Chapter 1INTRODUCTION1.1 GeneralTerrorism is a great concern for the world today. With the development of science and technology the capability of the terrorists to destroy a structure has also been increased. At the same time to cause maximum hazard, they aim at the vital structures which can breakdown the bone marrow of the nation or the locality. So to protect our existence, the vital structures should be resistant to a certain level of terroristic activity.Water tanks play the most vital role in the existence of population. Because if a water tank is destroyed, it affects a huge population. Similarly the petroleum tanks and chemical tanks play very important role in the transportation, production, economy of a country. But if they fail due to terroristic attack, then it may be catastrophic and at the same time paralyzing also. So these structures should be equipped through proper research in order to prevent the destruction.Though research has been carried out in the field of blast response of tanks, no research work has been done in the base isolation of these tanks. It has already been proved that the isolators reduce the response of structures due to dynamic loadings, so the isolation will definitely have some effect on the response of tank. The research proceeds in the track line as:-study of explosives, process of detonation, evaluation of blast pressure, response of single degree of freedom system to impact and blast pressure, numerical analysis of response of isolated and non- isolated tank using three lumped mass model system, finite element modeling of tank and liquid mass and finding the response of fixed base and isolated tank and validating the results. The dynamics of explosion on the basis of conservation of energy is studied by Henrych (1979). Different blast wave front parameters have been studied by Rankine (1870) and Brode (1955) independently. Baker (1983) and Kingery (1984) have represented the blast wave parameters graphically. Blast wave effects on plates have been studied by Rajendran and Lee (2009). Remennikov (2003) has studied blast pressure on buildings. United states army has developed a manual TM-5-1300, now known as UCF 3-340-02, which gives details of blast resistant design. Tawadros and Glockner (1972) carried out experiment to get dynamic response data of shells under simulated blast wave. Ruiz et al. (1989) studied the elastic response of thin walled cylindrical shell to blast loading; analyzed both rotationally symmetrical and sideways loadings; the results obtained from both loadings were compared with those obtained experimentally. Gefken et al. (1988) studied the response modes of thin cylindrical shells when impulsively loaded with radial axisymmetrical loads; both internally pressurized and unpressurised shells were analyzed. Xi et al (2000) gave theoretical and finite element solution for an orthotropic thick cylinder under arbitrary impact loading. Jhung et al (2006) investigated the dynamic response characteristics of a structure impacted by high speed projectile and validated using LS-DYNA and ANSYS. Housner (1963) has given two-degree of freedom liquid lumped mass model. Haroun (1983) has modeled the liquid as three-degree-of freedom liquid lumped mass model. Shrimali and Jangid (2003, 2004) studied the response of base isolated liquid storage tanks to seismic excitation. Jaiswal et al. (2007) reviewed seismic codes on liquid containing tanks.1.2 ObjectivesThe aim of the present project is to study the blast response of fixed and isolated liquid tank numerically and by rigorous finite element modelling and to compare the results. In specific, objectives are:1. To numerically find the response of isolated liquid filled tank under blast loading using three-degree-of-freedom liquid lump mass model. 2. To compare the results with non-isolated liquid filled tank. 3. To find the response of tank with and without liquid. 4. Validating the results.5. To model the liquid mass and the tank using rigorous finite element analysis6. To do parametric study1.1Organization of reportThe report consists of four chapters including this introductory chapter. The chapter 2 consists of the review the state of art of explosion process, evaluation of blast pressure and other blast wave parameters, liquid mass modeling, blast response of non-isolated tank and earthquake response of isolated tank. Chapter 3 consists of response of single degree of freedom system to impact loading and blast loading. Chapter 4 consists of response of isolated and non-isolated tank using three degree of freedom lumped mass model. Chapter 2LITERATURE REVIEW2.1 GeneralThe study of blast effect is a combination of physical and chemical phenomenon. When