FAA level II Composite Structural Engineering Safety Awareness Course

Materials and Process Control ModuleA Historical Perspective

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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 -----

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Stable distribution up to ~ 350 0F, TG ~ 375 to 400 0F

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)

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Stable distribution up to ~ 350 0F, TG ~ 400 0F

The Matrix Dominated Response: Boron-epoxy Lamina, Effect of Temperature on [90] Lamina Properties; Boron-Epoxy, RTD (Kaminski 1971)

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Characterization of Temperature Dependence for Statistical Parameters, Weibull

• Variable estimates for shape parameter,

• Analysis suggest that fiber and matrix had distinctive Weibull shape parameters,

– (f) and – (m)

• Stable to (TG ~ 50 to 70 0F)– Applied to early USAF

development a/c– Adopted in to AC20-107A

• Property retention curves for translation of (T); (T)f & (T)m

• Rational: minimize viscoelastic effects in design and certification

Curing Temp.

[90]C

[0]C

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Material allowabls (Dry) Reflect Temperature Dependence, the Influence of the Glass Transition and the Environmental Compensation Factor, ECF,

Property Retention Curves Evolved to

Become the Basis of the ECLF.

Temperature,0F

% R

eten

tion

E11-ε22γ12ν12

DRY

G12-ε11

100

80

60

40

20

0

50 100 150 200 250 300 350 400 4500

ETDRTD

NARMCO T300/5208

Historical Data

ECF(AMB + ∆T) = RTD/ETD

ε22

E22ε22

Early Measurement Of The Glass Transition Temperature

Why The in-service T <Tg – 50 (0F).

• The guidance for the Tg- 50 (0F) was first applied to the B-2 Composite structures;– Uncertainty in Tg measurement– Provide a Tg 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 > 212 0F (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, F33615-76-C-5324, GrummanPART 2 DISCUSSION

Influence of operational usage on temperature and moistureImplications for tailored DESIGN allowable and ECLF

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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 350 0 F Tg Then The Historical Material.)

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Examples Of Failure Mode and Application Correlation

• Fiber Tension:– Laminates – Open and filled hole in tension

(Relatively insensitivity to Fatigue, Temperature and Moisture)

• Matrix Tension:– Transverse strain, unidrictional– Interlaminar Tension Strength, ILTS– Open and filled hole in Compression– Stringer terminations

(Sensitive to Fatigue, Temperature and Moisture)

• Shear:– Interlaminar Shear Strength, ILSS– In-plain– Bonded (adhesive) joints

(Sensitive to Fatigue, Temperature and Moisture)

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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)

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α , WeibullPerameter

Static Fatigue LEF

Laminates and Bolted Joints 20 - 25 1.3 - 2.0 --

Stringer Terminations

75 Deg. Angles~ 23 1.45 1.13

ILTS ~ 14 -- --

Bonded Joints 10 - 12 1.4 – 1.8 1.15

ILSS > 30 -- --

Unidirectional IM7/8552 TapeData by; A. Makeev, Y. Nikishkov, G. Seon; ICCM 2009

ILT and ILS Fatigue

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

3.00 3.70 4.00 4.70 5.00 5.70 6.00 6.70 7.00 7.70 8.00

Frac

tion

Stat

ic S

tren

gth

Log N

ILT and ILS Fatigue

Log. (LS fatigue) Log. (ILT fatifue)

No-growth Designs Typically Limited The Matrix Sensitive ILTS & ILSS Strengths:

ILTS ~ < 45% RT static & ILSS ~ < 55% RT staticSHOULD ILT AND ILS DRIVE LEF & ECLF?

Detail design features include ply drop off rates, local curvature, buckling or no buckling, internally pressurized structure, --, positive process control

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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

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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 – increasedpotential for in-service delamination

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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

Tool for curved surface

Curved cured skin

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.

Inter radius of curvature

required for fit to internal

frame.

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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

.

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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.

The panel does not fit to the frame! WHY?

IS THE CURED PLY THICKNESS DIFFERENT THAN ASSUMED OR

SPECIFIED? 9/14/10 16JCHalpin

The Curved Panel is Assembled On A Curved Frame: Does Not Fit, What Are The Options?

