Assessments of Structural Health Monitoring for Fatigue Cracks in Metallic Structures by Using Lamb Waves Driven by Piezoelectric Transducers

In this paper, the assessments of structural health monitoring (SHM) for metallic structures are presented. Tests were conducted on three specimens for fatigue crack testing and damage index measurements. In each specimen, a transducer array of six Stanford Multiactuator-Receiver Transduction (SMART) Layers, each of which contains a piezoelectric transducer, was mounted near the fatigue crack. The Lamb wave was driven by one SMART Layer and received by another. The transducer array generated nine forward and nine backward propagating paths. The damage index defined in the first arrival window (FAW) was obtained for each path. In this work, we propose an averaging process to obtain average damage index from the transducer array. The Lamb waves with various driving frequencies were tested to conduct the performance of the damage monitoring. The results show that the plot of the average damage index versus the actual crack length reveals a narrow band, indicating a small standard deviation of the measurement data. The data of numerous crack lengths and Lamb wave paths are processed statistically to determine the probability of detection (POD) of this damage detection method. It was found that the Lamb wave with the driving frequency of $f=450\mathrm{kHz}$ performed best in the crack monitoring. By using an threshold damage index ${\mathrm{DI}}_{\mathrm{th}}=0.01$, the detectable crack sizes $a_{50/50}$, $a_{90/50}$, and $a_{90/95}$ for the case of $f=450\mathrm{kHz}$ are 4.83, 5.79, and 5.92 mm, respectively. Based on the proposed approach, the relations between the damage index and actual crack length and the POD curves can be applicable to a real structure with the same geometry.