A non contact technique using Zero-Group Velocity (ZGV) Lamb modes is developed to probe the bonding between two solid plates coupled by a thin layer. The layer thickness is assumed to be negligible compared with the plate thickness and the acoustic wavelength. The coupling layer is modeled by a normal and a tangential spring to take into account the normal and shear interfacial stresses. Theoretical ZGV frequencies are determined for a symmetrical bi-layer structure and the effect of the interfacial stiffnesses on the cut-off and ZGV frequencies are evaluated. Experiments are conducted with two glass plates bonded by a drop of water, oil, or salol, leading to a few micrometer thick layer. An evaluation of normal and shear stiffnesses, is obtained using ZGV resonances locally excited and detected with laser ultrasonic techniques.PACS numbers: 43.20. Gp, 43.35.Zc, 62.30.+d, 68.60.Bs, 79.20.Ds The increasing use of adhesive bonding in the industry has been motivated by the need of stronger and lighter structures. This technique is more suitable for providing continuous adhesion properties and easier to process than other ones like welding, riveting or screwing. The bonding between two solid plates can be probed by various ultrasonic techniques: longitudinal and shear waves reflection or transmission at the interface, 1,2 thickness resonances, 3 ultrasonic guided waves propagating along the interface. 4-8 All these methods require a modelisation of the ultrasonic wave interaction with the interface supposed homogeneous. 9 Recently it was shown that Zero-Group Velocity (ZGV) Lamb modes associated with laser ultrasonic techniques, allow a local and non contact measurement of mechanical properties of isotropic or anisotropic plates and shells. 10,11 These ZGV modes, corresponding to a minimum frequency of dispersion curves, also exist in layered plate structures. 12 Local ZGV resonances have been used to image the lack of adhesive bond between two plates. 13In this letter a non contact method, based on the measurement of ZGV resonance frequencies is proposed to probe interfacial stiffnesses between two plates. An interfacial behavior model 9,16 is used to calculate the dispersion curves and Zero-Group Velocity Lamb modes in a symmetrical structure composed of two plates coupled by a thin layer. The ultrasonic wave interaction with the interface is described by spring boundary conditions. Firstly, the effect of longitudinal and transverse stiffnesses on cut-off and ZGV frequencies is studied. Secondly, experimental results with liquid or solid compliant layers are obtained by laser ultrasonic techniques. Then, the values of spring stiffnesses, estimated from ZGV resonance frequencies, are discussed.Theoretical model -The structure is composed of two identical, isotropic and homogeneous plates with a coupling layer in-between. 7 The plate thickness is denoted h and lateral dimensions are supposed infinite. Their mass density is denoted ρ and their bulk wave velocities c l and c t . The coupling layer thickness d is a...

Zero-group-velocity (ZGV) waves have the peculiarity of being stationary, and thus locally confining energy. Although they are particularly useful in evaluation applications, they have not yet been tracked in two dimensions. Here we image gigahertz zero-group-velocity Lamb waves in the time domain by means of an ultrafast optical technique, revealing their stationary nature and their acoustic energy localization. The acoustic field is imaged to micron resolution on a nanoscale bilayer consisting of a silicon-nitride plate coated with a titanium film. Temporal and spatiotemporal Fourier transforms combined with a technique involving the intensity modulation of the optical pump and probe beams gives access to arbitrary acoustic frequencies, allowing ZGV modes to be isolated. The dispersion curves of the bilayer system are extracted together with the quality factor Q and lifetime of the first ZGV mode. Applications include the testing of bonded nanostructures.