Sheet Thinning Research Articles

The glacial geomorphology of the Mackenzie Mountains region, Canada

ABSTRACT During the Last Glacial Maximum, the Mackenzie Mountains region was glaciated by three distinct ice sources; the Laurentide Ice Sheet, the Cordilleran Ice Sheet, and independent montane glaciers. Rapid ice sheet thinning of the Laurentide-Cordilleran ice saddle in the south of this region contributed to rapid sea level rise events and influenced the style of deglaciation to the north. The current understanding of the glacial history of the broader region has been established through mapping from aerial imagery and early surveys between the early 1970s to the 2010s. The central portions of the Mackenzie Mountains have not yet been mapped. We present a new glacial geomorphological map for the Mackenzie Mountains region covering over 220,000 km2. This updated geomorphological map will form the basis of future work to reconstruct the former maximum ice extents, flow dynamics, and retreat pattern.

MEASUREMENTS OF SINUOUS WAVES ON MECHANICALLY FORCED RADIALLY EXPANDING FREE PLANAR LIQUID SHEETS USING CHROMATIC CONFOCAL MEASUREMENTS

The chromatic confocal measurement technique is used to investigate the dynamics of radially expanding free liquid sheets generated by liquid jet impingement onto the flat end of a harmonically excited cylinder. Measurements of the sinuous-mode capillary waves are compared to experimental results reported by Bremond et al. (2007) and analytical solutions from two inviscid linear theories: The "aerodynamic (wave-growth)" theory by Bremond et al. (2007), which considers the interaction between the liquid film and the surrounding gas phase, and the linear theory by Tirumkudulu and Paramati (2013) which neglects the gas phase surrounding the liquid sheet but predicts growth of forced sinuous disturbances due to higher-order inertia terms associated with the thinning of the radially expanding sheet (“thinning” theory). For liquid Weber numbers Wed based on impinging jet velocity and jet diameter which are larger than 800 and aerodynamically stable forcing conditions, agreement with "thinning" theory is found in terms of radial positions of nodes and anti-nodes of the sheet disturbance envelope as well as growth of envelope maxima and minima. However, absolute amplitude values are significantly overpredicted in this case. For all other investigated operating conditions where instability due to film thinning is expected to dominate film dynamics (according to "thinning theory"), no such instability was observed experimentally. For most investigated cases, "aerodynamic wave-growth" theory agreed better with the experimentally observed film behavior. The importance of the naturally most-amplified aerodynamic mode was considered but could not be quantified as part of the present investigation.

Applying Magnetic Curvature to MMS Data to Identify Thin Current Sheets Relative to Tail Reconnection.

Thin current sheets (TCSs) have been postulated to be a necessary precondition for reconnection onset. Magnetic reconnection X-lines in the magnetotail have been observed to be more common duskward of midnight. We take advantage of the MMS tetrahedral formation during the 2017-2020 MMS tail seasons to calculate the thickness of the cross-tail neutral sheet relative to ion gyroradius. While a similar technique was applied to Cluster data, current sheet thickness over a broader range of radial distances has not been robustly explored before this study. We compare our analysis to recent theories regarding mechanisms of tail current sheet thinning and to recent simulations. We find MMS spent more than twice as long in ion-scale TCSs in the pre-midnight sector than post-midnight, despite nearly even plasma sheet dwell time. The dawn-dusk asymmetry in the distribution of Ion Diffusion Regions, as previously reported in relation to regions of TCSs, is also analyzed.

The last Fennoscandian Ice Sheet glaciation on the Kola Peninsula and Russian Lapland (Part 1): Ice flow configuration

