Brittle Fragmentation Research Articles

Unusual fluidal behavior of a silicic magma during fragmentation in a deep subaqueous eruption, Havre volcano, southwestern Pacific Ocean

Magma responds to applied stresses in either a viscous or elastic manner, depending on the time scales over which strain is accommodated. For silicic magmas, high strain rates of explosive volcanism cause brittle fragmentation and produce abundant small particles (ash). The A.D. 2012 Havre (Kermadec arc, southwestern Pacific Ocean) eruption at ∼900 m water depth deposited a unit of silicic ash with features indicative of syn- and/or post-fragmentation viscous deformation. Viscously deformed ash makes up 3%-35% of the two main ash subunits, S1 and S2, with the remaining ash formed by brittle fragmentation. Viscous behavior of melt during production of fine ash is unexpected for the silicic Havre magma, and for the high strain rates typical of fine fragmentation. The occurrence together of viscous and brittle ash grains suggests local and/or short-term variations in eruption conditions. We infer an explosive eruption mechanism modified by magma-water interaction, during which multi-source steam-veiled fragmentation sites permitted rapid viscous deformation of ash prior to contact with water.

Orogenic gold formation in an evolving, decompressing hydrothermal system: Genesis of the Samut gold deposit, Eastern Desert, Egypt

Samut is a vein-type gold mineralization in the Eastern Desert of Egypt, which shares many geologic features with other orogenic deposits in this region. The deposit has a long history of mining activity dating back to ancient times, but so far has received little attention from the standpoint of geologic study. Here, we present new field, petrographic, microthermometric and microanalytical data from the Samut gold deposit to evaluate its genesis and fluid evolution. Vein quartz at Samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time. In some samples, brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation. This brittle style of deformation culminates with latest-stage brittle opening and loss of confining pressure, resulting in open-space filling textures.Backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins (Py I, Py II and Py III). Laser ablation ICPMS analysis of pyrites from mineralized quartz veins reveals that the earlier Py I is low in arsenic and gold, whereas Py II and Py III are enriched in both elements. Anhedral Py II is particularly gold-rich, and gives way to blocky, euhedral Py III that is rich in arsenic but not as rich in gold compared to Py II. Fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of ∼300 °C, early in the hydrothermal system. The calculated fluid pressures at homogenization of the aqueous-carbonic inclusions furnish a wide range throughout the paragenesis, perhaps on the order of ∼1 to 3 kbar. The δ34S values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of +0.6 to +4.9‰, overlapping with the broad range of δ34S values calculated for ore fluids in orogenic gold deposits worldwide. Together, these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn-deformational vein formation, and tending towards the late stages of vein growth prior to loss of confinement. Free-milling gold appears to be mostly related to the later stages of brittle fracturing and re-sealing of the veins during repeated fluid release, as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions.

Physical Origin of the Fine-Particle Problem in Blasting Fragmentation

Reducing the amount of fine particles (fines) in dynamic brittle fragmentation is important within the mining industry, to save energy and reduce environmental hazard. Blasting-round design to achieve this has been held back by a lack of understanding of the fundamental physics, and by the technical complexity of blasting-process measurements. The authors use numerical simulations and experimental data to study the physical origin of fines generated in civil-engineering blasts. Surprisingly, the fragments can be classified according to universal mechanisms. This insight has the potential to generate innovative engineering solutions that may save resources and protect the environment.

Fragmentation and Plinian eruption of crystallizing basaltic magma

Basalt is the most ubiquitous magma on Earth, erupting typically at intensities ranging from quiescently effusive to mildly explosive. The discovery of highly explosive Plinian eruptions of basaltic magma has therefore spurred debate about their cause. Silicic eruptions of similar style are a consequence of brittle fragmentation, as magma deformation becomes progressively more viscoelastic. Magma eventually crosses the glass transition and fragments due to a positive feedback between water exsolution, viscosity and decompression rate. In contrast to silicic eruptions, the viscosity of basaltic magmas is thought to be too low to reach conditions for brittle fragmentation. Pyroclasts from several basaltic Plinian eruptions, however, contain abundant micron-size crystals that can increase magma viscosity substantially. We therefore hypothesize that magma crystallization led to brittle fragmentation during these eruptions. Using combined oscillatory and extensional rheometry of concentrated particle-liquid suspensions that are dynamically similar to microcrystalline basaltic magma, we show that high volume fractions of particles and extension rates of about 1 s−1 or greater result in viscoelastic deformation and brittle fracture. We further show that for experimentally observed crystallization rate, basaltic magma can reach the empirical failure conditions when erupting at high discharge rates.

