managing impact risk for composite structures: unifying durability and ... · 10 may 2007 jchalpin...

10 May 2007 JCHalpin & Hyonny Kim 1

Managing Impact Risk for CompositeStructures:

Unifying Durability and Damage TolerancePerspective.

J C Halpin, JCH Consultants, Dayton, OH

Hyonny Kim, U of California at San Diego


Presentation focus• Airframes operate in a variety of discrete threat

environments.– Composite airframes have different capabilities than metallics

and they also have increased sensitivity to a variety of impactthreats.

– Threats include:• bird strike,• hail,• runway debris,• tool-drop,• tire rupture,• incidental contact with ground vehicles• panels lost in-flight &• other (lighting strike).

– Focus on a group of discrete threats, like hail, outlining aDurability & Damage Tolerance technology and safetymanagement approach for impact events in operational fleets.


Presentation Objective:Consistent Methodology for Various Classes of Impact Conditions.

1.Threat Environments need to be quantified, agreed to, and kept up to date.They are independent of platforms. (Good databases for US but need data forinternational locations.)

2. Damage Thresholds should be the bases for the dimensioning of laminatesexposed to impact threats (the damage thresholds will depend on the natureof the threat, FOD is different than hail ice, than bird-strike, than rubber puckfor tire damage, etc.).

3. Defense of a design is the responsibility of the OEMs including test &analysis methods. (Realistic test protocols & modeling are important tomanage the different impact threats)

4. Inspection for Cause triggered by extreme events, and a defined operatingperiod of Unrepaired Service Usage allows short-term operations before repairdependent upon severity of damage as defined by the Pxx internal loadconditions within the traditional Durability & Damage Tolerance concepts.

This perspective would utilize a “Self Evident Event” instead of “Self EvidentDamage” as a criteria condition. The dent has been a “Self Evident Damage”criteria condition BUT is not a sufficient basis for a preventive design &maintenance concept.


Structural Integrity• The structural strength, rigidity, damage tolerance,

durability and functioning of structural parts of the airframeas affecting the safe use and cost-of-ownership of the airvehicle– DAMAGE TOLERANCE: The ability of the airframe to resist

failure due to -- damage for a specified period of unrepairedservice usage.

– DURABILITY: the ability of the airframe to resist --corrosion,thermal degradation, delamination, wear, and the effects offoreign object damage for a specified period of time.

• ASIP: Aircraft structural integrity program is all of theactions required to assure the structural integrity of theairframe.


Critical Parts• A critical part is one, the single failure of which could cause

loss of the aircraft --- or result in major economic impact onthe aircraft, --.– FATIGUE/FRACTURE CRITICAL PARTS: Primary structural

components that are designed by durability and/or damagetolerance requirements --. These parts generally call forspecial fatigue/fracture toughness controls, quality controlprocedures, NDT/I practices, and analytical requirements.

– MAINTENANCE CRITICAL PARTS: Airframe (structural)components designed by durability requirements. The failureof which may result in functional impairment of, or majoreconomic impact on an aircraft or subsystem performance.The failure of the part requires costly maintenance and/or partrepair or replacement, which if not performed wouldsignificantly degrade performance or operational readiness(dispatch reliability). ---


Recognition of and Tolerance forPreexisting and/or Service Induced Damage

• SLOW CRACK GROWTH STRUCTURE: --consists of thosedesign concepts where flaws or defects (Preexisting orService Induced Damage) are not allowed to attain the critical sizerequired for unstable rapid crack (delamination) propagation.– Safety is assured through slow crack growth for periods of usage

depending on the degree of inspectability.– The strength of slow crack growth structure with subcritical damage

presence shall not degrade below a specified limit for the period ofunrepaired service usage.

• MINIMUM PERIOD OF UNREPAIRED SERVICE USAGE: is thatperiod of of time during which the appropriate level of damage(assumed initial or in-service) is presumed to remain unrepairedand allowed to grow within the structure.– Allowable Damage Limits– Aircraft availability– Dispatch reliability


Early ASIP Criteria:“B Threat Allowables” - Design to Threat,

Bound Dent Inspection Requirement - Avoiding Penalizing Better Materials & Designs.