A detailed reconstruction of palaeo-ice flow configuration is lacking for the Kola Peninsula and Russian Lapland, northwest Arctic Russia. This study presents, for the first time, a 14-stage reconstruction of the last Fennoscandian Ice Sheet (FIS) flow configuration on the Kola Peninsula and Russian Lapland from build-up to complete deglaciation. Flowsets (n = 102) and cross-cutting bedform assemblages identified from a high-resolution subglacial bedform record (subglacial lineations and subglacial ribs) are combined with subglacially streamlined bedrock data to define ice flow patterns, ice divides, ice margins, and glaciation styles of the last glaciation through relative time.The results demonstrate that the FIS flow configuration was not static. The FIS advanced eastwards across the region from Scandinavia, rather than expanding from local upland areas, and established a predominantly cold-based ice mass on the Kola Peninsula with an extensive adjacent warm-based ice lobe in the White Sea. An east-west aligned ice divide was located on the Kola Peninsula and Russian Lapland during the ice sheet build-up stages. However, this divide was short-lived as the White Sea lobe dominated ice flow on the peninsula during the local-Last Glacial Maximum, and ice predominantly flowed across the region from the main ice dispersal zone that was centred over the Gulf of Bothnia during deglaciation. Deglaciation on the peninsula was initially characterised by cold-based ice sheet retreat on the central eastern Kola Peninsula and warm-based ice lobe retreat in the White Sea. Continued deglaciation was characterised by ice sheet thinning, exposing mountain summits as nunataks behind the ice sheet margin, with topography constraining ice flow around upland areas. Ice streams in the region were drivers and consequences of continued ice sheet configuration change throughout glaciation. Four palaeo ice streams – (i) the Imandra Ice Stream; (ii) the Lovozero Ice Stream; (iii) the Kanozero Ice Stream; and (iv) the Kuusamo Ice Stream – and three possible palaeo-ice stream pathways are identified.We consider our ice flow configuration reconstruction to be the simplest palaeo-glaciological interpretation of the bedform record of the Kola Peninsula and Russian Lapland. The empirically-based reconstruction of ice flow geometry provides a regional framework and context for interpreting results from local-scale fieldwork, and is presented in a format that can be utilised by numerical ice sheet modellers to test and verify their models. The results underpin a new data-driven reconstruction of the last FIS in northwest Arctic Russia that we present in Part 2 of this study.

Current Sheet Thinning in the Wake of a Bubble Injection

AbstractA crucial property of the substorm growth phase is the current sheet thinning, which is often attributed to adiabatic convection. Injecting low‐entropy bubbles reduce pressure‐balance inconsistencies and restore current sheet thickness to its initial value. Recent observations from Cluster and MMS showed additional thinning of the current sheet in the wake of a transitory bubble injection compared with the configuration before the injection. We employ the Rice Convection Model‐MHD coupled code to investigate how the transport of bubbles causes the fast thinning. The simulation results reconstructed the observations' most prominent characteristics. We find more earthward transport of the magnetic flux in the bubble than its wake, therefore depleting magnetic flux and stretching the magnetic field lines there. Interestingly, additional R0‐sense field‐aligned currents close the enhanced dawn‐to‐dusk current behind the bubble.

Rapid southeastern Laurentide Ice Sheet thinning during the last deglaciation revealed by elevation profiles of in situ cosmogenic 10Be

Accurate reconstruction of Laurentide Ice Sheet volume changes following the Last Glacial Maximum is critical for understanding ice sheet contribution to sea-level rise, the resulting influence of meltwater on oceanic circulation, and the spatial and temporal patterns of deglaciation. Here, we provide empirical constraints on Laurentide Ice Sheet thinning during the last deglaciation by measuring in situ cosmogenic 10Be in 81 samples collected along vertical transects of nine mountains in the northeastern United States. In conjunction with 107 exposure age samples over five vertical transects from previous studies, we reconstruct ice sheet thinning history. At peripheral sites (within 200 km of the terminal moraine), we find evidence for ∼600 m of thinning between 19.5 ka and 17.5 ka, which is coincident with the slow initial margin retreat indicated by varve records. At locations >400 km north of the terminal moraine, exposure ages above and below 1200 m a.s.l. exhibit different patterns. Ages above this elevation are variable and older, while lower elevation ages are indistinguishable over 800−1000 m elevation ranges, a pattern that suggests a subglacial thermal boundary at ∼1200 m a.s.l. separating erosive, warm-based ice below and polythermal, minimally erosive ice above. Low-elevation ages from up-ice mountains are between 15 ka and 13 ka, which suggests rapid thinning of ∼1000 m coincident with Bølling-Allerød warming. These rates of rapid paleo-ice thinning are comparable to those of other vertical exposure age transects around the world and may have been faster than modern basin-wide thinning rates in Antarctica and Greenland, which suggests that the southeastern Laurentide Ice Sheet was highly sensitive to a warming climate.

