Managing Impact Risk for Composite Structures: Unifying Durability and Damage Tolerance Perspective. J C Halpin, JCH Consultants, Dayton, OH Hyonny Kim, U of California at San Diego 10 May 2007 JCHalpin & Hyonny Kim 1
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 impact threats. 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 a Durability & Damage Tolerance technology and safety management approach for impact events in operational fleets. 10 May 2007 JCHalpin & Hyonny Kim 2
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 for international locations.) 2. Damage Thresholds should be the bases for the dimensioning of laminates exposed to impact threats (the damage thresholds will depend on the nature of the threat, FOD is different than hail ice, than bird-strike, than rubber puck for tire damage, etc.). 3. Defense of a design is the responsibility of the OEMs including test & analysis methods. (Realistic test protocols & modeling are important to manage the different impact threats) 4. Inspection for Cause triggered by extreme events, and a defined operating period of Unrepaired Service Usage allows short-term operations before repair dependent upon severity of damage as defined by the Pxx internal load conditions within the traditional Durability & Damage Tolerance concepts. This perspective would utilize a Self Evident Event instead of Self Evident Damage 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. 10 May 2007 JCHalpin & Hyonny Kim 3
Structural Integrity The structural strength, rigidity, damage tolerance, durability and functioning of structural parts of the airframe as affecting the safe use and cost-of-ownership of the air vehicle DAMAGE TOLERANCE: The ability of the airframe to resist failure due to -- damage for a specified period of unrepaired service usage. DURABILITY: the ability of the airframe to resist --corrosion, thermal degradation, delamination, wear, and the effects of foreign object damage for a specified period of time. ASIP: Aircraft structural integrity program is all of the actions required to assure the structural integrity of the airframe. 10 May 2007 JCHalpin & Hyonny Kim 4
Critical Parts A critical part is one, the single failure of which could cause loss of the aircraft --- or result in major economic impact on the aircraft, --. FATIGUE/FRACTURE CRITICAL PARTS: Primary structural components that are designed by durability and/or damage tolerance requirements --. These parts generally call for special fatigue/fracture toughness controls, quality control procedures, NDT/I practices, and analytical requirements. MAINTENANCE CRITICAL PARTS: Airframe (structural) components designed by durability requirements. The failure of which may result in functional impairment of, or major economic impact on an aircraft or subsystem performance. The failure of the part requires costly maintenance and/or part repair or replacement, which if not performed would significantly degrade performance or operational readiness (dispatch reliability). --- 10 May 2007 JCHalpin & Hyonny Kim 5
Recognition of and Tolerance for Preexisting and/or Service Induced Damage SLOW CRACK GROWTH STRUCTURE: --consists of those design concepts where flaws or defects (Preexisting or Service Induced Damage) are not allowed to attain the critical size required 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 of unrepaired service usage. MINIMUM PERIOD OF UNREPAIRED SERVICE USAGE: is that period of of time during which the appropriate level of damage (assumed initial or in-service) is presumed to remain unrepaired and allowed to grow within the structure. Allowable Damage Limits Aircraft availability Dispatch reliability 10 May 2007 JCHalpin & Hyonny Kim 6
Early ASIP Criteria: B Threat Allowables - Design to Threat, Bound Dent Inspection Requirement - Avoiding Penalizing Better Materials & Designs. Damage Source Zone Damage Level Requirements Tool Drop 0.5 in. Dia. Solid Impactor low velocity Normal to surface 1 High probability of impact Impact energy smaller of 6 Ft.-lbs or visible damage (0.1 in. deep) with a minimum of 4 Ft-lbs No functional impairment or structural repair required for 2 design lifetimes. No water intrusion No visible damage from a single 4 ft-lb impact Same as Zone 1 2 Low probability of impact Impact energy smaller of 6 ft-lbs or visible damage (0.1 in. deep) No functional impairment after 2 design lifetimes. No water intrusion after repair if damage is visible INITIAL FLAW - DAMAGE ASSUMPTION IMPACT DAMAGE Damage From a 1.0 in. Dia. Hemispherical Impactor With 100 Ft-lbs of Kinetic Energy or With That Kinetic Energy Required to Cause a Dent 0.10 Inches Deep, Whichever Is Less 10 May 2007 JCHalpin & Hyonny Kim 7