Damage From a 1.0 in. Dia. HemisphericalImpactor With 100 Ft-lbs of Kinetic Energy orWith That Kinetic Energy Required to Cause aDent 0.10 Inches Deep, Whichever Is Less

IMPACTDAMAGE

INITIAL FLAW -DAMAGEASSUMPTION

•No functional impairment after2 design lifetimes.•No water intrusion after repairif damage is visible

Impact energy smallerof 6 ft-lbs or visibledamage (0.1 in. deep)

2Low probability ofimpactSame as Zone 1

•No functional impairment orstructural repair required for 2design lifetimes.•No water intrusion•No visible damage from asingle 4 ft-lb impact

Impact energy smallerof 6 Ft.-lbs or visibledamage (0.1 in. deep)with a minimum of 4Ft-lbs

1High probability ofimpact

Tool Drop•0.5 in. Dia.Solid Impactor•low velocity•Normal to surface

RequirementsDamage LevelZoneDamage Source


Hail & Runway Debris Were Recognized in the Initial Adv. CompositesDurability Damage Tolerance Guidance in Mid 1980’s; Two Assumptions:

1- Intent Was to Size Structure Compatible With Damage Thresholds &2 - That a Hard Impactor Was Valid for All Damage Sources

90%MilHBK210


Generic Sequence for an Impact Event:Impacting Mass May or May Not Be Conserved Through Penetration

(example ice fragmentation on contact)

KE0 = KER+ W(material damage) + W(target material loss) + W(impactor material loss, ice) + W(other)

FTE is threshold energy for initiation of material damage, i. e. delamination

Ballistic (& usually Dent) threshold energies are associated with initiation offiber rupture & back side “break-out.”

Initial Impact

Hydrostatic Compression

V0mBefore Contact

Compression-Shear

Tension-Shear

Penetration & Vibration

Lost Material, mL

VRmRResidual Velocity

KE0

FTE

Dent Threshold

Ballistic Threshold

Shear-Plug formation

Ballistic Penetration

WR

Lo V

Hi V


Initial Studies in the 70’s & 80’s Were Focused on Hard Object Impact.Micrographic Examination of Impacted Laminates Verified the Damage Threshold Concept

and Nature of the Induced Damage Above the Damage Thresholds.


Impact Damage Thresholds & Areas Were Recognized to BeDependent on Thickness in the Mid 1970’s.

Hard Object Impact Characterization for FOD & Tool Drop.Initial Focus on Thickness Scaling


Variation of Damage Size Between FTE & Penetration (laminate breakdown): Is the Impact Damage Sub-critical?

Is It Self Evident?What is the Allowable Damage Limit?


Dam

age

Dia

met

er

Energy, Force or Size of Impactor

BVID?

BVID Requires Significant LaminateDamage to Remain Persistent.


A Tool to Manage Maintenance and Schedule Departure Reliability:Minimum Period of Unrepaired Service Usage Applied to Impact

Damage.D

amag

e D

iam

eter

Pxx Specified Damage Limit Allowable Damage Limit

Energy or Size of Impactor

Could provide limited period ofusage to schedule repairs?


Optimized ASIP-like Procedure &Risk Assessment (FAR 25.571 & AC 20-107A)

• Severe events -- Rogue Events• Impact capability at different structural locations

(or zones) defined;– No damage– Localized damage, or– Penetration & broad area damage (~ 2 X in KE levels)

• No inspection below FTE capability• Above FTE capability;

– Severity triggers action• Category 2 damage requiring NDI• FAR 25.571 & AC 20-107A

– What, where, how, when, & margin to operate awaitingmaintenance & repair.

– Focused & defined process


Characterizing the Hail Threat•Understanding Hail•Ground & in-flight

–Operational usage•MIL HDBK data ?

