NAV Advanced Composites Airframe Technology Branch North Island

NAV Advanced Composites Airframe Technology Branch North Island AIR NAV Minimum Bondline Length Study 22 -24 August 2017, Hill AFB Do. D Advanced Composite Maintainers Technical Interchange Meeting Prepared By: Alexandru Popescu Aerospace Engineer NAVAIR North Island ISSC Advanced Composites, 43352 (619) 545‐ 0716 alexandru. p. popescu@navy. mil NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. AIR

Advanced Composites Airframe Technology Branch North Island NAV AIR Summary ‐ Motivation ‐ Review of “Introduction to Adhesive Bonding, ” by Pierre Cyr, L‐ 3 MAS (09 June 2010) ‐ 6/β Bondline Length Input Trends ‐ A 4 EI Doubler vs. Lap Method: Doubler Breakdown ‐ A 4 EI Doubler Forcing Zero Shear Stress 2 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR Motivation ‐ Speed brake well repair patches required relatively short bondline. A 4 EI breakdown with short bondline observed. ‐ “Preferred minimum patch overlap is 1. 50 inches” according to the BJAM manual (pg. 2. 1‐ 2 Revision Date: 25 September 2015) ‐ Why 1. 5 in. ? ‐ Can smaller patch overlap be safely utilized? ‐ Can A 4 EI analyze smaller bondlines? ‐ Is there an explanation for the shear stress mismatch at joint center? 3 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR Impact on Repairability ‐ Reducing the patch overlap length may be necessary in order to satisfy repair parameters, including geometric restrictions ‐ Providing a check for bondline lengths less than 1. 25 in. means that parts that previously may have been unrepairable can now be analyzed and possibly repaired, reducing scrap rates 4 NAVAIR Public Release 17 -0011 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island “Introduction to Adhesive Bonding” by Pierre Cyr, L‐ 3 MAS NAV AIR • Cyr Fig. 4 (Hart‐Smith) Cyr Fig. 5 5 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV Advanced Composites Airframe Technology Branch North Island AIR “Introduction to Adhesive Bonding”, cont. • Cyr Fig. 8 6 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR “Introduction to Adhesive Bonding”, cont. Elastic region (slope) moves inward as the applied load increases. Plasticity begins at joint ends. Elastic region has fixed length until the regions begin to overlap as severe plasticity occurs. Central portion of joint can have zero shear stress. Dr. Kim at UCSD discusses in his SE 171 course. Cyr Fig. 9 7 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR “Introduction to Adhesive Bonding”, cont. ‐ Further information can be found online in several recorded lectures from Dr. Kim’s SE 171 course from UCSD (Winter 2017) ‐ 27 Feb 17 ‐ 01 Mar 17 8 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR “Introduction to Adhesive Bonding”, cont. • A 4 EI. pdf Fig. 3‐ 2 6/β is completely independent of joint loading. It is solely dependent on adhesive and adherend material properties and thicknesses 9 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR “Introduction to Adhesive Bonding”, cont. • 10 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR “Introduction to Adhesive Bonding”, cont. Conclusions & Recommendations: 6/β bondline length for elastic load transfer is intended to improve fatigue resistance and damage tolerance. Good starting point for determining alternative bondline length. Could be problematic if this bondline check recommends a length shorter than the BJAM manual recommended 1. 5 in. Provides some insight as to how A 4 EI calculates joint strength. 11 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV Advanced Composites Airframe Technology Branch North Island AIR 6/β Bondline Length Input Trends Goal is to study bondline length trend for different patch‐skin combinations and adhesive inputs. Chose skin‐patch configuration for four adhesives (EA 9394 (5 day, 4 hr, 1 hr), and FM 300‐ 2 (2 hr)) then performed the following analyses: Changing Skin Thickness: Changing Patch Thickness: 16 -ply AF/ 3 -ply RB 14 -ply AF/ 3 -ply RB 12 -ply AF/ 3 -ply RB 10 -ply AF/ 3 -ply RB 8 -ply AF/ 3 -ply RB 12 -ply AF/ 9 -ply RB 12 -ply AF/ 8 -ply RB 12 -ply AF/ 7 -ply RB 12 -ply AF/ 6 -ply RB 12 -ply AF/ 5 -ply RB 12 -ply AF/ 4 -ply RB 12 -ply AF/ 3 -ply RB Thickness Ratio Thickness Ratio Stiffness Ratio Parenthesis Value Variable Studied: Parenthesis Value 12 Parenthesis Value NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 6 -ply AF/ 3 -ply RB 4 -ply AF/ 3 -ply RB Thickness Ratio Parenthesis Value

