Multiple wavemode scanning for near and far-side defect characterisation

The combination of ultrasonic inspections using different wavemodes can give more information than is available with single mode inspection. In this work, the response of shear and Rayleigh waves to surface-breaking defects propagating on the near-side and far-side of a sample is investigated. The directivity of shear waves generated by a racetrack coil electromagnetic acoustic transducer (EMAT) is identified and used to set an ideal separation for a pair of transmit-receive EMATs. Defects are indicated by a reduction in the transmitted Rayleigh wave amplitude, and by blocking of the shear wave. Used together, these can identify features in the bulk wave behaviour which are due to near-face surface-breaking defects, and give a full picture of both surfaces. By using a combination of the two wavemodes, the angle of propagation and length of any near-side defects can additionally be identified. A scanning method for samples is proposed

Phased electromagnetic acoustic transducer array for Rayleigh wave surface defect detection

A phased electromagnetic acoustic transducer (EMAT) array system has been developed for detection and characterisation of surface breaking defects. An array of four linear coils which are individually controlled are used to generate a Rayleigh wave. The high current electronics combined with the coil designs enables the array to generate either narrowband or broadband signals, and controlling the phase delay between the channels makes it possible to change the ultrasound wavelength without requiring the physical separation of the coils to be changed. Experimental results show that the four-coil phased array is able to generate a wavelength range from 3.0 mm to 11.7 mm. Surface breaking defects were characterised using a transmit-receive set-up with a broadband EMAT detector being used to detect the Rayleigh wave. Machined surface slots with different depths were used for technique validation. The results show that the array is sensitive to surface defects and that a wide depth sensitivity range for defect sizing can be easily achieved by applying phasing to tune the wavelength of operation. A large increase in detection flexibility is immediately shown

Predictions of entry heating for lower surface of shuttle orbiter

A broad base of thermocouple and phase change paint data was assembled and correlated to the nominal design 14414.1 and proposed STS-1 (first flight of the space transportation system) entry trajectories. Averaged data from phase change paint tests compared favorably with thermocouple data for predicting heating rates. Laminar and turbulent radiation equilibrium heating rates were computed on the lower surface of the Shuttle orbiter for both trajectories, and the lower surface center line results were compared both with aerodynamic heating design data and with flight values from the STS-1 and STS-2 trajectories. The peak laminar heating values from the aerodynamic heating design data book were generally 40 to 60 percent higher than the laminar estimates of this study, except at the 55 percent location of maximum span where the design data book values were less than 10 percent higher. Estimates of both laminar and turbulent heating rates compared favorably with flight data

The interaction of polymer dispersed liquid crystal sensors with ultrasound

Polymer dispersed liquid crystals (PDLCs) have been shown to be sensitive to ultrasound through the acousto-optic effect. The acousto-optic response of PDLCs was studied over a broad frequency range (0.3–10 MHz). We demonstrate that the displacements required to produce acousto-optic clearing of PDLC films can be as low as a few nanometers, which is at least 103 times smaller than the PDLC droplet size, is 105 times smaller than the PDLC layer thickness, and of the order of the molecular size of the liquid crystal constituents. This suggests that the acousto-optic effect in PDLCs is due to the microscopic effects of the LC reorientation under torques or flows rather than the LC reorientation through macroscopic droplet deformation. The displacement required for clearing is related to the frequency of operation via an exponential decay. We attribute the observed frequency response to a freezing out of the rotational motion around the short axis of the liquid crystal. The reported frequency dependence and displacements required indicate that the effects and materials described here could be used for ultrasound visualization in a non-destructive testing context

UK open source crime data: accuracy and possibilities for research

In the United Kingdom, since 2011 data regarding individual police recorded crimes have been made openly available to the public via the police.uk website. To protect the location privacy of victims these data are obfuscated using geomasking techniques to reduce their spatial accuracy. This paper examines the spatial accuracy of the police.uk data to determine at what level(s) of spatial resolution – if any – it is suitable for analysis in the context of theory testing and falsification, evaluation research, or crime analysis. Police.uk data are compared to police recorded data for one large metropolitan Police Force and spatial accuracy is quantified for four different levels of geography across five crime types. Hypotheses regarding systematic errors are tested using appropriate statistical approaches, including methods of maximum likelihood. Finally, a “best-fit” statistical model is presented to explain the error as well as to develop a model that can correct it. The implications of the findings for researchers using the police.uk data for spatial analysis are discussed

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

Playful Platonist : The development of ideas in the novels of Iris Murdoch

This thesis examines Iris Murdoch's novels in the light of her philosophical thinking. It places her ethical thinking in the context of twentieth century moral philosophy and shows that her approach to the problems of the subject is out of key with the general run of contemporary philosophical thinking. It traces her debt to G. E. Moore, and the modification which she offers of the Intuitionist view of Ethics, as she develops the significance, for her, of learning to see the appropriateness of concepts to the facts of the case, and of the value of 'attention' in the development of moral behaviour. The thesis Outlines the way in which Iris Murdoch's early interest in the existentialist philosophy of Sartre becomes a firm point of philosophical objection. She finds that Sartre's disgust with the contingency of the world is based on a defensive solipsism, and argues that the world beyond the self should be a matter for delight and instruction in the insignificance of the individual. It further shows how her fundamental philosophical beliefs are indebted to Plato, in that she follows him in attributing to beauty a significance as a guide to the good, and also in equating love with knowledge of the external world. It points out that Iris Murdoch's comments on the novel have often been used to attack her own fictional practice, and suggests that this practice is mistaken. It points out the significance of the essay 'Existentialists and Mystics', which has been neglected by critics, both in terms of illuminating her interests and purposes and also in providing appropriate terminology with which to discuss her novels. The thesis then examines the individual novels and traces the evolution of Iris Murdoch's ideas in them from an interest in, but philosophical hostility to, existentialism, towards the 'mystical' novel which celebrates reality beyond the self, and sees freedom as obedience to that external reality. It suggests that Iris Murdoch consistently attempts to develop secular concepts which will take the place of the religious concepts which she feels are no longer appropriate

Elastic Energy, Fluctuations and Temperature for Granular Materials

We probe, using a model system, elastic and kinetic energies for sheared granular materials. For large enough $P/E_y$ (pressure/Young's modulus) and $P/\rho v^2$ ($P/$kinetic energy density) elastic dominates kinetic energy, and energy fluctuations become primarily elastic in nature. This regime has likely been reached in recent experiments. We consider a generalization of the granular temperature, $T_g$, with both kinetic and elastic terms and that changes smoothly from one regime to the other. This $T_g$ is roughly consistent with a temperature adapted from equilibrium statistical mechanics.Comment: 4 pages, 4 figure

Volume fluctuations and geometrical constraints in granular packs

Structural organization and correlations are studied in very large packings of equally sized acrylic spheres, reconstructed in three-dimensions by means of X-ray computed tomography. A novel technique, devised to analyze correlations among more than two spheres, shows that the structural organization can be conveniently studied in terms of a space-filling packing of irregular tetrahedra. The study of the volume distribution of such tetrahedra reveals an exponential decay in the region of large volumes; a behavior that is in very good quantitative agreement with theoretical prediction. I argue that the system's structure can be described as constituted of two phases: 1) an `unconstrained' phase which freely shares the volume; 2) a `constrained' phase which assumes configurations accordingly with the geometrical constraints imposed by the condition of non-overlapping between spheres and mechanical stability. The granular system exploits heterogeneity maximizing freedom and entropy while constraining mechanical stability.Comment: 5 pages, 4 figure