Classification of volcanic ash particles using a convolutional neural network and probability

Analyses of volcanic ash are typically performed either by qualitatively classifying ash particles by eye or by quantitatively parameterizing its shape and texture. While complex shapes can be classified through qualitative analyses, the results are subjective due to the difficulty of categorizing complex shapes into a single class. Although quantitative analyses are objective, selection of shape parameters is required. Here, we applied a convolutional neural network (CNN) for the classification of volcanic ash. First, we defined four basal particle shapes (blocky, vesicular, elongated, rounded) generated by different eruption mechanisms (e.g., brittle fragmentation), and then trained the CNN using particles composed of only one basal shape. The CNN could recognize the basal shapes with over 90% accuracy. Using the trained network, we classified ash particles composed of multiple basal shapes based on the output of the network, which can be interpreted as a mixing ratio of the four basal shapes. Clustering of samples by the averaged probabilities and the intensity is consistent with the eruption type. The mixing ratio output by the CNN can be used to quantitatively classify complex shapes in nature without categorizing forcibly and without the need for shape parameters, which may lead to a new taxonomy.

The pumice raft-forming 2012 Havre submarine eruption was effusive

A long-standing conceptual model for deep submarine eruptions is that high hydrostatic pressure hinders degassing and acceleration, and suppresses magma fragmentation. The 2012 submarine rhyolite eruption of Havre volcano in the Kermadec arc provided constraints on critical parameters to quantitatively test these concepts. This eruption produced a >1 km3 raft of floating pumice and a 0.1 km3 field of giant (>1 m) pumice clasts distributed down-current from the vent. We address the mechanism of creating these clasts using a model for magma ascent in a conduit. We use water ingestion experiments to address why some clasts float and others sink. We show that at the eruption depth of 900 m, the melt retained enough dissolved water, and hence had a low enough viscosity, that strain-rates were too low to cause brittle fragmentation in the conduit, despite mass discharge rates similar to Plinian eruptions on land. There was still, however, enough exsolved vapor at the vent depth to make the magma buoyant relative to seawater. Buoyant magma was thus extruded into the ocean where it rose, quenched, and fragmented to produce clasts up to several meters in diameter. We show that these large clasts would have floated to the sea surface within minutes, where air could enter pore space, and the fate of clasts is then controlled by the ability to trap gas within their pore space. We show that clasts from the raft retain enough gas to remain afloat whereas fragments from giant pumice collected from the seafloor ingest more water and sink. The pumice raft and the giant pumice seafloor deposit were thus produced during a clast-generating effusive submarine eruption, where fragmentation occurred above the vent, and the subsequent fate of clasts was controlled by their ability to ingest water.

Advancement of magma fragmentation by inhomogeneous bubble distribution

Decompression times reported in previous studies suggest that thoroughly brittle fragmentation is unlikely in actual explosive volcanic eruptions. What occurs in practice is brittle-like fragmentation, which is defined as the solid-like fracture of a material whose bulk rheological properties are close to those of a fluid. Through laboratory experiments and numerical simulation, the link between the inhomogeneous structure of bubbles and the development of cracks that may lead to brittle-like fragmentation was clearly demonstrated here. A rapid decompression test was conducted to simulate the fragmentation of a specimen whose pore morphology was revealed by X-ray microtomography. The dynamic response during decompression was observed by high-speed photography. Large variation was observed in the responses of the specimens even among specimens with equal bulk rheological properties. The stress fields of the specimens under decompression computed by finite element analysis shows that the presence of satellite bubbles beneath a large bubble induced the stress concentration. On the basis of the obtained results, a new mechanism for brittle-like fragmentation is proposed. In the proposed scenario, the second nucleation of bubbles near the fragmentation surface is an essential process for the advancement of fragmentation in an upward magma flow in a volcanic conduit.