NAV Advanced Composites Airframe Technology Branch North Island AIR 6/β Bondline Length Input Trends, cont. Varied adherend thickness ratio (by changing skin thickness, patch thickness) Varying Skin Thickness Varying Patch Thickness EA 9394 5 day RTD EA 9394 4 hr RTD EA 9394 1 hr RTD FM 300‐ 2 2 hr RTD 1. 8000 Bondline Length: 6/β 1. 3000 1. 2000 1. 1000 1. 0000 0. 9000 0. 8000 0. 450 0. 650 0. 850 1. 050 1. 250 1. 450 1. 650 Skin-Patch Thickness Ratio 1. 850 1. 6000 1. 4000 1. 2000 1. 0000 0. 8000 0. 450 0. 650 0. 850 1. 050 1. 250 Skin-Patch Thickness Ratio Conclusion: No observable trend due to stiffness summation. FM 300‐ 2 requires larger bondline length. 13 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 450

NAV Advanced Composites Airframe Technology Branch North Island AIR 6/β Bondline Length Input Trends, cont. Varied adherend stiffness ratio (by changing skin thickness, patch thickness) Varying Patch Thickness EA 9394 5 day RTD EA 9394 4 hr RTD EA 9394 1 hr RTD FM 300‐ 2 2 hr RTD 1. 4500 1. 5500 1. 3500 1. 4500 Bondline Length: 6/β Varying Skin Thickness 1. 2500 1. 1500 1. 0500 0. 9500 0. 8500 2. 000 1. 3500 1. 2500 1. 1500 1. 0500 0. 9500 2. 500 3. 000 3. 500 Skin-Patch Stiffness Ratio 4. 000 0. 8500 2. 000 2. 500 3. 000 3. 500 Skin-Patch Stiffness Ratio Conclusion: No observable trend due to stiffness summation. FM 300‐ 2 requires larger bondline length. 14 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 4. 000

