An accurate mathematical representation of the mechanical behaviour of human skin is essential when simulating deformations occurring in the skin during body movements or clinical procedures. In this study constitutive stress-strain relationships based on experimental data from human skin in vivo were obtained. A series of multiaxial loading experiments were performed on the forearms of four age- and gender matched subjects. The tissue geometry, together with recorded displacements and boundary forces, were combined in an analysis using finite element modelling. A non-linear optimization technique was developed to estimate values for the material parameters of a previously published constitutive law, describing the stress-strain relationship as a non-linear anisotropic membrane. Ten sets of material parameters where estimated from the experiments, showing considerable differences in mechanical behaviour both between individual subjects as well as mirrored body locations on a single subject. The accuracy of applications that simulate large deformations of human skin could be improved by using the parameters found from the in vivo experiments as described in this study.
The polar ocean's sea ice cover is an unconventional and challenging geophysical target. Helicopter-electromagnetic (HEM) sea-ice thickness mapping is currently limited to 1D interpretation due to traditional procedures and systems. These systems are mainly sensitive to layered structures, ideally set for the widespread flat (level) ice type. Because deformed sea ice (e.g., pressure ridges) is 3D and usually also heterogeneous, ice thickness errors up to 50% can be observed for pressure ridges using 1D approximations for the interpretation of HEM data. We researched a new generation multisensor, airborne sea ice explorer (MAiSIE) to overcome these limitations. Three-dimensional finite-element modeling enabled us to determine that more than one frequency is needed, ideally in the range 1-8 kHz, to improve thickness estimates of grounded sea-ice pressure ridges that are typical of 3D sea ice structures.With the MAiSIE system, we found a new electromagnetic concept based on one multifrequency transmitter loop and a 3C receiver coil triplet with active digital bucking. The relatively small weight of the EM components freed enough payload to include additional scientific sensors, including a cross-track lidar scanner and high-accuracy inertial-navigation system combined with dual-antenna differential GPS. Integrating the 3D ice-surface topography obtained from the lidar with the EM data at frequencies from 500 Hz to 8 kHz in x-, y-, and z-directions, significantly increased the accuracy of sea-ice pressure-ridge geometry derived from HEM data. Initial test flight results over open water showed the proof-of-concept with acceptable sensor drift and receiver sensitivity. Noise levels were relatively high (20-250 parts-per-million) due to unwanted interference, leaving room for optimization. The 20 ppm noise level at 4.1 kHz is sufficient to map level ice thickness with 10 cm precision for sensor altitudes below 13 m.
A multiaxial testing rig has been designed to investigate mechanical properties of soft tissue membranes. This approach has the advantage over biaxial loading in that it can be used to investigate soft tissue membranes with complex structural architecture. A finite element model of tissue mechanics has been used to analyze the experimental data in order to evaluate the stress-strain relationship, and a forward solve algorithm developed to estimate material parameters values for a given constitutive law. The multiaxial testing rig and the finite element analysis have been used to evaluate the constitutive properties of in-vivo human skin.
In recent years, there has been an increased focus on environmental issues near oil and gas production sites and pipelines. The trend is likely to continue as future oil and gas production is commencing in environmentally vulnerable areas, and farther north near the polar ice cap. The exploration in more remote areas implies that online in situ monitoring has great advantages over expedition based monitoring.The Norwegian Geotechnical Institute (NGI) is an independent research and consultancy foundation having over 40 years of worldwide offshore instrumentation experience. NGI has recently launched a research initiative to develop new methods for in situ and on line environmental monitoring offshore. In 2012, NGI designed and installed an integrated environmental monitoring system for subsea leakage detection. It was installed on the seabed beneath an operating oil and gas platform in the North Sea. We give here an overview of the instrumentation system and the motivation for the different design choices. The system is an example of an integrated monitoring system, where different types of sensors complement each other and gives a more thorough understanding of methane transport at the site of interest. After installation, NGI has analyzed monitoring data and provided decision support to operator personnel. Nearly two years of production site data collection represents a unique data set that enables a comprehensive analysis of methane transport.From the operator side, the motivation for environmental monitoring usually arises from the need to answer clearly defined questions such as 'Is there a gas leakage in the production system? Where is it? How large is the leakage?' The leakage detection system is expected to answer these questions, preferrably in an unambiguous manner. However, natural gas can be released to the water phase from several sources e.g. organic matter on the seabed. If the leakage detection monitoring data shall be interpreted in a control room that operates with alarm states, these sources of background noise must be identified and separated from a real leak situation. We look at which circumstances to be aware of and how to maximize the operational value of a leakage detection system.
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