533 Prediction of Initial and Striking Velocity of Primary Fragments from Cased Spherical Explosive inside Steel Cubical Structure Prahalad Srinivas Joshi * and S.K. Panigrahi Department of Mechanical Engineering, DIAT, Girinagar, Pune – 411 025, India * Email: prahjo20904@gmail.com AbStrACt Usually, energy generated from an explosive’s detonation is transferred partly in the form of the blast impulse and some in the form of the kinetic energy of casing fragments. When detonation occurs in an explosive casing, it breaks the casing into fragments of diferent weights with varying velocities. The extent of destruction by these energized fragments depends upon the initial velocity they gain after an explosion. The momentum gained by the fragments decides the capability to perforate a barrier or propagate an explosion. A three-dimensional non-linear FEA method is used to model a box-shaped steel structure. This box-shaped structure is subjected to an internal cased explosion for estimating the initial and striking velocities of primary fragments. The efect of varying charge weight and the efect of the sacrifcial wall on the initial and striking velocity of fragments via numerical simulations are also carried out. The initial and striking velocity values obtained through simulation are compared with the design guidelines of the code-based approach, and a good agreement is reported. Keywords: Explosive casing; Gurney equation; Initial velocity; Primary fragments; Striking velocity Received : 25 March 2022, Revised : 02 July 2022 Accepted : 15 July 2022, Online published : 26 August 2022 Defence Science Journal, Vol. 72, No. 4, July 2022, pp. 533-542, DOI : 10.14429/dsj.72.18001 © 2022, DESIDOC 1. IntroduCtIon Usually, the high explosives (HE) are enclosed in metal casings while in service. Upon detonation, HE produces exceptionally high pressure and hot gasses, which in turn pressurizes the damaged inner walls of the metal casing to form primary fragments. When they impact surrounding structures, these primary fragments develop secondary fragments and infuence the design of the armament and other protective structures. Thus, studying the fragment velocity could beneft design guidelines for the efectiveness of weapons consisting of HE in a closed container like a warhead or bomb. A mathematical model was developed by assuming an infnitely long cylindrical casing, allowing only the radial motion of fragments and nullifying the efect of detonation in the longitudinal direction. 1 Based on this, initial velocities of fragments due to the detonation of metal casings flled with HE was calculated using Eqn. (1) and verifed with experimental results. 2 The initial velocity of fragments is determined by the equation given below: 0 2 1 0.5 β = + β v E (1) where v 0 is the initial velocity of fragments, √2E is Gurney energy, E is contribution to the explosion’s kinetic energy due to HE’s unit mass, and β is the ratio of casing mass to explosive mass. Relation between stress state and thermo-plasticity of cylindrical metal casings flled with HE with a slight modifcation of Taylors 1 hypothesis of radial fracture mode was proposed by Hoggatt and Recht. 3 It has been observed that the walls of the metal casing accelerated radially outward due to compressive hoop stress produced during the detonation of the HE. It was also attempted to fnd speeds of metal layers after detonation in the case of multi-layered warheads by Jones. 4 Jones-Wilkins-Lee equation 5-7 is widely used to study the reaction processes of detonation of HE. Gas leakage in an internal blast event was identifed, and the Gurney equation was modifed by multiplying by a factor of 0.8 to overcome. The factor accounted for the efect of gas leakage, which was verifed by experimental results obtained by Charron. 8 The material properties of the metal casings and HE characteristics primarily afect the detonation outcome. Pearson 9 observed that the behavior of fragments is not a separate event and can infuence the total system. A series of aluminium and copper cylinder expansion tests were carried out for a varied mass of explosive to mass of cylinder ratio and explosives. These were further verifed by performing the expansion tests for evaluating and describing the acceleration characteristics of the metal fragments by Bola, et al. and Kury, et al. 10-11 Subsequently, many researchers 12-14 calculated the fragment velocities by modifying Gurney energy as mentioned in Eqn. (2), given below. G D 2E 3.08 = (2) where, D is detonation velocity, and E G is Gurney energy.