Ιntermetallic β-Ti3Au hard thin films prepared by magnetron sputtering

In the current study, we report on a deposition method to synthesize the intermetallic β-Ti3Au thin films by magnetron sputtering. We examine thin films crystallinity and structure-property relationships in terms of deposition conditions. The study is motivated by the vast application potential of β-Ti3Au, known to possess high hardness, interesting scratch and wear behaviors, and very good biocompatibility. The Ti1−xAux thin films (x=0–1) were deposited onto the Si(100) substrates by triple target magnetron sputtering under an Ar atmosphere and temperature varying from room temperature and to 650°C. The samples were characterized by X-ray diffraction (XRD) under grazing incidence and Bragg-Brentano geometries, scanning and transmission electron microscopy (SEM, TEM) equipped with an energy dispersive spectroscopy detector (EDS), and depth-sensing nanoindentation to investigate the dependence of the phase configuration, microstructure and mechanical properties on the Au/Ti ratio and deposition temperature. It was shown that upon varying the Au content the coatings develop a variety of single-phases or mixed structures composed of cubic, hexagonal and/or tetragonal phases of the binary Au-Ti system. At x=0.25, the compound Ti3Au developed a quasi-amorphous or defective nanocrystalline structures when deposited at room or low-temperatures, while a fully crystalline β-Ti3Au with an A15-type structure and lattice parameter of a=0.50925nm was obtained when deposited at 350–400 °C. The β-phase films exhibited a hardness of 10.4–12.1GPa, which is among the upper limits for intermetallic compounds, while the Young's modulus (173–201GPa) is fairly comparable to 209GPa predicted by first principles theoretical calculations. Thin film crystallinity and hardness strongly influenced both the deformation mode and the wear behavior under nanoscratch testing. The wear mechanisms changed from ploughing with ductile displacement, to microcutting with the formation of curly debris, to cracking with brittle fragmentation.

Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes

The Tungurahua volcano (Northern Andean Volcanic Zone) has been erupting since 1999, with at least four eruptive phases up to present. Although a dozen of research focuses in tephra fall deposits during this period, none of them cover the full eruptive cycle. We investigated the eruptive mechanisms and tephra fall deposition processes at Tungurahua between 1999 and 2014, through systematic analyses of tephra samples collected westward of the volcano using mechanical sieving grain size analysis, lithology, scanning electron microscopy, X-Ray fluorescence and X-Ray diffraction. Tephra is compounded by varying amounts of scoria (black and brown), lithics, hydrothermally altered fragments, pumice, glass shards and free crystals. Textural analyses of juvenile grains (scoria, pumice and glass shards) revealed a diversity of features concerning to their vesicularity, shape and surface/perimeter. Initially, tephra was characterized by hydrothermally altered fragments related to a phreatic phase which then evolved to a pure magmatic activity with Strombolian eruptions. A homogeneous andesitic composition was observed between 1999 and 2003; however silicarich compositions occurred later in 2006. Similarly, the mineral assemblage contained plagioclase, pyroxene and olivine, but magnetite and akermanite were then included during 2006, thus indicating the eruption of a new, probably mixed magma. As consequence, Plinian activity occurred in August 2006. Further activity in 2007 ejected notable amounts (40-65%) of recycled material during Vulcanian eruptions. New eruptions occurred between 2008 and 2010, and juvenile ash revealed the interplay between brittle and ductile fragmentation through ash explosions, jetting events and Strombolian activity. The activity between 2010 and 2012 incorporated hydrothermally altered material at time that eruptive silences became longer and frequent, thus suggesting the development of a sporadic hydrothermal system. Finally, between 2013 and 2014 a series of Vulcanian events occurred. Observed grain size distributions allow us to propose three different processes occurring during tephra deposition: 1) deposition of multiple ash plumes, 2) contributions from elutriated pyroclastic density currents or grain size mixing due to major eruptions, and 3) the aggregation of particles due to rain and/or lighting. From mineralogy and grain size we infer that exposition to ash may produce acute human health effects.