New 10Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica

Abstract. Evidence for the timing and pace of past grounding line retreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE) of Antarctica provides constraints for models that are used to predict the future trajectory of the West Antarctic Ice Sheet (WAIS). Existing cosmogenic nuclide surface exposure ages suggest that Pope Glacier, a former tributary of Thwaites Glacier, experienced rapid thinning in the early to mid-Holocene. There are relatively few exposure ages from the lower ice-free sections of Mt. Murphy (<300 m a.s.l.; metres above sea level) that are uncomplicated by either nuclide inheritance or scatter due to localised topographic complexities; this makes the trajectory for the latter stages of deglaciation uncertain. This paper presents 12 new 10Be exposure ages from erratic cobbles collected from the western flank of Mt. Murphy, within 160 m of the modern ice surface and 1 km from the present grounding line. The ages comprise two tightly clustered populations with mean deglaciation ages of 7.1 ± 0.1 and 6.4 ± 0.1 ka (1 SE). Linear regression analysis applied to the age–elevation array of all available exposure ages from Mt. Murphy indicates that the median rate of thinning of Pope Glacier was 0.27 m yr−1 between 8.1–6.3 ka, occurring 1.5 times faster than previously thought. Furthermore, this analysis better constrains the uncertainty (95 % confidence interval) in the timing of deglaciation at the base of the Mt. Murphy vertical profile (∼ 80 m above the modern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 to 6.3 ± 0.4 ka). Taken together, the results presented here suggest that early- to mid-Holocene thinning of Pope Glacier occurred over a shorter interval than previously assumed and permit a longer duration over which subsequent late Holocene re-thickening could have occurred.

Regional sea-level highstand triggered Holocene ice sheet thinning across coastal Dronning Maud Land, East Antarctica

The East Antarctic Ice Sheet stores a vast amount of freshwater, which makes it the single largest potential contributor to future sea-level rise. However, the lack of well-constrained geological records of past ice sheet changes impedes model validation, hampers mass balance estimates, and inhibits examination of ice loss mechanisms. Here we identify rapid ice-sheet thinning in coastal Dronning Maud Land from Early to Middle Holocene (9000–5000 years ago) using a deglacial chronology based on in situ cosmogenic nuclide surface exposure dates from central Dronning Maud Land, in concert with numerical simulations of regional and continental ice-sheet evolution. Regional sea-level changes reproduced from our refined ice-load history show a highstand at 9000–8000 years ago. We propose that sea-level rise and a concomitant influx of warmer Circumpolar Deep Water triggered ice shelf breakup via the marine ice sheet instability mechanism, which led to rapid thinning of upstream coastal ice sheet sectors.

Pin Extrusion for Mechanical Joining from Orbital Formed Tailored Blanks with Local Material Pre-Distribution

Pin extrusion is a common process to realise pin structures in different geometrical dimensions for a subsequent joining operation. Nevertheless, the process of pin extrusion offers process limits regarding sheet thinning as a consequence of the punch penetration depth into the sheet. Thereby, cracks at the residual sheet thickness can occur during strength tests, resulting in a failure of the complete joint due to severe thinning. Therefore, measures have to be taken into account to reduce the thinning. One possibility is the application of orbital formed tailored blanks with a local material pre-distribution, which allows a higher sheet thickness in the desired area. Within this contribution, the novel approach of a process combination of orbital forming and pin extrusion is investigated. To reveal the potential of a local material pre-distribution, conventional specimens are compared with previously orbital formed components. Relevant parameters such as the residual sheet thickness, the pin height as well as the average hardness values are compared. The results show a significant positive influence of a local material pre-distribution on the residual sheet thickness as well as the resulting pin height. Furthermore, the strain hardening during orbital forming can be seen as an additional advantage. To conclude the results, the process limits of conventional pin extrusion can be expanded significantly by the application of specimens with a local material pre-distribution.

