Damage detection and monitoring in composite pipes using piezoelectric sensors

Composite materials have become more and more attractive for use in structural applications over recent years, but assessment of damage remains a challenge since this is barely visible, especially when subjected to relatively low energy impact events. The use of piezoelectric transducers for structural health monitoring (SHM) of composites is increasing due to their ability to be embedded without disturbing the structure, low cost, small size, durability, and low power consumption. There is a wealth of research supporting their use for passive and active SHM, yet few studies combine the two. In this work, a multi-layered carbon fibre/epoxy composite pipe is subjected to multiple cycles of mechanical loading/unloading in a three point bending configuration. The specimen is instrumented with eight piezoelectric wafer active sensors (PWAS), used as passive receivers of acoustic emission signals during loading. It is possible to track the location of damage as the test progresses, by triangulation of AE signals. Active monitoring of the specimen is performed using piezoelectric sensors successively as transmitters and receivers of guided waves, in a pitch-catch configuration. Signals are recorded between successive loadings of the specimen to assess the state of damage at each stage, and compare against the ‘pristine’ condition. Cross-comparison of tuning curves obtained from the pristine condition and test data show attenuation in amplitude of the L(0,2) mode. A damage index is proposed based on this amplitude reduction.