An Efficient Procedure for Bonding Piezoelectric Transducers to Thermoplastic Composite Structures for SHM Application and Its Durability in Aeronautical Environmental Conditions

• Published in: Sensors (Basel, Switzerland) Switzerland), 2023-05, Vol.23 (10), p.4784
• Main Authors: Sofi, Tasdeeq, Gude, Maria R, Wierach, Peter, Martin, Isabel, Lorenzo, Eduardo
• Format: Article
• Language: English
• Subjects: acousto-ultrasonic transducers; Adhesives; Aeronautics; Aircraft; Aircraft structures; Bond strength; Carbon fiber reinforced plastics; Circuit components; Composite materials; Composite structures; Durability; durability and integrity; electro-mechanical susceptance; Environmental conditions; Environmental quality; Epoxy adhesives; Epoxy resins; Fiber optics; High temperature; Ketones; Literature reviews; Manufacturing; Mechanical impedance; Moisture absorption; Piezoelectric transducers; Polyether ether ketones; Schedules; sensor bonding; Sensors; Shelf life; Structural health monitoring; Susceptance; Thermal cycling; thermoplastic adhesive films; Ultrasonic testing
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s23104784

Piezoceramic transducers (PCTs) bonded to carbon fiber-reinforced plastic (CFRP) composite structures must be durable as well as remain properly bonded to the structure in order to provide reliable data for accurate guided-wave-based structural health monitoring (SHM) of aeronautical components. The current method of bonding transducers to composite structures through epoxy adhesives faces some shortcomings, such as difficult reparability, lack of weldability, longer curing cycles, and shorter shelf life. To overcome these shortcomings, a new efficient procedure for bonding the transducers to thermoplastic (TP) composite structures was developed by utilizing TP adhesive films. Application-suitable TP films (TPFs) were identified and characterized through standard differential scanning calorimetry (DSC) and single lap shear (SLS) tests to study their melting behavior and bonding strength, respectively. Special PCTs called acousto-ultrasonic composite transducers (AUCTs) were bonded to high-performance TP composites (carbon fiber Poly-Ether-Ether-Ketone) coupons with a reference adhesive (Loctite EA 9695) and the selected TPFs. The integrity and durability of the bonded AUCTs in aeronautical operational environmental conditions (AOEC) were assessed in accordance to the standard Radio Technical Commission for Aeronautics DO-160. The AOEC tests performed were operating low and high temperatures, thermal cycling, hot-wet, and fluid susceptibility tests. The health and bonding quality of the AUCTs were evaluated by the electro-mechanical impedance (EMI) spectroscopy method and ultrasonic inspections. The AUCT defects were created artificially and their influence on the susceptance spectra (SS) was measured to compare them with the AOEC-tested AUCTs. The results show that a small change occurred in the SS characteristics of the bonded AUCTs in all of the adhesive cases after the AOEC tests. After comparing the changes in SS characteristics of simulated defects with that of the AOEC-tested AUCTs, the change is relatively smaller and therefore it can be concluded that no serious degradation of the AUCT or the adhesive layer has occurred. It was observed that the most critical tests among the AOEC tests are the fluid susceptibility tests, which can cause the biggest change in the SS characteristics. Comparing the performance of the AUCTs bonded with the reference adhesive and the selected TPFs in the AOEC tests, it was seen that some of the TPFs, e.g., Pontacol 22.1