Crystal-rich lava dome extrusion during vesiculation: An experimental study

Lava dome-forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide. The extrusion mechanics of crystal-rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear. Understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential. Here, we present new experimental results on the rheology of synthesised, crystal-rich (50 to 80vol% quartz crystals), hydrous (4.2wt% H2O in the glass) dacite samples, which vesiculate from 5 to 27vol% gas bubbles at high temperatures (from glass transition temperature to 797°C) during deformation conducted in a parallel plate viscometer (constant stress at 0.63–0.64MPa, and variable strain-rates ranging from 8.32·10−8 to 3.58·10−5s−1). The experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt, instigated in the experiments by increasing temperature. During gas exsolution (i.e. nucleation and growth of gas-pressurised bubbles) and volume inflation, we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity. At crystal contents <60vol%, gas bubbles form and coalesce during expansion and viscous deformation, favouring strain localisation and gas permeability within shear bands, which control the overall sample rheology. At crystallinities of 60 to 70vol%, gas exsolution generates pressurisation (i.e. pore pressure increase) within the bubbles trapped in the solid crystal clusters, and embryonic formation of microscopic fractures through melt and crystals drives the system to a brittle behaviour. At higher crystallinity (80vol%) vesiculation leads to large pressurisation, which then triggers extensive brittle fragmentation. Through macroscopic fractures, outgassing determines the rheological stalling of the system. In the light of these results we propose a rheological description of crystal-rich lava dome mechanics. The contrasting experimental behaviours at different crystallinities have implications for the style of slow-ascending dome-forming eruptions. All other factors being equal, our experiments suggest that crystal-poor magmas will undergo efficient outgassing, reducing the potential for an explosive eruption. Conversely, crystal-rich magmas may experience limited outgassing and larger gas overpressures during vesiculation, therefore increasing the potential for an explosive eruption.

An estimation model for the fragmentation properties of brittle rock block due to the impacts against an obstruction

Mountain hazards with large masses of rock blocks in motion–such as rock falls, avalanches and landslides–threaten human lives and structures. Dynamic fragmentation is a common phenomenon during the movement process of rock blocks in rock avalanche, due to the high velocity and impacts against obstructions. In view of the energy consumption theory for brittle rock fragmentation proposed by Bond, which relates energy to size reduction, a theoretical model is proposed to estimate the average fragment size for a moving rock block when it impacts against an obstruction. Then, different forms of motion are studied, with various drop heights and slope angles for the moving rock block. The calculated results reveal that the average fragment size decreases as the drop height increases, whether for free-fall or for a sliding or rolling rock block, and the decline in size is rapid for low heights and slow for increasing heights in the corresponding curves. Moreover, the average fragment size also decreases as the slope angle increases for a sliding rock block. In addition, a rolling rock block has a higher degree of fragmentation than a sliding rock block, even for the same slope angle and block volume. Finally, to compare with others’ results, the approximate number of fragments is estimated for each calculated example, and the results show that the proposed model is applicable to a relatively isotropic moving rock block.

Rock-like brittle material fragmentation under coupled static stress and spherical charge explosion

Theoretical analysis and laboratory tests were performed to investigate the fragmentation of rock-like materials under coupled static stress and spherical charge explosion. The variations of the explosion crater shape and volume were studied. The experimental results indicate that uniaxial static stress changes the shape of the explosion crater at the free surface from circular to elliptical and increases the volume of the explosion crater. In addition, a higher material strength corresponds to a smaller explosion crater volume. Under biaxial equal stress condition, the failure zone on the free surface is always circular and the volume of the explosion crater decreases and subsequently increases. The influence mechanisms of static stress on the explosion crater are discussed in detail, and the experimental phenomena are well explained.

