Improving transient analysis technology for aircraft structures

Aircraft dynamic analyses are demanding of computer simulation capabilities. The modeling complexities of semi-monocoque construction, irregular geometry, high-performance materials, and high-accuracy analysis are present. At issue are the safety of the passengers and the integrity of the structure for a wide variety of flight-operating and emergency conditions. The technology which supports engineering of aircraft structures using computer simulation is examined. Available computer support is briefly described and improvement of accuracy and efficiency are recommended. Improved accuracy of simulation will lead to a more economical structure. Improved efficiency will result in lowering development time and expense

Rigidity of the free factor complex

We establish the following non-abelian analogue of the Fundamental Theorem of Projective Geometry: the natural map from ${\rm{Aut}}(F_n)$ to the automorphism group of the free-factor complex $\mathcal{AF}_n$ is an isomorphism. We also prove the corresponding theorem for the action of ${\rm{Out}}(F_n)$ on the complex of conjugacy classes of free factors

Probabilistic analysis of bearing capacity of piles with variable parameters in cpt test and calculation according to the requirements

A probabilistic concept for determining pile bearing capacity is presented, taking into account the variability of CPT test parameters and methodology of calculation according to the requirements of Eurocode 7 (EN 1997-1: 2004). Based on a single initial (real) CPT test, a larger number of generated (simulation) CPT tests are introduced drawn from solutions of statistics and probability theory. Research has found that the best solutions are achieved using the DA 2 design approach for n(CPT) > 10 tests. Taking into account the deterministic and probabilistic approach in the analysis of pile bearing capacity, it is found that for the DA 2 design approach, the ratio of pile bearing capacity obtained from simulation and the capacity as determined through three methods (Mazurkiewicz, Van der Veen and hyperbolic approximation) is Rcd, /Pu = 1.148. Using the reliability index, the following values of partial resistance factors are obtained: λ, s /P 1.1, λ, b /P 1.1, which also points to the DA 2 design approach

Evolution of seismic layer 2B across the Juan de Fuca Ridge from hydrophone streamer 2-D traveltime tomography

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q05009, doi:10.1029/2010GC003462.How oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the Earth's interior and the oceans. Although considerable work has been done characterizing the evolution of seismic layer 2A, there has been little analysis of the processes that affect layer 2B after formation. Here we present high-resolution 2-D tomographic models of seismic layer 2B along ∼300 km long multichannel seismic transects crossing the Endeavour, Northern Symmetric, and Cleft segments of the Juan de Fuca Ridge. These models show that seismic layer 2B evolves rapidly following a different course than layer 2A. The upper layer 2B velocities increase on average by 0.8 km/s and reach a generally constant velocity of 5.2 ± 0.3 km/s within the first 0.5 Myr after crustal formation. This suggests that the strongest impact on layer 2B evolution may be that of mineral precipitation due to “active” hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber. Variations in upper layer 2B velocity with age at time scales ≥0.5 Ma show correlation about the ridge axis indicating that in the long term, crustal accretion processes affect both sides of the ridge axis in a similar way. Below the 0.5 Ma threshold, differences in 2B velocity are likely imprinted during crustal formation or early crustal evolution. Layer 2B velocities at propagator wakes (5.0 ± 0.2 km/s), where enhanced faulting and cracking are expected, and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2B. Analysis of the layer 2B velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2B velocities, respectively. Additionally, the distance between present up-flow and down-flow zones is related to the amount of sediment cover because, as sediment cover increases and basement outcrops become covered, direct pathways from the igneous basement through the seafloor are cut off, forcing convective cells to find alternate paths.This research was supported by National Science Foundation grants OCE0002488 and OCE0648303 to S.M.C. and M.R.N., NSERC Discovery grant to M.R.N., and a Bruce C. Heezen Graduate Research Fellowship (Office of Naval Research grant N00014‐02‐1‐0691) to K.R.N

Upper crustal seismic structure along the Southeast Indian Ridge: Evolution from 0 to 550 ka and variation with axial morphology

The seismic structure of uppermost crust evolves after crustal formation with precipitation of alteration minerals during ridge-flank hydrothermal circulation. However, key parameters of crustal evolution including depth extent and rates of change in crustal properties, and factors contributing to this evolution remain poorly understood. Here, long-offset multichannel seismic data are used to study the evolution of seismic layer 2A and uppermost 2B from 0 to 550 ka at three segments of the intermediate spreading rate Southeast Indian Ridge. The segments differ in on-axis morphology and structure with crustal magma bodies imaged at axial high and rifted high segments P1 and P2, but not at axial valley segment S1 and marked differences in thermal conditions within the upper crust are inferred. One-dimensional travel time modeling of common midpoint supergathers is used to determine the thickness and velocity of layer 2A and velocity of uppermost 2B. At all three segments, layer 2A velocities are higher in 550 ka crust than on-axis (by 7–14%) with the largest increases at segments P1 and P2. Velocities increase more rapidly (by 125 ka) at P1 with spatial variations in velocity gradients linked to location of the underlying crustal magma body. We attribute these differences in crustal evolution to higher rates of fluid flow and temperatures of reaction at these ridge segments where crustal magma bodies are present. Layer 2A thickens off-axis at segments P1 and P2 but not at S1; both off-axis volcanic thickening and downward propagation of a cracking front linked to the vigor of axial hydrothermal activity could contribute to these differences. In zero-age crust, layer 2B velocities are significantly lower at segments P1 and P2 than S1 (5.0, 5.4, and 5.8 km/s respectively), whereas similar velocities are measured off-axis at all segments (5.7–5.9 km/s). Lower on-axis 2B velocities at segments P1 and P2 can be partly attributed to thinner layer 2A, with lower overburden pressures leading to higher porosities in shallowest 2B. However, other factors must also contribute. Likely candidates include subaxial deformation due to magmatic processes and enhanced cracking with axial hydrothermal activity at these segments with crustal magma bodies

