Strength and Damage Tolerance of Adhesively Bonded Joints: Stress-Based and Fracture-Based Approaches Hyonny Kim Assistant Professor School of Aeronautics and Astronautics, Purdue University presented at the FAA Workshop on Key Characteristics for Advanced Material Control September 16-18, 2003, Chicago, IL Funding Acknowledgements: FAA, NASA AGATE, WSU/NIAR, NASA Langley Research Center, ASEE, Purdue Research Foundation FAA Workshop Chicago 2003-09 1
Outline Introduction Bonded Joint Stress Analyses in-plane shear loading combined shear + tension loading plastic strain-based ultimate load variable bondline thickness tension loading in general unbalanced single lap joint Damage Tolerance of Disbonded Joints Buckling/Disbond Growth experiments models New Directions FAA Workshop Chicago 2003-09 2
Introduction General Joint Loading Shear Due to Fuselage Torsion Tension Due to Internal Pressure Segment of Joint Loaded by Biaxial Tension/Compression and Shear N y Bonded Doubler Compression Due to Fuselage Bending N x N xy N xy N x N y in-plane shear + tension produces adhesive shear stress FAA Workshop Chicago 2003-09 3
Introduction Damage Tolerance: Buckling/Disbond Growth compression causes buckling subsequent disbond growth possible FAA Workshop Chicago 2003-09 4
Introduction Damage Tolerance: Buckling/Disbond Growth Stringer-Stiffened Panel Wing Assembly P FAA Workshop Chicago 2003-09 5
Overarching Objectives bridge knowledge gap between coupon data and full scale structural performance provide engineering and scientific community with advancements in: the mechanics of bonded joints damage tolerance design and analysis philosophy assist FAA and GA companies by providing research support FAA Workshop Chicago 2003-09 6
In-Plane Shear Loaded Lap Joint Outer Adherend 5 z y 4 Adhesive x-z Shear for Aluminum Joint FEA SLBJ Theory N xy x N xy, MPa a τxz 3 2 Inner Adherend 1 in-plane shear load transfer across joint produces τ xz shear stress in adhesive 0-1.0-0.5 0.0 0.5 1.0 y/c at x = L/2 Kim, H. and Kedward, K.T., Stress Analysis of Adhesive Bonded Joints Under In-Plane Shear Loading, Journal of Adhesion, Vol. 76, No. 1, 2001, pp. 1-36. FAA Workshop Chicago 2003-09 7
In-Plane Shear Loaded Doubler inherently 2D problem 2 τ o xy 2 o λ τxy + C o = 0 τ a xz = t o τ o xy y τ yz shear peaks along x = 0, a τ a yz = t o τ o xy x z b y Bonded Doubler - Outer Adherend N xy 0 0 N xy Base Structure - Inner Adherend a x τ xz shear peaks along y = 0, b (not shown) Kim, H. and Kedward, K.T., Stress Analysis of Adhesive Bonded Joints Under In-Plane Shear Loading, Journal of Adhesion, Vol. 76, No. 1, 2001, pp. 1-36. FAA Workshop Chicago 2003-09 8
Combined Loading: Shear + Tension 800 Von-Mises Based Failure Envelope Outer Adherend N (lb/in) y 600 400 t a = 0.01 200 N y N xy z y N xy 0 0 200 400 600 800 1000 1200 N (lb/in) xy x N y Inner Adherend Kim, H. and Kedward, K.T., The Design of In-Plane Shear and Tension Loaded Bonded Composite Lap Joints, Journal of Composites Technology and Research, Vol. 24, No. 2, 2002, pp 297-307. Invited paper to special issue in honor of Don Oplinger. FAA Workshop Chicago 2003-09 9
Plastic Strain-Based Ultimate Load in-plane shear loading ductile adhesive joint carries greater load than elastic-limit design more conservative than Hart-Smith elastic-perfectly plastic model 30 25 Shear Stress (MPa) 20 15 10 adhesive test data from J. Tomblin, WSU 5 Data Fitting Curve 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Shear Strain Kim, H. and Lee, J., Adhesive Nonlinearity and the Prediction of Failure in Bonded Composite Lap Joints, Joining and Repair of Composite Structures, ASTM Special Technical Publication STP1455, submitted January 2003. FAA Workshop Chicago 2003-09 10
Failure Prediction vs. Test Data box beam torsion lap shear coupon experiments conducted by Wichita State University (John Tomblin) Max. Shear Flow (lbf/in) 3000 2500 2000 1500 1000 500 0 * Experimental Data Purdue Analysis Maximum Shear Flow Constitutive behavior for 0.20 was not avail. Used 0.12 data, with reduced failure strain. 0.00 0.05 0.10 0.15 0.20 0.25 Bondline Thickness (in) Tomblin, J., Seneviratne, W., Kim. H., and Lee, J., Characterization of In-Plane Shear Loaded Adhesive Lap Joints: Experiments and Analysis, FAA Final Report, DOT/FAA/AR-03/21, May 2003. FAA Workshop Chicago 2003-09 11
Variable Bondline Thickness (VBT) FAA Workshop Chicago 2003-09 12
VBT: Increased Stress Due to Bondline Thinning 8 7 6 Min. f Max. f f = 1 5 4 3 2 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 t 1 / t unif Kim, H. The Influence of Adhesive Bondline Thickness Imperfections on Stresses in Composite Joints, Journal of Adhesion, Vol. 79, No. 7, 2003, pp 621-642. FAA Workshop Chicago 2003-09 13
