Through-wall Phased Array Imaging of Small Surface-Breaking Cracks

Accurate and reliable measurements of small surface-breaking cracks are becoming increasingly important in safety-critical components such as in nuclear power plants, because of the desire for increased operational lifetimes. With great coverage and low cost, the use of imaging to find and size surface-breaking cracks with data from ultrasonic phased arrays is becoming more attractive. For the phased array imaging, both the array configuration and the technique used to generate the images from the measurements are key factors in the image quality and sizing accuracy. Thus, the selection of a proper imaging technique is essential to ensure accurate results for a specific situation of inspection. Additionally, a particular challenge in imaging surface-breaking cracks using through-wall measurement so see cracks at the remote wall (backwall) is that the reflection from the backwall is very strong, often masking the presence of the defect itself; this presents a particular challenge to the imaging algorithms used to reconstruct an image. In this study, the performance of three algorithms, the Total Focusing Method (TFM), the Half-Skip TFM (HSTFM) and the Factorisation Method (FM) will be compared for surface-breaking cracks. These three imaging algorithms will be used on simulated and experimental array data. Their performances will be compared in both the accuracy of sizing the defects and the quality of the images produced

Solution to Reconstruct the Height Correlation Function of a Randomly Rough Surface Using the Diffuse Scattered Elastic Waves

The scattering of elastic waves provides a way for subsurface sensing of surface conditions, which may be applied in NDE to characterize crack-like defects within a component or corrosion surfaces on a remote surface such as the inside of a pipe. In [1], a methodology has previously been suggested to estimate the surface RMS value using a single transducer performing the pulse echo inspection. The conventional formulae representing the exponential decay of the expected coherent intensity in the specular direction as a function of the RMS value was applied in that work. Apart from the RMS roughness, the surface correlation function is also very important as it describes the lateral variation of surface height. However, so far there has been no solution found to reconstruct the height correlation function, since it is strongly related with the diffuse field whose analytical form was not accessible until recently. In [2] we have developed an analytical expression to represent the elastic wave diffuse field, on the basis of which we proceed in this study to develop a solution to reconstruct the complete correlation function from the diffuse field. The methodology relies on the angular measurement of the expected diffuse intensity within a narrow frequency band using a phased array. In practice, multiple scans of different parts of the surface need to be performed using a phased array, so that the expected intensity at different angles can be obtained by averaging the intensities from all scans. The diffuse intensity is then extracted by subtracting the coherent intensity from the mean total intensity. By applying the developed inverse algorithm, the correlation function can be recovered from the diffuse intensity. In this study, numerical simulations via the Monte Carlo approach using multiple surface realizations are run to obtain the expected angular distribution of the diffuse intensity, which is then used in the inverse algorithm to reconstruct the correlation function. Results show that the developed method successfully reconstructs the height correlation function for a range of RMS values from low to high roughness

Identification of leaky Lamb waves for waveguides sandwiched between elastic half-spaces using the Spectral Collocation Method

In non-destructive evaluation guided wave inspections, the elastic structure to be inspected is often embedded within other elastic media and the ensuing leaky waves are complex and non-trivial to characterise; we consider the canonical example of an elastic waveguide surrounded by other elastic materials that demonstrates the fundamental issues with characterising the leaky waves in such systems. Due to the complex wavenumber solutions required to represent them, leaky waves pose significant challenges to existing numerical methods, while methods that spatially discretise the field to retrieve them suffer from the exponential growth of their amplitude far into the surrounding media. We present a spectral collocation method yielding an accurate and efficient identification of these modes, leaking into elastic half-spaces. We discretise the elastic domains and, depending on the exterior bulk wavespeeds, select appropriate mappings of the discretised domain to complex paths, in which the numerical solution decays and the physics of the problem are preserved. By iterating through all possible radiation cases, the full set of dispersion and attenuation curves are successfully retrieved and validated, where possible, against the commercially available software DISPERSE. As an independent validation, dispersion curves are obtained from finite element simulations of time-dependent waves using Fourier analysis

