Influence of Ni Interlayers on the Mechanical Properties of Ti6Al4V/(WC-Co) Friction Welds

Ni interlayers were introduced prior to dissimilar friction welding of Ti6Al4V base material to three cemented carbide substrates. The fracture strength of Ti6Al4V/(WC-6 wt% Co) welds were poor and were markedly improved when 20-µm thick Ni interlayers were introduced prior to dissimilar friction welding. These results were only produced when the (WC-6 wt% Co) cermet was electroplated prior to friction welding. When the Ti6Al4V alloy was electroplated prior to friction welding, fractured WC particles and cracking were observed in the (WC-Co) carbide substrate. The fracture strengths of Ti6Al4V/(WC-11 wt% Co) and Ti6Al4V/(WC-24 wt% Co) welds were not improved when 20-µm thick Ni interlayers were introduced prior to friction welding. During mechanical testing, the Ni layer retained at the dissimilar joint interface created a region of weakness.     

Characterizing 3D vegetation structure from space: Mission requirements

Human and natural forces are rapidly modifying the global distribution and structure of terrestrial ecosystems on which all of life depends, altering the global carbon cycle, affecting our climate now and for the foreseeable future, causing steep reductions in species diversity, and endangering Earth's sustainability.To understand changes and trends in terrestrial ecosystems and their functioning as carbon sources and sinks, and to characterize the impact of their changes on climate, habitat and biodiversity, new space assets are urgently needed to produce high spatial resolution global maps of the three-dimensional (3D) structure of vegetation, its biomass above ground, the carbon stored within and the implications for atmospheric green house gas concentrations and climate. These needs were articulated in a 2007 National Research Council (NRC) report (NRC, 2007) recommending a new satellite mission, DESDynI, carrying an L-band Polarized Synthetic Aperture Radar (Pol-SAR) and a multi-beam lidar (Light RAnging And Detection) operating at 1064 nm. The objectives of this paper are to articulate the importance of these new, multi-year, 3D vegetation structure and biomass measurements, to briefly review the feasibility of radar and lidar remote sensing technology to meet these requirements, to define the data products and measurement requirements, and to consider implications of mission durations. The paper addresses these objectives by synthesizing research results and other input from a broad community of terrestrial ecology, carbon cycle, and remote sensing scientists and working groups. We conclude that:

(1)

Current global biomass and 3-D vegetation structure information is unsuitable for both science and management and policy. The only existing global datasets of biomass are approximations based on combining land cover type and representative carbon values, instead of measurements of actual biomass. Current measurement attempts based on radar and multispectral data have low explanatory power outside low biomass areas. There is no current capability for repeatable disturbance and regrowth estimates.

(2)

The science and policy needs for information on vegetation 3D structure can be successfully addressed by a mission capable of producing (i) a first global inventory of forest biomass with a spatial resolution 1 km or finer and unprecedented accuracy (ii) annual global disturbance maps at a spatial resolution of 1 ha with subsequent biomass accumulation rates at resolutions of 1 km or finer, and (iii) transects of vertical and horizontal forest structure with 30 m along-transect measurements globally at 25 m spatial resolution, essential for habitat characterization.

We also show from the literature that lidar profile samples together with wall-to-wall L-band quad-pol-SAR imagery and ecosystem dynamics models can work together to satisfy these vegetation 3D structure and biomass measurement requirements. Finally we argue that the technology readiness levels of combined pol-SAR and lidar instruments are adequate for space flight. Remaining to be worked out, are the particulars of a lidar/pol-SAR mission design that is feasible and at a minimum satisfies the information and measurement requirement articulated herein.     