Hail & Runway Debris Were Recognized in the Initial Adv. Composites Durability 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% Mil HBK 210 10 May 2007 JCHalpin & Hyonny Kim 8
Generic Sequence for an Impact Event: Impacting Mass May or May Not Be Conserved Through Penetration (example ice fragmentation on contact) KE 0 m V 0 Before Contact Lo V FTE Dent Threshold Initial Impact Hydrostatic Compression Shear-Plug formation Compression-Shear Hi V Ballistic Threshold Tension-Shear Penetration & Vibration Ballistic Penetration Lost Material, m L W R m R V R Residual Velocity KE 0 = KE R + 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 of fiber rupture & back side break-out. 10 May 2007 JCHalpin & Hyonny Kim 9
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. 10 May 2007 JCHalpin & Hyonny Kim 10
Impact Damage Thresholds & Areas Were Recognized to Be Dependent on Thickness in the Mid 1970 s. Hard Object Impact Characterization for FOD & Tool Drop. Initial Focus on Thickness Scaling 10 May 2007 JCHalpin & Hyonny Kim 11
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? 10 May 2007 JCHalpin & Hyonny Kim 12
BVID Requires Significant Laminate Damage to Remain Persistent. Damage Diameter BVID? Energy, Force or Size of Impactor 10 May 2007 JCHalpin & Hyonny Kim 13
A Tool to Manage Maintenance and Schedule Departure Reliability: Minimum Period of Unrepaired Service Usage Applied to Impact Damage. Damage Diameter P xx Specified Damage Limit Allowable Damage Limit Could provide limited period of usage to schedule repairs? Energy or Size of Impactor 10 May 2007 JCHalpin & Hyonny Kim 14
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 awaiting maintenance & repair. Focused & defined process 10 May 2007 JCHalpin & Hyonny Kim 15
Characterizing the Hail Threat Understanding Hail Ground & in-flight Operational usage MIL HDBK data? Need international data base 10 May 2007 JCHalpin & Hyonny Kim 16
Hail Stone Growth Is Enhanced in Strong Storm Systems, Vertical Wind Gusts Carrying Small Hail Upward Becoming Large Stones. Sometimes These Storms Generate Tornados. Extreme Sizes Are Typically Associated With Weather Dangerous for Aviation Larger Hail Stones are Aggregates of Smaller Ice Particles. Four Different Size Distributions Suggested by Data from Storm Chasers & NOAA. 10 May 2007 JCHalpin & Hyonny Kim 17
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 United States, 7 inches in diameter and 18.75 inches in circumference. The second largest recorded hailstone fell in Coffeyville, Kansas on September 3, 1970. It measured about 17.5 inches in circumference (over 5.6 inches in diameter) and weighed more than 26 ounces (almost 2 pounds)! 10 May 2007 JCHalpin & Hyonny Kim 18
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; ) 10 May 2007 JCHalpin & Hyonny Kim 19
NOAA Severe Hail Data Base for Observed for the Hub Airport Areas 2,829 Severe Hail Reports Between 1955 &2006. Fitted to a Truncated Weibull Distribution, P(n w) ( ) = exp! % D! 0.53 P n w # $ " & ( ' ) 10 May 2007 JCHalpin & Hyonny Kim 20
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 10 May 2007 JCHalpin & Hyonny Kim 21
There is an EU Severe Hail Data Base: Example 577 reports in 2006; diameter of 2.0 cm or more. 10 May 2007 JCHalpin & Hyonny Kim 22
International Hail Database? The principle zone of hail occurrence is between 30-50 0 N 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 be quantified, agreed to, and kept up to date. It is independent of platforms. Basic to design & risk assessments 10 May 2007 JCHalpin & Hyonny Kim 23
Damage Thresholds Characterization of Failure Threshold Energy, FTE Realistic Test protocols & Modeling FTE dependent on threat Influence on Laminate sizing 10 May 2007 JCHalpin & Hyonny Kim 24
Observations of Non-Damaging (84.1 M/sec. & 125 J) and Penetration (106 M/sec. & 199 J) of 42.7mm SHI Impacts On 1.22 mm Panels. Note: SHI fragmentation sensitivity to impacting velocity 10 May 2007 JCHalpin & Hyonny Kim 25
In the 1990 s a Focus on Hail Testing Emerged. Failure Modes Observed for Simulated Hail Ice, SHI, Impacts, an example. Gas gun technology evolved from P&W experience with birdstrike & roofing test STD s. 10 May 2007 JCHalpin & Hyonny Kim 26
Adhesive Bonded Composite Lap Joint: Distinct Failure Threshold Energy, FTE, Identified Impacting hail ice ball on a bonded lap shear specimen Resultant damage area as a function of Impact KE. Bases for: FTE ABAQUS Digital modeling Scaling of FTE s Ply Delamination adjacent to bond observed & verified by simulation 10 May 2007 JCHalpin & Hyonny Kim 27
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. 10 May 2007 JCHalpin & Hyonny Kim 28
H. Kim Has Demonstrated a Procedure to Correlate a Range of Impact Parameters That Are Useful for Preliminary Sizing. 10 May 2007 JCHalpin & Hyonny Kim 29