NAV Advanced Composites Airframe Technology Branch North Island AIR 6/β Bondline Length Input Trends, cont. Varied “parenthesis value” (by changing skin thickness, patch thickness) EA 9394 5 day RTD EA 9394 4 hr RTD EA 9394 1 hr RTD FM 300‐ 2 2 hr RTD 1. 8000 Bondline Length: 6/β 1. 6000 1. 4000 1. 2000 1. 0000 0. 8000 1. 500 2. 000 2. 500 3. 000 3. 500 Skin-Patch Parenthesis Value 4. 000 4. 500 Conclusion: No observable trend due to stiffness summation. FM 300‐ 2 requires larger bondline length. 15 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV Advanced Composites Airframe Technology Branch North Island AIR 6/β Bondline Length Input Trends, cont. Conclusions & Recommendations: We cannot predict the bondline length for elastic load transfer from just the skin‐patch stiffness ratios or any of the ratios I analyzed. ‐The individual adherend stiffnesses are required This means that we have to calculate this bondline length for each proposed repair. However, A 4 EI has been observed to exhibit a breakdown at short bondline lengths. Several examples have been studied to determine if this event happens below certain bondline lengths 16 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline Advanced Composites Airframe Technology Branch North Island A 4 EI has been observed to exhibit a breakdown at short bondline lengths. Several examples have been studied to determine if this event happens below certain bondline lengths. Performed a study with an example from DED‐F 18‐ 4575‐ 17 with Nxy’ RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond). Studied all 4 adhesives in A 4 EI: RTD EA 9394 5 day cure cycle 1150 0. 6 in. AFM Cyr 6/β bondline length: 0. 805 in. Strength (lb/in) 1100 1050 1000 D Elastic (lb/in) L Elastic (lb/in) 950 Greater than 1% difference between Doubler and Lap methods 900 850 0. 25 17 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD EA 9394 1, 4 hr cure cycle 1400. 0 1350. 0 AFM 1300. 0 Strength (lb/in) AFM 0. 6 in. 1250. 0 D Elastic (lb/in) 1200. 0 L Elastic (lb/in) 1150. 0 Cyr 6/β bondline length: 0. 805 in. Greater than 1% difference between Doubler and Lap methods 1100. 0 1050. 0 0. 25 18 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD EA 9394 all cure cycles 1300 1250 CFM 1200 Strength (lb/in) CFM 0. 6 in. 1150 D Elastic (lb/in) 1100 L Elastic (lb/in) Greater than 1% difference between Doubler and Lap methods 1050 CFM 1000 950 Cyr 6/β bondline length: 0. 805 in. 19 0. 25 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD FM 300 -2 2 hr cure cycle 2050 1950 0. 8 in. AFM Cyr 6/β bondline length: 1. 05 in. Strength (lb/in) 1850 1750 1650 D Elastic (lb/in) L Elastic (lb/in) 1550 Greater than 1% difference between Doubler and Lap methods 1450 1350 1250 0. 25 20 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD FM 300 -2 2 hr cure cycle 1800 1700 Cyr 6/β bondline length: 1. 05 in. Strength (lb/in) CFM 0. 8 in. 1600 1500 D Elastic (lb/in) 1400 L Elastic (lb/in) 1300 NOTE: Elastic‐Plastic is N/A for this CFM case Greater than 1% difference between Doubler and Lap methods 1200 1100 0. 25 21 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: ETW EA 9394 5 day cure cycle 690 1. 2 in. AFM = CFM Cyr 6/β bondline length: 1. 554 in. Strength (lb/in) 670 650 630 D Elastic (lb/in) 610 L Elastic (lb/in) 590 Greater than 1% difference between Doubler and Lap methods 570 550 0. 25 22 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: ETW FM 300 -2 2 hr cure cycle 1175 1125 Strength (lb/in) AFM Cyr 6/β bondline length: 2. 059 in. 1. 6 in. 1075 1025 975 D Elastic (lb/in) L Elastic (lb/in) 925 875 Greater than 1% difference between Doubler and Lap methods 825 775 0. 50 23 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: CTD EA 9394 5 day cure cycle 1675 0. 55 in. 1625 Cyr 6/β bondline length: 0. 701 in. Strength (lb/in) AFM 1575 1525 D Elastic (lb/in) L Elastic (lb/in) 1475 Greater than 1% difference between Doubler and Lap methods 1425 1375 0. 25 24 0. 50 0. 75 1. 00 Bondline Length (in) 1. 25 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 50

