Early-stage interaction between settlement and temperature-gradient metamorphism

ABSTRACTSnow metamorphism and settlement change the microstructure of a snowpack simultaneously. Past experiments investigated snow deformation under isothermal conditions. In nature, temperature gradient metamorphism and settlement often occur together. We investigated snow settlement in the first days after the onset of temperature-gradient metamorphism in laboratory experiments by means of in-situ time-lapse micro-computed tomography. We imposed temperature gradients of up to 95 K m−1 on samples of rounded snow with a density of ~230 kg m−3 and induced settlement by applying 1.7 kPa stress with a passive load on the samples simultaneously. We found that snow settled about half as fast when a temperature gradient was present, compared with isothermal conditions. The change in specific surface area after 4 days caused by temperature-gradient metamorphism was only a few percent. The viscosity evolution correlated with the amount of the temperature gradient. Finite element simulations of the snow samples revealed that stress-bearing chains had developed in the snow structure, causing the large increase in viscosity. We could show that a small change in microstructure caused a large change in the mechanical properties. This explains the difficulty of predicting snow mechanical properties in applications such as firn compaction or snow avalanche formation.

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Orientation selective grain sublimation-deposition in snow under temperature gradient metamorphism observed with Diffraction Contrast Tomography

10.5194/tc-2021-22 ◽

2021 ◽

Author(s):

Rémi Granger ◽

Frédéric Flin ◽

Wolfgang Ludwig ◽

Ismail Hammad ◽

Christian Geindreau

Keyword(s):

Temperature Gradient ◽

Horizontal Plane ◽

Time Lapse ◽

Snow Sample ◽

Crystalline Orientation ◽

Diffraction Contrast ◽

Crystallographic Facets ◽

Over Time ◽

Material Fluxes

Abstract. In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favorable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in-situ time-lapse Diffraction Contrast Tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 °C m−1 at mean temperatures in the range between −4.1 °C and −2.1 °C for three days. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution and from the analysis of the material fluxes, we indeed observe higher sublimation-deposition rate for grains with their c-axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. That latter observation is explained in terms of geometrical interactions between grains.

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Time-series within automatically generated ROIs from wildlife cameras are well able to explain variability in forest phenology on a temperature gradient

10.5194/egusphere-egu2020-16006 ◽

2020 ◽

Author(s):

Lars Uphus ◽

Annette Menzel

Keyword(s):

Time Series ◽

Temperature Gradient ◽

Time Lapse ◽

Relevant Information ◽

High Temporal Resolution ◽

Deciduous Species ◽

Multiple Thresholds ◽

In Situ Observations ◽

Wildlife Cameras

Using RGB camera data (e.g. webcams, wildlife cameras) has great potential to measure forest phenology over climate gradients, because of its very high temporal resolution, while at the same time being more objective and less time consuming than in situ observations. To make images useful for the purpose of measuring phenological events, such as Start of Season (SOS) and End of Season (EOS), there is need to derive Regions of Interest (ROI) objectively and (semi-)automatically. In order to answer this need, Bothmann et al. (2017) proposed a method which randomly sets a number of pinpricks in the image and calculates how greenness over time from all other pixels correlates to these different pinpricks. Subsequently, ROIs are created by discarding the pixels with low correlation, using multiple thresholds.&#160;Despite its advantage of being automated and more objective compared to prevailing expert-based ROIs, and therefore its potential applicability for phenological research using a large amount of cameras, the method has not been reproduced for this purpose so far. Therefore, we assess here how well this method is able to separate foliage of different deciduous species from evergreens and phenologically irrelevant components in time-lapse wildlife camera data and in that way how suitable it is in explaining variation in phenology over a temperature gradient. We used 73 Cuddleback wildlife cameras troughout Bavaria which were installed within nine quadrants of 6*6 kilometers spanning a temperature gradient of 2.5&#176;C. Hourly taken images of deciduous forests in spring, summer and autumn 2019 were analysed. Half of them were facing canopy, and half of them were facing understory. We applied the principles of the method from Bothmann et al. (2017) and assigned the best matching ROI to foliage of Fagus sylvatica or other deciduous species. Within this ROI, mean Green Chromatic Coordinate (GCC), a greenness index, over all pixels within the ROI, was derived per time-stamp. Afterwards, a time-series was calculated on these GCC values and with a suitable combination of curve-fitting techniques, SOS and EOS were derived, expressed in Day of Year (DOY). We compared these SOS and EOS dates with weekly in situ observations of spring and autumn phenology, which were taken in the same quadrants. Despite that Bothmann's method was developed on a single tower-mounted scientific webcam which viewed on canopy from above, while we made use of wildlife cameras at 73 different locations facing either understory perpendicular or canopy from below, it was able to distinguish F. sylvatica and other deciduous foliage from phenologically less relevant information. Time-series derived from these ROIs were able to explain variability in phenology between understory and canopy and over the temperature gradient similarly and supplementary to in situ observations.&#160;

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Snowbreeder 5: a Micro-CT device for measuring the snow-microstructure evolution under the simultaneous influence of a temperature gradient and compaction

Journal of Glaciology ◽

10.1017/jog.2016.143 ◽

2017 ◽

Vol 63 (238) ◽

pp. 355-360 ◽

Cited By ~ 5

Author(s):

MAREIKE WIESE ◽

MARTIN SCHNEEBELI

Keyword(s):

