AgingAging ofof CompositeComposite AircraftAircraft StructuresStructures BeechcraftBeechcraft StarshipStarship andand BB--737737 HorizontalHorizontal StabilizerStabilizer John Tomblin, PhD Executive Director, NIAR Lamia Salah Sr. Research Engineer, NIAR June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence FAA Sponsored Project Information ¾ Principal Investigators & Researchers ¾ Dr. John Tomblin, Wichita State University ¾ Lamia Salah, Wichita State University ¾ FAA Technical Monitor ¾ Curtis Davies ¾ Other FAA Personnel Involved ¾ Larry Ilcewicz, Peter Sheprykevich ¾ Industry Participation ¾ Mike Mott, Ric Abbott Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 2 Motivation/ Objective ¾ Current market/ economic conditions are requiring the use of aircraft structures beyond their DSO ¾ Industry is relying on existing inspection standards to ensure aging aircraft airworthiness ¾ Most aging studies are focused on metallic structures, need to address composite aging as well ¾ To evaluate the aging effects on a Beechcraft starship (NC-8) main wing after 12 years of service (1827 hours) Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 3 Background ¾ Program was launched in 1982 ¾ Objectives were to produce the most advanced turboprop business airplane feasible at the time and to promote the use of composites in a business aircraft ¾ Benefits: to achieve elaborate contours through composite molding, lower part count, manufacturing simplicity, composite’s resistance to corrosion, good fatigue properties, weight savings. ¾ 70% of the airframe by weight is composite ¾ FAA certification was obtained on June 14th 1988 to FAA part 23 regulations and special conditions ¾ A total of 53 airframes were built but only a handful ever sold. In 2003, the OEM decided to retire the entire Starship fleet except five that were still flying as of Summer of 2007. Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 4 Approach/ Methodology ¾ Non-Destructive Inspection to identify flaws induced during manufacture/ service (delamination, disbonds, impact damage, moisture ingression, etc…) ¾ Coupon level static and fatigue tests to investigate any degradation in the mechanical properties of the material (comparison with OEM tests) ¾ Physical and thermal tests to validate design properties, identify possible changes in the chemical/ physical/ thermal properties of the material ¾ Full scale static, durability and damage tolerance tests to evaluate the structural health/performance of the main wing 19 years since manufacture (12 years in service) Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 5 Test Article Description (Main Wing) ¾ Monococque structure with three spars and five full-chord ribs symmetric wrt about the aircraft centerline ¾ The wing skins are cured in one piece 54 feet tip to tip ¾ The wing skins are secondarily bonded to the spars and ribs using paste adhesive Root Rib MLG Spar Inboard Flap Rib MLG Rib FWD Curved Spar Mid Flap Rib FWD Spar Outboard Flap Rib Mid Elevon Rib Aft Spar Inboard Center Spar Aft Spar Middle Aft Spar Outboard Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 6 Test Article Description (Main Wing) Skin ¾H-Joint: used to join the upper and Paste Adhesive lower skins to the spars ¾A cutout is first routed in the skin prior to bonding the joint to the skin. ¾The joint is then secondarily bonded to the skin using paste and film Film Adhesive adhesive Paste Adhesive ¾ The spars are finally bonded to the assembly using paste adhesive Spar BL 199 US H-Joint Cross Section Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 7 Test Article Description (Main Wing) Skin ¾ V-Joint: used to bond the upper and Paste lower wing skins to sections of the Adhesive forward and aft spars ¾ The pre-cured graphite epoxy joint is secondarily bonded to the wing skin first using paste adhesive ¾ After this process is completed, the assembly is subsequently joined to the Paste spars using paste adhesive Adhesive BL 78 LS Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 8 Test Article Description (Main Wing) ¾ Lightning Protection achieved through the use of hybrid woven graphite/ aluminum fabric as the surface ply in all exterior surfaces ¾ Materials used was