1. Shear Actuation of Piezoelectric Transducers for Ultrasonic Evaluation of Adhesive Joints
Hussain Altammar
Mechanical Engineering Department, University of Wisconsin-Milwaukee

2. Objective
The objective of this study is to investigate the feasibility of using shear mode piezoelectric (PZT) transducers in structural health monitoring for evaluation of adhesive joints.

3. Introduction
A major reason that limits wider use of adhesively bonded structures is that they are susceptible to various kinds of hard to detect damage such as voids, cracks, disbond, delamination, and kissing bonds.
A nondestructive evaluation technique that can accurately monitor and assess the integrity of adhesive joints is of interest in order to boost their use in critical applications.

4. Introduction Cont. 𝑇 5 = 𝑐 55 𝐸 𝑆 5 − 𝑒 35 𝐸 3
It is proven that specific modes of lamb waves, the antisymmetric modes, are sensitive to adhesive joint defects including kissing bonds.
Shear mode PZT transducers are considered in this study to excite ant-symmetric lamb waves in the adhesive joint of a composite specimen.
𝑇 5 = 𝑐 55 𝐸 𝑆 5 − 𝑒 35 𝐸 3
𝐷 3 = 𝑒 35 𝑆 5 + 𝜀 33 𝑇 𝐸 3

5. Method Step 1: Specimen Preparation
The Shear mode PZT transducers generate directional waves.
An alignment frame was used to accurately attach the shear mode PZT transducers on the aluminum sheet at a certain location and orientation.
Before adhering the aluminum substrates together, two PTFE wedges coated with graphite were implanted in the middle of the adhesive joint to create an artificial damage in the specimen.

6. Method

7. Method Step 2: Testing and Signal processing
The pristine specimen was tested in pitch- catch configuration by 5-tone and 7-tone burst signals.
The damage was introduced to the specimen by pulling out one of the PTFE wedges and then testing with the same actuation signals.
The voltage signals received from the transducer and the sensor were de-noised using wavelet transform.

8. Results
Comparative response of a pristine and damaged sample generated by exciting PZT 1 with 7-peak signal at 30 kHz
a) The actuation signal, 5-peak tone burst with a central frequency of 30 kHz; b) received signal after propagating 200 mm from PZT 1 to PZT 2.

9. Results Cont.
Comparative response of a pristine and damaged sample generated by exciting PZT 2 with 5-peak signal at 40 kHz.
Comparative response of a pristine and damaged sample generated by exciting PZT 1 with 5-peak signal at 40 kHz.

10. Main Findings
Shear mode PZT transducers has the capability to excite and sense antisymmetric lamb waves.
Embedding the PZT transducers in the adhesive joint could be a promising approach to monitor the integrity of multilayered structures.

11. Future Work
Investigate testing samples with other common joint defects such as cracks, disbond, and kissing bond.
Develop an algorithm to identify the location of damage and estimate the remaining useful life.

12. References
[1] Nagy, P., (1991) “Ultrasonic Detection of Kissing Bonds at Adhesive Interface” Journal of Adhesion Science Technology, No. 8, pp
[2] Kundu, T., Maji, A., Ghosh, T., Maslov, K., (1997) “Detection of Kissing Bonds by Lamb Waves” Ultrasonic, Vol. 35, pp
[3] Giurgiutiu, V. (2014), Structural Health Monitoring with Piezoelectric Wafer Active Sensors, Amsterdam: Academic Press, an imprint of Elsevier.
[4] Yang, J., (2006), Analysis of Piezoelectric Devices, Hackensack, NJ: World Scientific.