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 projectiletarget 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 U shear in shear plug: 2 2 * 1.. 2. 2 2! # xz * ave & ( D U shear = ' dvol = ) H$ Vol G " xz Gxz % Initiation of damage occurs when critical value reached, i.e., τ xz. ~ τ ILS, defining FTE: 2 Panel Thickness H ILS Stress at Center Depth " xz!d ILS Stress Contours Ice Sphere Diameter D Contours of " xz DYNA3D Profile Average " ave. x where τ ILS = 100 MPa is interlaminar shear strength, G ILS = 5.17 GPa is interlaminar shear modulus ref: * Kim H, Kedward KT. Modeling Hail Ice Impacts and Predicting Impact Damage Initiation in Composite Structures. AIAA Jo. 2000;38(7):1278-1288. 10 May 2007 JCHalpin & Hyonny Kim 30
Illustration of the Influence of of Hail Size & Test Simulation Methodology on Laminate Thickness, ASTM Characterization Increases Laminate Thickness. (@ Constant KE) ROM Estimates, limited data base Simple Laminate, Final Dimensions Dictated by Variety of Requirements ASTM 10 May 2007 JCHalpin & Hyonny Kim 31
Illustration of the Influence of Test Simulation Methodology on the Thickness of 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 Function of Incident Angle, Thickness, inches 0.275 0.25 0.225 0.2 0.175 0.15 0.125 0.1 0.075 0.05 0.025 0 SHI ASTM 0 10 20 30 40 50 60 70 80 90 Incident Angle, (90 degrees is normal) impact Glancing impact on curved surface - solid laminate Minimum gage dictated by other A/C requirements 10 May 2007 JCHalpin & Hyonny Kim 32
Summary: Ice Impacts on Composites 10 May 2007 JCHalpin & Hyonny Kim 33
Incidental Contact With Ground Vehicles: Blunt Impact Damage: Background 10 May 2007 JCHalpin & Hyonny Kim 34
Plan of Work Three tasks addressing each of the objectives: Task 1. Identification of Common Blunt Impact Scenarios and Establishment 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 (if permitted) and written notes outcome of this task will be summarized in a written document/faa report 10 May 2007 JCHalpin & Hyonny Kim 35
Tasks 2 and 3 Impact Threat Characterization Blunt Impact Damage Characterization and Testing Conduct Lab and Large Scale Experiments Also: Simulation of Impact Damage Formation Simple Analytical Tools Development Damage Tolerance Assessment 10 May 2007 JCHalpin & Hyonny Kim 36
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 three levels: 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 ongoing advisement, particularly in Task 2 methodology development, and in defining parameters for experiments to be conducted in Task 3. Level 3. Provide material support for Task 3. Make available test panels, substructure components, and/or impactors (e.g., a cargo cart) for full-scale blunt impact investigations. 10 May 2007 JCHalpin & Hyonny Kim 37
Request From Today s Audience feedback on proposed activities wish list from industry/agency perspective what should this tool look like? be capable of? what quantities are most important to you? additional participants 10 May 2007 JCHalpin & Hyonny Kim 38
Managing & Explaining The Risk: Hail Example Self evident Rogue EVENTS The damaging storm events are so severe that their infrequent occurrence cannot go unnoticed! Hail sizes >0.75 inches only produced by SEVERE weather conditions Rough Events, beyond B or A Allowable or esign to threat size P xx & Inspection criteria in original D&DT guidance seems appropriate. Functional impairment, aero-elastic tolerance to a disbond Dispatch Reliability for civil fleet operations Convert self evident damage to self evident events in the legacy ASIP 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 maintenance actions. Apply to both Primary & Reliability critical structure. 10 May 2007 JCHalpin & Hyonny Kim 39
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) 1955, 5 B757 s & 50 other Jets Damaged Hail 3.5 inches 1992, 1 B767 Damaged 10 May 2007 JCHalpin & Hyonny Kim 40
Self Evident Rogue Events 10 May 2007 JCHalpin & Hyonny Kim 41
Providing a Consistent Methodology Some level of severe Rogue hail exposure will require OEM guidance for an inspection by the operators 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 design guidance for; NDI procedures Schedule departure reliability Implied ASIP requirements Potentially reduced inspection requirements? Economic risk? 10 May 2007 JCHalpin & Hyonny Kim 42