Towards quantifying tropical tree species richness in tropical forests

This letter reports tests of whether the normalized difference vegetation index (NDVI) from Landsat Enhanced Thematic Mapper Plus (ETM+) and radar backscatter (C‐, L‐ and P‐bands) from Airborne Synthetic Aperture Radar (AIRSAR) imagery can be used to estimate tree species richness from 25 1‐ha plots within continuous lowland forest types (dry, moist, wet) in the Panama Canal Zone Watershed. Species richness for trees ranged from 49 to 98 species per ha. There was no evidence that density, basal area, above‐ground biomass or P‐backscatter was correlated with tree species richness. Mean and standard deviations (SDs) of the NDVI could explain 39% and 37% of the variance in tree species richness, respectively (41% when combined), while mean radar backscatter in the shorter wavelength C‐ and L‐bands could explain 44% and 33% of the variance in tree species richness, respectively (50% when combined).     

The Global Rain Forest Mapping project - A review

The Global Rain Forest Mapping (GRFM) project is an international endeavour led by the National Space Development Agency of Japan (NASDA), with the aim of producing spatially and temporally contiguous Synthetic Aperture Radar (SAR) data sets over the tropical belt on the Earth by use of the JERS-1 L-band SAR, through the generation of semi-continental, 100 m resolution, image mosaics. The GRFM project relies on extensive collaboration with the National Aeronautics and Space Administration (NASA), the Joint Research Centre of the European Commission (JRC) and the Japanese Ministry of International Trade and Industry (MITI) for data acquisition, processing, validation and product generation. A science programme is underway in parallel with product generation. This involves the agencies mentioned above, as well as a large number of international organizations, universities and individuals to perform field activities and data analysis at different levels. The GRFM project was initiated in 1995 and, through a dedicated data acquisition policy by NASDA, data acquisitions could be completed within a 1.5-year period, resulting in a spatially and temporally homogeneous coverage to encompass the entire Amazon Basin from the Atlantic to the Pacific; Central America up to the Yucatan Peninsular in Mexico; equatorial Africa from Madagascar and Kenya in the east to Sierra Leone in the west; and south-east Asia, including Papua New Guinea and northern Australia. Over the Amazon and Congo river basins, the project aimed to provide complete cover at two different seasons, featuring the basins at high and low water. In total, the GRFM acquisitions comprise some 13000 SAR scenes, which are currently in the course of being processed and compiled into image mosaics. In March 1999, SAR mosaics over the Amazon Basin (one out of two seasonal coverages) and equatorial Africa (both seasonal coverages) were completed; the data are available on CD-ROM and, at a coarser resolution, via the Internet. Coverage of the second-season Amazon and Central America will be completed during 1999, with the south-east Asian data sets following thereafter. All data are being provided free of charge to the international science community for research and educational purposes.     

Evaluation of Metal Dusting Behavior in Reformer Tube Made of HP–Nb Alloy on the Outer Surfaces

The metal dusting behavior of Iron–Chromium–Nickel heat resistant HP–Nb steel specimen was investigated at the outer surfaces while the methane gas was passed inside the hole of the specimen. After an exposure of 130 h in a flowing methane (CH₄) gas at 680 °C, different dispersed corrosion products were formed on the outer surface of the specimen near the hole. Conventional metallography and scanning electron microscopy were used to identify the microstructure of the reaction products. Energy dispersive X-ray spectroscopy was used for microchemical analysis. The phases produced on the surface were identified by X-ray diffraction. Some of reaction products found as surface deposits on the outer surfaces of specimen near the hole contained Fe and Cr carbides, Fe, Cr and Ni oxides, scale of Ni, Fe particles and free C. Results revealed that carbon nano-filaments materials could be formed during disintegration of heat resistant HP–Nb steel under metal dusting environment.     

Influence of Heat Input in Pulsed Current GTAW Process on Microstructure and Corrosion Resistance of Duplex Stainless Steel Welds

 The high corrosion resistance of duplex stainless steel (DSS) is due to elements such as Cr, Mo and N, but also depends on the microstructure. The best general properties are obtained with approximately equal amounts of austenite and ferrite and the absence of third phases such as σ (sigma) and Cr2N. In the present work the effect of heat input variations on the microstructure and corrosion resistance of a DSS UNS S32760 in artificial sea water media were studied. The corrosion resistance in 3.5% of NaCl solution was evaluated by potentiostatic polarization tests at room temperature. It is found that the presence of sigma phase and Cr2N decreases the corrosion potential. The specimen with heat input of approximately 0.95 kJ/mm have the best corrosion characteristics, which is the result for the lack of deleterious phases such as sigma and Cr2N and balanced ferrite-austenite proportion.     