Variations in upper crustal structure due to variable mantle temperature along the Southeast Indian Ridge

There is a systematic variation in axial morphology and axial depth along the Southeast Indian Ridge (SEIR) with distance away from the Australian Antarctic Discordance, an area of cold uppermost mantle. Since spreading rate (72–76 mm/yr) and mantle geochemistry appear constant along this portion of the SEIR, the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature. In this study, we report results from a multichannel seismic investigation of on-axis crustal structure along this portion of the SEIR. Three distinct forms of ridge crest morphology are found within our study area: axial highs, rifted axial highs, and shallow axial valleys. Axial highs have a shallow (~ 1500 m below seafloor (bsf)) magma lens and a thin (~ 300 m) layer 2A along the ridge crest. Rifted axial highs have a deeper (~ 2100 m bsf) magma lens and thicker (~ 450 m) layer 2A on-axis. Beneath shallow axial valleys, no magma lens is imaged, and layer 2A is thick (~ 450 + m). There are step-like transitions in magma lens depth and layer 2A thickness with changes in morphology along the SEIR. The transitions between the different modes of axial morphology and shallow structure are abrupt, suggesting a threshold-type mechanism. Variations in crustal structure along the SEIR appear to be steady state, persisting for at least 1 m.y. Portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank, shallower ridge flank depths, and at the location of along-axis Mantle Bouguer Anomaly (MBA) lows. The long-wavelength variation in ridge morphology along the SEIR from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature. Superimposed on the long-wavelength trend are segment to segment variations that are related to the absolute motion of the SEIR to the northeast which influence mantle melt production and delivery to the ridge

Dry Juan de Fuca slab revealed by quantification of water entering Cascadia subduction zone

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Nature Geoscience 10 (2017): 864-870, doi:10.1038/ngeo3050.Water is carried by subducting slabs as a pore fluid and in structurally bound minerals, yet no comprehensive quantification of water content and how it is stored and distributed at depth within incoming plates exists for any segment of the global subduction system. Here we use seismic data to quantify the amount of pore and structurally bound water in the Juan de Fuca plate entering the Cascadia subduction zone. Specifically, we analyse these water reservoirs in the sediments, crust and lithospheric mantle, and their variations along the central Cascadia margin. We find that the Juan de Fuca lower crust and mantle are drier than at any other subducting plate, with most of the water stored in the sediments and upper crust. Variable but limited bend faulting along the margin limits slab access to water, and a warm thermal structure resulting from a thick sediment cover and young plate age prevents significant serpentinization of the mantle. The dryness of the lower crust and mantle indicates that fluids that facilitate episodic tremor and slip must be sourced from the subducted upper crust, and that decompression rather than hydrous melting must dominate arc magmatism in central Cascadia. Additionally, dry subducted lower crust and mantle can explain the low levels of intermediate-depth seismicity in the Juan de Fuca slab.This research was funded by the US NSF

Two-Dimensional Magnetic Resonance Tomographic Microscopy using Ferromagnetic Probes

We introduce the concept of computerized tomographic microscopy in magnetic resonance imaging using the magnetic fields and field gradients from a ferromagnetic probe. We investigate a configuration where a two-dimensional sample is under the influence of a large static polarizing field, a small perpendicular radio-frequency field, and a magnetic field from a ferromagnetic sphere. We demonstrate that, despite the non-uniform and non-linear nature of the fields from a microscopic magnetic sphere, the concepts of computerized tomography can be applied to obtain proper image reconstruction from the original spectral data by sequentially varying the relative sample-sphere angular orientation. The analysis shows that the recent proposal for atomic resolution magnetic resonance imaging of discrete periodic crystal lattice planes using ferromagnetic probes can also be extended to two-dimensional imaging of non-crystalline samples with resolution ranging from micrometer to Angstrom scales.Comment: 9 pages, 11 figure

Analysis of causes of degradation of parent material and weld metal of breeches pipe at hydro power plant 'Perućica'

Presented are results of non-destructive tests performed on the parent material and welded joints of structural parts of a breeches pipe (collar and anchor), and results of destructive tests performed on the parent material of the anchor. Non-destructive testing included visual (VT), mag-netic particle (MT), and ultrasonic testing (UT), while destructive tests included determination of chemical com-position, tensile properties, impact energy and hardness. Based on test results it is established that the major cause for the occurrence of damage in the carrying structure of the breeches pipe no.1 of pipeline III, or in other words, the degradation of anchor parent material and welded joints between the collar and anchor, and between the pipeline and anchor, is in the fact that the breeches pipe started to lean, not on the collar, as specified by design, but on the anchor that could not endure all loads occurring in service