Y.S. Karinski

On Blast Pressure Analysis Due to a Partially Confined Explosion: I. Experimental Studies

ABSTRACT The paper presents a second part of a study aiming at understanding some characteristics of an interior explosion within a room with limited venting. Internal explosions may occur as a result of an ammunition storage explosion, or a charge explosion within a room in a terrorist action or a warhead explosion following its penetration into a closed space. This paper follows an experimental study and validation of the problem and aims at an extended numerical investigation of several aspects of the problem. The analysis has been performed for a room with rigid walls having limited venting by using AUTODYN 12 commercial program implementing the Eulerian multi-material approach, and the results were compared with the experimental measurements. An effective simplified model with lumped parameters based on the Bernoulli equation has been developed for the quasi-stationary phase of the detonation products outflow from the room through the venting openings.

An analytical model to evalulate the static soil pressure on a buried structure

Abstract A discrete-continuous model to analyze a buried structure response to static surface loading as well as the soil gravitational load at ‘service-state’ conditions is presented. A two-degree-of-freedom model represents the structure above which a continuous vertical column represents the soil. The proposed model simulates the soil-buried structure interaction affected by the structure’s roof displacement as well as the rigid body displacement of the whole structure relative to that of the free field. The model can represent positive and negative arching and provides an understanding of the effects that various variables have on the arching type and on the structure response. Other soil-structure parameters that are included in the model are the soil and structure material properties, roof span and thickness, the structure’s height, and the depth of burial and external pressure. Simulations of a rectangular buried conduit performed by both the proposed model and by a finite element analysis yielded similar interface loads and similar influence of the problem parameters on the results. This example demonstrates the effect of the structure’s stiffness and height on the soil arching above it and on the average interface load acting on its roof. Thus, the proposed model can be used in preliminary stages of the design process to easily evaluate the effect of variables such as the structure properties on the response.

A simple model to assess the effect of soil shear resistance on the response of soil-buried structures under dynamic loads

The response of a buried structure to a surface loading is analyzed by a relatively simplistic model, yet comprehensive enough to delineate both wave propagation phenomena and effects of soil arching. Analysis of a buried circular plate response under surface impulsive loading, according to this model (which shows a good agreement with the theoretical predictions to the experimental results) enables an insight to the problems full range, from very short impulse load to long, quasi-static loading. Application of this solution to a study case shows that both wave propagation related phenomena and the arching phenomenon (related to relative displacements in the soil above the structure) may be involved in the systems response to surface impact loading. Hence, a general analysis of a buried structures response to dynamic surface loading should consider not only the wave propagation effects, but also take into account the soil arching effect.

On Blast Pressure Analysis Due to a Partially Confined Explosion: III. Afterburning Effect

This paper aims at extending our understanding with regard to some characteristics of an interior explosion within a room with limited venting. An interior explosion may be the result of an ammunition storage explosion, or an explosive charge as part of a terrorist action or a warhead explosion that follows its penetration into a closed space in a military action. Full scale experiments have been performed with a TNT charge detonated at the center of a single room sized space with rigid boundaries. The room has a limited size opening for venting at the ceiling. Numerical simulations of the problem have been performed using AUTODYN Ver. 12.1 and compared with the experimental measurements. Some deviations between the measured pressure and the predicted pressure motivated the present study in an attempt to study the effect of the additional energy released due to the burning of the detonation products reacting with the surrounding oxygen. The study that is described in this paper enhanced our understanding. Incorporation of this effect considerably improved the predictions. The present study clarified when, how and to what extent the afterburning should be introduced in the analysis.

Afterburning Aspects in an Internal TNT Explosion

The afterburning is a complex chemical process which stems from the reaction of the detonation products with the oxygen in the air when appropriate conditions exist. TNT is a very fuel-rich explosive as indicated by the large negative oxygen balance value of −74%. It means that there is not enough oxygen in its initial chemical compound and extra oxygen is needed to make the afterburning energy release possible. This article describes in details the calculation process for evaluating the amount of energy release in a confined TNT explosion. Moreover, partial afterburning energy release is also calculated for cases of oxygen deficiency. Commonly, numerical simulations take into account only the detonation energy in blast pressure analysis and it is responsible for under prediction of blast pressures in a confined explosion. Accounting for the afterburning energy as well considerably improves the predictions and yields pressures that are in good correspondence with measured data. The calculation time however increases by an order of magnitude. An afterburning coefficient was defined as the relation between the total energy released and the detonation energy. This coefficient was found useful for correcting numerical simulation results of TNT confined explosion which take into account only the detonation energy. This correction can be achieved by multiplying the pressure-time history with the afterburning coefficient. In addition, an analytic method, based on thermodynamic rules, was developed for calculating the gas pressure resulted by TNT confined explosion. This unique method takes into account the variation of the total energy released and the heat capacity ratio depending on the ratio between the charge weight divided by the confined air volume. The gas pressure obtained using this method was shown to be in good agreement with experimental results that are published in the literature.