The effect of surface morphology on pitting corrosion resistance of Ni nanocrystalline coatings

In this study, pitting corrosion resistance of nano and microcrystalline Ni coatings was evaluated based on polarization diagrams and pit morphology observations. Two copper substrates with higher {400} and {111} peak intensity were used. In the case of direct current deposition, a mixed morphology consisting of pyramids and blocks appeared on both copper substrates, but a spherical morphology was detected in the case of pulse current deposition. Breakdown potential of the nanocrystalline coatings was higher than that of microcrystalline coating. In the case of nanocrystalline deposits, coating with higher grain size showed higher resistance to pitting corrosion, which is in contrary to the results reported by others. The morphology of corroded surface revealed that the pits were confined on the pyramids and therefore the blocky morphology was responsible for higher resistance to pitting corrosion.     

Analysis of scattering mechanisms in SAR imagery over borealforest: results from BOREAS '93

As part of the intensive field campaign (IFC) for the Boreal forest ecosystem-atmosphere research (BOREAS) project in August 1993, the NASA/JPL AIRSAR covered an area of about 100 km×100 km near the Prince Albert National Park in Saskatchewan, Canada. At the same time, ground-truth measurements were made in several stands which have been selected as the primary study sites. This paper focuses on an area including jack pine stands in the Nipawin area near the park. Upon examining the AIRSAR data from stands of old and young jack pine (OJP and YJP), distinct signatures are observed for each of the forest types at various frequencies and polarizations, in particular, at P-band HH. The authors use a forest scattering model in conjunction with the ground-truth measurements to explain such behavior. The forest model includes the major scattering mechanisms by taking the forest component interactions into account. The contribution from each of the scattering mechanisms to the total backscatter is calculated and their differences for OJP and YJP stands are evaluated. The results are used to discuss the effect of the physical properties of the forest components in each stand on radar backscatter. They are also used to show that it is not only the backscatter level but also the relative contribution from various scattering mechanisms that will help in quantitative interpretation of SAR data. This work is mainly intended as a precursor to the authors ongoing work which uses a mechanism-specific inversion technique to retrieve forest parameters from SAR data for these BOREAS sites     

Microwave backscattering and emission model for grass canopies

Microwave radar and radiometer measurements of grasslands indicate a substantial reduction in sensor sensitivity to soil moisture in the presence of a thatch layer. When this layer is wet it masks changes in the underlying soil, making the canopy appear warm in the case of passive sensors (radiometer) and decreasing backscatter in the active case (scatterometer). A model for a grass canopy with thatch is presented in order to explain this behavior and for comparison with observations. The canopy model consists of three layers: grass, thatch, and the underlying soil. The grass blades are modeled by elongated elliptical discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneous and flat. The distorted Born approximation is used to compute the radar cross section of this three layer canopy and the emissivity is computed from the radar cross section using the Peake formulation for the passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moisture, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-band radiometer measurements at these grasslands     

Rock fraction effects on the interpretation of microwave emissionfrom soils

The effects of the rock fraction were investigated through a combination of laboratory dielectric measurements and field observation of emissivity. A series of field measurements were conducted which included soils with (35% by volume) and without rocks. Analysis focused on the use of a 21-cm wavelength, although some field observations at 6 cm were also made. For the rock samples, the average values of the dielectric constant were 4.7 and 0.07 for the real and imaginary parts, respectively. The effects of rock fraction are not significant in estimating the sample soil moisture when 21-cm data are used, for the rock fraction examined. Data collected at 6 cm clearly showed that the presence of rocks will make this and shorter wavelengths useless as soil moisture sensors