Are Interlaminar Shear and Tension Compromised?

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Pull down skin to stringertypically

using fasteners?

Adhesively bonding

skin panel to stringer;

Leave fasteners

in?

Remove fasteners

and fill holes?

Damage Tolerance?Discard skin and build another

panel?

SHIM to FIT?

An Example of selected Data: But Not-Uncommon(6 data points per set)

0.179 0.186 0.193

1

2

3

4

5

6

7

8

9

10

11

12

0.175 0.180 0.185 0.190 0.195 0.200 0.205

Batc

h N

umbe

r

Thickness (mm)

Average Cured Thickness

Spec.

Data indicates to sift in mean and increased variability to the thicker side.

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Fabrication of C and H section

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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

Fabrication of H Section

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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.

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.

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Verification of Process Simulation Models, Process Window and Recurring “As-Cured” Quality Requires Practical Sensors

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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?

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?

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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

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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, F33615-76-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

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Some other examples of historical data demonstrating

consistent material failure mode and statistics.

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Fiber Controlled Lamina to Laminate, Boron-epoxy, RTD (Kaminski 1971)

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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

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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).

Historically fiber tensile strength and variability have translated into tensile and flexural strength; Example; Narmco 5208/T300, O0 RTD test

(McKague, Bullock and Head, J Comp. Mat 1973)

Stress, KSI

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Matrix Controlled Lamina to Laminate Transition, Boron-epoxy, RTD (Kaminski 1971)

FAA level II Composite Structural Engineering Safety Awareness Course

Part 2Recommendation For Application-specific Environmental Design Criteria For Design Allowable And ECLF Definition

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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-ACE100-2001-006 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 amoisture absorption in equilibrium with an ambient 85% relativehumidity, RH. Tailored, end of life, moisture content is acceptable forthick locations when supported by a proper analysis.

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Historical Origin of Design Moisture Content Section 2.02 Moisture Content, page 8.

“Wood is a hygroscopic material, continuallygiving off and taking up moisture in accordancewith the relative humidity and temperature towhich it is exposed. Thus, while the strength ofa piece of wood may be increased to a relativelyhigh value by drying to a low moisture content,some of that increase may be lost, if, in use, it isexposed to atmospheric conditions which tendto increase the moisture content. ---it isdesirable, therefore, to design a structure on thebasis of the strength corresponding to theconditions of use. ---It is for this reason that thedesign values for wood used in aircraft are basedon a moisture content of 15 percent which hasbeen found to be about the highest of theaverage moisture contents prevailing in thevarious regions of the United States.”

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Equilibrium Moisture Content: EMC Airframe White Spruce: Historical Moisture Requirement

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)

ANC-1815% Moisture

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Environmental Exposure• PS-ACE100-2001-006, 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, F33615-76-C-5324, Grumman

AFFDL-TR-78-92 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

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The USAF And NASA ACE Programs Provided The Background For Policy Statement PS-ACE100-2001-006 “Static Strength Substantiation Of Composite Airplane Structure“.

Illustration Of An Early “Hot” Structural Requirement.

Ambient Still Air Temperature = 112 0F

(44 0C)

Vertical Fin 192 0F (89 0C)

Inboard Aileron 209 0F (98 0C)

Wing Box: no Fuel 2050F (96 0C) & w Fuel 175 0F (79 0C)

Runway Temperature ~ 138 0F (58 0C)

Lower Fuselage, 165 0F (74 0C)

Upper Fuselage 223 0F (106 0C)

Dark Painted Surfaces;

Solar Absorptivity = 0.5

Emissivity = 0.2

G. W. Davis and I.F. Sakata, Design Considerations for Composite Fuselage Structure of Commercial Transport Aircraft NASA CR-1559296, March 1981

Maximum Surfaces Temperatures Due to Solar Radiation, Lockheed L 1011 (1981) Composite Airframe Study (Temperatures approach max after 20 minutes, 60 minutes soak shown)

20 May 2010 Jchalpin 7

Transient Convective Thermal and Moisture Simulation Status

•Transient analysis was a 4-min taxi run with aconstant relative wind velocity of 20 kn followedby constant takeoff, acceleration, and climb to190 kn in 1.2 min.

