Spallation Phenomena Research Articles

Energy Amplifier Systems as Sustainable Nuclear Reactors: An Overview

One of the present major issues for industrialised societies is the environmental impact of energy production. Nuclear power is identified as a possible sustainable opportunity to provide a technological answer to this problem. This review aims to overview the physical bases of accelerator-driven systems, focusing on spallation and transmutation phenomena. A discussion on the possible use of these nuclear devices is developed in the context of sustainable energy production, showing a possible new approach to nuclear energy, based on the developments of accelerator physics and technology during the last century.

Study on ballistic mechanism of B4C/Al gradient armor

AbstractIn order to improve the anti‐penetration performance of gradient armor, the constitutive models of B4C/Al composites with different compositions were determined according to the bending curves. The anti‐bullet simulation of B4C/Al gradient armor was carried out by ANSYS‐DYNA finite element software, and the stress state of B4C/Al gradient material under 7.62‐mm bullet penetration was analyzed. Meanwhile, the propagation law of stress wave in armor was studied by Hopkinson bar simulation and the improved internal stress wave model, which further revealed the ballistic mechanism of B4C/Al gradient armor. The simulation results showed that, compared with the traditional laminated armor, the toughness of B4C/Al gradient armor material increased with the change of layer thickness, resulting in the fact that the whole armor could withstand greater stress without breaking, and the anti‐penetration time was prolonged. In addition, the performance difference between the layers of the gradient armor was slight, and the spallation phenomenon of the relative double‐layer armor decreases, which enhanced the multiple hit performance of the armor and the absorption capacity of the stress wave. The performance of B4C/Al gradient armor specimens and double‐layer specimens were tested by drop hammer impact test. The test results were consistent with the simulation results.

Surface Forming Criteria of Ti-6AL-4V Titanium Alloy under Laser Loading

In order to study the spallation phenomenon of titanium alloy under the shock of nanosecond laser, the Neodymium-Yttrium-Aluminum Garnet laser was used to carry out laser shock experiments on the surface of titanium alloy. By observing and measuring the surface morphology of the target material, the forming factors and the changes of the surface morphology under different parameter settings, the forming criteria of the titanium alloy were obtained. The results show that under the single variable method, the change of laser energy can affect the target shape variable, and there is a positive correlation between them. When the thickness was greater than or equal to 0.08 mm, no obvious cracks were found in the targets. Moreover, the number of impact times was the key factor for the target deformation; with the growth of impact times, the target deformation gradually became larger until the crack appeared. The larger the diameter of the spot, the more likely the target was to undergo plastic deformation. The surface of titanium alloy with a thickness of 0.08 mm appeared to rebound under specific laser shock condition. The changes in the back of the target material were observed in real time through a high-speed camera, and the plasma induced by the laser was observed in the process. This study is based on the results of previous studies to obtain the titanium alloy forming criteria, which provides a basis for the setting of laser parameters and the thickness of the target when the nanosecond laser impacts the Ti-6AL-4V target.

Thermal jet drilling of granite rock: a numerical 3D finite-element study.

This paper presents a numerical study on thermal jet drilling of granite rock that is based on a thermal spallation phenomenon. For this end, a numerical method based on finite elements and a damage-viscoplasticity model are developed for solving the underlying coupled thermo-mechanical problem. An explicit time-stepping scheme is applied in solving the global problem, which in the present case is amenable to extreme mass scaling. Rock heterogeneity is accounted for as random clusters of finite elements representing rock constituent minerals. The numerical approach is validated based on experiments on thermal shock weakening effect of granite in a dynamic Brazilian disc test. The validated model is applied in three-dimensional simulations of thermal jet drilling with a short duration (0.2 s) and high intensity (approx. 3 MW m-2) thermal flux. The present numerical approach predicts the spalling as highly (tensile) damaged rock. Finally, it was shown that thermal drilling exploiting heating-forced cooling cycles is a viable method when drilling in hot rock mass. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'.

