Acousto-Ultrasonic Structural Health Monitoring of Aerospace Composite Materials

Piezoelectric transducers have a long history of applications in nondestructive evaluation (NDE) of material and structure integrity owing to their ability of transforming mechanical energy to electrical energy and vice versa. From an acoustic point of view, there is no difference between structural health monitoring (SHM) and conventional NDE since both rely on the same physics in the sense that in either case acoustic waves are generated and then detected. SHM was ‘born’ from the conjunction of several techniques and has a common basis with NDE. In fact, several NDE techniques can be converted into SHM techniques, by integrating sensors and actuators inside the monitored structure. For instance, traditional ultrasonic testing can be easily converted into an acousto-ultrasonic SHM system, using embedded or surface-mounted piezoelectric wafer active sensors (PWAS). These sensors should be affordable, lightweight, and unobtrusive such as to not impose cost and weight penalty on the structure and to not interfere with the structural strength and airworthiness. Other damage measuring methods based on large area measurements have been used in SHM development for verification and validation of damage and/or for understating the proposed SHM approach; however, they do not seem appropriate for permanent installation onto the monitored structure and will not be discussed under the heading of ‘SHM sensors’. This paper presents and discusses an overview of ultrasonic SHM techniques for composite materials. After a brief introduction, it presents the PWAS-based SHM principle, which is followed by a discussion of the passive and active ultrasonic SHM techniques. The paper identifies advantages and disadvantages of these in-situ NDE methods and guidelines for future work on heterogeneous, anisotropic materials, like aerospace composite polymer.