–Need international data base


Hail Stone Growth Is Enhanced in Strong Storm Systems, Vertical Wind Gusts CarryingSmall Hail Upward Becoming Large Stones. Sometimes These Storms Generate Tornados.

Extreme Sizes Are Typically Associated With Weather Dangerous for AviationLarger Hail Stones are Aggregates of Smaller Ice Particles.

Four Different Size Distributions Suggested by Data from Storm Chasers & NOAA.


How Large Can Hail Get?Fortunately Most Hail Is Small - Usually Less Than 2 Inches in Diameter.

Large Hail “Balls” Are Typically Aggregates.

•One storm near Aurora NE 22 June 2003 produced the largest sized stone to fall in the UnitedStates, 7 inches in diameter and 18.75 inches in circumference.

•The second largest recorded hailstone fell in Coffeyville, Kansas on September 3, 1970. Itmeasured about 17.5 inches in circumference (over 5.6 inches in diameter) and weighed morethan 26 ounces (almost 2 pounds)!


The Average Number of Severe Hail Days for US CONS:Days >0.50,0.50-1.0,1.1-2.0,2.1-3.0,3.1-4.0,4.1-5.0,5.1-6.0>6.0 Days/yr

for Hail greater than 0.75 inches (11 Airport Locations Examined; ⊗)

⊗

⊗⊗

⊗

⊗

⊗

⊗

⊗

⊗

⊗

⊗


P n |w( ) = exp!D ! 0.53

"

#

$%

&

'(

)

NOAA Severe Hail Data Base for Observed for the Hub Airport Areas2,829 Severe Hail Reports Between 1955 &2006.

Fitted to a Truncated Weibull Distribution, P(n|w)


Summary of Weibull Parameters and Hail Diameters in inches for Probabilities of 90, 95 &99% for Individual Locations and Estimates for the Composite Data Base.

2,829 Severe Hail Reports Between 1955 &2006


There is an EU Severe Hail Data Base:Example 577 reports in 2006; diameter of 2.0 cm or more.


International Hail Database?• The principle zone of hail occurrence is between 30

- 500N latitude.• North India may be area of most severe hail storms

-- limited records• Western Great Planes of the US used for

determination of design guidance– Current study– MIL HDBK 310 -- needs to be updated

• The hail threat environment needs to bequantified, agreed to, and kept up to date.– It is independent of platforms.– Basic to design & risk assessments


Damage Thresholds

•Characterization of Failure ThresholdEnergy, FTE•Realistic Test protocols & Modeling

–FTE dependent on threat•Influence on Laminate sizing


Observations of Non-Damaging (84.1 M/sec. & 125 J) and Penetration (106M/sec. & 199 J) of 42.7mm SHI Impacts On 1.22 mm Panels.

Note: SHI fragmentation sensitivity to impacting velocity


In the 1990’s a Focus on Hail Testing Emerged. Failure ModesObserved for Simulated Hail Ice, SHI, Impacts, an example.

Gas gun technology evolved from P&W experience with birdstrike & roofing test STD’s.


Adhesive Bonded Composite Lap Joint:Distinct Failure Threshold Energy, FTE, Identified

• Impacting hail ice ball on abonded lap shearspecimen

• Resultant damage area asa function of Impact KE.

• Bases for:– FTE– ABAQUS Digital

modeling– Scaling of FTE’s

• Ply Delamination adjacentto bond observed &verified by simulation


Competing Test Protocols Evolved. At Comparable Incoming Kinetic Energy the ASTM Cotton Composite Reinforced Ice

Produces a Different (2X larger) Dynamic Impulse Than Un-reinforced Ice

Simulated Natural Ice Fragments on Contact ASTM After Impact

Comparison of 100% vs. Unfilled (0%) Ice; all 60.96 mm Diaz.


