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Durability of Adhesively Bonded Joints in Aerospace Structures

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Durability of Adhesively Bonded Joints in Aerospace Structures Durability of adhesive bonded joints in aerospace structures Washington State University Date: 11/08/2017 AMTAS Fall meeting Seattle, WA Durability of Bonded Aircraft Structure • Principal Investigators & Researchers • Lloyd Smith • Preetam Mohapatra, Yi Chen, Trevor Charest • FAA Technical Monitor • Ahmet Oztekin • Other FAA Personnel Involved • Larry Ilcewicz • Industry Participation • Boeing: Will Grace, Peter VanVoast, Kay Blohowiak 2 Durability of adhesive bonded joints in aerospace structures 1. Influence of yield criteria Yield criterion 2. Biaxial tests (Arcan) Plasticity 3. Cyclic tests Hardening rule 4. FEA model Nonlinearity in Bonded Joints 1. Creep, non-linear response Tension (closed form) 2. Ratcheting, experiment Time Dependence 3. Creep, model development Shear (FEA) 4. Ratcheting, model application Approach: Plasticity vPrimary Research Aim: Modeling of adhesive plasticity to describe nonlinear stress-strain response. ØSub Task: 1. Identify the influence of yield criterion and hardening rule on bonded joints (complete) 2. Characterizing hardening rule (complete Dec 2017) 3. Characterizing yield criterion (complete Dec 2017) 4. Numerically combining hardening rule and yield criterion (complete Feb 2017) Approach: Plasticity 1. Identify yield criterion and hardening rule of bonded joints • Input properties: Bulk and Thin film adhesive properties in Tension. 60 Characterization of Tensile properties 55 50 45 40 35 30 25 Bulk Tension: Tough Adheisve 20 Tensile Stress [MPa] Bulk Tension: Standard Adhesive 15 10 Thin film Tension: Tough Adhesive 5 Thin film Tension: Standard Adhesive 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Axial Strain Approach: Plasticity 1. Identify the influence of yield criterion and hardening rule on bonded joints • Background: Investigate yield criterion and hardening rule of bonded joints. • Outcome type: Comparison of Test results vs different FEA models Scarf Experiment: bonded joints in shear 45 40 35 30 25 20 Tough: scarf WALS Shear Stress [MPa] 15 Tough: lap shear 10 Standard: scarf 5 Standard: lap shear 0 0 0.005 0.01 0.015 0.02 Axial Strain Approach: Plasticity • Validation of Material model: Isotropic vs Kinematic hardening • Choice of Input property: Bulk vs. Thin film 45 Exp vs. FEA: Tough Scarf 45 Exp vs. FEA: Tough WALS 40 40 35 35 30 30 25 25 Experiment 20 Experiment 20 FEA: Kinematic hardening with thin film 15 FEA: Kinematic hardening with thin film 15 Shear Stress [MPa] FEA: Isotropic hardening with thin film 10 FEA: Isotropic hardening with thin film Shear Stress [MPa] 10 FEA: Kinematic hardening with bulk adhesive FEA: Kinematic hardening with bulk adhesive 5 5 FEA: Isotropic hardening with bulk adhesive FEA: Isotropic hardening with bulk adhesive 0 0 0 0.005 0.01 0.015 0.02 0.025 0 0.003 0.006 0.009 0.012 0.015 Axial Strain Axial Strain • Status: Completed, Journal paper submitted • Task outcome: Adhesive follows a von Mises yield criterion and kinematic hardening Approach: Plasticity • Validation of Material model: Isotropic vs Kinematic hardening • Choice of input property: Bulk = thin film (for Standard adhesive) 45 45 Exp vs. FEA: Standard Scarf Exp vs. FEA: Standard WALS 40 40 35 35 30 30 25 25 20 20 15 Experiment 15 Shear Stress [MPa] Experiment 10 Shear Stress [MPa] FEA: Kinematic hardening 10 5 FEA: Kinematic hardening FEA: Isotropic hardening 5 0 FEA: Isotropic hardening 0 0.001 0.002 0.003 0.004 0.005 0 Axial Strain 0 0.003 0.006 0.009 0.012 0.015 Axial Strain • Status: Completed, Journal paper submitted • Task outcome: Adhesive follows a von Mises yield criterion and kinematic hardening Approach: Plasticity 2. Characterize hardening rule • Background: Similar to metals, adhesives could follow isotropic, kinematic or combined hardening rule. • Lot of research done with metals, but none with bonded joints. • This has to be verified by studying the movement of the yield surface under cyclic tension compression loading in a biaxial stress space. Ref: Muransky O. et al [Metal Plasticity] Approach: Plasticity • Outcome type: Cyclic ten-comp tests for scarf joint. Plot of yield surface translation/expansion/both, Journal paper σ_� τ Scarf Joint in cyclic test fixture 〖−σ〗_� σ_� Black = Original Red = Isotropic • Scarf joint being tested in ten-comp. -τ Blue = Kinematic 〖−σ〗_� • DIC is used for calculating shear Orange = Combined strain Approach: Plasticity • Outcome type: Cyclic ten-comp tests for scarf joint. Plot of yield surface translation/expansion/both, Journal paper 5000 Blue = Red (all cycles) 4500 Stress controlled Exp: Tension: Max 4000 Peaks [+4800 to -4800 psi] 3500 Increasing Strain 3000 2500 2000 1500 Cycle # Yield surface size [psi] 1000 Tension : Yield 