The Detection of Fatigue Damage Accumulation in a Thick Composite Beam Using Acousto Ultrasonics

The Acousto Ultrasonics (AU) technique is a Non-Destructive Testing (NDT) technique, widely used for thin, plate-like composite structures. The application of this technique to thick structures, such as the spar cap of rotor blades of wind turbines is considered promising. A problem for the spar caps is fatigue damage. This paper therefore focusses on the fatigue damage detection in a thick composite beam. Two laboratory specimens with a thickness of 56 mm, width of 60 mm and length of 900 mm are equipped with piezo-electric transducers on the top and bottom surface. Short ultrasonic burst waves with varying actuation frequencies are sent by one transducer and measured with the other transducers. Preliminary tests are executed to assess the damage detection capability. The damage is initially simulated by drilling a hole at one location with a stepwise increasing depth of 10 to 56 mm. The number of actuator-sensor paths crossing the simulated damage increases for increasing hole depth. Various Damage Indicator (DI) algorithms and the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) are used for damage assessment and visualisation. A correlation between the DI values and the severity and location of the damage is found. This result is a positive indication for the applicability of AU for damage detection in thick composite structures. A second identical beam is currently placed in a three-point bending fatigue setup