Center for Aircraft Structural Life Extension Providing Structural Integrity Technology to the Aerospace Community Compilation of Damage Findings from Multiple Recent Teardown Analysis Programs 25 th International Conference on Aeronautical Fatigue Symposium, Rotterdam, Netherlands May 2009 Gregory A. Shoales, Scott A. Fawaz and Molly R. Walters , USAF Academy/CAStLE, Colorado, USA DISTRIBUTION STATEMENT A: Approved for public release: distribution is unlimited
Overview � Purpose � Aircraft Structural Teardown Programs � Common goals � Primary tasks � Subject Aircraft � Findings � Future Work 2
Purpose � To present an overview of failure analysis (FA) findings from a variety of teardown analysis programs � Conducted 2005-2007 � Three aircraft categories � Eight total aircraft � Aircraft production years between 1957 and 1968 � All findings presented are from CAStLE analysis � 711 total failure analysis � 395 from light trainer/attack aircraft (1957-1968) � 282 from medium transport aircraft (1968) � 34 from heavy transport aircraft (1963) 3
Teardown Program Goals � Assess damage state after a period of known usage � Evaluate and/or revise damage prediction models � Assist in the validation of inspection methods � Other � Input to help determine inspection intervals (an output of damage prediction models) � Prepare for future repair action or redesign 4
Teardown Program Tasks Select Vendors Identify Purpose and Requirements Identify Subjects and Extract Disassemble Extracted Components Clean Parts and Remove Coatings Nondestructive Inspection of Parts Prioritize NDI Indications Perform Failure Analysis Analyze and Report Findings 5
Subject Aircraft � Light trainer/attack class aircraft � All wing structure in four aircraft � Wing to fuselage attach structure in four aircraft � All fatigue critical structure throughout two aircraft � Flight hours (FH) ranging between 16K and 23K � Medium transport aircraft � Center wing from a single aircraft � 22K FH, 46K equivalent hours � Heavy transport aircraft � Fatigue critical structure throughout a single aircraft � 18K FH, 12K landings, 3.5K pressure cycles 6
Findings � Finding type � NDI implications � Operational usage damage scale � Corrosion damage � Damage location � Initiation site size distribution 7
Finding Type 1000 Environmental damage Environmental damage _ Number of Findings 100 10 1 Unknown Non-operational Damage Exfoiliation IGC SCC In-plane Cracks Fatigue No Damage Bore Corrosion Corrosion Mechanical Damage Overstress Material Defect Damage findings resulting from operational usage Damage findings resulting from operational usage Finding Type 8
Finding Type by Aircraft Category Percentage for Each Category __ 45% Light Trainer/attack 40% Medium Transport 35% Heavy Transport 30% 25% 20% 15% 10% 5% 0% Environmental Environment operational Unknown Stress No Damage & Stress Non- Primary Damage Source Newest aircraft, least corrosion 9
Finding Type by Aircraft Category Percentage for Each Category __ 45% Light Trainer/attack 40% Medium Transport 35% Heavy Transport 30% 25% 20% 15% 10% 5% 0% Environmental Environment operational Unknown Stress No Damage & Stress Non- Primary Damage Source Production and maintenance quality indicator and programmatic decisions 10
Finding Type by Aircraft Category Multiple vendors, less Percentage for Each Category __ 45% oversight, varied NDI Light Trainer/attack 40% expertise Medium Transport 35% Heavy Transport 30% 25% 20% 15% 10% 5% 0% Environmental Environment operational Unknown Stress No Damage & Stress Non- Primary Damage Source One highly skilled/experienced inspector, high degree of oversight 11
Finding Type by Aircraft Category Percentage for Each Category __ 45% Light Trainer/attack 40% Medium Transport 35% Heavy Transport 30% 25% 20% 15% 10% 5% 0% Environmental Environment operational Unknown Stress No Damage & Stress Non- Primary Damage Source We usually find the root cause 12
NDI Implications fatigue crack size vs. indication strength 100 All damage found at sites Fatigue Crack Size (mm) _ Light Trainer/attack below 70% FSH are smaller Medium Transport 10 than 1.27 mm Heavy Transport 1 0.1 0.01 0 20 40 60 80 100 Relative BHEC Indication Strength (%FSH) 13
NDI implications damaged component type In-Plane Component Type Corrosion SCC Cracks Fatigue Overstress Unknown Skin 20 8 0 50 0 4 Skin Stiffenner 0 0 0 16 0 3 Rib Cap 11 0 0 14 1 0 Spar Cap 140 1 36 27 2 1 Fitting 4 2 2 13 1 1 � 357 total findings � Most corrosion and most fatigue cracks are in hidden, 2 nd layer, unreliable operational NDI available, if any � SCC cracks in skin; 4 in each of the transport aircraft � In-plane cracks; no available inspection 14