an explosive is ignited, it decomposes violently with evolution of heat and gases. The rapid expansion of these gases results in shock pressure on the surface of solid bodies. The detail process of explosion, explosives used, and the evaluation of blast pressure are discussed in this chapter. The state of art is studied under the headings of liquid mass modeling, blast effect of tank without isolation and earthquake response of tanks with isolation.2.2 ExplosionsExplosions can be physical, nuclear or chemical. When explosion takes place due to sudden failure of physical structure is called physical explosion e.g. catastrophic failure of a cylinder of compressed gas. When explosion takes place due to formation of different atomic nuclei by redistribution of the protons and neutrons within the interacting nuclei is called nuclear explosion and when an explosion involves rapid combustion of different chemical compounds is called chemical explosion. Usually in terroristic attacks, explosives used are chemical in nature. So we will study the chemical explosion in more detail. A chemical explosion involves the rapid oxidation of fuel elements composed of hydrocarbon compounds. The oxygen required for combustion is contained within the compound. Most explosives are condensed under high pressure. When explosives react, they decompose violently with evolution of heat and gases. The rapid expansion of these gases results in generations of shock pressure in any solid materials which are in near vicinity. 2.2.1ExplosivesThe explosives which are usually used in military and terroristic activities are organic by origin. The elements which constitute the explosives are hydrogen, oxygen, carbon, nitrogen, and sulphur. Some commonly used explosives are 1- Pentaerythritol tetranitrate (C5H8O12N4), commonly known as PETN is used as a main charge in munitions and also in demolition detonators. The heat of explosion or mass specific energy Q of this material is 5940kJ/kg and generates 0.79m3 of gas per kg of the material.2- 1,3,5-trinitro-1,3,5-triaza-cyclohexene (C3H6O6N6), commonly known as cyclonitrite or RDX is used as a filling for military munitions and special applications such as linear cutting charges.3- 1,3,5,7-tetranitro-1,3,5,7-tetra-aza-cyclo-octane (C4H8O8N8), commonly known as HMX, which is a white crystalline solid used as a filling for high performance munitions.4- N-methyl-N-nitro-2,4,6-trinitroaniline (C7H5O8N8), commonly known as tetryl or CE which is used in military detonators.5- 2,4,6-trinitrotoluene (C7H5O6N3), commonly known as TNT. Probably it is the mostly widely used explosive. It is a pale yellow-brown crystalline solid having mass specific energy 4520kJ/kg and produces 0.73m3 of gas per kg of TNT. It is used in military munitions being mixed with other explosives such as RDX, HMX or ammonium nitrate.6- Nitroglycerine (C3H5N3O9), an oily liquid, commonly known as blasting oil. It explodes with great violence producing 6700kJ/kg of heat and 0.74m3/kg of gas.7- Nitroglycol (C2H4O6N2) explodes releasing very high energy i.e. 6730kJ/kg of material and liberates 0.74m3 of gas.Among all the materials TNT is the most widely used and very efficient explosive. So it has been accepted as the basis of conversion for all other explosives. An equivalent conversion factor for all the explosives has been decided according to their mass specific energy. The factors are given in tabular form below. For calculations of different blast parameters the explosive mass is converted to TNT mass by multiplying the TNT equivalent. Table 2.1 2.2.2 TerminologyCombustion is the term used to describe oxidation process in which oxygen required may be consumed from outside or that contained within.ExplosiveMass specific energyQ(kJ/kg)TNT equivalentQ/QTNTAmatol 80/20(80% ammonium nitrate, 20%TNT)2650 0.586Compound B(60%RDX, 40% TNT)5190 1.148HMX 5680 1.256RDX 5360 1.185Lead azide 1540 0.340Mercury fulminate 1790 0.390Nitroglycerine 6700 1.481Pentolite 50/50 (50% PETN, 50% TNT)5110 1.129PETN 5800 1.282Tetryl 4520 1.00TNT 4520 1.00Torpex (42% RDX, 40% TNT, 18% aluminium)7540 1.667Blasting gelatin(91% nitroglycerine, 7.9% nitrocellulose, 0.9% antiacid, 0.2%water)4520 1.0060% nitroglycerine dynamite 2710 0.600Deflagration is the combustion process in which the rate of decomposition is much lower than the speed of sound in the material. Deflagration is propagated by the liberated heat of reaction. The flow direction of reaction products is in opposition to the direction of decomposition.Detonation is the form of reaction of an explosive which produces a high intensity shock wave. The reaction rate is described by detonation velocity lies between 1500m/s to 9000m/s which is much faster