A Coupled Approach to Simulate the Explosion Response of a Buried Structure in a Soil-Rock Layered Medium:

The paper presents an investigation on the response of a buried structure in a soft soil layer above a rock bed that is subjected to blast loading. A comprehensive approach to simulate the behavior of soft soil with a buried structure laying over a rock mass due to the dynamic (explosive) action in the rock mass is presented. The numerical algorithm was developed to simulate the shock wave propagation within the medium, considering both the bulk and deviatoric damage and taking into account the possible shear damage accumulation. It takes into account the contact conditions between the layers to simulate the shock wave transmission and the soil-structure interaction including their possible separation. The soil-lining interaction problem is solved by a combination of the variational-difference method (for the lining) and of the Godunovs method (for the soil). The coupling of these two approaches is performed by calculation of the contact stresses and velocities on the soil-lining boundary. Two types of the soil models were investigated – an ideal elastic plastic medium and an irreversible compressible medium with failure in bulk tension – and the results were compared with each other. The analysis of the free surface and interface velocities and displacements as well as the contact stresses and the damage at the media interface were carried out. The effects of the relative cover thickness on the interface and free surface behavior is studied. Analysis of the contact stresses at the soil-tunnel interface, was carried out. The effect of the charge location as well as the effect of the soil-rock interface on the tunnels response was investigated.

An Experimental Study on the Equation of State of Cementitious Materials Using Confined Compression Tests

The behavior of concrete under severe loading is of interest, especially for problems like ballistic impact and penetration and near distance explosions, where very high pressures are developed. For these problems the behavior of concrete at very high hydrostatic pressures is of importance. There is very little data available on concrete behavior at that high pressure level. Therefore there is much need for an extensive experimental work in order to provide necessary data and illuminate the rather obscure area of concrete behavior at high pressures. However high pressure controlled testing requires special and expensive equipment, and the testing is associated with a wide variety of technical problems. Recently published experimental data, obtained by utilizing a high-capacity tri-axial press, indicates that concrete that is subjected to high pressures behaves differently than concrete under low uniaxial loading. When uniaxial loading is applied, without any confining pressure, the concrete specimen demonstrates a well-known brittle behavior where failure is caused by a localized damage. Quite to the contrary, at high levels of confining pressures, the concrete behaves like a ductile material, and its failure is associated with diffuse material damage. The experimental data at the very high pressure range is most important to understand the processes of damage evolution that governs the characteristics of the equation of state. This paper presents the development of an experimental setup that is capable of performing confined compression tests of mortar and concrete specimens at high pressures up to 400MPa. The experimental study aims at investigating the effect of water/cement ratio as well as the ratio of fine aggregate on the different branches of the equation of state: active loading and unloading/reloading. The paper presents some of the test results as well as a new equation of state that is based on the multi scale approach. The model is applicable for dry materials; cementitious paste and concrete in which the pores are filled with water should be treated differently to account for the liquid phase.

Blast Pressure Distribution on a Buried Obstacle in a Porous Wet Soil

The paper presents a comprehensive approach to simulate the blast pressure distribution on a rigid or flexible planar or curved obstacle buried in a porous soil medium. The Lyakhov three-phase model is adopted to take into account the volumetric components of the soil medium (air, water, and solid matrix) and their properties, in the formulation of all the branches of the equation of state that simulates the soil. The present approach considers both the bulk and the shear elastic-plastic behavior, including the effect of soil pressure on the yield strength for the stress tensor deviator. Both Godunov and the variational-difference methods are applied to solve the problem of blast wave propagation within the soil medium. An example problem of an explosion in a porous medium is presented, and the analysis of the soil-obstacle interaction under the blast action using the proposed method shows good correspondence with available experimental results. The pressure distribution along the envelope of a rigid obstacle has been investigated for different standoff distances of the explosive from the obstacle as well as for different levels of soil water contents up to saturation. In addition, the plane problems of blast response of a thin-walled lining that is buried in this medium has been studied for different levels of water content and the linings extreme deformation including its dynamic buckling under the blast high pressures was investigated.

Response of a buried structure to surface repetitive loading

Analysis of the dynamic response of a buried structure subjected to steady-state vibrations at the soil surface is analyzed by a relatively simple model, yet comprehensive enough to delineate both wave propagation phenomena and effects of soil arching. Application of the model to a study case shows that there exists a range of external load periods which may cause significant increase of the structures deflection, and that as the frequency of the surface repetitive load increases it is more likely that it would match one of the natural periods of the soil-structure system, and special care would be needed in the design process, either to change the systems properties or to verify a proper amount of damping for the system. The analysis demonstrates the effect on the response, of the surface load period, of the depth of burial, and of the soil arching, and it shows the importance of the soil-structure systems parameters to form a basis for the design procedure of similar problems.

The Effect of an Intermediate Inclusion in Soil on a Buried Lined Tunnel due to a Nearby Explosion

The paper presents the analysis of a buried explosion in the proximity of a tunnel in a soil medium. The soil is modeled as an elastic plastic material with irreversible bulk and deviatoric strains. The tunnel lining is modeled as an elastic plastic Timoshenko shell. An inclusion is placed in soil between the explosion source and the tunnel and its effect on the tunnel response is investigated. A coupled approach is proposed to allow to perform stable calculations of that complex problem and to follow the development of the large deformations and distortions of the tunnel linings shape, as well as the large deformations of the explosive cavity. The effect of the rigid single inclusion or of the set of rigid inclusions located between the charge and the lined tunnel on the tunnel linings response has been studied. When the inclusion is relatively distant from the lining, even a small inclusion improves the state of displacement and pressure of the lining, while when the inclusion is placed close to the lining, the linings displacements increase. When the inclusion is small, its size slightly affects the lining shape while for middle sized inclusion the permanent displacement sharply decreases. Compared to the case of a single inclusion, a set of intermediate inclusions of the same size slightly decrease the peak stress at the front point and more significantly protects the periphery zone. The usage of larger inclusions yields better protection of the lining front part.