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Example from A 4 EI. pdf pg. 172. Reduced the bondline and recorded the elastic and potential strengths calculated by A 4 EI for both the doubler and lap methods: Cyr 6/β bondline length: 1. 39 in. 6000 1. 5 in. Strength (lb/in) 5000 4000 D Elastic (lb/in) D Potential (lb/in) L Elastic (lb/in) 3000 L Potential (lb/in) 1. 2 in. 2000 Greater than 1% difference between Doubler and Lap methods 1000 0. 25 25 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Example from A 4 EI. pdf pg. 172 with adherend thicknesses reduced to 0. 046”. Reduced the bondline and recorded the elastic and potential strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): Cyr 6/β bondline length: 1. 19 in. 4975 4475 1. 3 in. Strength (lb/in) 3975 3475 D Elastic (lb/in) 2975 D Potential (lb/in) L Elastic (lb/in) 2475 1975 1475 Greater than 1% difference between Doubler and Lap methods 975 0. 25 26 L Potential (lb/in) 1. 0 in. 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example H‐stab patch‐skin combination with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD FM 300 -2 2 hr cure cycle 1875 0. 7 in. 1775 Strength (lb/in) AFM 1675 1575 D Elastic (lb/in) L Elastic (lb/in) 1475 Greater than 1% difference between Doubler and Lap methods 1375 1275 0. 25 27 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example H‐stab patch‐skin combination with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD FM 300 -2 2 hr cure cycle 1700 CFM Strength (lb/in) NOTE: Elastic‐Plastic is N/A for this CFM case 0. 7 in. 1600 1500 1400 D Elastic (lb/in) L Elastic (lb/in) 1300 Greater than 1% difference between Doubler and Lap methods 1200 1100 0. 25 28 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example speed brake well repair patch with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD EA 9394 5 day cure cycle 890 0. 5 in. 870 Strength (lb/in) AFM 850 830 D Elastic (lb/in) L Elastic (lb/in) 810 Greater than 1% difference between Doubler and Lap methods 790 770 0. 25 29 0. 50 0. 75 1. 00 1. 25 Bondline Length (in) 1. 50 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example speed brake well repair patch with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD EA 9394 5 day cure cycle 990 0. 5 in. 970 Strength (lb/in) CFM 950 930 D Elastic (lb/in) 910 L Elastic (lb/in) 890 Greater than 1% difference between Doubler and Lap methods 870 850 0. 25 30 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 75 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline: Data Summary Advanced Composites Airframe Technology Branch North Island Example L_6/β, in. Bondline Length at % Deviation at 1. 25 1% deviation, in. Bondline Length DED‐F 18‐ 4575‐ 17 RTD AFM with EA 9394 5 day cure cycle 0. 805 0. 6 0. 01 DED‐F 18‐ 4575‐ 17 RTD CFM with all EA 9394 cure cycle 0. 805 0. 6 0. 01 DED‐F 18‐ 4575‐ 17 RTD AFM with EA 9394 1 & 4 cure cycles 0. 805 0. 6 0. 02 DED‐F 18‐ 4575‐ 17 RTD AFM with FM 300‐ 2 2 hr cure cycle 1. 054 0. 8 0. 09 DED‐F 18‐ 4575‐ 17 RTD CFM with FM 300‐ 2 2 hr cure cycle 1. 054 0. 8 0. 09 DED‐F 18‐ 4575‐ 17 ETW with EA 9394 5 day cure cycle 1. 554 1. 2 0. 85 DED‐F 18‐ 4575‐ 17 ETW AFM with FM 300‐ 2 2 hr cure cycle 2. 059 1. 6 2. 91 DED‐F 18‐ 4575‐ 17 CTD AFM with EA 9394 5 day cure cycle 0. 701 0. 55 0. 00 A 4 EI Stepped‐Lap Joint Analysis PDF pg. 172 1. 388 1. 2 0. 93 A 4 EI Stepped‐Lap Joint Analysis PDF pg. 172 Modified 1. 186 1. 0 0. 37 H‐stab RTD AFM with FM 300‐ 2 2 hr cure cycle 0. 896 0. 7 0. 04 H‐stab RTD CFM with FM 300‐ 2 2 hr cure cycle 0. 896 0. 7 0. 04 Speed Brake Well RTD AFM with EA 9394 5 day cure cycle 0. 646 0. 5 0. 00 Speed Brake Well RTD CFM with EA 9394 5 day cure cycle 0. 646 0. 5 0. 00 31 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline: Discussion and Thoughts Advanced Composites Airframe Technology Branch North Island ‐ The BJAM 1. 5 in. preferred minimum patch overlap may be due to this A 4 EI limitation. ‐ For all but one example, the 1 % deviation between the doubler and lap methods happened at less than 1. 25 in. Furthermore, even at 1. 25 in. , only one example had greater than a 1% deviation ‐ Because the A 4 EI Doubler method calculates either EOH or EOP depending on the patch‐ skin selection, the output may display the non‐critical end of the joint. ‐ This shortcoming is resolved by switching the patch‐skin configurations ‐ We have seen evidence that a relatively short bondline yields inaccurate results with AFM (EOH) properties. ‐ When Doubler and Lap methods in A 4 EI diverge, the Doubler results become invalid as evidenced by increasing strength with shorter bond length. ‐ We lose our ability to differentiate between CFM and AFM because Doubler method can model both CFM and AFM if patch‐skin configuration is swapped, while Lap shows the worst‐case 32 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

