MEASUREMENTS OF CHARACTERISTICS FOR FRAGMENTATION – BURSTING HEADS

The paper deals with questions appearing at investigation of high explosive-fragmentation heads of medium calibre when the blast waves (BW) generated by the flying fragments affect the results of velocity of the BW generated by the head itself (HBW). A method used by the authors for protection of measurement sensors against destructive action of the fragments is described. Distribution of produced fragments regarding the sizes and in function of scattering angle was studied. Mean velocity of highest speed (2100 m/s) fragments was measured on the base of first 10 m. Distribution of HBW velocities in function of radius of propagation was measured.

Features of intensive evaporation of liquid-metal steel drops heated in a high-frequency inductor

The paper presents the results of experimental studies of the processes of intense melting in air of samples (solid balls) made of metals, primarily various steels. It is shown that the heating of some steels is accompanied by intense sparking - the ejection of small secondary droplets (sparks) from the primary droplet heated up to 2500 K into the surrounding space. A possible mechanism of this process is proposed and described at a qualitative level. Possible reasons for the explosive fragmentation of secondary droplets are indicated and experimentally confirmed. The vibration process of molten samples shell, caused by the vortex motion and evaporation of the melt inside the droplet, is described. The influence of spark formation on the stability of the induction melting process is demonstrated.

The good, the bad, and the bubbly: Textural controls on secondary vesiculation of silicic magma through thermal heating

<p>Volcanic eruptions are driven by magma buoyancy caused by volatiles exsolving to form a separate gas phase. Gas overpressurization within the melt and subsequent fragmentation determines whether an eruption will be explosive (fragmentation) or effusive (no fragmentation). Therefore, a thorough understanding of factors which affect the ability of a melt to exsolve and retain or lose these volatiles effectively is required. Bubble nucleation and subsequent growth are the processes by which volatiles exsolve from the melt into an exsolved gas phase. As continued vesiculation occurs, bubbles may begin to interact and coalesce at a point called the percolation threshold, permeable pathways through the melt will then form until the permeability threshold is reached allowing volatiles to outgas, reducing overpressure. Currently, research has focused on the effects that pressure, temperature, and composition have on volatile solubility and eventual vesiculation. However, erupted bombs consisting of assemblages of heterogenous pyroclasts that were subsequently sintered and welded into a conduit-forming plug during the Cordón Caulle 2011-2012 eruption display vesiculation trends within naturally occurring obsidian pyroclasts formed by sintering of particles or quenched melt that cannot be resolved by volatile solubility effects alone. The erupted products have consistently low volatile contents with little variability present across erupted samples (0.07-0.32 wt. % H2O). Within these pyroclasts, the heterogeneity of internal textures is visible when viewed using backscattered electron (BSE) imaging or X-ray computed tomography (XCT) as clear borders exist between regions that are of a clastogenic (sintered) origin, of a quenched melt origin, or formed by variable forms of vesiculation. The heterogeneity of internal textures present within even a obsidian pyroclastic domain led to the hypothesis that foaming discrepancies observed within individual clasts were due to pre-existing textures promoting or inhibiting secondary vesiculation in the shallow conduit. This secondary vesiculation occurs through near isobaric temperature increases in the shallow conduit, following primary vesiculation and volatile exsolution associated with isothermal decompression from storage at depth. To test this hypothesis, obsidian samples of 1cm x 1-2cm x 1-3cm were heated above their glass transition temperature (Tg), between 850-910°C, to allow obsidians to vesiculate as the melt would have in the conduit. The textures of the samples were characterised before and after heating using x-ray computed tomography. The results show that within the slightly volatile supersaturated samples variable foaming was observed for each independent texture within obsidian pyroclasts, with foaming preferentially occurring within regions that contained pre-existing, isolated bubbles. These experiments show that limited thermally driven in situ foaming of relatively dense clasts containing small isolated bubbles, can increase overpressure if the domain doesn’t expand as bubbles form without increasing permeability leading to gas overpressure within this smaller region and localised explosions in order to clear this blockage, explaining the hybrid effusive-explosive eruptions observed at Cordón Caulle.</p>

The good, the bad, and the bubbly: Textural controls on secondary vesiculation of silicic magma through thermal heating

