Yichao Fan

Surface-breaking crack gauging with the use of laser-generated Rayleigh waves

This paper studies crack depth gauging using wideband Rayleigh waves generated by laser in the thermoelastic regime. A finite element method (FEM) is used to calculate the scattering of the Rayleigh waves. The reflected and transmitted Rayleigh waves have different propagation paths and have very different frequency spectra. The arrival times of these scattered waves are crack depth related and can be used for crack depth gauging. Experimental measurements agree well with FEM predictions and validate the usefulness of the crack depth gauging method.

Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point. The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.

Signal enhancement of the in-plane and out-of-plane Rayleigh wave components

Several groups have reported an enhancement of the ultrasonic Rayleigh wave when scanning close to a surface-breaking defect in a metal sample. This enhancement may be explained as an interference effect where the waves passing directly between source and receiver interfere with those waves reflected back from the defect. We present finite element models of the predicted enhancement when approaching a defect, along with experiments performed using electromagnetic acoustic transducers sensitive to either in-plane or out-of-plane motion. A larger enhancement of the in-plane motion than the out-of-plane motion is observed and can be explained by considering ultrasonic reflections and mode conversion at the defect.

Non-linear enhancement of laser generated ultrasonic Rayleigh waves by cracks

Laser generated ultrasound has been widely used for detecting cracks, surface and sub-surface defects in many different materials. It provides a non-contact wideband excitation source, which can be focused into different geometries. Previous workers have reported enhancement of the laser generated Rayleigh wave, when a crack is illuminated by pulsed laser beam irradiation. We demonstrate that the enhancement observed is due to a combination of source truncation, the free boundary condition at the edge of the crack and interference effects. Generating a Rayleigh wave over a crack can lead to enhancement of the amplitude of the Rayleigh wave signal, a shift in the dominant frequency of the wideband Rayleigh wave and strong enhancement of the high frequency components of the Rayleigh wave.

Thermoelastic laser generated ultrasound using a ring source

A pulsed Nd : YAG laser is focused to a sharp ring on an aluminium sample in the thermoelastic regime. This is achieved by firstly expanding the collimated laser beam before passing it through a bi-convex lens and an axicon lens. The addition of the extra convex lens provides a significant focusing improvement. On thick samples, this shape of acoustic source generates simultaneously a surface Rayleigh wave and bulk waves. The surface waves generated by the ring source travel outwards from the ring and also inwards to focus at a point in the centre of the ring. Large amplitude Rayleigh waves are thus generated at the centre of the ring. The absolute, out-of-plane displacement caused by the acoustic waves travelling along a sample surface is measured using a Michelson interferometer with a bandwidth of approximately 80 MHz. The experimental results have been correlated with those obtained by numerical modelling to show good agreement and less successfully with finite element modelling. The ring acoustic source has various applications for defect detection and measurement, and material characterization.

Shear wave generation using a spiral electromagnetic acoustic transducer

A spiral electromagnetic acoustic transducer (EMAT) is efficient in eddy current generation and has been used for surface defect inspection using Rayleigh waves or thickness gauging based on plane waves in echo mode. Measured and calculated particle velocities and directivities are presented. It is found that the shear wave is not predominantly a plane wave. It has zero amplitude on the axis of the generation EMAT and has maximum amplitude at the critical angle. The shear wave could be used in the steel industry for both internal and surface defect inspections together with Rayleigh wave.

Ultrasonic metal sheet thickness measurement without prior wave speed calibration

Conventional ultrasonic mensuration of sample thickness from one side only requires the bulk wave reverberation time and a calibration speed. This speed changes with temperature, stress, and microstructure, limiting thickness measurement accuracy. Often, only one side of a sample is accessible, making in situ calibration impossible. Non-contact ultrasound can generate multiple shear horizontal guided wave modes on one side of a metal plate. Measuring propagation times of each mode at different transducer separations, allows sheet thickness to be calculated to better than 1% accuracy for sheets of at least 1.5mm thickness, without any calibration. (Some figures may appear in colour only in the online journal)

The wave-field from an array of periodic emitters driven simultaneously by a broadband pulse

The use of phased array methods are commonplace in ultrasonic applications, where controlling the variation of the phase between the narrowband emitters in an array facilitates beam steering and focusing of ultrasonic waves. An approach is presented here whereby emitters of alternating polarity arranged in a one-dimensional array are pulsed simultaneously, and have sufficiently wide, controlled bandwidth to emit a two-dimensional wave. This pulsed approach provides a rapid means of simultaneously covering a region of space with a wave-front, whereby any wave that scatters or reflects off a body to a detector will have a distinct arrival time and frequency. This is a general wave phenomenon with a potential application in radar, sonar, and ultrasound. The key result is that one can obtain a smooth, continuous wave-front emitted from the array, over a large solid angle, whose frequency varies as a function of angle to the array. Analytic and finite element models created to describe this phenomenon have been validated with experimental results using ultrasonic waves in metal samples.

ULTRASONIC DETECTION OF SURFACE-BREAKING RAILHEAD DEFECTS

We recently presented measurements of defects on the railhead, using a novel pitch‐catch ultrasonic system comprising of two electro‐magnetic acoustic transducers (EMATs) generating and detecting Rayleigh waves. Current systems used on the UK rail network for detecting surface breaking defects are limited in speed ( 5 mm). The non‐contact EMAT system has the potential to operate at higher line speed, improving network inspection coverage. The current system detects signals and performs an FFT in less than 1 ms, and changes in the detected signal amplitude and frequency content are used to characterise defects. A new set of simulated defects on sections of rail have been produced, including half‐face slots machined normal to the railhead surface, clusters of angled slots, and pocket defects more typical of real defects. The smallest pocket defects are difficult to detect, with changes in signal amplitude and cut‐off falling close to the noise level. However, at chosen higher f...

LIFT‐OFF PERFORMANCE OF FERRITE ENHANCED GENERATION EMATS

Electromagnetic Acoustic Transducers (EMATs) are non‐contact ultrasonic transducers capable of generating wide‐band ultrasonic waves on electrically conductive and magnetostrictive samples. The lack of physical contact makes EMATs particularly suitable for online inspection applications, or situations where samples may be moving or hot. The generation efficiency of a given EMAT on a given sample is dependent on the “lift‐off”, which is the distance between the EMAT and the sample surface. Efficiency dramatically reducing with increased lift‐off. This requirement to be in close proximity to the sample imposes a practical limit of operation and changes in lift‐off due to phenomena such as sample vibration can have practical implications in certain NDE applications. This paper describes some results from experiments comparing the performance of a ferrite enhanced EMAT design to one of our ‘standard’ EMATs, where we have substituted the permanent magnet from the standard EMAT with a suitable ferrite material....