In situ health monitoring of fuel weep holes using ultrasonic guided waves

This thesis presents an investigation into the scattering of the fundamental Lamb, shear horizontal and edge-guided Rayleigh-type wave modes from sub-surface and hard-to-inspect damage on cavities representative of fuel weep holes in integral stiffeners. Detection, localisation and characterisation of scattered elastic guided wave fields can assist with improving the reliability and probability of detection of health monitoring systems through informed placement of actuators and sensors. The combination of the experimental-computational-analytical approach gives rise to new insights and guidance for the quantification of defects located in hard-to-inspect regions with actuators and sensors placed in easily accessible locations. Three-dimensional laser vibrometry is used to confirm that ultrasonic guided plate waves, generated by permanently bonded low-profile surface-mounted piezoceramic disc transducers, are able to form edge-guided Rayleigh-type waves on the free surface of open cavities. The behaviour of the propagating ultrasonic guided waves around cavities and their interaction with simulated fatigue damage emanating from the cavities is visualised. In situ non-destructive evaluation of the airframe stiffener is shown to be viable using symmetric guided waves given that the strength of the characteristic scattered spiralling guided wave field maintains its form and grows in amplitude as the damage length in increased. In contrast to the high frequency bulk-wave wedge transducer methodology traditionally used for weep hole inspection, the length-scales of the in situ technique are such that the wavelength of the incident guided waves are comparable with the hole diameter, and larger than the defect length. The monitoring of upward growing hidden damage from circular weep holes in integral stiffeners using symmetric guided waves is unique in that the received energy, that is the measured surface velocity on the accessible surface, monotonically increases with respect to increasing damage length. A series of computational studies with a three-dimensional finite element model is conducted to understand how and why scattered plate waves from the defect can be detected on the accessible surface. The ability to accurately model acousto-ultrasonic elastic wave dynamics is crucial for future developments in this research field. The computational results are compared with the experimental laser vibrometry data for model validation. It is concluded that the boundary conditions for wave scattering from the hidden notch are a complex combination of the incident wave field and the dispersive edge-guided Rayleigh-type wave that is formed on the free surface of the cavity.