Simulation of femtosecond laser ablation and spallation of titanium film based on two-temperature model and molecular dynamics

In order to study the interaction between a femtosecond laser and a titanium film, the method of combining the two-temperature model and molecular dynamics was used to simulate the ablation and spallation of the titanium film by the femtosecond laser with a wavelength of 800 nm. The spallation of the titanium film was explained from the perspective of the temperature and the stress wave. It is found that the femtosecond laser with a pulse width of 300 fs loads the titanium film, and the ablation threshold is around 0.05 J/cm2. The effects of different pulse widths in the range of 100–500 fs and different energy densities in the range of 0.10–0.20 J/cm2 on the spallation results of titanium films were also studied. We found that the femtosecond laser with a pulse width of 100 fs promotes the spallation phenomenon to occur more quickly. The surface spallation products of the titanium film were large clusters of atoms when the energy density is 0.10 J/cm2, but most of the surface spallation products of the titanium film were single atoms when the energy density is 0.20 J/cm2. These indicated that the energy density is an important factor affecting the spallation products of the titanium film. The evolution of maximum electron temperature, electron-lattice equilibrium temperature, and coupling time with pulse width and energy density are also given and analyzed. These results have important reference significance for studying the fracture and damage of titanium, and they help us to understand the mechanism of femtosecond laser processing materials.

Thermal shock assisted percussive drilling: A numerical study on the single-bit axisymmetric case

This paper presents a numerical study on thermal shock assisted percussive drilling. For this end, a finite element based numerical approach for solving the governing thermo-mechanical problem explicitly in time is presented. Rock fracture is described in the continuum sense by a damage-viscoplasticity model with separate damage variables for tensile and compressive failure types. In the numerical simulations, an axisymmetric numerical rock is dynamically indented with a spherical tool in cases of intact and heat shocked heterogeneous rocks. In realistic simulations with an initial borehole, the heat shock treatment of rock enhances the mechanical breakage, increasing especially the tensile damaging, in unconfined and confined conditions with and without down-the-hole pressure. However, according to the simulations, down-the-hole pressure effectively prevents the thermal spallation phenomenon crucial for thermal jet drilling.

Numerical study on the effect of confinement on thermal spallation drilling of hard rock

This paper presents a numerical study on the effect of confining pressure (down-to-hole pressure) on the performance of the thermal jet drilling on rock. For this end, a numerical method including a viscoplastic-damage model for rock and an explicit scheme for solving the governing thermo-mechanical problem, which is assumed uncoupled. Moreover, rock heterogeneity, which facilitates the thermal spallation phenomenon, is taken into account. Numerical simulations under axisymmetric conditions demonstrate that the efficiency of thermal spallation based drilling is reduces drastically due to confinement.

Spallation Analysis of Concrete Under Pulse Load Based on Peridynamic Theory

Spallation analysis is one of important research directions in impact dynamics. By combining the newly developing Peridynamics (PD) theory, the spallation phenomenon of concrete is numerically simulated using C language and MATLAB programming. The factors that may affect the spalling are verified: the type of pulse load, the geometric size of the model and the action time of pulse load. The dynamic response of spallation of three-dimensional concrete columns under different pulse loading forms (rectangular pulse, triangular pulse and exponential pulse) is analyzed. (1) Under the same impulse effect, only one spalling occurs in the rectangular pulse, and no multiple spallation occurs when the pulse amplitude increase. Exponential and triangular pulses can produce multiple spallation phenomena, and the time for the first spallation phenomenon is rectangular pulse < triangle pulse < exponential pulse. (2) The effect of the same linear triangle pulse on spalling of concrete columns with different lengths (100 mm, 200 mm and 300 mm) is analyzed. The triangle pulse can cause single or multiple spallation, which is related to the length and size of the model. (3) Finally, by changing the number of time steps of the pulse load, the different spalling phenomena of triangular pulses are analyzed. The thickness of the first layer increases significantly with the increase of the action time.

