Analytical model of lap-joint adhesive with embedded piezoelectric transducer for weak bond detection

In recent years, the aerospace and automotive industries experienced a growing interest in the implementation of structural members’ bonding using adhesive films. Adhesive joints contribute to manufacturing savings and are advantageous from a structural prospective as they provide a relatively uniform stress distribution at the bonded region, hence reducing local stress concentration. Regretfully, the adhesive bonds’ strength is vulnerable to the presence of defects which limits their use for structural purposes. A particularly dangerous class of defects—generically referred to as “kissing bond”—consists of localized imperfections that do not provide adequate bonding between the adherent and the adhesive yet maintaining the two surfaces in contact. Past attempts at detecting kissing bonds using ultrasonics were only marginally successful stressing the need to search for more reliable methods. This article describes a novel, in situ, impedance-based method for bond-line degradation detection during the service life of a structure. The method relies on piezoelectric transducers embedded within the adhesive bond-line and actuated by a low-voltage harmonic signal. The article focuses on deriving an approximate analytical model describing the dynamic behavior of the actuator embedded in the homogeneous bond-line. A study describing the sensitivity of the electro-mechanical response of the transducer to the changes in various bond-line properties and adhesion conditions is also presented.