than thermal conduction and radiation.Blast wave is the highly compressed layer of air which contains high pressure energy and so it propagates or expands at a very high speed in order to balance its pressure difference it possesses as compared to the atmosphere.2.2.3Generation and propagation of blast waveWhen a condensed high explosive is ignited, the explosives produce gases at a high rate as a product of detonation which is can be at a pressure of 10-30Gpa and a temperature of 3000-4000o C. A violent expansion of gases takes place and the surrounding air is forced out of the volume it occupies. As a consequence the layer of air surrounding the gaseous products is compressed, and this layer, which contains high pressure energy, is called the blast wave. As disequilibrium is established between the blast wave and surrounding air, the blast wave travels outwards from the centre of the explosion (expansion of blast wave). The pressure in the wave front decreases as it moves away from the centre of explosion. Due to the momentum of the gases, it over-expands and the pressure decreases below atmospheric pressure. This creates a negative or suction phase, which thus applies a decelerative force to the surrounding gas molecules, resulting in a backward flow of air towards the explosion centre. The variation of blast pressure with time at a fixed point is shown in the fig. 2.1.ps is peak over static pressure, p0 is ambient pressure or atmospheric pressure, Ts is the time of positive pressure, ts- is time of negative pressure and ta is time of arrival. The negative pressure is much less than positive pressure. The plot shows that after the time ta the structure exhibits the maximum pressure ps and then the pressure decreases exponentially to a negative pressure and then attains equilibrium. The positive pressure is of our concern. psp0Peak overpressureAmbient pressurePositive phase durationNegative phase durationp(t) Tsts-tatp Fig. 2.1 variation of pressure with time 2.2.4Blast wave parameters During the detonation process, a detonation wave generates and propagates in the explosive. The parameters that are used to assess the detonation performance of an explosive are the Chapman-Jouguet (C-J) detonation pressure, the temperature of detonation and the detonation velocity (Rajendran and Lee, 2009). Typically for an explosive(TNT) with a density of 1650kg/m3, the Chapman-Jouguet (C-J) detonation pressure is 21,000MPa, the detonation temperature is 3720k and the detonation velocity is 6950m/s. once the process of detonation is completed, the interaction of product gases with the surrounding medium takes place. The product gases with high pressure and temperature expand outward by generating pressure wave. The gaseous products are assumed to be in viscid at high temperature and thus the viscous forces are not considered for explosive modeling. In air explosion, the shock wave moves with the gas-air interface. An equation of state (EOS) of the explosive relating energy, pressure and volume is essential for the numerical modeling of the detonation process (Rajendran and Lee,2009).the most commonly used EOS to describe the state of detonation products is Jones-Wilkins-Lee (JWL),which is given as 1 2jwl in1 2( , ) 1 e 1 eRV R Vp V U A B ERV R V V | ` | ` + + . , . , (2.1) Where A, B, 1R, 2Rand are material constants, jwlpis the pressure, Vis the relative volume compared to the initial volume of the explosive and inUin is the internal energy per volume and E is the initial energy per unit mass of explosive. The first in the JWL equation is known as the high pressure term dominates first for close V to 1, the second term is influential for Vclose to 2 and last term corresponds to the expanded state.The blast pressure time history is given by Friedlanders equation (Smith and Hetherington, 1994)as s( )( ) 1 expsb t t tp t pT T ] ] ] ] ] ] (2.2) Where, spis peak over static pressuresTis time of positive blast pressure atis time of arrival bis decay coefficientPeak over static pressure is the maximum pressure at a particular point which may occur for a larger distance with a larger intensity of blast and can also occur for less distance and less intensity of blast. So, scaled distance is used to evaluate the intensity of blast pressure at a point considering the distance and weight of explosive used in terms of TNT (Smith and Hetherington, 1994).1 3=RZW(2.3) Where, R is the radial distance from the center of explosion W is the weight of TNT in kilogram For chemical explosions, the peak overpressure is expressed as (Brode, 2009):(a) Near field explosion condition, 36.71spZ + bar (ps> 10 bar) (2.4) (b)Far field explosion condition, 2 30.975 1.455 5.850.019spZ Z Z + + bar (0.1