Ultrasonic stress wave characterization of composite materials

The work reported covers three simultaneous projects. The first project was concerned with: (1) establishing the sensitivity of the acousto-ultrasonic method for evaluating subtle forms of damage development in cyclically loaded composite materials, (2) establishing the ability of the acousto-ultrasonic method for detecting initial material imperfections that lead to localized damage growth and final specimen failure, and (3) characteristics of the NBS/Proctor sensor/receiver for acousto-ultrasonic evaluation of laminated composite materials. The second project was concerned with examining the nature of the wave propagation that occurs during acoustic-ultrasonic evaluation of composite laminates and demonstrating the role of Lamb or plate wave modes and their utilization for characterizing composite laminates. The third project was concerned with the replacement of contact-type receiving piezotransducers with noncontacting laser-optical sensors for acousto-ultrasonic signal acquisition

A study on the near-field interactions of ultrasonic surface waves with surface-breaking defects

This thesis is concerned with the detection of surface-breaking defects, such as stress corrosion cracking, using an ultrasonic scanning approach in which a laser source and detector are scanned over the near-field of a defect. Large increases in the amplitude and frequency content of an incident ultrasonic wave are present when either the source or the detector is very close to the defect, leading to a phenomenon known as ultrasonic near-field enhancement. The extent of the ultrasonic enhancement varies with defect characteristics such as defect depth and angle to the surface. Ultrasonic enhancement is observed in both experiment and finite element simulations using Rayleigh waves for both scanning laser detection and scanning laser source methods. The near-field enhancement is shown to vary as a function of the angle of the defect to the horizontal for Rayleigh wave enhancements, allowing the positioning and characterisation of artificial angled defects that are similar to rolling contact fatigue defects in railtrack. The mechanisms behind the near-field enhancement of Rayleigh waves at angled defects are identified, and this aids in the understanding of the behaviour of ultrasound as it interacts with surface-breaking defects. Ultrasonic enhancements are also reported to be present in individual Lamb wave modes for interactions with artificial open-mouthed defects in thin plates, which are similar to the open end of stress corrosion defects. The mechanisms behind both the scanning laser detection and scanning laser source enhancements are identified and used to explain the variation in the enhancement as a function of increasing defect severity. Positioning of these defects is also achieved by identification of the enhancement location. Finally, the scanning laser technique is applied to real stress-driven defects, and both scanning approaches are shown to be capable of detecting partially-closed defects in a variety of sample geometries. The position, geometric alignment and an estimate of the defect depth are obtained for real defects in thin plates, pipework sections and in irregularly shaped engine components

DUAL MODE SENSING OF CRACK GROWTH IN STEEL BRIDGE STRUCTURES

ABSTRACT Monitoring of fatigue cracking in steel bridge structures using a combined passive and active scheme has been approached by the authors. Passive acoustic emission (AE) monitoring is able to detect crack growth behavior by picking up the stress waves resulting from the breathing of cracks while active ultrasonic pulsing can quantitatively assess structural defect by sensing out an interrogating pulse and receiving the structural reflections. The dual-mode sensing functionality is pursued by using the R15I ultrasonic transducers. In the paper, we presented the subject dual-mode sensing on steel compact tension (CT) specimens in a laboratory setup. Passive AE sensing was performed during fatigue loading and showed its capability to detect crack growth and location. At selected intervals of loading cycles, the test was paused to allow for active sensing by pulsing the transducers in a roundrobin pattern. Plate waves were excited, propagated and interacted within the structure. Several approaches were proposed to analyze the interrogation data and to correlate the data features with crack growth. Root means square deviation (RMSD) damage index (DI) was found as a good indicator for indicating the overall crack development. Short time Fourier transform (STFT) provided both time and frequency information at the same time. Moreover, wave velocity analysis showed interesting results when crack developed across the transmitter-receiver path

Surface defect characterisation using non-contact ultrasound

Electromagnetic acoustic transducers (EMATs) have been used as a non-contact ultrasound approach for detecting and characterising surface defects in aluminium bars and billet. The characterisation was made from understanding the interaction of broadband Rayleigh surface waves with surface crack growing normal or inclined to the sample surface, based on rolling contact fatigue (RCF) cracks in rail tracks. The interaction with normal cracks have been previously reported. For inclined cracks, mode conversion of Rayleigh waves to Lamb-like waves occur in the wedge section formed by the crack, resulting in strong and prominent enhancement in the signal detected. This is confirmed by finite element analysis (FEA) models and Lamb waves arrival times calculation. Signal enhancement from the interaction creates features in B-scan images, and they have been used for initial crack classifications. Then, a number of analyses were performed to estimate the crack inclination, and accurately determine the crack vertical depth. A feature extraction and image classification program based on genetic programming have been developed (through a collaboration work) to perform automated classification on the B-scans. The program produces more than 90% accuracy using the experimental data set. The viability of EMATs to detect and fully characterise narrow cracks have been investigated through experiments using laser interferometer and comparison with EMATs measurements. The results confirmed that narrow cracks can be detected with EMATs, with initial classification (in B-scans) to normal/inclined. However, the depth sizing may not be accurate, and suggestion for better designs of EMATs have been made. FEA models have been used to study the interaction of the Rayleigh waves with branched cracks. Interesting results are observed in terms of Rayleigh waves reflections, which helps to determine the presence of a branch on RCF-like cracks. A method has been proposed for calculating the length of the branch, following a number of analyses