Actividad explosiva del turrialba (Costa Rica) en el 2010-2016

La más reciente actividad eruptiva del volcán Turrialba se inició el 5 de enero de 2010, después de más de un siglo de estar dormido. El proceso de formación de cenizas emitidas (i.e., tipo de fragmentación) y el comportamiento aerodinámico de las mismas, se investigaron mediante la combinación de análisis granulométricos, petrografía óptica, análisis mineralógico, microscopía electrónica de barrido (SEM) y sistema de energía dispersiva (EDS). La ceniza estuvo compuesta por porcentajes variables de líticos accidentales, frescos (no necesariamente juveniles) a alterados hidrotermalmente (15 - 50 %), con minerales hidrotermales (1 - 8 % vol.: anhidrita, yeso, bassanita?, alunita, hexahidrita, pirita, heulandita, azufre nativo), minerales de arcilla (8 - 17 %: montmorillonita, haloisita, alofana) y una menor cantidad de vidrio fresco (5 - 49 % vol.) como esquirlas y fragmentos (3 - 20% vol. taquilita y 2 - 26 % vol. sideromelana), fenocristales (3 - 13 % vol.), que pueden ser tanto juveniles como accesorios/reciclados (1 - 5 % vol. plagioclasa, 1 - 7 % vol. piroxeno, 0 - 1 % vol. olivino, 0 - 6 % vol. opacos, cristobalita y tridimita), y xenocristales (≤ 1 % vol.: riebeckita y biotita). Los minerales secundarios son el producto de la alteración hidrotermal profunda y de su expresión en el campo de fumarolas superficial. Las características texturales identificados en partículas de ceniza finas (90 - 350 µm) sugieren que se formaron por la fragmentación debido a la interacción explosiva magma/agua. Las estructuras fundidas parecen estar relacionada con el comportamiento dúctil de las partículas sometidas a altas temperaturas (&gt; 600 ° C) en el sistema de fumarólico/magmático. El porcentaje del componente fresco fue bajo (1 - 2 % vol.) en la apertura del conducto eruptivo del 2010, incrementando paulatinamente hasta el presente (ca. 12 - 18 % vol. en 2013 - 2015). Las erupciones en el cráter Oeste, durante el 2014 al 2016, estuvieron relacionadas con una boca al inicio y después por dos o tres bocas simultáneamente activas. La alternancia de explosiones volcánicas (VEI: 0 - 2), desde conducto cerrado con la formación de nuevos cráteres, hasta un conducto abierto, y los intervalos de reposo (desgasificación exhalativa inter-eruptiva) fue controlado, posiblemente, por la velocidad a la que el magma poco vesiculado ascendió y se mantuvo en el edificio volcánico. Las tefras consistes en una sucesión compleja de capas generadas por corrientes de densidad piroclástica diluidas (principalmente oleadas húmedas), baja temperatura (&lt; 300 ºC), originadas por pulsos simples y cortos, por el colapso de columnas eruptivas freatomagmáticas, que viajaron cortas distancias (&lt; 1000 m) desde las bocas cratéricas, superando obstáculos, ocurridas simultáneamente con depósitos de caída piroclástica y emisión de proyectiles balísticos. El material fino en suspensión, en la parte superior de la columna convectiva, fue dispersado en la atmósfera y se sedimentó sobre el Valle Central. Los períodos de reposo podrían estar relacionada con un enfriamiento temporal del sistema de diques magmático o por una disminución de la actividad magmática en profundidad. La teoría de la fragmentación / transporte secuencial fue utilizada para deconvolucionar (desagregar) las granulometrías en 5 subpoblaciones, que fueron luego asociadas a mecanismos de fragmentación. Los casos en que se dio traslape de las modas fueron resueltos con una nueva manera de utilizar el coeficiente de fragmentación. Por primera vez se muestra que los resultados obtenidos, por el método mencionado, concuerdan con lo esperado de la erupciones freatomagmáticas, y con lo derivado a partir del modelo ab initio de fractura fractal.

Predicting the mineral composition of dust aerosols: Insights from elemental composition measured at the Izaña Observatory.

Regional variations of dust mineral composition are fundamental to climate impacts but generally neglected in climate models. A challenge for models is that atlases of soil composition are derived from measurements following wet sieving, which destroys the aggregates potentially emitted from the soil. Aggregates are crucial to simulating the observed size distribution of emitted soil particles. We use an extension of brittle fragmentation theory in a global dust model to account for these aggregates. Our method reproduces the size-resolved dust concentration along with the approximately size-invariant fractional abundance of elements like Fe and Al in the decade-long aerosol record from the Izaña Observatory, off the coast of West Africa. By distinguishing between Fe in structural and free forms, we can attribute improved model behavior to aggregation of Fe and Al-rich clay particles. We also demonstrate the importance of size-resolved measurements along with elemental composition analysis to constrain models.

Investigations on deformation and fracture behavior of workpiece material during high speed machining of 7050-T7451 aluminum alloy

Research on material deformation and fracture behavior under large strain rate has great practical significance. The dynamic property of 7050-T7451 aluminum alloy under the strain rate ranging from 1.2×105s−1 to 2.0×106s−1 is investigated with high speed orthogonal cutting experiments. The microstructures on fracture surface of serrated and fragmented chips are observed and analyzed. The results indicate that the workpiece material in the primary deformation zone undergoes severe plastic deformation causing the formation of ductile fracture and dimples within the cutting speed ranging from 1000m/min to 5000m/min (i.e. the strain rate is between 1.2×105s−1 and 1.4×106s−1), during which the serrated chips are produced. The fragmented chips are generated due to brittle fracture during the cutting speed ranging from 5000m/min to 7000m/min (i.e. the strain rate ranges from 1.4×106s−1 to 2×106s−1). The Energy Dispersive X-ray Analysis (EDAX) results at different positions in chips show that the content of oxygen element into chip has positive relationship with the cutting temperature raised due to material deformation. Brittle tearing and fragments adhesion can be seen on the finished surface produced under ultra high cutting speed because the fragmented chips are formatted in the mode of brittle fracture. The research shows that material deformation and fracture behavior changes greatly under ultra large strain rate, which then affects the material removal mechanism and finished surface quality during machining process.