H. Kim Has Demonstrated a Procedure to Correlate a Range ofImpact Parameters That Are Useful for Preliminary Sizing .


Failure Threshold Energy, FTE, Shear Plug Analysis• Initial damage mode following FTE is observed to

be delamination for SHI impact• Delamination formation associated with

interlaminar shear stress (ILS)• Peak ILS develops very early after initial projectile-

target contact– 6-12 µs per numerical simulation*– energizes a shear plug within this time– plug has high ILS throughout its volume– plug size related to ice diameter D– Vol is volume of impacting ice ball

• Strain energy density Ushear in shear plug:

• Initiation of damage occurs when critical valuereached, i.e., τxz. ~ τILS , defining FTE:

where τILS = 100 MPa is interlaminar shear strength, GILS =5.17 GPa is interlaminar shear modulus

22

.

.

2

2

1

22. !

"

#$%

&== '

DH

GdVol

GU

xz

ave

Volxz

xz

shear

()

**

ref: * Kim H, Kedward KT. Modeling Hail Ice Impacts and Predicting ImpactDamage Initiation in Composite Structures. AIAA Jo. 2000;38(7):1278-1288.

Ice Sphere

Diameter

D

!D

"xz

x

DYNA3D Profile

Average "ave.ILS Stress

at Center

Depth

Panel

Thickness

H Contours

of "xz

ILS Stress Contours


Illustration of the Influence of of Hail Size & Test Simulation Methodology onLaminate Thickness,

ASTM Characterization Increases Laminate Thickness. (@ Constant KE)ROM Estimates, limited data base

Simple Laminate, Final Dimensions Dictated by Variety of Requirements

ASTM


Illustration of the Influence of Test Simulation Methodology on the Thicknessof a Typical Pressurized Structure: The ASTM Cotton Composite Reinforced

Test Ball Is Significantly More Damaging That Simulated Hail Ice.

Hail Failure Threshold Withstanding Thickness as

•Glancing impact on curved surface - solid laminate•Minimum gage dictated by other A/C requirements

Function of Incident Angle,

0

0.025

0.05

0.075

0.1

0.125

0.15

0.175

0.2

0.225

0.25

0.275

0 10 20 30 40 50 60 70 80 90Incident Angle, (90 degrees is normal) impact

Th

ickn

ess

, in

ches

SHI ASTM


Summary: Ice Impacts on Composites


Incidental Contact With Ground Vehicles:Blunt Impact Damage: Background


Plan of Work• Three tasks – addressing each of the objectives:

• Task 1. Identification of Common Blunt Impact Scenarios andEstablishment of Partner Relationships– initial phase of the project– active communication with the following organizations

• airline maintenance depots• aircraft manufacturers• OEM suppliers• FAA and EASA

– identify blunt impact scenarios that are:• most commonly occurring• of major concern• e.g., source of impact, location, and type of damage formed

– visits to maintenance depots will be made and documented via photographs (ifpermitted) and written notes

– outcome of this task will be summarized in a written document/FAA report


Tasks 2 and 3Impact Threat Characterization

Blunt Impact Damage Characterization and Testing• Conduct Lab and Large Scale Experiments

Also:

• Simulation of ImpactDamage Formation

• Simple Analytical ToolsDevelopment

• Damage ToleranceAssessment


Industry/Agency Participation• key component of this research activity is participation from

industry/agency partners– research focus and activities should be relevant to the user

community

• requested industry/agency participation broken down into threelevels:– Level 1. Initial Guidance. Contribution to Task 1. Hyonny Kim will visit

the facility to learn and see first hand about the companies’experiences in this topic and document experiences.

– Level 2. Advisory. Provide initial guidance, as well as ongoingadvisement, particularly in Task 2 methodology development, and indefining parameters for experiments to be conducted in Task 3.

– Level 3. Provide material support for Task 3. Make available testpanels, substructure components, and/or “impactors” (e.g., a cargocart) for full-scale blunt impact investigations.


Request From Today’s Audience

– feedback on proposed activities

– “wish list” from industry/agencyperspective• what should this tool look like? be

capable of?• what quantities are most important to

you?

– additional participants


Managing & Explaining The Risk: Hail Example“Self evident Rogue EVENTS”

The damaging storm events are so severe that their infrequent occurrencecannot go unnoticed!