Temperature Gradient ◽

Constant Temperature ◽

Snow Sample ◽

Micro Computed Tomography ◽

Peltier Element ◽

Constant Temperature Gradient ◽

New Device ◽

Isothermal Conditions ◽

Snow Metamorphism ◽

Simultaneous Influence

ABSTRACTThe instrumented sample holder Snowbreeder 5 is used to investigate the simultaneous influence of settlement on temperature-gradient snow metamorphism in time-lapse micro-computed tomography experiments. So far, experiments have only been done on temperature-gradient snow metamorphism without settlement or settlement under isothermal conditions. With the new device we can impose a constant temperature gradient on a snow sample and induce settlement by placing a passive load on top of the snow sample. The weight of the load can be varied, simulating various snow heights on top of the snow sample. Snow-temperature measurements on the passive load are possible due to wireless data transfer via Bluetooth. The temperature gradient is set by controlling the air temperature inside the computer tomograph and by a Peltier element at the bottom of the snow sample. First experiments under isothermal conditions and a constant temperature gradient of 43 K m−1 showed that the settlement was reduced to almost half as soon as a temperature gradient was applied under otherwise almost equal snow conditions. The compactive viscosity in the isothermal experiment was in the range of literature values.

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Three-point bending study on the microscopic fracture behavior of pre-oxidized Cr-coated Zr-4 alloys

Journal of Physics Conference Series ◽

10.1088/1742-6596/2076/1/012051 ◽

2021 ◽

Vol 2076 (1) ◽

pp. 012051

Author(s):

Mingyue Du ◽

Chenxue Wang ◽

Jishen Jiang ◽

Xianfeng Ma

Keyword(s):

Mechanical Properties ◽

Plastic Deformation ◽

Fracture Behavior ◽

Early Stage ◽

Bending Test ◽

Cracking Behavior ◽

Three Point Bending ◽

Crack Penetration ◽

Cr Coating

Abstract In this study, an in situ three-point bending test was carried out to study the mechanical properties and cracking behavior of the Cr-coated Zr-4 alloy considering the effect of pre-oxidation. The results showed that high temperature pre-oxidation led to the formation of intermetallic ZrCr2 at the coating/substrate interface and an α-Zr(O) layer beneath the interface. During the three-point bending test, the Cr coating and Zr-4 substrate showed good plastic deformation. However, the brittle intermetallic ZrCr2 diffusion layer exhibited cracks in the early stage, which accelerated the crack penetration to the Cr coating and the Zr-4 substrate, leading to the pre-failure of the pre-oxidized sample.

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Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography

The Cryosphere Discussions ◽

10.5194/tcd-6-1673-2012 ◽

2012 ◽

Vol 6 (3) ◽

pp. 1673-1714 ◽

Cited By ~ 3

Author(s):

B. R. Pinzer ◽

M. Schneebeli ◽

T. U. Kaempfer

Keyword(s):

Temperature Gradient ◽

Physical Properties ◽

Time Lapse ◽

Steady Temperature ◽

Data Set ◽

External Temperature ◽

Vapor Flux ◽

Snow Metamorphism ◽

Micro Tomography

Abstract. Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during metamorphism under a steady temperature gradient (STGM) of 50 K m−1, using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, and in addition the exact locations of these phase changes. From the four-dimensional data set, we calculated the average time that an ice volume stayed in place before it sublimated, and found a characteristic residence time of 2–3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snow pack, where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60 % of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of STGM that is produced by directly observing the microstructure of snow in situ sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow.

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Orientation selective grain sublimation–deposition in snow under temperature gradient metamorphism observed with diffraction contrast tomography

The Cryosphere ◽

10.5194/tc-15-4381-2021 ◽

2021 ◽

Vol 15 (9) ◽

pp. 4381-4398

Author(s):

Rémi Granger ◽

Frédéric Flin ◽

Wolfgang Ludwig ◽

Ismail Hammad ◽

Christian Geindreau

Keyword(s):

Temperature Gradient ◽

Horizontal Plane ◽

Time Lapse ◽

Deposition Rates ◽

Crystalline Orientation ◽

Diffraction Contrast ◽

Crystallographic Facets ◽

Over Time ◽

Material Fluxes

Abstract. In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favourable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in situ time-lapse diffraction contrast tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 ∘Cm-1 at mean temperatures in the range between −4.1 and −2.1 ∘C for 3 d. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution, and from the analysis of the material fluxes, we observe higher sublimation–deposition rates for grains with their c axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. The latter observation is explained in terms of geometrical interactions between grains.

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Dynamics of Nanoparticle Chain Aggregates of Carbon Under Tension

MRS Proceedings ◽

10.1557/proc-778-u4.4 ◽

2003 ◽

Vol 778 ◽

Author(s):

Rajdip Bandyopadhyaya ◽

Weizhi Rong ◽

Yong J. Suh ◽

Sheldon K. Friedlander

Keyword(s):

Mechanical Properties ◽

Carbon Black ◽

Polymer Film ◽

Elastic Behavior ◽

Small Strains ◽

Transmission Electron ◽

Chain Aggregates ◽

Nanoparticle Chains ◽

Reinforcing Filler

AbstractCarbon black in the form of nanoparticle chains is used as a reinforcing filler in elastomers. However, the dynamics of the filler particles under tension and their role in the improvement of the mechanical properties of rubber are not well understood. We have studied experimentally the dynamics of isolated nanoparticle chain aggregates (NCAs) of carbon made by laser ablation, and also that of carbon black embedded in a polymer film. In situ studies of stretching and contraction of such chains in the transmission electron microscope (TEM) were conducted under different maximum values of strain. Stretching causes initially folded NCA to reorganize into a straight, taut configuration. Further stretching leads to either plastic deformation and breakage (at 37.4% strain) or to a partial elastic behavior of the chain at small strains (e.g. 2.3% strain). For all cases the chains were very flexible under tension. Similar reorientation and stretching was observed for carbon black chains embedded in a polymer film. Such flexible and elastic nature of NCAs point towards a possible mechanism of reinforcement of rubber by carbon black fillers.

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