Castaings (Ultrasonics 54:1760–1775, 2014) showed that shear-horizontal (SH) guided waves generated by electromagnetic acoustic transducers and transmitted across aluminium adhesive lap joints, interpreted through a spring-interface finite-element model, can separately infer the interfacial shear stiffness and the adhesive shear modulus — but reported that the identification is non-unique, that in-joint modal conversions were unexploited, that composite adherends remained out of reach, and that no relation to mechanically measured bond strength had been established. Later work implemented the proposed higher-order-SH-mode remedy on aluminium–epoxy–aluminium trilayers (quantifying thickness, interfacial stiffness, and modulus), analyzed and exploited the SH0-to-SH1-like mode conversion inside the joint overlap for adhesion quantification, and extended shear-guided-wave bond assessment to carbon-fibre-reinforced-polymer lap joints via direction-independent cutoff frequencies. What remains open is the problem's surviving core: an inversion that is unique for realistic joint configurations — viscoelastic adhesives, multiple weak interfaces, anisotropic adherends with imperfectly known properties — and, above all, an experimentally validated quantitative relation between the ultrasonically inferred interfacial and cohesive parameters and the destructively measured strength of the joint, a relation that reviews as recent as 2024 still treat as an open investigative question rather than a settled capability.

Background

Adhesive bonding joins two members (adherends) through a thin polymer layer, and bonded structures would ideally be qualified for service without destroying them. A joint can fail cohesively (inside the adhesive layer) or adhesively (at the adhesive–adherend interface), and the two weaknesses have different causes and remedies, so a useful nondestructive evaluation (NDE) must separate them. Shear-horizontal (SH) ultrasonic guided waves — plate waves whose particle motion is parallel to the plate surface and perpendicular to the propagation direction — are natural probes of the shear response of a bond: the fundamental mode SH0 is nondispersive and travels across the joint, while higher-order modes SH1, SH2, … are dispersive and exist only above cutoff frequencies set by the plate thickness. Electromagnetic acoustic transducers (EMATs) generate and detect these waves without couplant through Lorentz forces in electrically conducting plates; periodic-permanent-magnet (PPM) EMATs imprint a fixed wavelength and thereby select particular SH modes. Imperfect interfaces are conventionally modeled by spring boundary conditions: the traction is continuous but the displacement jumps in proportion to an interfacial stiffness KK (units N/m3^3), with KTK_T the tangential (shear) stiffness; a perfect bond corresponds to a plateau value above which the ultrasonic response no longer changes. Given measured transmitted waveforms, one solves an inverse problem — adjusting the adhesive shear modulus GadhG_{adh} and the interfacial stiffnesses KTK_T in a finite-element or semi-analytical forward model until simulated and measured signals agree. Castaings (2014) demonstrated this program on aluminium lap joints with engineered bond qualities (sand-blasted versus untreated interfaces, oil pollution, an embedded tissue layer), inferring, for example, KT≈1.5×1015 N/m3K_T \approx 1.5\times10^{15}\ \mathrm{N/m^3} for a good interface versus ≈3×1013 N/m3\approx 3\times10^{13}\ \mathrm{N/m^3} for an untreated one, and Gadh≈1.0 GPaG_{adh} \approx 1.0\ \mathrm{GPa} for the cured epoxy. The method's standing limitations, stated in the source itself, are fourfold: the inversion is not unique, since different (GadhG_{adh}, KTK_T) pairs produce nearly identical simulated signals and adhesive-thickness changes can mimic strength loss; the modal conversions the waves undergo inside the joint overlap, which carry information about the interfaces, were not analyzed; the approach had been shown only for isotropic metal adherends, whereas carbon-fibre-reinforced-polymer (CFRP) adherends are anisotropic and their imperfectly known, spatially variable elastic properties can produce ultrasonic effects comparable to those of the bond itself; and the inferred parameters had never been validated against mechanical failure tests — the standard destructive test being the single-lap shear test to failure — so the link between the stiffness seen by ultrasound and the strength measured in the testing machine was unestablished. This last gap belongs to a long-standing problem of adhesive NDE: joints with intimate contact but no chemical adhesion (so-called kissing bonds) can appear intact to many linear wave measurements yet fail at low load, and no generally accepted nondestructive predictor of adhesive joint strength exists.

Problem Statement

Can shear-horizontal ultrasonic guided waves deliver a nondestructive evaluation of adhesive lap joints that is both uniquely invertible and predictive of mechanical strength? Concretely: (1) Unique inversion for realistic joints. For joint configurations beyond the idealized metal trilayer — viscoelastic adhesive layers, multiple imperfect interfaces, and adherends whose properties may be anisotropic and spatially variable, such as carbon-fibre-reinforced polymer — establish whether multimode SH measurements (SH0 transmission together with higher-order SH-like modes and their in-joint conversions) determine the interfacial shear stiffnesses and the adhesive shear modulus uniquely within tolerances the respondent states and justifies, or else exhibit and quantify the residual ambiguity, taking account of the fact that imperfectly known adherend properties can produce ultrasonic effects comparable to those of the bond itself. (2) Strength correlation. Establish, or rule out by controlled experiment, a quantitative relation between the ultrasonically inferred parameters and the joint's mechanically measured failure strength, such that bond strength can be predicted nondestructively for the studied class of joints. The common objective of both parts is a validated nondestructive assessment of adhesive joint quality that separates interfacial from cohesive weakness and carries demonstrated mechanical significance.

References

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