Shock‐induced deformation phenomena in magnetite and their consequences on magnetic properties

AbstractThis study investigates the effects of shock waves on magnetic and microstructural behavior of multidomain magnetite from a magnetite‐bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Changes in apparent crystallite size and lattice parameter were determined by X‐ray diffraction, and grain fragmentation and defect accumulation were studied by scanning and transmission electron microscopy. Magnetic properties were characterized by low‐temperature saturation isothermal remanent magnetization (SIRM), susceptibility measurements around the Verwey transition as well as by hysteresis parameters at room temperature. It is established that the shock‐induced refinement of magnetic domains from MD to SD‐PSD range is a result of cooperative processes including brittle fragmentation of magnetite grains, plastic deformation with shear bands and twins as well as structural disordering in form of molten grains and amorphous nanoclusters. Up to 10 GPa, a decrease of coherent crystallite size, lattice parameter, saturation magnetization (Ms), and magnetic susceptibility and an increase in coercivity, SIRM, and width of Verwey transition are mostly associated with brittle grain fragmentation. Starting from 20 GPa, a slight recovery is documented in all magnetic and nonmagnetic parameters. In particular, the recovery in SIRM is correlated with an increase of the lattice constant. The recovery effect is associated with the increasing influence of shock heating/annealing at high shock pressures. The strong decrease of Ms at 30 GPa is interpreted as a result of strong lattice damage and distortion. Our results unravel the microstructural mechanisms behind the loss of magnetization and the modification of magnetic properties of magnetite and contribute to our understanding of shock‐induced magnetic phenomena in impacted rocks on earth and in meteorites.

Confort 15 model of conduit dynamics: applications to Pantelleria Green Tuff and Etna 122 BC eruptions

Numerical simulations are useful tools to illustrate how flow parameters and physical processes may affect eruption dynamics of volcanoes. In this paper, we present an updated version of the Conflow model, an open-source numerical model for flow in eruptive conduits during steady-state pyroclastic eruptions (Mastin and Ghiorso in A numerical program for steady-state flow of magma-gas mixtures through vertical eruptive conduits. U.S. Geological Survey Open File Report 00-209, 2000). In the modified version, called Confort 15, the rheological constraints are improved, incorporating the most recent constitutive equations of both the liquid viscosity and crystal-bearing rheology. This allows all natural magma compositions, including the peralkaline melts excluded in the original version, to be investigated. The crystal-bearing rheology is improved by computing the effect of strain rate and crystal shape on the rheology of natural magmatic suspensions and expanding the crystal content range in which rheology can be modeled compared to the original version (Conflow is applicable to magmatic mixtures with up to 30 vol% crystal content). Moreover, volcanological studies of the juvenile products (crystal and vesicle size distribution) of the investigated eruption are directly incorporated into the modeling procedure. Vesicle number densities derived from textural analyses are used to calculate, through Toramaru equations, maximum decompression rates experienced during ascent. Finally, both degassing under equilibrium and disequilibrium conditions are considered. This allows considerations on the effect of different fragmentation criteria on the conduit flow analyses, the maximum volume fraction criterion (“porosity criterion”), the brittle fragmentation criterion and the overpressure fragmentation criterion. Simulations of the pantelleritic and trachytic phases of the Green Tuff (Pantelleria) and of the Plinian Etna 122 BC eruptions are performed to test the upgrades in the Confort 15 modeling. Conflow and Confort 15 numerical results are compared analyzing the effect of viscosity, decompression rate, temperature, fragmentation criteria (critical strain rate, porosity and overpressure criteria) and equilibrium versus disequilibrium degassing in the magma flow along volcanic conduits. The equilibrium simulation results indicate that an increase in viscosity, a faster decompression rate, a decrease in temperature or the application of the porosity criterion in place of the strain rate one produces a deepening in fragmentation depth. Initial velocity and mass flux of the mixture are directly correlated with each other, inversely proportional to an increase in viscosity, except for the case in which a faster decompression rate is assumed. Taking into account up-to-date viscosity parameterization or input faster decompression rate, a much larger decrease in the average pressure along the conduit compared to previous studies is recorded, enhancing water exsolution and degassing. Disequilibrium degassing initiates only at very shallow conditions near the surface. Brittle fragmentation (i.e., depending on the strain rate criterion) in the pantelleritic Green Tuff eruption simulations is mainly a function of the initial temperature. In the case of the Etna 122 BC Plinian eruption, the viscosity strongly affects the magma ascent dynamics along the conduit. Using Confort 15, and therefore incorporating the most recent constitutive rheological parameterizations, we could calculate the mixture viscosity increase due to the presence of microlites. Results show that these seemingly low-viscosity magmas can explosively fragment in a brittle manner. Mass fluxes resulting from simulations which better represent the natural case (i.e., microlite-bearing) are consistent with values found in the literature for Plinian eruptions (~106 kg/s). The disequilibrium simulations, both for Green Tuff and Etna 122 BC eruptions, indicate that overpressure sufficient for fragmentation (if present) occurs only at very shallow conditions near the surface.