Thinning of the Magnetotail Current Sheet Inferred From Low‐Altitude Observations of Energetic Electrons

AbstractThe magnetospheric substorm is a key mode of flux and energy transport throughout the magnetosphere associated with distinct and repeatable magnetotail dynamical processes and plasma injections. The substorm growth phase is characterized by current sheet thinning and magnetic field reconfiguration around the equatorial plane. The global characteristics of current sheet thinning are important for understanding of magnetotail state right before the onset of magnetic reconnection and of the key substorm expansion phase. In this paper, we investigate this thinning at different radial distances using plasma sheet (PS) energetic (>50 keV) electrons that reach from the equator to low altitudes during their fast (∼1 s) travel along magnetic field lines. We perform a multi‐case study and a statistical analysis of 34 events with near‐equatorial observations of the current sheet thinning by equatorial missions and concurrent, latitudinal crossings of the ionospheric projection of the magnetotail by the low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats at approximately the same local time sector. Energetic electron fluxes thus collected by ELFIN provide near‐instantaneous (<5 min duration) radial snapshots of magnetotail fluxes. Main findings of this study confirm the previously proposed concepts with low‐altitude energetic electron measurements: (a) Energy distributions of low‐altitude fluxes are quantitatively close to the near‐equatorial distributions, which justifies the investigation of the magnetotail current sheet reconfiguration using low‐altitude measurements. (b) The magnetic field reconfiguration during the current sheet thinning (which lasts ≥ an hour) results in a rapid shrinking of the low‐altitude projection of the entire PS (from near‐Earth, ∼10RE, to the lunar orbit ∼60RE) to 1–2° of magnetic latitude in the ionosphere. (c) The current sheet dipolarization, common during the substorm onset, is associated with a very quick (∼10 min) change of the tail magnetic field configuration to its dipolar state, as implied by a poleward expansion of the PSPS at low altitudes.

Spontaneous rupture of surfactant-covered thin liquid sheets

The rupture of thin free films of Newtonian fluids are analyzed theoretically and computationally when the two free surfaces are covered with insoluble surfactant and surface rheological (viscous) effects are important. For highly viscous fluids in the Stokes limit, we show that the dynamics near the rupture singularity exhibits self-similarity of the second kind and the dominant balance involves van der Waals pressure and bulk and surface viscous stresses. For moderately viscous fluids inertia is also important and self-similarity is of the first kind. Closed-form expressions for the sheet's thinning rate are found and can be used to determine values of surface viscosity from experiments.

Tailoring Titanium Sheet Metal Using Laser Metal Deposition to Improve Room Temperature Single-Point Incremental Forming.

Typically, due to their limited formability, elevated temperatures are required in order to achieve complex shapes in titanium alloys. However, there are opportunities for forming such alloys at room temperature using incremental forming processes such as single-point incremental forming (SPIF). SPIF is an innovative metal forming technology which uses a single tool to form sheet parts in place of dedicated dies. SPIFs ability to increase the forming limits of difficult-to-form materials offers an alternative to high temperature processing of titanium. However, sheet thinning during SPIF may encourage the early onset of fracture, compromising in-service performance. An additive step prior to SPIF has been examined to tailor the initial sheet thickness to achieve a homogeneous thickness distribution in the final part. In the present research, laser metal deposition (LMD) was used to locally thicken a commercially pure titanium grade 2 (CP-Ti50A) sheet. Tensile testing was used to examine the mechanical behaviour of the tailored material. In addition, in-situ digital image correlation was used to measure the strain distribution across the surface of the tailored material. The work found that following deposition, isotropic mechanical properties were obtained within the sheet plane in contrast to the anisotropic properties of the as-received material and build height appeared to have little influence on strength. Microstructural analysis showed a change to the material in response to the LMD added thickness, with a heat affected zone (HAZ) at the interface between the added LMD layer and non-transformed substrate material. Grain growth and intragranular misorientation in the added LMD material was observed. SPIF of a LMD tailored preform resulted in improved thickness homogeneity across the formed part, with the downside of early fracture in a high wall angle section of the sheet.