E7K8 12K/ 280 and 145 tape and AS4 E7K8 3K/195 PW fabric. Material qualification was conducted per Military Handbook 17 specifications. ¾ Lamina and Laminate testing was conducted to generate tension, compression, shear strength and strain allowables in various environmental conditions Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 9 Disassembly ¾ Main components disassembled (fuselage, forward wing, main wing, nacelles, fuel tanks) ¾ Main wing cut in two pieces for ease of transportation Coupon Full Scale Article Destructive Evaluation Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 10 NDI-LH Main Wing Preliminary TTU Non-Destructive inspection showed no evidence of flaws induced during manufacture or service in the skins. OEM records suggest that porosity levels in the upper skin LE flanges exceed 2.5% Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 11 Thermal Analysis-DMA ¾ Storage Modulus is an indication of the stiffness of the material, tanδ is a measure of the damping of the material ¾ DMA curves with a shallow storage modulus transition and a narrow tanδ indicate a highly cross linked material Lower Skin LF BL 260 DMA results Storage Modulus 159°C 318°F Tan δ 193°C 379°F Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 12 Thermal Analysis-DSC Lower Skin UF BL 260 DSC results Very small heat of reaction values indicative of a highly cross linked material Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 13 Full Scale Structural Test ¾ A baseline Non-Destructive Inspection scan has been generated prior to conducting the full scale test in order to identify possible manufacturing flaws or defects induced during service ¾ NDI grid has been drawn on the structure for ease of inspection and flaw growth monitoring ¾ Visual inspection, TTU and tap testing were used for the inspection ¾ A few areas in both the upper and lower skins have been identified as disbonds by the inspectors Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 14 Full Scale Structural Evaluation Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 15 Full Scale Structural Evaluation Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 16 Wet Lay-Up Repair ¾ Wet lay-up repair per BS 24204 and BS23727 using EA 956 resin ¾ Repair Analysis conducted by the OEM and demonstrated positive margins for axial loading ¾ Repair Stacking sequence: PW45/T0/8HS0/T0/8HS90/T0/PW45 Scarfed repair area Ply 1 application Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 17 Wet Lay-Up Repair Ply 2 application Cured Repair Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 18 Strain Gage Identification/ NDI Inspection Grid Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 19 Test Article Instrumentation SG R11 Lower Skin RBL 294.2 FS 477.5 Repair Instrumentation Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 20 Limit Load Test- Upbending Case Cond 4A ¾ Limit Load Cond 4A- (Max Positive Moment) ¾ During certification, wing suffered damage at 122%LL 135%LL and 141%LL before sustaining UL ¾ Shear/ moment/ torque introduced matched the static 4A values from RBL 100 to RBL 360 BENDING MOMENT VS LSTA 3000000 Proposed Test 2500000 FT-575 Test FT-575 Requested Static Up-Bending 4A 2000000 max Env min Env 1500000 1000000 500000 Bending Moment (in-lbs) Moment Bending 0 -500000 -1000000 -1500000 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 LSTA (in) Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 21 Limit Load Test ¾ Wing sustained 100% Up-bending Limit Load Test Wichita State University June 17th, 2008 The Joint Advanced Materials and Structures Center of Excellence 22 Limit Load Test ¾ Strain vs % LL comparison between current test and wing max upbending certification test (Cond 4A) ¾ No major change in compliance, certification data correlates very well with aged structure limit load test results Strain vs % LL (Current Test vs Static Max Strain vs % LL (Current Test vs Static Max Up-Bending Test to failure) Up-Bending Test to failure) 0 3500 R6A_test_USRBL174FS424.5 R10_LS_RBL208_FS439.5 R30A_4A_To failure 3000 R62A_to_failure -500 2500 -1000 2000 -1500 1500 -2000 1000 Strain (microstrain) Strain Strain (microstrain) -2500 500 -3000 0 0 20406080100 020406080100 %Limit Load % Limit