•Point selected as the earliest possible time thatthe aircraft could be subjected to significantmaneuver or gust loads and would occur at 1.2min after brake release.

•At the end of 1.2 min of flight, a stabilizer stringerwould be at 82 0C (180 0F). [~ 71 0C (160 0F) recentanalysis]

•The minimum temperature of -59 0C (-75 0F) wasbased on the lowest ambient temperatureexperienced in flight modified by the effect ofaerodynamic heating.

DEFAULT Moisture Guidance; absorbedmoisture in equilibrium with 85% RH.

•Independent of thickness•Independent of utilization and operations

•PRELIMINARY SIMULATIONS in 1975 to1980 time period indicated medium andlong haul airframes would see about 75 to60% of this value.

•LOCAL THICK SECTION structural couldbe between 45 to 50% of the 85% guidance.

•WEIGHT driven by “Hot – Wet” allowableand Environmental Compensation TestFactor, 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

Sample Ground Environment AFFDL-TR-78-92 Vol. 1: HSTAB At Charleston SC

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Southeastern US CIVIL Hubs Are Similar Annual Temperature And Humidity To Other Worst Case MIL Short Haul Locations

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Example of a “Worst Case” Hub location:Extra Long Haul Operations

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Charleston SC

Example of a “Worst Case” Hub location:Medium Haul Operations

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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

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HSTAB Skin Simulation: AS/3501-6 “Like” Moisture Properties Assumed

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Alenia Material• Resin system a blended epoxy-thermoplastic matrix; UD IMS-977/2• Moisture equilibrium @ 85% RH

– 3501-6 like simulation ~ 1.1% moisture– 977-2 matrix Preliminary ~ 0.76%

• Expected 977-2 result– 0.69 reduction in moisture levels for the 3501-6 like resin– ~ 0.50 to 0.54% moisture weight gain at end of life

• 977-2 simulation – waiting for data: – equilibrium Moisture – RH– Diffusion data (preliminary available at 1 temperature)– Tg moisture reduction (preliminary in material specification)

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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

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Comparison Of OHC Wet And Dry Property Retention. (Moisture Reduces The Tg Shifting The Retention Curve To Lower Temperatures.)

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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)

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Equilibrium Moisture Wt. Gain & Glass Transition Transition, TG0C

(3501-6 type matrix)

Laminate Weight in Laminate Resin

Epoxy Resin Tg in Laminate

Illustration Of Historical Relationship

For Basic Resin Systems: ~25% Weight Fraction in Composite

Toughened Matrix Are Resins Blends,

Some Resin Components Do Not

Absorb Moisture BUT The Epoxy Phase

Will Have The Same Tg Moisture Relationship

9/14/10 JCHalpin

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.

Prop

erty

Matrix - Wet Fiber -Dry

Temperature, T (K0)

Ambient Hot

Matrix -Dry

X = Xref – [a (T – TR)]*[exp-(T/TG )b]

20

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 21JCHalpin

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Static Strength Assessment: Test and Analysis for Multiple Structural Locations and Loading conditions

(Non-linear load-displacement response)

Test

ECF Margin By Analysis

22

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-78-92

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

HSTAB Skin Simulation Ground Exposure for Medium Range Usage Comments:

• AS/3501-6 “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

JCH comment in Discussion Period:Resin Glass Transition Temperature

Rational for Tg – 50 0F GuidelineFAA Safety Awareness Workshop Discussion Topic

9/27/10 JCHalpin 1

Comment• Chart 21 of the Part 2 presentation by JCHalpin has a

table for the legacy 3501-6 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 0F guideline was introduced to:– encourage a 350 0F (177 0F) cure to provide some

Damage Tolerance for local surface heating, and– RT(amb) structural verification testing.

9/27/10 JCHalpin 2

Comparison Of Laminate To Internal Resin Moisture Weight Gain, Resultant Tg Depression And For a

Generic Modified Toughened Matrix (Reduced Epoxy Content)

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