Current sheet thinning in the wake of a bubble injection

Current sheet thinning in the wake of a bubble injection

Investigation of Kinetic Ballooning Instability in 2D Harris Sheet Equilibrium With Finite Normal B z Field

Substorm onset is believed to be a fundamental process for magnetic and plasma energy transport in both magnetosphere and ionosphere. In this regard, the ballooning instability is considered to be one of the key trigger mechanism for the substorm onset scenario in the near-Earth plasma sheet. Here, the kinetic ballooning instability (KBI) in the near-Earth magnetotail is analyzed using the two-dimensional (2D) generalized Harris sheet and Voigt equilibrium models. The corresponding kinetic ballooning mode is unstable in the intermediate range of perpendicular wave number (ky) and the equatorial beta (βeq). We further note that the growth rate of the ballooning mode reduces significantly with the increase in electron-to-ion temperatures ratio (Te/Ti) and wave number ky. The kinetic ballooning mode is found to be most unstable in a thin current sheet region at the equatorial location xe ∼ (8.5 − 10)RE, where the ballooning drive term (βeq/LpRc) is dominant on stiffening effect due to minimum in normal magnetic field. Because of the stabilizing effect as caused by the field line stiffening and the strong field line stabilization, the ballooning mode is stable (marginally unstable) close (away) from the Earth. The magnitude of the KBI growth rates as a function of the equatorial beta βeq and location xe for a wide and thin current sheets using the Voigt equilibrium model are smaller than the Harris sheet counterpart, which are used for modeling the near-Earth plasma sheet configuration for the slow and fast substorm growth phases, respectively. The results suggest that the excitation of KBI through the local current sheet thinning may also be an effective trigger mechanism for the substorm onset in the near-Earth magnetotail. This article is protected by copyright. All rights reserved.

A novel approach to the conventional deep drawing process

In this study, the mechanical formability of Al6061-T6 sheet material using a pre-bulging process was experimentally investigated. Therefore, an innovative die entrance design was applied to conventional deep drawing dies. In addition to the formability of the Al6061-T6 material, the effects of the innovative design on the energy consumption and thickness distribution were also investigated. For the experimental studies, deep drawing dies capable of mechanically pre-bulging at 0°, 15°, and 30° angles were designed and experimental studies were carried out at a 10 mm/sec forming speed with constant blank holder force. It was observed that the Al6061-T6 material, which was drawn at the highest ratio of ß = 2.0 in a single operation with the conventional deep drawing method, can be deep drawn at a drawing ratio of ß = 2.2 in a single operation with this new method, and the forming force was reduced by 24.45%. It was found that the deep-drawn Al6061-T6 sample, which was drawn at the highest ß = 2.2 drawing ratio, exhibited a maximum sheet thinning of 15%, and it could be formed with a single die in a single operation without forming defects such as cracking and wrinkling.

Cordilleran Ice Sheet Stability During the Last Deglaciation

AbstractWe report 20 10Be exposure ages from glacial erratics and bedrock on the west coast of British Columbia, Canada, which add to existing chronologies of Cordilleran Ice Sheet retreat along ∼600 km of coastal North America. Collectively, these data show the western ice limit reached the present coast by 18–16 ka then retreat slowed for ∼4,000 years until 14–13 ka. We attribute initial retreat to destabilization and grounding line retreat resulting from rising sea level and/or ocean warming in the northern Pacific. Subsequent stability of the ice sheet at the present coastal margin was likely due to the transition from marine to terrestrial margins despite increasing temperatures that may have driven ice sheet thinning. Our findings demonstrate the importance of understanding both climatic and nonclimatic drivers of ice sheet change through time.