Operational Usage Damage Scale crack damage only _ 80 Number of Findings 60 40 20 0 < 0.6 < 1.27 to 0.6 < 3 to 1.27 > 3 Maximum Damage Dimension (mm) � 205 crack findings attributed to operational usage � 42% are smaller than 1.27 mm � 4 findings are smaller than 0.127 mm 15
Operational Usage Damage Scale crack damage by source 35 Environment Env & Stress 30 Stress Number of Findings_ Unknown 25 20 15 10 5 0 < 0.6 < 1.27 to 0.6 < 3 to 1.27 > 3 Maximum Damage Dimension (mm) � Majority of damage is due to stress � “Unknown” only exists in the very small scale damage 16
Operational Usage Damage Scale by aircraft category � Lower two bins represent part through cracks for all three aircraft categories � Lower three bins are part through cracks for medium transport 40 Number of Findings _ Light Trainer/attack 35 Medium Transport 30 Heavy Transport 25 20 15 10 5 0 < 0.6 < 1.27 to 0.6 < 3 to 1.27 > 3 Maximum Damage Dimension (mm) 17
Corrosion Damage Surface Area (mm 2 ) 10000 1000 100 10 require maintenance 1 action 0.1 0 10 20 30 40 50 60 % Thickness Loss � Light Trainer/attack category only � No corrosion damage evaluated in medium transport � Severe corrosion in one region attributed to retirement decision � No additional corrosion found during teardown program � Corrosion damage ignored in heavy transport � Most damage broad but not deep � Only 10 of 138 require maintenance action 18
Damage Location light trainer/attack aircraft wing Fuselage Station Left Side Right Side aft aft aft aft outboard outboard outboard outboard Wing Station � Damage is concentrated on front and aft spar and along two ribs near main gear attachment fitting � No significant difference right to left � Data permits analysis for MSD, MED, WFD 19
Damage Location medium transport aircraft wing Fuselage Station _ Left Side Right Side aft outboard Wing Station � Damage concentrated near two critical wing details; aft wing to body attach point and outboard wing fitting � No significant difference left to right � Data permits analysis for MSD, MED, WFD 20
Fatigue, SCC & Corrosion-Fatigue Initiating Site Size Distribution � What is “initiation feature size”? Feature identifiable as mechanical damage, pit or pit cluster Initiation site size Last resolvable striation � Even with conservative approach, the largest site is 0.624 mm � 90% are less than 0.254 mm � 48% are less than 0.127 mm Dimensions (mm) Minimum Maximum Average Initiation Feature % Corrosion Pit 80% 0.022 0.624 0.135 Mechanical Damage 20% 0.040 0.326 0.156 Percentage of Initiation Sites on Faying Surface 31% 21
Initiation Site Size Distribution literature compared to the present work As-Built As-Is To-Be Too Late! Probability of Occurrence Predicted range of Corrosion flaws at and Bulk Corrosion future Fatigue Material and/or Fatigue depot Quality intervals Manufacturing Defects Flaw Size Similar distributions of corrosion (pits) and manufacturing defects (mechanical damage) Distribution of initiation site sizes from the present work Number of Occurances Corrosion Pits Mechanical Damage 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Maximum Dimnesion (mm) 22
Initiation Site Size Distribution compared to damage finding scale 16 14 EIFS is dependent 12 Occurances Corrosion Pits 10 upon the specific Mechanical Damage 8 structural detail 6 4 2 0 0.05 0.15 0.25 0.35 0.45 0.55 0.65 Initiation Site Size (mm) What does the data show? 16 14 46 mm 12 Occurances There is a notion in the literature that EIFS 10 distributions track well with continuing damage 8 27 mm 6 From Corrosion Pits distributions and are independent of structural detail 4 From Mechanical Damage 2 0 0.2 0.6 1 1.4 1.8 2.2 2.6 3 3.4 3.8 4.2 4.6 5 5.4 5.8 Damage Size (mm) Distribution of damage size does not track with distribution of corresponding initiation site size 23
Future Work � Holistic life data has historically not been the analysis emphasis of teardown programs � Future programs shall place special emphasis on: � Identifying initiating feature characteristics (type, location and dimensions) � Tracking the progression of damage from each identified feature � CAStLE’s current program represents significantly more teardown data than the combination of the eight aircraft discussed herein 24
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