High temperature oxidation of alloy 617, 740, HR6W and Sanicro 25 in supercritical water at 650°C

ABSTRACTIn this study, the oxidation behaviour of alloy 617, alloy 740, alloy HR6W and Sanicro 25 was investigated in SCW and vapour environment at 650°C for 320 h. The weight gain and the thickness of the oxide films of samples after exposure in SCW are higher than that in vapour. Inner oxidation phenomena were observed on alloy HR6W and Sanicro 25, which have higher content of Fe. The structure of the oxide films formed on alloy 617, alloy 740, alloy HR6W and Sanicro 25 after exposure in SCW at 650°C for 320 h is composed by a Fe-rich (mainly hematite) outer layer and a Cr-rich inner layer. The thickness of the outer Fe-rich layer is related with the Fe content in alloy matrix. Oxide films spallation phenomena were found on alloy HR6W after exposure in SCW.

Fatigue and Mechanical Behavior of Ti-6Al-4V Alloy with CrN and TiN Coating Deposited by Magnetic Filtered Cathodic Vacuum Arc Process

Coatings of 3 μm CrN and TiN were prepared by a magnetic filtered cathodic vacuum arc process (MFCVA) on Ti-6Al-4V substrates, respectively. Rotating bending tests and uniaxial tests were conducted for investigating the effect of the thin and uniformly distributed hard CrN and TiN coatings on the fatigue and mechanical properties of Ti-6Al-4V substrate. During both tests, no coating spallation phenomenon was observed, which indicated that the hard coating bound well with the substrate. The fatigue test results showed that the fatigue strength of the coated sample was decreased in both the low- and high-cycling fatigue regimes compared with the uncoated Ti-6Al-4V substrate. Compared with the TiN coating, the CrN coating caused a more significant reduction on the fatigue property of the uncoated Ti-6Al-4V substrate due to its inferior plastic deformation capacity. Furthermore, the tensile test results showed that the coated sample had a relative higher ultimate strength, yield strength, and lower elongation compared with the uncoated Ti-6Al-4V substrate. This may be due to the fact that the hard coating could suppress the initiation of cracks, and so higher stress was needed for crack initiating. During the crack propagation period, the hard coating cracked at a relative higher velocity, which led to cracking of the ductile substrate and elongation reduction.

Metallographic Investigation of Structural Changes upon Collapse of Cylindrical Copper Shells

Metallography has been used in this study to investigate copper samples cut from cylindrical shells collapsed under shock wave loading. Spallation phenomena (either the shell is divided into concentric rings, or a wide zone with spallation cavities and cracks is formed), which occur in shells under shock wave loading are considered. High-rate deformation during inertial convergence heals pores and cracks. Traces of healing remain in the microstructure. Various cases of nonradial convergence, when a stable circular deformation front is lost, are observed. Bends appear on the thin-walled shell and the shell crumples during its converging. Periodically arranged protrusions form on the inner surface of the thick-walled shells. They subsequently join together, converging to the center of the shell. A wave-shaped relief, protrusions, dents, and corrugation on the outer surface indicate the loss of stability.

A new gas–solid reaction model for voloxidation process with spallation

Abstract A new methodology, the crack–spallation model, has been developed to analyze gas–solid reactions dominated by crack growth inside of the solid reactant and spallation phenomena. The new model physically represents three processes of the reaction progress: (1) diffusion of gas reactant through pores; (2) growth of product particle in pores; and (3) crack and spallation of solid reactant. The validation of this method has been conducted by comparison of results obtained in an experiment for oxidation of UO2 and the shrinking core model. The reaction progress evaluated by the crack–spallation model shows better agreement with the experimental data than that evaluated by the shrinking core model. To understand the trigger point during the reaction progress, a detailed analysis has been conducted. A parametric study also has been performed to determine mass diffusivities of the gas reactant and volume increase constants of the product particles. This method can be appropriately applied to the gas–solid reaction based on the crack and spallation phenomena such as the voloxidation process.

