Embedded Active Sensors for In-situ Structural Health Monitoring of Aging Aircraft Structures

The aging of aerospace structures is a major current concern of civilian and military aircraft operators. Piezoelectric active sensors offer special opportunities for developing sensor arrays for in-situ health monitoring of aging aircraft fleet, because they are small, non-invasive, inexpensive and easily wired into sensor arrays. This paper presents work done on developing and utilizing piezoelectric active sensor arrays to detect near-field and far-field damage in aircraft-grade metallic plates as well as realistic builtup panels representative of aging aircraft structures. Damage due to structural cracks and corrosion was considered. For near-field damage detection, the electro-mechanical (E/M) impedance method was selected as the health-monitoring indicator. Systematic experiments conducted on statistical samples of incrementally damaged specimens were used to fully understand and calibrate the investigative method. Subsequent experiments on realistic aging aircraft panels confirmed our findings. For far-field damage detection, guided Lamb wave ultrasonic techniques were considered. The need for high-frequency wave propagation for achieving the necessary time and space resolution in the given structural damage detection conditions was articulated and exemplified. A complete active sensor array installation and transmitter-receiver instrumentation utilizing standard laboratory equipment and PC/microcontroller software was constructed. Systematic experiments conducted on aircraftgrade metallic plates were used to test the pulse-echo and acousto-ultrasonic wave propagation capabilities of the system. Preliminary experiments conducted on realistic aging aircraft panels proved promising. PIEZOELECTRIC ACTIVE SENSORS Piezo-electric active sensors are small, non-intrusive, and inexpensive piezoelectric wafers that are intimately affixed to the structure and can actively interrogating the structure (Giurgiutiu and Zagrai, 2001a). Piezoelectric active sensors are non-resonant *) Associate Professor, Member ASME **) Graduate Research Assistant, Student Member ASME devices with wide band capabilities. They can be wired into sensor arrays that are connected to data concentrators and wireless communicators. Piezoelectric active sensors have captured the interest of academic and industrial community due to their low cost and small non-intrusive nature (Bartkowicz et al., 1996; Boller et al., 1999). The general constitutive equations of linear piezo-electric material behavior, given by ANSI/IEEE Standard 176-1987, describe a tensorial relation between mechanical and electrical variables (mechanical strain, Sij, mechanical stress, Tkl, electrical field, Ek, and electrical displacement Dj) in the form: , E ij ijkl kl kij k T j jkl kl jk k S s T d E