test 9/14/10 JCHalpin 22

53 Summary Operational defined end-of-life moisture content can be defined for worst Case operations: Hot and humid locations Operational usage Environmental definition to define End of Service moisture and temperatures was recognized as a need in the 1970 s; Environmental Sensitivity of Advanced Composites program AFFDL-TR Vol. 1 provides basis for analysis Runway storage environments, exposure without flight usage Basis for analysis. Coordination of hub locations and operational usage Expanded material data base and interpolation Recommendation For Application-specific Environmental Design Criteria For Design Allowable And ECLF Definition 9/14/10 JCHalpin 23

54 HSTAB Skin Simulation Ground Exposure for Medium Range Usage Comments: AS/ like moisture properties assumed. 85% moisture = 1.10 %M absorber at equilibrium. Upper surface is worst case; 70% of 85% RH value for thickness simulated. Most moisture absorber during hot-humid nights. Drying during the day. Overnight long haul operations further reduce absorbed moisture - most ground time during sunlight heating Less M absorbed in lower surfaces 9/14/10 JCHalpin 24

55 JCH comment in Discussion Period: Resin Glass Transition Temperature Rational for Tg 50 0 F Guideline FAA Safety Awareness Workshop Discussion Topic 9/27/10 JCHalpin 1

56 Comment Chart 21 of the Part 2 presentation by JCHalpin has a table for the legacy resin systems. Note that at 100% RH equilibrium saturation the wet Tg is close to the boiling point of water. When the resin Tg comes close to the boiling point of water there is the potential for blistering or a local delamination when hot exhaust from ground equipment impinges on the surface. The Tg 50 0 F guideline was introduced to: encourage a F (177 0 F) cure to provide some Damage Tolerance for local surface heating, and RT(amb) structural verification testing. 9/27/10 JCHalpin 2

57 Comparison Of Laminate To Internal Resin Moisture Weight Gain, Resultant Tg Depression And For a Generic Modified Toughened Matrix (Reduced Epoxy Content) 9/27/10 JCHalpin 3