NAV Advanced Composites Airframe Technology Branch North Island AIR A 4 EI Doubler Forcing Zero Shear Stress • (Cyr Eq. 13‐ 16) τ These two boundary conditions are commonly used x 33 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR Discussion & Thoughts ‐ Will CFM have a different bondline length according to 6/β elastic load transfer length? ‐ CFM will have same bondline length because the formula does not consider ‐ How do we account for a discontinuity in shear stresses mismatching at the joint center when we have CFM and AFM adhesive properties? ‐ True solution would be to model the z‐axis stresses and failure mode limited by adherend surface ply failure (due to peel stresses). ‐ This was the motivation behind studying Cyr’s paper and 6/β bondline length. ‐ Some non‐A 4 EI investigation since CFM properties apply to half of the joint may be useful stop‐gap solution until advanced modeling that accounts for peel stresses. ‐ Possible future investigation: Can they be equal and not equal to zero? ‐ 34 It seems like the A 4 EI doubler method forces them to be equal by assuming the shear stress goes to zero near the joint center as a boundary condition. NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR Other Strategies ‐ Wung, E. C. , and O’Callaghan, P. J. , “Adhesively Bonded Composite Doubler Loaded by Remote In‐Plane Loads, ” CAMX, NG 16‐ 0916, May 2016. ‐ Solution presented for in‐plane loading is superposition of Volkersen/Hart‐ Smith for bi‐axial, Levy for shear loading ‐ Employs trigonometric series from Timoshenko to approximate shear stress ‐ Wung, E. C. , and O’Callaghan, P. J. , “A Greatly Improved A 4 EI Code for the Analysis of Bonded Composite Joints, ” SAMPE, SE 17‐ 0866, May 2017. ‐ Created Excel VBA Macro that matches A 4 EI results and eliminates A 4 EI convergence issues by employing closed‐form solutions to solve for shear stress 35 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited.

Advanced Composites Airframe Technology Branch North Island NAV AIR

Advanced Composites Airframe Technology Branch North Island NAV Backup Slides 37 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. AIR