Carbon Ablation in Hypervelocity Air and Nitrogen Shock Layers

Experiments studying carbon ablation were performed with electrically heated graphite strips embedded into a steel base structure. The test articles were exposed to hypervelocity air and nitrogen flows to investigate a previously proposed coupling effect of surface carbon monoxide production on the levels of cyanogen via gas phase reactions. Based on the relative levels of cyanogen emissions between the two conditions, it was concluded that, although carbon monoxide interactions were a nonnegligible contributor to cyanogen production, the majority was due to direct surface nitridation. In terms of overall trends, the nitrogen condition results showed continually increasing cyanogen emissions with surface temperature. For air, cyanogen emissions reduced with the temperature above 2500 K, which was consistent with the previous experiments in air. The combination of these observations suggested that the oxidation rate decreased above 2500 K. This phenomenon is well known, but it has not previously been observed for an ablating body with the additional influence of a realistic hypervelocity shock layer. High-speed video recordings for these experiments allowed observation of spallation phenomena in much greater detail than for previous work. It was clearly seen that spallation can significantly alter the flowfield, and it is a phenomenon that must be given greater consideration in future studies.

Spallation occurrence from polyamide materials irradiated by thermal plasma with water absorption

This paper first describes the effect of water absorption in polyamide material irradiated by thermal plasmas on the occurrence of spallation phenomena. The interaction between polyamide materials and arc plasmas occurs particularly in the low voltage circuit breaker and aerospace fields. Spallation phenomena are those in which polymer particles are ejected from polymer bulk materials irradiated by high heat flux. To confirm the effect of water absorption into the polyamide material on spallation phenomena, polyamide specimens with and without water absorption were irradiated by Ar inductively coupled thermal plasma. The results show that the polyamide specimen with water absorption ejected spallation particles, whereas the polyamide specimen without water absorption were only slightly ejected, indicating that water absorption promotes the occurrence of spallation. The cooling effects of the spallation polyamide 66 (PA66) particles ablation were also estimated in hot air to assess the arc quenching ability from the spallation particle inclusion. This estimation showed that 10 and more PA66 particles inclusion might decrease the air temperature by 3000 K effectively, which can be useful to enhance arc quenching in circuit breakers working in air.

Study of extreme states of matter at high energy densities and high strain rates with powerful lasers

In this paper, a review of the most important results of the experimental studies of thermonuclear plasma in conical targets, the generation of shock waves and of spallation phenomena in different materials is presented, all of which have been carried out at the laser facilities of the A M Prokhorov General Physics Institute RAS since 1977.

Numerical and experimental analysis of spallation phenomena

The spallation phenomenon was studied through numerical analysis using a coupled Lagrangian particle tracking code and a hypersonic aerothermodynamics computational fluid dynamics solver. The results show that carbon emission from spalled particles results in a significant modification of the gas composition of the post-shock layer. Results from a test campaign at the NASA Langley HYMETS facility are presented. Using an automated image processing of short exposure images, two-dimensional velocity vectors of the spalled particles were calculated. In a 30-s test at 100 W/cm2 of cold-wall heat flux, more than 722 particles were detected, with an average velocity of 110 m/s.

The dynamical response characteristics of elastic–plastic coal under dynamic load

