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The purpose of this paper is to study the mechanical response of polyurea, soda-lime glass (glass, for short), polyurea/glass/polyurea and glass/polyurea/glass sandwich structures under dynamic-loading conditions involving propagation of planar longitudinal shockwaves.

The problem of shockwave generation, propagation and interaction with material boundaries is investigated using non-equilibrium molecular dynamics. The results obtained are used to construct basic shock Hugoniot relationships associated with the propagation of shockwaves through a homogeneous material (polyurea or glass, in the present case). The fidelity of these relations is established by comparing them with their experimental counterparts, and the observed differences are rationalized in terms of the microstructural changes experienced by the shockwave-swept material. The relationships are subsequently used to predict the outcome of the interactions of shockwaves with polyurea/glass or glass/polyurea material boundaries. Molecular-level simulations are next used to directly analyze the same shockwave/material-boundary interactions.

The molecular-level simulations suggested, and the subsequent detailed microstructural analyses confirmed, the formation of topologically altered interfacial regions, i.e. polyurea/glass and glass/polyurea interphases.

To the authors’ knowledge, the present work is a first attempt to analyze, using molecular-level simulation methods, the interaction of shockwaves with material boundaries.

In the recent work, a new blast-wave impact-mitigation concept involving the use of a protective structure consisting of bimolecular reactants (polyvinyl pyridine+cyclohexyl chloride), capable of undergoing a chemical reaction (to form polyvinyl pyridinium ionic salt) under shockwave loading conditions, was investigated using all-atom reactive equilibrium and non-equilibrium molecular-dynamics analyses. The purpose of this paper is to reveal the beneficial shockwave dispersion/attenuation effects offered by the chemical reaction, direct simulations of a fully supported single planar shockwave propagating through the reactive mixture were carried out, and the structure of the shock front examined as a function of the extent of the chemical reaction (i.e. as a function of the strength of the incident shockwave). The results obtained clearly revealed that chemical reactions give rise to considerable broadening of the shockwave front. In the present work, the effect of chemical reactions and the structure of the shockwaves are investigated at the continuum level.

Specifically, the problem of the (conserved) linear-momentum accompanying the interaction of an incident shockwave with the protective-structure/protected-structure material interface has been investigated, within the steady-wave/structured-shock computational framework, in order to demonstrate and quantify an increase in the time period over which the momentum is transferred and a reduction in the peak loading experienced by the protected structure, both brought about by the occurrence of the chemical reaction (within the protective structure).

The results obtained clearly revealed the beneficial shock-mitigation effects offered by a protective structure capable of undergoing a chemical reaction under shock-loading conditions.

To the authors’ knowledge, the present manuscript is the first report dealing with a continuum-level analysis of the blast-mitigation potential of chemical reactions.

The purpose of this paper is to introduce and analyze a new blast-wave impact-mitigation concept using advanced computational methods and tools. The concept involves the use of a protective structure consisting of bimolecular reactants displaying a number of critical characteristics, including: a high level of thermodynamic stability under ambient conditions (to ensure a long shelf-life of the protective structure); the capability to undergo fast/large-yield chemical reactions under blast-impact induced shock-loading conditions; large negative activation and reaction volumes to provide effective attenuation of the pressure-dominated shockwave stress field through the volumetric-energy storing effects; and a large activation energy for efficient energy dissipation. The case of a particular bimolecular chemical reaction involving polyvinyl pyridine and cyclohexyl chloride as reactants and polyvinyl pyridinium ionic salt as the reaction product is analyzed.

Direct simulations of single planar shockwave propagations through the reactive mixture are carried out, and the structure of the shock front examined, as a function of the occurrence of the chemical reaction. To properly capture the shockwave-induced initiation of the chemical reactions during an impact event, all the calculations carried out in the present work involved the use of all-atom molecular-level equilibrium and non-equilibrium reactive molecular-dynamics simulations. In other words, atomic bonding is not pre-assigned, but is rather determined dynamically and adaptively using the concepts of the bond order and atomic valence.

The results obtained clearly reveal that when the chemical reactions are allowed to take place at the shock front and in the shockwave, the resulting shock front undergoes a considerable level of dispersion. Consequently, the (conserved) linear momentum is transferred (during the interaction of the protective-structure borne shockwaves with the protected structure) to the protected structure over a longer time period, while the peak loading experienced by the protected structure is substantially reduced.

To the authors’ knowledge, the present work is the first attempt to simulate shock-induced chemical reactions at the molecular level, for purposes of blast-mitigation.

In the present work, a new blast-/ballistic-impact mitigation concept is introduced and its efficacy analyzed using advanced computational methods and tools. The concept involves the use of a zeolite protective layer separated by air from the structure being protected and in contact with a water layer in front. The paper aims to discuss these issues.

To properly capture the attendant nano-fluidics phenomena, all the calculations carried out in the present work involved the use of all-atom molecular-level equilibrium and non-equilibrium molecular-dynamics simulations.

Under high-rate loading, water molecules (treated as a nano-fluidic material) are forced to infiltrate zeolite nanopores wherein, due to complex interactions between the hydrophobic nanopore walls and the hydrogen bonds of the water molecules, water undergoes an ordering-type phase transition and acquires high density, while a significant portion of the kinetic energy of the water molecules is converted to potential energy. Concomitantly, a considerable portion of this kinetic energy is dissipated in the form of heat. As a result of these energy conversion/dissipation processes, the (conserved) linear momentum is transferred to the target structure over a longer time period, while the peak loading experienced by the structure is substantially reduced.

