the bond looks perfect

but might be empty

Problem statement

Adhesive bonding can produce joints that are lighter, more fatigue-resistant, and more evenly stressed than riveted or bolted connections. However, a "kissing bond" — where surfaces are in intimate contact but have little or no adhesion — is virtually indistinguishable from a good bond using any current non-destructive evaluation (NDE) technique. Kissing bonds can substantially weaken a joint while producing no acoustic or visual indication — in controlled contamination studies, joints contaminated with mold-release agent lost roughly 27% of their shear strength yet formed interfaces that pass conventional inspection (Jeenjitkaew & Guild 2017). Because no NDE method has demonstrated it can ensure the strength of a finished bond, FAA AC 20-107B (mirrored by EASA AMC 20-29) requires that any bonded joint whose failure would be catastrophic be substantiated by one of three options: design features that limit disbond size so the structure still carries limit load, proof testing of every production article, or reliable NDI that ensures joint strength. With the NDI option out of reach and per-article proof testing prohibitively expensive, manufacturers default to disbond-arresting design features — in practice redundant mechanical fasteners ("chicken rivets") — negating much of the weight savings; EASA's CM-S-005 applies the same logic to cap the allowable size of bonded repairs on critical structure.

Why this matters

Industry engineers make the case that eliminating bondline fasteners would allow thinner components, more efficient load paths, and the removal of thousands of stress concentrations — significant weight savings and airframe optimization beyond the fastener hardware itself (CompositesWorld 2014). Beyond aerospace, adhesive bonding in automotive (mixed-material lightweighting), wind turbine blades (spar cap bonding), and composite infrastructure repair all face the same certification barrier. The inability to verify bond quality non-destructively means either over-designing with fasteners, accepting uninspectable joints, or destructively testing coupons from each production lot — all costly compromises.

What’s been tried and why it hasn’t worked

Conventional ultrasonics can detect disbonds (air gaps) but not kissing bonds, because the intimate surface contact transmits acoustic waves nearly identically to a properly cured bond. Nonlinear ultrasonics (harmonic generation, sub-harmonic resonance) show laboratory promise for distinguishing kissing from good bonds via contact acoustic nonlinearity, but sensitivity depends on closure stress, contamination type, and surface roughness — and these methods haven't demonstrated field reliability. Laser shearography detects deformation differences under load but requires controlled loading and has resolution limits for thin bondlines. Thermography, X-ray CT, and guided wave methods all struggle with the same fundamental problem: kissing bonds produce no measurable geometric or elastic discontinuity. Process monitoring (cure sensing, surface energy measurement before bonding) addresses root cause but doesn't verify the finished joint.

What would unlock progress

Either (1) a physics-based NDE technique sensitive to adhesion strength rather than geometric defects — candidate mechanisms include interface-specific nonlinear acoustic response, electromagnetic coupling changes at unbonded interfaces, or terahertz spectroscopy of bondline chemistry; or (2) embedded sensors (fiber optics, RFID stress sensors) that continuously monitor bond stress in service, shifting from inspection-based to health-monitoring-based certification; or (3) traceable surface preparation certification that makes kissing bonds physically impossible, removing the need for post-bond inspection.

Entry points for student teams

A team could fabricate controlled kissing bond specimens (contaminated aluminum or composite coupons) and compare the sensitivity of multiple NDE methods (linear ultrasonic, nonlinear harmonic, shearography) under identical conditions. Alternatively, a team could prototype an embedded fiber-optic sensor array for a bonded composite joint and demonstrate strain anomaly detection at a known kissing defect. Relevant skills: materials testing, ultrasonics, signal processing, composite manufacturing.