At present, it has been confirmed that tensile stress can be generated in coal bodies through instantaneous unloading, which leads to severe damage in certain areas of the coal. However, the process for the generation of this tensile stress is still unclear. In this study, by taking the elastic–plastic coal in the front of the tunneling face and the rock cross-cut coal uncovering region as examples, and by applying the rules of reflection and transmission to the stress waves, the concrete propagation process of stress waves in coal under dynamic loading was examined. At the same time, the formation and acting mechanisms of the tensile stress waves under the interaction of the loading and unloading stress waves were studied. The results indicated that compression-shear, tension, and tension-shear failures in the elastic–plastic coal were evident, under the role of the instantaneous loading and unloading. Due to the fact that the wave impedance of the coal tended to distribute unevenly in the front of the tunneling face, the loading and unloading stress waves were thereby transmitted and reflected in the propagation process of the coal body, and formed unloading stress waves. The tensile stress generated in the coal under the unloading waves, which were prone to severe damage to coal body, led to dramatic increases of the gas desorption rate, as well as a significant rising of the gas pressure. These results led to the occurrences of the coal and gas outburst accidents. Therefore, the development process of the coal and gas outbursts was determined to have begun on the inside of the coal, then to gradually develop towards the tunneling face. Multi-beam unloading waves were formed by the instantaneous unloading of the blast-hole wall, as well as the pressure released around the coal. Radial tensile stress waves in the coal were prominent, under the role of the unloading waves, which led to tension failure in the coal, which in turn generated circumferential cracks. This caused the spallation phenomena of the coal around the outburst-hole wall during the process of the coal and gas outbursts. The conclusions of this study have important theoretical guiding significance for the prevention of coal and rock dynamic disasters.

Investigation of the spall strength of graphite using nano- and picosecond laser pulses

Spallation phenomena in graphite targets were investigated experimentally under nano- and picosecond shock-wave action at laser facilities “Kamerton-T” (GPI RAS) and PHELIX (GSI). In the range of strain rates of 1 to 10 μs−1 at the first time, data of dynamic tensile strength of the material were obtained. At maximal realized strain rate of 14 μs−1, the spall strength value 2.1 GPa has been achieved that is 64% of the theoretical ultimate tensile strength of the graphite. Spallation was observed not only on the backside of the target, but also on its front (irradiated) surface. The morphology of the front and rear surfaces of the targets was studied using the optical and scanning electron microscopy. The structure of the graphite in irradiated area on the facial side as well as in the spallation zone on the rear side of the target was investigated by Raman scattering method. A comparison of the dynamic strength of the graphite with the dynamic strength of a synthetic diamond is done.

Microstructures and Properties of B&lt;sub&gt;4&lt;/sub&gt;C-Based Composites with Laminar Structure

Three kinds of ceramic composites with laminar structure, B4C-B4C/Ti3SiC2, B4C-B4C/Ti2AlC and B4C-B4C/8%Si were synthesized by hot pressing sintering, respectively. The microstructures and properties of the composites were investigated in detail. The experimental results showed the two layers of each composite were in good combination and no spallation phenomenon was observed during the process of fracture. The fracture mechanism of three kinds of composites was similar with each other. The fracture mode of ceramic layers was mainly transgranular fracture. The ductile layers which showed good ductile fracture characteristic exhibited a mixed mode of intergranular and transgranular fracture. The results of mechanical properties showed the comprehensive performance of B4C-B4C/8%Si was the best, the flexural strength and fracture toughness of which were 524 MPa and 3.5 MPa∙m1/2 respectively.

Negative pressure and spallation in graphite targets under nano- and picosecond laser irradiation

We present the results of experiments on the spallation phenomena in graphite targets under shock-wave nano- and picosecond irradiation, which have been performed on Kamerton-T (GPI, Moscow, Russia) and PHELIX (GSI, Darmstadt, Germany) laser facilities. In the range of the strain rates of , the data on the dynamic mechanical strength of the material at rapure (spallation) have been for the first time obtained. With a maximal strain rate of , the spall strength of is obtained, which constitutes 64 % of the theoretical ultimate tensile strength of graphite. The effect of spallation is observed not only on the rear side of the target, but also on its irradiated (front) surface. With the use of optical and scanning electron microscopes, the morphology of the front and rear surfaces of the targets is studied. By means of Raman scattering of light, the graphite structure both on the target front surface under laser exposure and on its rear side in the spall zone is investigated. A comparison of the dynamic strength of graphite and synthetic diamond is performed.