Lamb wave near field enhancements for surface breaking defects in plates

Near field surface wave ultrasonic enhancements have previously been used to detect surface breaking defects in thick samples using Rayleigh waves. Here, we present analogous surface wave enhancements for Lamb waves propagating in plates. By tracking frequency intensities in selected regions of time-frequency representations, we observe frequency enhancement in the near field, due to constructive interference of the incident wave mode with those reflected and mode converted at the defect. This is explained using two test models; a square based notch and an opening crack, which are used to predict the contribution to the out-of-plane displacement from the reflected and mode converted waves. This method has the potential to provide a reliable method for the near field identification and characterisation of surface breaking defects in plates

Scanning laser source and scanning laser detection techniques for different surface crack geometries

Standard test samples typically contain simulated defects such as slots machined normal to the surface. However, real defects will not always propagate in this manner; for example, rolling contact fatigue on rails propagates at around 25º to the surface, and corrosion cracking can grow in a branched manner. Therefore, there is a need to understand how ultrasonic surface waves interact with different crack geometries. We present measurements of machined slots inclined at an angle to the surface normal, or with simple branched geometries, using laser ultrasound. Recently, Rayleigh wave enhancements observed when using the scanning laser source technique, where a generation laser is scanned along a sample, have been highlighted for their potential in detecting surface cracks. We show that the enhancement measured with laser detector scanning can give a more significant enhancement when different crack geometries are considered. We discuss the behaviour of an incident Rayleigh wave in the region of an angled defect, and consider mode-conversions which lead to a very large enhancement when the detector is close to the opening of a shallow defect. This process could be used in characterising defects, as well as being an excellent fingerprint of their presence

Non-contact ultrasonic detection of angled surface defects

Non-destructive testing is an important technique, and improvements are constantly needed. Surface defects in metals are not necessarily confined to orientations normal to the sample surface; however, much of the previous work investigating the interaction of ultrasonic surface waves with surface-breaking defects has assumed cracks inclined at 90° to the surface. This paper explores the interaction of Rayleigh waves with cracks which have a wide range of angles and depths relative to the surface, using a non-contact laser generation and detection system. Additional insight is acquired using a 3D model generated using finite element method software. A clear variation of the reflection and transmission coefficients with both crack angle and length is found, in both the out-of-plane and in-plane components. The 3D model is further used to understand the contributions of different wavemodes to B-Scans produced when scanning a sample, to enable understanding of the reflection and transmission behaviour, and help identify angled defects. Knowledge of these effects is essential to correctly gauge the severity of surface cracking

Electron spin relaxation and tunnelling methyl groups

The energy transfer has been studied between free radicals and nearby tunnelling methyl groups near the resonant condition that the electron Larmor frequency and the methyl tunnelling frequency are equal. The measurements were made in a gamma -irradiated single crystal of 4-methyl-2,6-ditertiarybutylphenol (MDBP) at 4.2K and consisted of saturating the ESR signal and observing its return to equilibrium. This has been done at eleven ESR frequencies in the range 8.97\u3c nu s\u3c0.63 GHz and for saturation times of 5, 50 and 200 s. A theoretical model has been developed for the non-exponential recovery which assumes that all the 4-methyl groups of the molecules surrounding a free radical have the same tunnelling rotation frequency nu t and that the only coupling between the electrons and the methyl groups is the intermolecular dipole-dipole interaction

Nuclear spin-lattice relaxation and activation energies of tunnelling methyl groups

The coupling of lattice vibrations with the rotational motion of hindered methyl groups is considered. It is shown that the apparent activation energy for methyl group rotation is determined at relatively high temperatures by the methyl group torsional energy splittings, but at low temperatures it depends on the frequencies of short-wavelength lattice modes which couple relatively strongly with methyl-group rotation. The temperature dependence of the proton spin lattice relaxation time from 6 to 50K is reported for three samples containing methyl groups whose tunnelling frequencies have previously been measured very precisely. They are 4-methyl-2,6-ditertiarybutylphenol (MDBP) and two deuterated derivatives. The activation energy measured at temperatures between 18 and 50K is in good agreement with the splitting between ground and first-excited methyl group torsional states, as computed from the known tunnelling frequencies assuming a three-fold hindering potential. At temperatures below 15K the observed activation energy is only about half this value

Working dogs for the farm : 4-H club project

October, 1952."Acknowledgement and thanks are due to Dr. A. H. Groth, Dean of the School of Veterinary Medicine, for help in preparing the circular."--Page 3."University of Missouri College of Agriculture and the United States Department of Agriculture Cooperating"--Page [27].Title from caption

Scanning laser techniques for characterisation of different surface breaking defect geometries

Measurements using a laser scanning system consisting of a pulsed Nd:YAG laser to generate surface ultrasonic waves and an interferometer to detect the surface displacement, are presented for different samples and defect geometries. We show, firstly, details of the interaction of Rayleigh waves in thick samples with machined slots inclined at an angle to the surface normal, or with simple branched geometries, scanning the generation source over the defect (SLLS) or scanning the detection point over the defect (SLD). Secondly, we discuss effects of Lamb waves interacting with V-shaped defects in thin samples. The results from these measurements have shown that the signal enhancement found in the near-field in both cases can be used to position the defect and gain an idea of its geometry, and have shown this to be a suitable fingerprint of the presence of the defect

Characterisation of hidden defects using the near-field ultrasonic enhancement of Lamb waves

Defects that propagate from the inside of a structure can be difficult to detect by traditional non-destructive inspection methods. A non-contact inspection method is presented here that uses the near-field interactions of ultrasonic Lamb waves to detect defects propagating into a 1.5 mm thick aluminium sheet from the opposite side to that which is inspected. Near-field interactions of the S0 Lamb waves with the defects are shown to give rise to a characteristic increase in the wave magnitude, which is used to position and characterise these hidden defects. It is shown that such defects are difficult to detect from a study of their influence on ultrasonic transmission alone. Single defects of different depths, and systems of multiple defects with varying separations and relative depths, are successfully detected in both experimental trials and FEM simulations. Reliable single defect detection is achieved for defects with a minimum depth of 30% of the plate thickness, and resolution of multiple defects is achieved for defect separations of 5 mm

The market pig : 4-H club project

At head of title: University of Missouri College of Agriculture, Agricultural Extension Service."August, 1946.""University of Missouri College of Agriculture and the United States Department of Agriculture Cooperating"--Page 12.Title from cover

The purebred gilt : 4-H club project

At head of title: University of Missouri College of Agriculture, Agricultural Extension Service."June, 1946.""University of Missouri College of Agriculture and the United States Department of Agriculture Cooperating"--Page [19].Title from cover