Bond Structure Characterization Work at UCSB

Bond Structure Characterization Work at UCSB

UCSB Bond Structure Characterization Work Curtis R. Davies FAA William J. Hughes Technical Center Atlantic City International Airport, NJ 2nd FAA Workshop on Composite Material Control September 16-18, 2003 Westin OHare Chicago, Il Effects of Surface Preparation on the Long-Term Durability of Composite Adhesive Bonds Keith T. Kedward & Jason Bardis Dept. of Mech. & Env. Engineering, UCSB Mechanical Testing Investigation DCB traveling) Wedge (static or Similar test geometriescan compare results DCB: pull apart adherends at constant rate Static wedge: insert wedge, place in environment, observe crack Traveling wedge: force wedge through bondline at slow velocity, verified against established DCB Static Wedge Test Same specimen as traveling wedge

Insert 1/8 steel dowel pin with hammer Soak in acidic, basic, or pH-neutral de-ionized room temperature (71 F) H2O Crack growth usually stabilized within several hours 8 preparation types, 4 specimens per group, up to 510 hours exposure Traveling Wedge Test Variant on DCB & static wedge test where wedge is forced slowly (0.125 in/min quasi-static loading condition) through specimen Simple specimen fixturing and testing Analysis based on fracture mechanics / Beam-On-ElasticFoundation Test results extremely sensitive to crack length measurement (4 th power term) Scanning Electron Microscopy FEP NAT SRB VLP (release film) (peel ply) (release fabric) (peel ply) No blast

Blast Qualitative visual assessment of surface preparation Conclusions: Test Methods test method values measured pros cons notes traveling wedge GIc, failure mode, cleavage force simple, straightforward crack measure, disturb frac. Surf. best overall for shortterm DCB GIc, failure mode, failure load straightforward, widely accepted

fixtures, short-term test good for measuring GIc static wedge durability, failure mode, GIa easy, $$$, no equipment, GIa test chamber, test time less distinction between preps avg shear strength, failure mode database, simple, real loading peel, poor service prediction poorly suited peel load, failure mode simple requires plastic deformation use for metal only

failure mode easy, $$$, fast no engineering value quick, qualitative feedback crack front shape see crack front in opaque sample $$$, qualitative confirm specimen configuration SEM surface morphology surface details, abrasion analysis $$$, qualitative, specimen prep good for morph. feedback XPS surface chemistry simple

$$$ good chem analysis EDX surface chemistry in conjunction with SEM $$$, specimen prep no useful results shear floating roller peel hammer/wedge X-ray photo Surface Preparation Effects on Long-Term Durability Motivation Technical Minimization or prevention of interfacial failure in adhesive bonds to polymer matrix composites Characterization of the effects of the following on bond durability: Chemical contamination from release fabric, peel ply, or release film Surface preparation Develop recommended practices for bagging, preparation, and bonding to ensure consistent bonds

Surface Preparation Variables Adherend Material Layup of Adherends Vary abrasive material Vary grit size Vary number of passes Vary applied pressure Vary material (nylon, polyester, etc.) Vary direction of removal (angle relative to outer ply) Vary time between removal and adhesive application Compressed Air Blowing Vary length of exposure Shop air vs. dry Nitrogen tank Paste Adhesive Preparation

Vary humidity Vary length of exposure Expose adherends before bonding Expose adherends after bonding Expose adherends while performing wedge test Water Bath Use vacuum bag or not Elevated temperature vs. room temperature cure Humidity Exposure Glass microbeadssilane treatment Wires Tabs/tape Film adhesive carrier cloth Applied pressure Adhesive Cure

Vary filler type Vary percentage of filler in adhesive Bondline Thickness Control Mix by hand or by machine Apply vacuum to remove trapped air Filler Material in Adhesive Release Fabric/Peel Ply/Release Film Variations Vary pressure Vary grit size Vary grit media type Vary frequency of changing out re-used media Vary number of passes Vary speed of passes Vary distance / angle of blaster from surface

Hand Abrasion Vary type of solvent (acetone, isopropyl alcohol, etc.) Vary number of wipes Vary applicator material Air dry vs. wipe dry Grit Blast 0[n]n] layup Alternate layup Solvent Wiping Composite matrix vs. metal Boeing / Lancair / Cirrus Design composites

Vary water temperature Vary length of exposure Soak adherends before bonding Soak adherends after bonding Soak adherends while performing wedge test Temperature Exposure Vary temperature Vary length of exposure Expose adherends before bonding Expose adherends after bonding Expose adherends while performing wedge test Surface Preparation Findings Results show that peel plies and release fabrics are entirely different materials that are not interchangeable Any surface to be secondarily bonded must be cured against a peel ply, not a release fabric Pre-bond abrasion of adherends is recommended but can be omitted in certain cases if process and material control is strict Evaluation of Bonded Lap Joints with Variable Thickness Adhesive Layers Keith T. Kedward & Yuqiao Zhu Dept. of Mech. & Env. Engineering, UCSB

Background and Motivation An appreciation of load transfer in adhesively bonded joints Potential utilization of extensive test database on bonded lap joint coupons ASTMs most commonly cited single lap joint, ASTM D1002/99 and comparison with double lap joint configuration Potential variation in shear strength for different thickness adhesive layers Tolerance to variations in thickness Importance of both adhesive shear and peel stress components Candidate methods of analysis for interpretation and evaluation Lap Shear Performance of 2-Part Epoxy Adhesives* Product 1838 B/A 2216 B/A

3569 B/A 9323 B/A DP 460 DP 420 Test Temperature (C) -55 25 82 Generation 2,200 3,000 800 0 2,400 3,300 500 1 4,500 4,500 2,200 3 5,000 5,500 3,200 3 4,500 4,500 2,500 3 4,500 4,500 2,500 3 *Typical lap shear values in psi Ref. Hartshorn (3M) Single Lap Coupons

Apparent Shear Strength (psi) 5000 4500 Non-variable bond thickness 4000 variable along width (0.018") 3500 variable along width (0.019") 3000 variable along width (0.020") 2500 variable along width (0.018") 2000 variable along lap length (0.017") 1500 1000 variable along lap length (0.017") 500 variable along lap length (0.017") 0 0.000

0.005 0.010 0.015 0.020 0.025 Average Bond Thickness (inches) 0.030 Work-in-Progress Complete series of titanium and composite tapered bondline tests (width, length) Selected comparisons of double lap configurations Nonlinear Evaluation (Adhesive nonlinearity only) Surface chemistry and morphology studies Cyclic loading tests

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