Mapping and explaining the productivity of Pinus radiata in New Zealand

Mapping Pinus radiata productivity for New Zealand not only provides useful information for forest owners, industry stakeholders and policy managers, but also enables current and future plantations to be visualised, quantified, and planned. Using an extensive set of permanent sample plots, split into fitting (n = 1,146) and validation (n = 618) datasets, models of P. radiata 300 Index (an index of volume mean annual increment) and Site Index (an index of height growth) were developed using a regression kriging technique. Spatial predictions were accurate and accounted for 61% and 70% of the variance for 300 Index and Site Index, respectively. Productivity predicted from these surfaces for the entire plantation estate averaged 27.4 m³ ha⁻¹ yr⁻¹ for the 300 Index and 30.4 m for Site Index. Surfaces showed wide regional variation in this productivity, which was attributable mainly to variation in air temperature and root-zone water storage from site to site

Perinatal Cat and Dog Exposure and the Risk of Asthma and Allergy in the Urban Environment: A Systematic Review of Longitudinal Studies

Background. The literature is contradictory concerning pet exposure and the risk of development of asthma and other allergic diseases. Using longitudinal studies, we aimed to systematically review the impact of pet ownership in the critical perinatal period as a risk factor for allergies in childhood. Methods. Medline database was searched for urban cohort studies with perinatal exposure to cats and/or dogs and subsequent asthma or allergic disease. Results. Nine articles, comprising 6498 participants, met inclusion criteria. Six found a reduction in allergic disease associated with perinatal exposure to dogs or, cats or dogs. One study found no association. Two found increased risk only in high-risk groups. Conclusion. Longitudinal studies in urban populations suggest that perinatal pets, especially dogs, may reduce the development of allergic disease in those without a family history of allergy. Other unmeasured factors such as pet-keeping choices in allergic families may be confounding the association seen in these high-risk families, and further study is required

Survival, Growth and Reproduction of Non-Native Nile Tilapia II: Fundamental Niche Projections and Invasion Potential in the Northern Gulf of Mexico

Understanding the fundamental niche of invasive species facilitates our ability to predict both dispersal patterns and invasion success and therefore provides the basis for better-informed conservation and management policies. Here we focus on Nile tilapia (Oreochromis niloticus Linnaeus, 1758), one of the most widely cultured fish worldwide and a species that has escaped local aquaculture facilities to become established in a coastal-draining river in Mississippi (northern Gulf of Mexico). Using empirical physiological data, logistic regression models were developed to predict the probabilities of Nile tilapia survival, growth, and reproduction at different combinations of temperature (14 and 30°C) and salinity (0–60, by increments of 10). These predictive models were combined with kriged seasonal salinity data derived from multiple long-term data sets to project the species\u27 fundamental niche in Mississippi coastal waters during normal salinity years (averaged across all years) and salinity patterns in extremely wet and dry years (which might emerge more frequently under scenarios of climate change). The derived fundamental niche projections showed that during the summer, Nile tilapia is capable of surviving throughout Mississippi\u27s coastal waters but growth and reproduction were limited to river mouths (or upriver). Overwinter survival was also limited to river mouths. The areas where Nile tilapia could survive, grow, and reproduce increased during extremely wet years (2–368%) and decreased during extremely dry years (86–92%) in the summer with a similar pattern holding for overwinter survival. These results indicate that Nile tilapia is capable of 1) using saline waters to gain access to other watersheds throughout the region and 2) establishing populations in nearshore, low-salinity waters, particularly in the western portion of coastal Mississippi

Mapping the productivity of radiata pine

Forest owners, investors and policy makers all want to know the spread and productivity of New Zealand’s current and future radiata plantation. David Palmer, a geo-spatial analyst at Scion, has combined advanced statistical techniques with mapping technology to predict radiata 300 Index and Site Index for any location in New Zealand. The 300 Index is an index of volume mean annual increment, and the Site Index is for height and growth. The map of Site Index and 300 Index was built using growth measurement data from trees in 1,146 radiata pine permanent sample plots, planted between 1975 and 2003. The data was combined with a number of climate, land use, terrain and environmental variables to predict forest productivity under a range of conditions