<p>Volcanic eruptions are driven by magma buoyancy caused by volatiles exsolving to form a separate gas phase. Gas overpressurization within the melt and subsequent fragmentation determines whether an eruption will be explosive (fragmentation) or effusive (no fragmentation). Therefore, a thorough understanding of factors which affect the ability of a melt to exsolve and retain or lose these volatiles effectively is required. Bubble nucleation and subsequent growth are the processes by which volatiles exsolve from the melt into an exsolved gas phase. As continued vesiculation occurs, bubbles may begin to interact and coalesce at a point called the percolation threshold, permeable pathways through the melt will then form until the permeability threshold is reached allowing volatiles to outgas, reducing overpressure. Currently, research has focused on the effects that pressure, temperature, and composition have on volatile solubility and eventual vesiculation. However, erupted bombs consisting of assemblages of heterogenous pyroclasts that were subsequently sintered and welded into a conduit-forming plug during the Cordón Caulle 2011-2012 eruption display vesiculation trends within naturally occurring obsidian pyroclasts formed by sintering of particles or quenched melt that cannot be resolved by volatile solubility effects alone. The erupted products have consistently low volatile contents with little variability present across erupted samples (0.07-0.32 wt. % H2O). Within these pyroclasts, the heterogeneity of internal textures is visible when viewed using backscattered electron (BSE) imaging or X-ray computed tomography (XCT) as clear borders exist between regions that are of a clastogenic (sintered) origin, of a quenched melt origin, or formed by variable forms of vesiculation. The heterogeneity of internal textures present within even a obsidian pyroclastic domain led to the hypothesis that foaming discrepancies observed within individual clasts were due to pre-existing textures promoting or inhibiting secondary vesiculation in the shallow conduit. This secondary vesiculation occurs through near isobaric temperature increases in the shallow conduit, following primary vesiculation and volatile exsolution associated with isothermal decompression from storage at depth. To test this hypothesis, obsidian samples of 1cm x 1-2cm x 1-3cm were heated above their glass transition temperature (Tg), between 850-910°C, to allow obsidians to vesiculate as the melt would have in the conduit. The textures of the samples were characterised before and after heating using x-ray computed tomography. The results show that within the slightly volatile supersaturated samples variable foaming was observed for each independent texture within obsidian pyroclasts, with foaming preferentially occurring within regions that contained pre-existing, isolated bubbles. These experiments show that limited thermally driven in situ foaming of relatively dense clasts containing small isolated bubbles, can increase overpressure if the domain doesn’t expand as bubbles form without increasing permeability leading to gas overpressure within this smaller region and localised explosions in order to clear this blockage, explaining the hybrid effusive-explosive eruptions observed at Cordón Caulle.</p>

Explosive Fragmentation of Rock Masses with Heterogeneous Structure

A method to optimize drilling and blasting parameters with account of the physical and technical properties of rocks within the blasted block is proposed to improve the quality of blasting in open pit mines characterized by complex geological settings. The results of laboratory tests are provided that confirm improvement in the quality of rock sample crushing by blasting charges with variable delays and locations, depending on the strength properties of the samples, relative to blasting charges with unchanged parameters. The proposed method can be used in combination with GPR surveys of the rock mass. Explosive fragmentation of the rock masses with complex structures is characterized with a number of features caused by changes in the strength properties within the blasted block. In order to optimize the fragmentation efficiency of rock masses with complex geological structure, it is required to assess physical and technical properties of rocks and to determine their location and variations of the strength properties within the blasted block. It is possible to quickly assess the physical and technical properties of the blasted rocks using the surface georadar method. The outcome of this method is georeferencing of the reoradar data to the location and properties of the rocks to be blasted, along with the methodology of applying the georadar surveys, selection of the areal assembly type depending on the size and properties of detected jointing, and economic justification of applying this method. The novelty consists in linking the georadar data on the rocks to be blasted with drilling and blasting parameters.

Explosive fragmentation of Prince Rupert’s drops leads to well-defined fragment sizes

Anyone who has ever broken a dish or a glass knows that the resulting fragments range from roughly the size of the object all the way down to indiscernibly small pieces: typical fragment size distributions of broken brittle materials follow a power law, and therefore lack a characteristic length scale. The origin of this power-law behavior is still unclear, especially why it is such an universal feature. Here we study the explosive fragmentation of glass Prince Rupert’s drops, and uncover a fundamentally different breakup mechanism. The Prince Rupert’s drops explode due to their large internal stresses resulting in an exponential fragment size distribution with a well-defined fragment size. We demonstrate that generically two distinct breakup processes exist, random and hierarchical, that allows us to fully explain why fragment size distributions are power-law in most cases but exponential in others. We show experimentally that one can even break the same material in different ways to obtain either random or hierarchical breakup, giving exponential and power-law distributed fragment sizes respectively. That a random breakup process leads to well-defined fragment sizes is surprising and is potentially useful to control fragmentation of brittle solids.

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions

Although gas exsolution is a major driving force behind explosive volcanic eruptions, viscosity is critical in controlling the escape of bubbles and switching between explosive and effusive behavior. Temperature and composition control melt viscosity, but crystallization above a critical volume (>30 volume %) can lock up the magma, triggering an explosion. Here, we present an alternative to this well-established paradigm by showing how an unexpectedly small volume of nano-sized crystals can cause a disproportionate increase in magma viscosity. Our in situ observations on a basaltic melt, rheological measurements in an analog system, and modeling demonstrate how just a few volume % of nanolites results in a marked increase in viscosity above the critical value needed for explosive fragmentation, even for a low-viscosity melt. Images of nanolites from low-viscosity explosive eruptions and an experimentally produced basaltic pumice show syn-eruptive growth, possibly nucleating a high bubble number density.