1 (Blue: ten peak) 6250 +4800 to -1450 500 0 2 (Red: comp peak) 6250 -4800 to +1450 -500 Shear Stress [psi] Compression : Yield 3 (Blue: ten peak) 6250 +4800 to -1450 -1000 -1500 4 (Red: comp peak) 6250 -4800 to +1450 -2000 Blue 1 5 (Blue: ten peak) 6250 +4800 to -1450 -2500 Red 1 -3000 Blue 2 -3500 Red 2 -4000 Blue 3 Compression : Min -4500 -5000 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 Shear Strain • Status: Documentation in progress • Task outcome: Yield surface showed translation (kinematic hardening). Approach: Plasticity • Outcome type: Cyclic ten-comp tests for scarf joint. Plot of yield surface translation/expansion/both, Journal paper st nd 5000 Blue ~ Red (1 and 2 cycle) Test 1 4500 Purple = Orange (3rd and 4th cycle) 4000 Blue 3500 Virtual Strain Red Test 1 Cycles# 3000 Controlled: Yield surface size [psi] (presented here) 2500 Purple Stress peaks [psi] 2000 Orange 4800 1 (Blue: ten peak) 6300 +4800 to -1500 1500 Black 1000 -4500 2 (Red: comp peak) 6250 -4500 to +1750 500 0 4000 3 (Purple: ten peak) 5750 +4000 to -1700 Shear Stress -500 -4000 4 (Orange: comp peak) 5750 -4000 to +1700 -1000 -1500 -2000 -2500 Decreasing -3000 Stress: Test 2 Cycles# Yield surface size [psi] -3500 Stress peaks [psi] -4000 4800 1 (Blue: ten peak) 6200 +4800 to -1400 -4500 -4500 2 (Red: comp peak) 6160 -4500 to +1660 -5000 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 Shear Strain 3425 3 (Purple: ten peak) 4756 +3425 to -1330 -3500 4 (Orange: comp peak) 4800 -3500 to +1300 • Status: Documentation in progress • Task outcome: Yield surface showed translation after stabilizing cycles in a strain controlled/decreasing stress experiment (Kinematic hardening). Approach: Plasticity 3. Characterizing yield criterion • Background: Assumption of yield criteria to avoid complex characterization with thin bonded joints • Test bonded joints with mixed mode Arcan fixture to plot yield surface in biaxial stress space to identify shape of yield surface. • Outcome type: Test results, Journal paper + 23, / Normal component: σ = 1 0 + ,-. / Shear Component: σ = ) 0 Prospected Test set up Approach: Plasticity Schematic Yield Surface in Biaxial Stress Space 8000 Solid Black line: Initial Yield Dotted black line: Failure surface 6000 4000 τ 90 67.5 45 2000 22.5 σ 0 Assembly: fixture and coupon [psi] -σ 0 0 τ -22.5 -2000 -45 -67.5 -90 -4000 -τ Tensile stress Compressive Stress -6000 +ve Shear stress -ve Shear stress -8000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 σ [psi] Approach: Plasticity 8000 Tough Adhesive 8000 Tough Adhesive Exp: yield surface 7000 7000 Exp: failure surface Drucker Prager criterion 6000 Exp: yield surface 6000 von Mises criterion 5000 5000 4000 4000 3000 3000 Shear Stress Shear Stress 2000 2000 1000 1000 0 0 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 Normal Stress Normal Stress • Status: Documentation in progress • Task outcome: yield surface shape and size to be plotted from experimental data. Approach: Plasticity Standard Adhesive Standard Adhesive 8000 7000 Exp: failure surface Exp: yield surface 7000 Exp: yield surface 6000 von Mises criterion Drucker Prager criterion 6000 5000 5000 4000 4000 3000 Shear Stress Shear Stress 3000 2000 2000 1000 1000 0 0 0 1000 2000 3000 4000 5000 6000 7000 8000 0 1000 2000 3000 4000 5000 6000 7000 8000 Normal Stress Normal Stress • Status: Documentation in progress • Task outcome: yield surface shape and size to be plotted from experimental data. Approach: Plasticity 4. Numerically modeling nonlinear hardening rule. • Background: Modeling of nonlinear hardening is common in metals. • Nonlinear hardening has not been characterized for bonded joints due to experimental complexities, assumed to be linear. • To model nonlinear hardening by including possible effects form both Isotropic and or kinematic hardening. • Outcome type: Numerical modeling, comparison of experiment and simulation, Journal paper • Status: Identified the modeling technique to use cyclic scarf test data to extract nonlinear hardening parameters by separating isotropic and kinematic hardening components (estimated Feb 2018) • Task outcome: Numerical modeling-FEA simulation of nonlinear hardening Approach: Time dependence (viscoelasticity/viscoplasticity) Background • The time-dependent behavior of adhesives is important for the durability • Seldom accurate models for adhesives to predict the ratcheting • The response of shear coupons is more complicated to test Objectives The final objective is to build a shear viscoelastic modeling on bonded joints for ratcheting.: • Viscoelastic response of bulk resin, closed form model (complete) • FEA Viscoelasticity model of bulk Resin (12/31/2017) • FEA viscoelastic model of bonded joints in creep (05/31/2018) • FEA viscoelastic model of bonded joints in ratcheting (12/31/2018) Approach: Time dependence (viscoelasticity/viscoplasticity) •FM300-2
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