• Hail sizes >0.75 inches only produced by SEVERE weatherconditions– Rough Events, beyond “B or A Allowable” or “Design to” threat size

• Pxx & Inspection criteria in original D&DT guidance seemsappropriate.– Functional impairment, aero-elastic tolerance to a disbond– Dispatch Reliability for civil fleet operations

• Convert “self evident damage” to “self evident events” in the legacyASIP discussions.

• Inspection for Cause• Limited unrepaired service usage

– Allowable damage limit– Less than 30 days?

• This was/is intended to provide a loads and schedule margin for maintenanceactions.

• Apply to both “Primary” & “Reliability” critical structure.


Number of Severe Hail Reports in the Vicinity of the DFW Airport(51 Years, 419 Severe Storms & 1157 individual Reports),

Reports of Aircraft Damaged on the Ground(2.X Inch Requirement Would Not Protect Against 99% Event)

1992, 1 B767 Damaged

1955, 5 B757’s &50 other Jets Damaged

Hail 3.5 inches


Self Evident Rogue Events


Providing a Consistent Methodology

• Some level of severe Rogue hail exposure willrequire OEM guidance for an inspection by theoperators after an infrequent severe storm event.– Hail greater than 2.X inches does occur.– Schedule & residual loads margins for maintenance will

provide a practical & safe departure reliability

• Risk assessments should guide evolving designguidance for;– NDI procedures– Schedule departure reliability– Implied ASIP requirements– Potentially reduced inspection requirements?– Economic risk?


Status Matrix of Service Induced Impact Damage:Composite Structures

------------------------Others?Lighting

Strike

YesSometimesJSSG-2006Structures?

Steel or AluminumHemisphereDrop-tower

Tool-drop

Sometimes?

Yes

Yes

Sometimes

Yes

Yes

Self EvidentEvent

Yes

Sometimes

Sometimes

Usually

YES

YES

Impact Location(s); Zones 1 & 2

TBDTBDTBDIncidental

Contact WithGroundVehicles

???Panels LostIn-flight

??Rubber PuckTire Rupture

“B”Up-date

JSSG-2006 ??Lead Ball ?

Drop-tower?RunwayDebris

“B”Up-date MIL HDBK 310

YesMaturingSimulated Hail Ice,

SHIHail

“B”FAR’s (Wt. & Vel.)

YesGel-packBird Strike

ThreatAllowable

SimulationModels

Test ProtocolThreat


“Sandwich” Construction:

Honeycomb Sandwich Construction Increases Vulnerability


SUMMARY1. Damage Thresholds should be the bases for the dimensioning of laminatesexposed to hail and other impact threats (the damage thresholds will dependon the nature of the threat, FOD is different that hail ice, than the ASTM ball,than the rubber puck for tire damage, ---).

2. The threat environments need to be quantified, agreed to, and kept up todate. They are independent of platforms.

(Good data base for the US But need data for international locations.)JOINT INTERNATIONAL EFFORT?

A data base need; hail & operational velocities, ---.

3. Defense of a design is the responsibility of the OEMs inc. test & analysismethods.

(Realistic test protocols are available.)

4. Inspection for Cause; Hail example, triggered by extreme Rogue weatherconditions, and a defined operating period allowing short term operationsbefore repair, dependent upon severity of damage as defined by the Pxxinternal load conditions or Allowable Damage Limits.

This perspective would utilize a “Self Evident Rogue Event” instead of “SelfEvident Damage” as a criteria condition. The dent has been a “Self EvidentDamage” criteria condition BUT is not a sufficient basis for a preventivedesign & maintenance concept.


Appendix


How Does Hail Form?•Inside of thunderstorms are strong updrafts of warmair and downdrafts of cold air.

