The Influence of Finite-size Sources in Acousto-ultrasonics

This work explores the effects that the finite normal axisymmetric traction loading of an infinite isotropic plate has on wave propagation in acousto-ultrasonics (AU), in which guided waves are created using two normal incidence transducers. Although the work also addresses the effects of the transducer pressure distribution and pulse shape, this thesis concentrates on two main questions: how does the transducer's diameter control the phase velocity and frequency spectrum of the response, and how does the plate thickness relate to the plate's excitability? The mathematics of the time-harmonic solution and the physical principles and the practical considerations for AU wave generation are explained. Transient sources are modeled by the linear superposition of the time-harmonic solutions found using the Hankel transform and they are then compared to experimental data to provide insight into the relation between the size of the transducer and the preferred phase velocity

Finite Size and Speciment Thickness Influence in Acousto-Ultrasonic NDE

Acousto-ultrasonics (AU) uses a pair of transducers to characterize distributed damage in composite plates. A transducer placed normal to the surface creates resonances which propagate as plate waves. Once the receiving transducer picks up the signal, simple analysis techniques, such as the zeroth or first moment of the power spectrum, are applied to create a Stress Wave Factor (SWF). The SWF is then used to quantify the damage state of the composite once the system has been properly trained

Leaky guided ultrasonic waves in NDT.

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A General Purpose Approach to Calculating the Longitudinal and Flexural Modes of Multi-Layered, Embedded, Transversely Isotropic Cylinders

Over a number of years, the authors have been developing a general purpose program for predicting the properties of guided elastic waves. In addition to the well known solutions such as Lamb modes, the program can model waves in complicated structures, including structures that are comprised of multiple layers, have flat or cylindrical geometries, and are immersed in a fluid or embedded in a solid. The methodology for the program, ‘Disperse’, which is based on a global matrix algorithm, has previously been presented at this meeting and elsewhere [1,2]. This paper presents the extension of the model to include transversely isotropic cylinders. Generality is pursued by allowing arbitrary numbers of layers, axially symmetric or non-axially-symmetric (nonzero circumferential order) modes, and free, immersed, or embedded structures.</p

Finite Size and Speciment Thickness Influence in Acousto-Ultrasonic NDE

Acousto-ultrasonics (AU) uses a pair of transducers to characterize distributed damage in composite plates. A transducer placed normal to the surface creates resonances which propagate as plate waves. Once the receiving transducer picks up the signal, simple analysis techniques, such as the zeroth or first moment of the power spectrum, are applied to create a Stress Wave Factor (SWF). The SWF is then used to quantify the damage state of the composite once the system has been properly trained.</p

Guided Ultrasonic Waves for the Inspection of Post-Tensioned Bridges

This paper investigates the use of guided waves to inspect the embedded tendons in post- tensioned bridges. The unexpected collapse of the Ynys-y-Gwas bridge in South Wales[1] has highlighted the need for an inexpensive, reliable inspection method for testing the thousands of existing post-tensioned bridges. Post-tensioned construction allows large spans to be constructed with a light, inexpensive design. Figure 1 summarizes the technique, which involves constructing the framework of the bridge out of concrete, leaving metal or plastic tubes called ducts at predetermined locations. Steel bars or multi-wire strands (collectively referred to as tendons) are threaded through the ducts once the concrete has hardened. The steel tendons are tensioned and anchored by small collets at their ends. The tensioned steel forces the concrete into compression so that is better able to support the required loads. Once the steel is tensioned, the ducts are filled with grout to provide corrosion protection. However, large air voids can be trapped in the grout, providing areas for corrosion to occur, which could lead to catastrophic failure such as the Ynys-y- Gwas collapse. The current inspection techniques have not proven to be entirely satisfactory [2], especially for inspecting the portion of the tendon near the anchorages, which is a location where the tendon is particularly prone to corrosion.</p

Prediction of Reflection Coefficients from Defects in Embedded Bars

There is a recognized need worldwide for improved methods for the detection of corrosion of the tendons in post-tensioned concrete bridges [1,2]. Post-tensioning is used to construct light, strong bridges with the possibility of long spans. The technique involves constructing the concrete spans, leaving hollow tubes, called ducts, in place in theformwork while the concrete cures. Steel bars or strands, collectively called tendons, are then fed through the ducts and tensioned to force the concrete into compression. Finally the ducts are filled with grout to provide corrosion protection for the steel tendons. However voids which can form during the grouting process can allow water to collect in contact with the tendons, promoting corrosion. The detection of the corrosion of the tendons is very difficult because they are embedded deep within the bridge structure and are shielded by the ducts. Current inspection is primarily visual, involving drilling through to the ducts from the exterior of the bridge [1].</p

Simulation of Lamb Wave Propagation Using Pure Mode Excitation

A combination of studies of dispersion behavior (wave mode properties) with time domain Finite Element modelling constitutes a very useful approach to developing guided wave NDE techniques. The dispersion curves and the mode shapes reveal a lot of information about how a guided wave will behave in a plate and can be used to judge modes and frequencies that are likely to be sensitive to defects. However, because of their modal nature, the dispersion curves cannot predict how a guided wave will react with a local geometric change in a plate such as a defect, which is of great importance to Lamb wave testing. Finite element modelling can fill in this information.</p

A Resonance Frequency Technique to Monitor the Integrity of Dental Implants

Titanium implants are being used increasingly to provide support for prostheses replacing missing teeth in edentulous and partially dentate patients. There is good histological evidence to show that in satisfactory implants bone forms in intimate contact with the implant surface during the healing process following fixture placement [1]. Currently, fixtures are left unloaded for a period of 3–6 months following placement in order to allow this healing process to occur. It would be very valuable to be able to monitor the healing process non-invasively in order to determine more accurately when it is safe to load the implant. Similarly, during service, it would be beneficial to be able to detect the onset of problems such as an increase in the mobility of the implant due to infection, or to a decrease in the height of the bone surrounding the implant.</p

Disperse: A General Purpose Program for Creating Dispersion Curves

The application of guided waves in NDT can be hampered by the lack of readily available dispersion curves for complex structures. To overcome this hindrance, we have developed a general purpose program that can create dispersion curves for a very wide range of systems and then effectively communicate the information contained within those curves. The program uses the global matrix method to handle multi-layered Cartesian and cylindrical systems. The solution routines cover both leaky and non-leaky cases and remain robust for systems which are known to be difficult, such as large frequency-thicknesses and thin layers embedded in much thicker layers. Elastic and visco-elastic isotropic materials are fully supported; anisotropic materials are also covered, but are currently limited to the elastic, non-leaky, Cartesian case.</p