Status Matrix of Service Induced Impact Damage: Composite Structures Threat Test Protocol Simulation Models Threat Allowable Self Evident Event Impact Location(s); Zones 1 & 2 Bird Strike Gel-pack Yes B FAR s (Wt. & Vel.) Yes YES Hail Simulated Hail Ice, SHI Yes Maturing B Up-date MIL HDBK 310 Yes YES Runway Debris Lead Ball? Drop-tower?? B Up-date JSSG-2006? Sometimes Usually Tire Rupture Rubber Puck?? Yes Sometimes Panels Lost In-flight??? Yes Sometimes Tool-drop Steel or Aluminum Hemisphere Drop-tower? JSSG-2006 Structures Sometimes Yes Incidental Contact With Ground Vehicles TBD TBD TBD Sometimes? Yes Others? Lighting Strike ---- ----- ----- ----- ----- 10 May 2007 JCHalpin & Hyonny Kim 43
Sandwich Construction: Honeycomb Sandwich Construction Increases Vulnerability 10 May 2007 JCHalpin & Hyonny Kim 44
SUMMARY 1. Damage Thresholds should be the bases for the dimensioning of laminates exposed to hail and other impact threats (the damage thresholds will depend on 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 to date. 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 & analysis methods. (Realistic test protocols are available.) 4. Inspection for Cause; Hail example, triggered by extreme Rogue weather conditions, and a defined operating period allowing short term operations before repair, dependent upon severity of damage as defined by the Pxx internal load conditions or Allowable Damage Limits. This perspective would utilize a Self Evident Rogue Event instead of Self Evident Damage 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. 10 May 2007 JCHalpin & Hyonny Kim 45
Appendix 10 May 2007 JCHalpin & Hyonny Kim 46
How Does Hail Form? Inside of thunderstorms are strong updrafts of warm air and downdrafts of cold air. Condensation & FREEZING: If a water droplet is picked up by the updrafts...it can be carried well above the freezing level. With temperatures below 32F the water droplet freezes. (SMALL - GRAUPEL) Growth by RIMING: As the frozen droplet begins to fall...carried by cold downdrafts...it may thaw as it moves into warmer air toward the bottom of the thunderstorm. But the little half-frozen droplet may also get picked up again by another updraft carrying it back into very cold air and re-freezing it. With each trip above and below the freezing level the frozen droplet adds 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 to form large hail sizes. Large vertical wind shears required. (Typical of 2 in. diam. and larger - LARGE SIZES) 10 May 2007 JCHalpin & Hyonny Kim 47
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 10 May 2007 JCHalpin & Hyonny Kim 48
Airbus Damage Tolerance Methodology - Hail FAA workshop for Composite Damage Tolerance & Maintenance July 19-21 2006 The Hailstorm data 10mm = 0.39 inches 25mm = 0.98 inches 50mm ~ 2 inches 10 May 2007 JCHalpin & Hyonny Kim 49
Comparison of US & (MIL-HDBK) Proposed Design-to Hail Diameter Data Base % Exceeding Airbus, ----------- Diameter US Data Base (inches)------- ------ 50 10 5 1 0.4.88 (Est..) 1 2 1 1.76 2 2.63 10 May 2007 JCHalpin & Hyonny Kim 50
Largest Hail Stone Event Report 7 Inches; Note Time. Record setting hail occurred on the north side of Aurora. Thunderstorms erupted during the evening in south-central Nebraska. One storm near Aurora produced very large hail. One stone measured by an NWS Storm Survey team was determined to be the largest sized stone to fall in the United States. It measured 7 inches in diameter and 18.75 inches in circumference. Many other large stones were noted on the northern side of Aurora. Given the lack of wind, damage was limited to roofs, a few windows and some crop land near town. The storm also produced a couple of brief small tornadoes as it slowly moved through northern Hamilton county. At nearly the same time, another very slow moving storm produced several tornadoes, hail and extremely heavy rains in Thayer county. One tornado moved through the town of Deshler going from the southeast 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'W End 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: $ 500.0K Crop Damage: $ 1.0M 10 May 2007 JCHalpin & Hyonny Kim 51
The Ground Hail Risk Is Determined by the Probability of a Storm Event Combined With the Number of A/C on the Ground The average number of severe storms occurring at a specific hour for a specific airport location is determined by the cross products of the two probability functions and the expected number of hail days for 20 Years at that location:!p( n w) x!p( hr _ day) x(# S.S.days) The number of aircraft at a specific time is determined by flight operations of specific airlines. 10 May 2007 JCHalpin & Hyonny Kim 52
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 for the 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 weather conditions. Economic Durability & Dispatch Reliability topic Today a focus on quantifying damage threshold for design sizing, inspection and maintenance decisions is evolving. Visual indications of damage, useful field inspection indicators 10 May 2007 JCHalpin & Hyonny Kim 53