Numerical analysis of the effects of ultrasonic vibrations and elevated temperature in incremental sheet forming

Incremental sheet forming (ISF) is one of the novel processes practised for more than the last decade in the field of sheet metal forming. ISF, although recognised as a promising manufacturing process over conventional forming, does not give good results in terms of geometrical accuracy, sheet thinning, surface finish and the forming forces. To make the ISF process more efficient, strategies to enhance the forming limits and capabilities are constantly being developed. Two such adaptations to the ISF process are ultrasonic vibration-assisted incremental sheet forming (UVaISF) and elevated temperature incremental sheet forming (ET-ISF). In the present work, investigations have been carried out to compare ISF, UVaISF and ET-ISF on 1 mm thick AA1050 and Titanium Grade 2 sheets with the help of numerical simulations using Abaqus/CAE. Response parameters like stress, strain, thickness distribution, the final shape of the geometry, strain energy, energy dissipated during plastic deformation and the forming forces are evaluated to measure the effectiveness of the processes. It is observed that there is a considerable reduction in the forming forces and spring-back with the UVaISF and ET-ISF processes, in comparison to the ISF at room temperature. This considerable reduction in the forming forces and spring-back is due to the softening behaviour of sheet material and the drop in yield strength during UVaISF and ET-ISF processes. The work overcomes some of the limitations of the ISF process and enlarges the scope of application of the process.

Improving the stamping formability of rolled titanium/iron composite sheets by annealing

Rolled sheets are not suitable for stamping due to their poor formability. In this research, the stamping formability of the rolled titanium/iron composite sheet was improved by annealing. The mechanism for improving the stamping formability was studied by analyzing the changes in the microstructure, texture, and interface. It was found that after annealing at 660 °C for 1 h, the stamping formability was significantly improved, namely plasticity was improved, anisotropy was decreased and the interface showed good bonding properties. Another important finding of this study was that the textural evolution of the metal layers also influenced the occurrence of interfacial delamination during stretching. The decrease in the angle of inclination of the normal grain in the titanium layer and the increase in the γ-fiber in the iron layer resulted in a decreasing trend in the thickness thinning of the composite sheet. Therefore, although a significant reduction in the interfacial peel strength occurred after annealing, no premature interfacial delamination occurred.

Configuration of Magnetotail Current Sheet Prior to Magnetic Reconnection Onset

AbstractThe magnetotail current sheet configuration determines magnetic reconnection properties that control the substorm onset, one of the most energetic phenomena in the Earth’s magnetosphere. The quiet‐time current sheet is often approximated as a two‐dimensional (2D) magnetic field configuration balanced by isotropic plasma pressure gradients. However, reconnection onset is preceded by the current sheet thinning and the formation of a nearly one‐dimensional (1D) magnetic field configuration. In this study, using particle‐in‐cell simulations, we investigate the force balance of such thin current sheets when they are driven by plasma inflow. We demonstrate that the magnetic field configuration transitions from 2D to 1D thanks to the formation of plasma pressure nongyrotropy and reveal its origin in the nongyrotropic terms of the ion distributions. We show that substorm onset may be controlled by the instability and dynamics of such nongyrotropic current sheets, having properties much different from the most commonly investigated 2D isotropic configuration.

An experimental investigation into single point incremental forming of glass fiber-reinforced polyamide sheet with different fiber orientations and volume fractions at elevated temperatures

Recent studies have proven the possibility of polymer sheet forming by incremental sheet forming (ISF) process, while there are limited studies on ISF of polymer matrix composites. Fiber-reinforced polymers (FRP) are significantly useful in industries due to their unique properties. In the current study, owing to the merits of ISF and the wide applicability of FRP, single point incremental forming (SPIF) of polyamide 6 (PA6) sheets reinforced by glass fibers (GF) is taken into account at various forming temperatures. In this regard, a fixture equipped with a ceramic infrared heating element was designed. The effect of some important parameters such as the volume fraction and orientation of fibers and the forming temperature on the formability, the thickness distribution, and the dimensional accuracy of PA6 and PA6/GF sheets is investigated. According to the results, an improvement in the formability of the composite sheet can be obtained using SPIF by increasing the forming temperature and decreasing the volume fraction of fibers. The formability of the composite sheet with the fiber orientation of [0/90] is far less than the one with unidirectional fibers. In the most of the successfully formed samples, the transverse compaction and the fracture of fibers at the part bottom, the buckling of fibers at the edge of the backing plate, and the draw-in of the sheet along the main direction of fibers can be detected. The thinning of the composite sheet is most affected by the volume fraction and orientation of fibers, while the forming temperature has the least effect. Moreover, the feasibility of SPIF of a circular flange with a wall angle of 90° from the composite sheet is assessed at elevated temperature.