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD EA 9394 5 day cure cycle 1550 1500 0. 6 in. AFM Cyr 6/β bondline length: 0. 805 in. Strength (lb/in) 1450 1400 1350 D Elastic‐Plastic (lb/in) L Elastic‐Plastic (lb/in) 1300 1250 Greater than 1% difference between Doubler and Lap methods 1200 1150 0. 25 38 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD EA 9394 1, 4 hr cure cycle 1600 0. 6 in. 1550 AFM Strength (lb/in) 1500 1450 1400 D Elastic‐Plastic (lb/in) L Elastic‐Plastic (lb/in) 1350 1300 Cyr 6/β bondline length: 0. 805 in. Greater than 1% difference between Doubler and Lap methods 1250 1200 0. 25 39 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD EA 9394 all cure cycles 1300 1250 0. 6 in. CFM Strength (lb/in) 1200 1150 D Elastic‐Plastic (lb/in) 1100 L Elastic‐Plastic (lb/in) 1050 Greater than 1% difference between Doubler and Lap methods 1000 CFM 950 0. 25 Cyr 6/β bondline length: 0. 805 in. 40 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: RTD FM 300 -2 2 hr cure cycle 2650 0. 8 in. AFM Cyr 6/β bondline length: 1. 05 in. Strength (lb/in) 2450 2250 D Elastic‐Plastic (lb/in) L Elastic‐Plastic (lb/in) 2050 1850 Greater than 1% difference between Doubler and Lap methods 1650 0. 25 41 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: ETW EA 9394 5 day cure cycle 1. 2 in. 1550 AFM = CFM Cyr 6/β bondline length: 1. 554 in. Strength (lb/in) 1450 1350 1250 D Elastic‐Plastic (lb/in) 1150 L Elastic‐Plastic (lb/in) 1050 Greater than 1% difference between Doubler and Lap methods 950 850 0. 25 42 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: FM 300 -2 2 hr cure cycle 6025 Greater than 1% difference between Doubler and Lap methods at all tested lengths AFM Cyr 6/β bondline length: 2. 059 in. Strength (lb/in) 5025 4025 D Elastic‐Plastic (lb/in) 3025 L Elastic‐Plastic (lb/in) 2025 1025 0. 50 43 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island DED‐F 18‐ 4575‐ 17: EA 9394 5 day cure cycle 1650 0. 55 in. AFM Cyr 6/β bondline length: 0. 701 in. Strength (lb/in) 1600 1550 D Elastic‐Plastic (lb/in) L Elastic‐Plastic (lb/in) 1500 1450 Greater than 1% difference between Doubler and Lap methods 1400 0. 25 44 0. 50 0. 75 1. 00 Bondline Length (in) 1. 25 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 1. 50

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example H‐stab patch‐skin combination with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD FM 300 -2 2 hr cure cycle 2500 AFM 0. 7 in. 2400 Strength (lb/in) 2300 2200 2100 D Elastic‐Plastic (lb/in) 2000 L Elastic‐Plastic (lb/in) 1900 1800 Greater than 1% difference between Doubler and Lap methods 1700 1600 0. 25 45 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example speed brake well repair patch with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD EA 9394 5 day cure cycle 1225 1205 AFM 0. 5 in. Strength (lb/in) 1185 1165 1145 1125 D Elastic‐Plastic (lb/in) 1105 L Elastic‐Plastic (lb/in) 1085 Greater than 1% difference between Doubler and Lap methods 1065 1045 1025 0. 25 46 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

NAV AIR A 4 EI Doubler vs. Lap Method: Doubler Breakdown at Short Bondline, cont. Advanced Composites Airframe Technology Branch North Island Performed a study with an example speed brake well repair patch with RTD material properties then reduced the bondline and recorded the elastic and elastic‐plastic strengths calculated by A 4 EI for both the doubler and lap methods (stepped, single bond): RTD EA 9394 5 day cure cycle 990 CFM 0. 5 in. Strength (lb/in) 970 950 930 D Elastic‐Plastic (lb/in) 910 L Elastic‐Plastic (lb/in) 890 Greater than 1% difference between Doubler and Lap methods 870 850 0. 25 47 0. 50 0. 75 1. 00 1. 25 1. 50 Bondline Length (in) 1. 75 NAVAIR Public Release 17 -0008 DISTRIBUTION A. Approved for public release; distribution is unlimited. 2. 00

Advanced Composites Airframe Technology Branch North Island NAV AIR A 4 EI Lap, Doubler Logic • BJAM pg. 59, 95: CFM is typically edge of patch and based on double lap shear • BJAM pg. 60, 95: AFM is typically edge of hole and based on adhesive capability • BJAM pg. 118: Lap joint uses doubler model but uses two material property configurations to capture both EOP/EOH 48 NAVAIR Public Release 17 -0008 Fig. 4. 3‐ 3 Fig. 4. 3‐ 4 DISTRIBUTION A. Approved for public release; distribution is unlimited.