To the authors’ knowledge, the present work constitutes the first reported attempt to utilize pure SiO₂ hydrophobic zeolites in blast-/ballistic-impact protection applications.

This unusual book gives a combined account of three different branches of dynamics, dealing with particles, fluids and solid bodies. The preface explains the history of the course at the Massachusetts Institute of Technology on which the book is based. When instruction in aeronautical engineering was started at M.I.T., some forty years ago, the branch of the subject that was most fully developed from a rational viewpoint was the theory of dynamic stability. The students, with only an elementary training in mathematics and mechanics, found this theory of stability too difficult to grasp, and a course was therefore started to bridge the gap between the mathematics and mechanics that the students already knew and the complex problems of aircraft stability.

The design of the Advanced Combat Helmet (ACH) currently in use was optimized by its designers in order to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface/head collisions. Since traumatic brain injury experienced by a significant fraction of the soldiers returning from the recent conflicts is associated with their exposure to blast, the ACH should be redesigned in order to provide the necessary level of protection against blast loads. The paper aims to discuss this issue.

In the present work, an augmentation of the ACH for improved blast protection is considered. This augmentation includes the use of a polyurea (a nano-segregated elastomeric copolymer) based ACH external coating. To demonstrate the efficacy of this approach, blast experiments are carried out on instrumented head-mannequins (without protection, protected using a standard ACH, and protected using an ACH augmented by a polyurea explosive-resistant coating (ERC)). These experimental efforts are complemented with the appropriate combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction finite-element analysis.

The results obtained clearly demonstrated that the use of an ERC on an ACH affects (generally in a beneficial way) head-mannequin dynamic loading and kinematic response as quantified by the intracranial pressure, impulse, acceleration and jolt.

To the authors’ knowledge, the present work is the first reported combined experimental/computational study of the blast-protection efficacy and the mild traumatic brain-injury mitigation potential of polyurea when used as an external coating on a helmet.

The bulk of jet engine noise developed at high powers arises from the turbulent mixing of the jet efflux in the surrounding air, as judged from model experiments, and has a continuous spectrum with a single flat maximum. The high frequency sound arises from fairly close to the orifice, and reaches its maximum intensity at fairly large acute angles to the jet direction. Lower frequency noise arises from lower down stream and its maxima make smaller acute angles with the jet axis. The possible origins are briefly discussed in view of Lighthill's theory and refraction effects. The most intensesound has a wave‐length of the order of three or four exit diameters, and originates between five and ten diameters from the orifice. A semi‐empirical rule of noise energy depending on the jet velocity to the eighth power and the jet diameter squared gives a rough estimate of the noise level for both cold and heated jets. Further noise from heated or supersonic jets may occur through eddies travelling at supersonic speed and so producing small Shockwaves. Model experiments have shown that interaction between shock‐wave configurations in choked jets and passing eddy trains generates sound and this initiates further eddies at the orifice. The directional properties of this sound are quite distinctive, the maximum being in the upstream direction. Methods of reducing jet noise are briefly discussed.

The stock market has shown fluctuating degrees of volatility because of the recent COVID-19 pandemic in India. The present research aims to investigate the effect of the COVID-19 on the stock market volatility, and whether the economic package can control the market volatility or not, measured by a set of certain sector-level economic features and factors such as resilience variables.

We examine the correlation matrix, basic volatility model and robustness tests to determine the sector-level economic features and macroeconomic factors helpful in diminishing the volatility rising because of the COVID-19.

The outcomes of this study are significant as policymakers and financial analysts can apply these economic factors to set policy replies to handle the unexpected fluctuation in the stock market in sequence to circumvent any thinkable future financial crisis.

The originality of the paper is to measure the variables affecting the stock market volatility due to COVID-19, and understand the impact of capital market macroeconomic variables and dummy variables to theoretically explain the COVID-19 impact on stock market volatility.

The purpose of this paper is to optimize the design of the advanced combat helmet (ACH) currently in use, by its designers in order to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface/head collisions. Since traumatic brain injury experienced by a significant fraction of the soldiers returning from the recent conflicts is associated with their exposure to blast, the ACH should be redesigned in order to provide the necessary level of protection against blast loads. In the present work, augmentations of the ACH for improved blast protections are considered. These augmentations include the use of a polyurea (a nano-segregated elastomeric copolymer)-based ACH external coating/internal lining.

To demonstrate the efficacy of this approach, instrumented (unprotected, standard-ACH-protected, and augmented-ACH-protected) head-mannequin blast experiments are carried out. These experimental efforts are complemented with the appropriate combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction analysis.

The results obtained indicated that: when the extent of peak over-pressure reduction is used as a measure of the blast-mitigation effectiveness, polyurea-based augmentations do not noticeably improve, and sometimes slightly worsen, the performance of the standard ACH; when the extent of specific impulse reduction is used as a measure of the blast-mitigation effectiveness, application of the polyurea external coating to the standard ACH improves the blast-mitigation effectiveness of the helmet, particularly at shorter values of the charge-detonation standoff distance (SOD). At longer SODs, the effects of the polyurea-based ACH augmentations on the blast-mitigation efficacy of the standard ACH are inconclusive; and the use of the standard ACH significantly lowers the accelerations experienced by the skull and the intracranial matter. As far as the polyurea-based augmentations are concerned, only the internal lining at shorter SODs appears to yield additional reductions in the head accelerations.

To the authors’ knowledge, the present work contains the first report of a combined experimental/computational study addressing the problem of blast-mitigation by polyurea-based augmentation of ACH.