Damage sensing by guided waves using direct-write piezoelectric transducers in composite structures

Abstract

The ability to detect and monitor barely visible impact damage (BVID) in composite structures is a major challenge for aircraft structural health monitoring (SHM) that could provide improved reliability and safety in addition to reduced inspection time and repair costs. This thesis examines the application of novel direct-write transducers (DWTs) on carbon fiber reinforced polymer (CFRP) flat plates and T-joints. The ultimate aim is to provide an active sensing system using guided waves generated and received by a network of piezoelectric DWTs, capable of identifying BVID. An active sensing system is preferred, while passive sensing is liable to various noise effects, making signal interpretation difficult. The DWTs were designed and fabricated by spraying a 25 um thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TRFE)) coating with a comb-shaped electrode on CFRP structures for exciting and receiving selective ultrasonic guided waves. The multiple fingers of the comb-shaped electrode have a period of one wavelength. The characteristics, performance, and impact damage detection capability of the lightweight polymeric DWTs were investigated in comparison with heavier discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same structure. The investigated impact damage was only visible on the back side of the plate, which is not always accessible. Theoretical and experimental dispersion and tuning curves were obtained for the plate and T-joint using discrete PZT transducers after identifying Lamb wave fundamental modes (A0 and S0). A region of interest corresponding to frequencies from 50 to 500 kHz was selected due to relatively lower attenuation, almost constant group velocities, and reduced number of wave modes present that simplifies signal processing and analysis. Guided waves signals were acquired using different combinations of actuator-sensor in pitch-catch configuration (DWT, PZT), and using laser ultrasonic excitation that ultimately avoids the need for PZT actuators. Bonding PZTs in relatively thin plates can interfere with the structural response. The DWTs exhibited improved A0 or S0 mode excitation over the discrete PZT transducers on both plate and T-joint structures, allowing consistency and easier signal interpretation due to the single mode presence at a specific frequency. High signal-to-noise ratio (SNR) was obtained by effectively cancelling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the small signal amplitudes. The enhanced directivity allowed lower amplitude attenuation, making DWT a good candidate for monitoring critical hot-spot locations in structures, such as a composite T-joint. The combination of DWT as sensor with PZT as actuator achieved balanced performances in both wave mode selection and SNR. Thus, this combination achieved efficient detection of a 31 J impact damage, involving cracks and a 34 mm long delamination, compared to other transducer combinations. The PZT-DWT combination showed the highest signal amplitude changes of A0 or S0 mode with wavelengths of 10 to 12 mm, which implies the occurrence of a structural change related to the impact. When the brittle ceramic actuators were replaced by a contactless broadband laser excitation (Nd:YAG laser of wavelength 1064 nm, 5.4 ns pulse) and used in combination with the novel DWTs, a reliable damage detection was obtained. In conclusion, the potential and benefits of employing DWTs in SHM are the reduced weight, the flexibility of lead-free polymeric transducers, the high consistency, and the simplified signal interpretation due to an effective mode selection. DWTs can be patterned in any shape on flat or curved structures in batch fabrication, avoiding laborious manual installation, where a thick glue layer can be problematic in signal interpretation. DWTs are good candidates for monitoring CFRP T-joint structures, where conventional non-destructive techniques (NDT) are challenging because of the complex geometry and interference with the web. Accurate detection and growth prevention of BVID can avoid premature structural failure.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alberto Saiani (Supervisor) & Constantinos Soutis (Supervisor)