Tension Loading of Unbalanced Single Lap closed-form stress analysis of generalized unbalanced joint Joint Specs: - glass/epoxy, - overlap 2c = 25.4 mm, -t o = 2.49 mm, t i = 2t o, t a = 0.33 mm FAA Workshop Chicago 2003-09 14
Damage Tolerance of Disbonded Joints damage tolerance is safety concern thin flanges typically 1 mm (0.04 in.) susceptible to buckling + disbond growth disbond not easily detectable adhesive properties affect growth critically processing of joint very important FAA Workshop Chicago 2003-09 15
Buckling/Debonding Experiments Construction: glass/epoxy & carbon/epoxy face sheets 6.3 mm foam core two halves secondarily bonded with PTM&W ES6292 paste adhesive FAA Workshop Chicago 2003-09 16
Experimental Measurements Kim, H., Kwon, H., and Keune, J., Buckling Initiation and Disbond Growth in Adhesively Bonded Composite Flanges, 44th AIAA / ASME / ASCE / AHS / ASC Structures, Structural Dynamics, and Materials (SDM) Conference, April 7-10, 2003, Norfolk, VA FAA Workshop Chicago 2003-09 17
Disbond Growth Model one-edge free flanges fracture-mechanics based compute strain energy release rate assumed post-buckled mode shape w( x, y ) = 1 2 [ c ( y b ) + c ( y b ) + c ( y b ) 2 c ( y b ) 4 4 1 +...c n 2 ( y b ) n 2mπx ] 1 cos a 3 3 + m = 1: m = 2: FAA Workshop Chicago 2003-09 18
Comparison of Model and Experiments Glass/Epoxy [0] 4 ; b* = 25.4 mm 0.8 mm Flanges Applied Farfield Strain, ε o 0.005 0.004 0.003 0.002 Buckling - SS Buckling - C Disbonding - SS Disbonding - C Experiment - Buckling Experiment - Disbond G c = 578 J/m 2 0.001 0.000 0 50 100 150 200 250 Disbond Length, a (mm) FAA Workshop Chicago 2003-09 19
Non-Uniform Disbond Growth Front FAA Workshop Chicago 2003-09 20
Strain Energy Release Rate Profile detailed view of G along disbond front predicts corner disbond initiation comparison with VCCT/FEA as programmed by R. Kreuger 500 400 Extrapolated G peak ε o = 2,450 με FEA Model G (J/m 2 ) 300 200 100 0 0 0.2 0.4 0.6 0.8 1 y/b* FAA Workshop Chicago 2003-09 21
New Directions: Intrinsic Material Properties vs. Joint Behavior* understand relationship between intrinsic material properties vs. properties inferred from structural (joint) behavior intrinsic material properties should be independent of joint configuration, e.g., bondline thickness, mode of loading resolve differences observed from different test methods: tensile test dogbone napkin ring Krieger Gage / ASTM 5656 use nonlinear analyses and correlation with tests to extract TRUE, consistent, description of intrinsic material behavior Shear Stress (MPa) 30 25 20 15 10 5 t a =2.08 mm t a =1.07 mm 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Shear Strain t a =0.33 mm * Adhesive test data from J. Tomblin and W. Seneviratne, WSU * this topic was raised at the ASTM Symposium on Joining and Repair of Composite Structures, March 2003, Kansas City, MO FAA Workshop Chicago 2003-09 22
New Directions: Bonded Joint Impact (a) (c) (b) Damage on 38.1 mm Overlap Joint Due to 40 J Impact; (a) Impact Side; (b) Back Side, (c) C-Scan Showing Disbond Area basic guidance studies needed: characterize damage modes identify damage formation mechanisms determine governing parameters advanced: model predictions FAA Workshop Chicago 2003-09 23
New Directions: Shear Buckling and Disbonding A B Disbond in Joint Between Fuselage Halves investigate in-plane shear buckling and subsequent disbond growth o N xy leads into combined compression + shear loading FAA Workshop Chicago 2003-09 24
New Directions: Health Monitoring of Bonded Joints real time (or time interval based) monitoring of joint health existing: ultrasonic scanning common method for discrete interval inspection real-time implementation difficult electrical resistance external or embedded sensors / crack gages / fiber optics acoustic waves useful for internal, non-accessible locations correlate measurements with damage shape and location input for damage tolerance prediction models FAA Workshop Chicago 2003-09 25
New Directions: Full-Scale Behavior Prediction Disbond P Path to get from Coupons & Elements to Full Scale Component Level Understand Experimental Data??? Other Factors Disbond in Joint Between Fuselage Halves A Refined Models: Adhesive Plasticity, Peel Stress Failure Under Multiaxial Stress, Fracture Mechanics B Side Load at 25% Chord o N xy Sandwich Construction - Halves Bonded Along Top and Bottom C-L Vertical Load develop predictive models: maintain balance between simplicity and complexity FAA Workshop Chicago 2003-09 26