Investigation of the effects of structure-sensor interaction on piezoceramic transducers for structural health monitoring applications

Abstract

Surface bonded piezoceramic transducers used for structural health monitoring that are subjected to high strain loading are at risk of being compromised by damage to the attaching bond line or to the piezoceramic sensing element. In consequence, when baseline sensor data for a healthy structure is compared to subsequent data obtained from damaged sensors, incorrect assessment of the state of the structure may occur. This project investigated and developed a novel method of attaching piezoceramic transducers to specially prepared thermoset composite surfaces using functional thermoplastic surfaces as bond line media with the aim to improve the protection of the transducer from the damaging effects of high strain structural loading. To evaluate the effectiveness of this new approach, the fatigue performance of thermoplastic weld bonded and cyanoacrylate adhesive bonded transducers were compared. Finite element analysis (FEA) of transducer-bondline-structure models showed that strain transfer from the structure to the piezoceramic element reduced as bond layer thickness increased or as the bond layer modulus decreased. The simulations indicated that within each material layer shear strain changes gradually and the rate of change is different for each material type. At dissimilar material interface boundaries shear strain has a common value but the change gradient differences within each material mean the interface is a likely location for transducer failure. For cyanoacrylate and thermoplastic bonding systems the availability of published data regarding bond strength is limited. Static tensile testing was undertaken to generate some of the bond strength data for different bond layer thicknesses and a range of surface roughness. The data showed that tensile bond strength increased with decreasing adhesive layer thickness and that for the range of surface roughness tested the relative performance of each bond line thickness was similar. To further examine the performance of the two different bonding systems, fatigue testing was undertaken. The relative performance of each system was compared using transducer output voltage and impedance data. Specimens were fabricated by bonding 10 mm diameter, 1 mm thick silver electroded piezoceramic transducers to appropriately prepared thermoset composite test coupons. Test samples were subjected to sinusoidal loading at 5 Hz and maximum strains of 1000 µɛ, and 2000 µɛ or 3000 µɛ. For both bond types, sensor voltage output showed an initial period of rapid decrease in amplitude before typically settling at an approximate steady state output. The lower voltage output from the thermoplastic bonded transducers indicated that the thermoplastic bond lines reduced strain transfer between the host structure and the piezoceramic sensing element. The fatigue test results showed that polyvinylidene fluoride (PVDF) bonded transducers experienced a reduced or similar decrease in performance relative to the cyanoacrylate bonded transducers for the same number of load cycles and strain range. Impedance analysis (IA) metrics showed that the thermoplastic bonded piezoceramic systems experienced less damage during high strain cyclic testing. Post-test disassembly of test specimens showed that cyanoacrylate bonded transducers had a higher rate of transducer fracture. This work demonstrates that transducers can be successfully welded to thermoset composite structures using PVDF thermoplastic. This bonding system confers improved protection of the transducers compared to cyanoacrylate adhesive bonding.