Monitoring fatigue-induced transverse matrix cracks in laminated composites using built-in acousto-ultrasonic techniques

This work investigated the use of ultrasonic wave propagation in a composite laminate to monitor transverse matrix cracking. It was found that the change in the signal's power spectral density correlates to crack density increase, and its sensitivity was dependent on the actuator to sensor orientation. This orientation dependence was found to be the same as that of the laminate's stiffness degradation. A crack density damage index model was developed that relates signal changes to laminate stiffness degradation due to transverse matrix cracks. With this damage index model, a matrix cracking monitoring system was proposed, implemented, and validated. The developed system requires a calibration test for the composite layup of interest and a minimum of one actuator and two sensors at different angle orientations. The calibration test and general approach steps are described and validated. The system's crack density estimates match experimentally measured crack density values within established error bounds.