On Compressive Brittle Fragmentation

Dynamic brittle fragmentation is typically described using analytical and computational approaches for tensile stress‐states. However, most fragmentation applications (e.g., impact, blast) involve very large initial compressive stresses and deformations. In this study, the compressive fragmentation of brittle materials is investigated experimentally across a range of materials: silicon carbide, boron carbide, spinel, basalt and a stony meteorite. Analysis of our experimental results suggests that there exists two different regimes in the fragment size distributions, based on two brittle fragmentation mechanisms. The first is a mechanism that produces larger fragments and is associated with the structural failure of the sample being tested. This mechanism is influenced by the loading conditions (rate, stress state) and sample geometry. The second fragmentation mechanism produces comparatively smaller fragments and arises from the coalescence of fractures initiating and coalescence between defects in regions of large stresses and contact forces (e.g., between two fractured surfaces from the larger fragments). A framework is developed for comparing experimental compressive fragmentation results with tensile fragmentation theories. The compressive experimental results are shown to be adequately described by the theories using the new framework.

The rapidest unloading in dynamic fragmentation events

Solids usually break (fragmentize) into many pieces under high rate loading. Grady and co-worker have proposed one-dimensional theoretical models to estimate the average size of the fragments created in a ductile or a brittle fragmentation process. Numerical simulations have shown that the formulae, albeit identical in appearance, work well for the ductile fragmentation event, but poorly for the brittle case. In this paper we seek the physical mechanism that describes both the ductile and the brittle fragmentation processes. In analyzing the formation of multiple adiabatic shear bands, Grady and co-worker have proposed a conjecture that the bands automatically arrange their spacing so that the stress within the material is unloaded at the shortest time. In this paper, we apply Grady’s “rapidest unloading” principle to three types of solid fragmentations: the multiple adiabatic shear localizations, the ductile tensile fragmentation, and the brittle tensile fragmentation. We analyzed the simultaneous formation and growth of an array of equally-spaced defects, and the unloading wave propagations in the defect-free region. The average stress across the region was determined, from which the critical fracture time, defined as the time when the average stress drops to zero, is evaluated. It appears that for a prescribed strainrate, there always exists an optimum defect spacing corresponding to the rapidest unloading process. Assuming that in a natural fragmentation process the solids is unloaded in the fastest way, this optimum spacing provides an estimate for the average fragment size. For the three types of fragmentation events, the fragment size evaluated by using “the rapidest unloading” principle compares fairly well with the other reasonable fragment size models.

Damage on fused silica optics caused by laser ablation of surface-bound microparticles.

High peak power laser systems are vulnerable to performance degradation due to particulate contamination on optical surfaces. In this work, we show using model contaminant particles that their optical properties decisively determine the nature of the optical damage. Borosilicate particles with low intrinsic optical absorption undergo ablation initiating in their sub-surface, leading to brittle fragmentation, distributed plasma formation, material dispersal and ultimately can lead to micro-fractures in the substrate optical surface. In contrast, energy coupling into metallic particles is highly localized near the particle-substrate interface leading to the formation of a confined plasma and subsequent etching of the substrate surface, accompanied by particle ejection driven by the recoil momentum of the ablation plume. While the tendency to create fractured surface pitting from borosilicate is stochastic, the smooth ablation pits created by metal particles is deterministic, with pit depths scaling linearly with laser fluence. A simple model is employed which predicts ~3x electric field intensity enhancement from surface-bound fragments. In addition, our results suggest that the amount of energy deposited in metal particles is at least twice that in transparent particles.