•Condensation & FREEZING: If a water droplet ispicked up by the updrafts...it can be carried well abovethe freezing level. With temperatures below 32F thewater droplet freezes. (SMALL - GRAUPEL)

•Growth by RIMING: As the frozen droplet begins tofall...carried by cold downdrafts...it may thaw as itmoves into warmer air toward the bottom of thethunderstorm. But the little half-frozen droplet mayalso get picked up again by another updraft carrying itback into very cold air and re-freezing it. With each tripabove and below the freezing level the frozen dropletadds another layer of ice. (MODERATE SIZE)

•Growth by AGGREGATION:Rimed & Graupel crystals -half-frozen - are picked up again by another updraft.They can collide and stick together before refreezing toform large hail sizes. Large vertical wind shearsrequired. (Typical of 2 in. diam. and larger - LARGESIZES)


The US Data• The US source is the NOAA data base:

– Between 1955 and 2006– Data used to support severe storm warning

• Truncated and censored– Not all hail sizes reported– Originally minimum size of 0.53 inches– Now 0.75 inches and larger

• Reporting intervals of 0.25 inches• Maximum size reported in interval

– Individual “Severe Storms” may have several reports• Other NOAA sources suggest that a hail storm has a large

variability of hail sizes.• Data available:

– Location (collection stations)– Time of Day– Month and year


Airbus Damage Tolerance Methodology - HailFAA workshop for Composite Damage Tolerance & Maintenance

July 19-21 2006

• The Hailstorm data– 10mm = 0.39

inches– 25mm = 0.98

inches– 50mm ~ 2

inches


Comparison of US & (MIL-HDBK) Proposed“Design-to” Hail Diameter Data Base

(inches)-------------

-----------Diameter__

2.6321

215

1.76.88(Est..)10

10.450

US DataBaseAirbus,%

Exceeding


Largest Hail Stone Event Report 7 Inches;Note Time.

Record setting hail occurred on the north sideof Aurora. Thunderstorms erupted during theevening in south-central Nebraska. One stormnear Aurora produced very large hail. Onestone measured by an NWS Storm Surveyteam was determined to be the largest sizedstone to fall in the United States. It measured 7inches in diameter and 18.75 inches incircumference. Many other large stoneswere noted on the northern side of Aurora.Given the lack of wind, damage was limited toroofs, a few windows and some crop land neartown. The storm also produced a couple of briefsmall tornadoes as it slowly moved throughnorthern Hamilton county. At nearly the sametime, another very slow moving storm producedseveral tornadoes, hail and extremely heavyrains in Thayer county. One tornado movedthrough the town of Deshler going from thesoutheast side of town to the west.

•Begin Date:22 Jun 2003,06:05:00 PM CST Begin•Location: Aurora Begin•LAT/LON: 40°52'N / 98°00'WEnd Date:22 Jun 2003,06:05:00 PM CST End•Location: Aurora End•LAT/LON: 40°52'N / 98°00'W•Magnitude: 7.00 inches•Fatalities: 0 Injuries: 0•Property Damage: $ 500.0KCrop Damage: $ 1.0M


The Ground Hail Risk Is Determined by theProbability of a Storm Event Combined With the

Number of A/C on the Ground

• The average number of severe storms occurringat a specific hour for a specific airport location isdetermined by the cross products of the twoprobability functions and the expected number ofhail days for 20 Years at that location:

• The number of aircraft at a specific time isdetermined by flight operations of specificairlines.

!P n |w( ) x!P hr _day( ) x(# S.S.days)


Comment: 1970 to 85 vs. Today• Hail and Runway Debris where understood to be a threat to

minimum gage structure.– Honeycomb sandwich construction has increased

vulnerability

• D&DT Hail size was an estimate of the central location forthe hail size distribution– Birdstrike & HAIL had been specified at the equivalent of a “B

allowable.”– Extreme hail sizes are/were known to occur but were assumed

to be “self evident” events as they require severe weatherconditions.

– Economic Durability & Dispatch Reliability topic

• Today a focus on quantifying damage threshold for designsizing, inspection and maintenance decisions is evolving.– Visual indications of damage, useful field inspection

indicators

managing impact risk for composite structures: unifying durability and ... · 10 may 2007 jchalpin...

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