Structural Design and Analysis of an Existing Aerodynamically Optimised Mortar Shell

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2008-01

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Addis Ababa University

Abstract

Mortar shell design is one of the most critical components in defence organization of projectile design. Unavilability of data and literature regarding artillery projectiles and rocket warheads, are considered to be one of the main contributors for the failure of manufacturing projectile in a local industry. The need for this study is to design and fabricate this type of mortar shell in Ethiopia itself. Thus, the primary objective of this thesis is to develop a ballistics model of mortar shell and predicting the performance of mortar casing by ANSYS a finite element analysis package. Initially, the model is designed as a simplistic ballistic model capable of predicting the pressure and thrust generated by a propellant from a given set of input parameters. Its structure was determined on the basis of gun design empirical, experimental investigations of ballistic research laboratories (BRL) and weapon design of ammunition and artilleries of high explosive calibers. High explosive warhead performances depend on its geometrical shape and dimensions, mass of explosive charge and explosive type, material of warhead case, initiation way and initiation point position, fuse type, round to round variations, etc. These are important parameters in determining the state of projectile features and stresses during the design of projectile. Thus, in this work a parametric study is conducted by varying the casing thickness and its length to study their effect on the fragmentation and stability of mortar projectile for the selected design parameters. Analysis of stresses and deformation in the casing is an important area of research for projectiles optimal design. This thesis investigates the characteristics of high explosives warhead fragmentation analysis This research not only gives fragmentation analysis of the shell by continuum method and trajectory calculations, but also gives the stress values for three different loading conditions correspond to launching, hitting the target and subsequent pyro blast. To estimate the structural stress, three-dimensional model of a mortar shell was made by finite element method using ANSYS. This thesis also considers the study of variation of primary and secondary propellant charge on range. The results obtained by this research are presented and compared with the available literature. This study not only validates the design of the mortar shell, but also helps the Defense Ministry go ahead with its production for its need. xiii Objectives of the thesis The specific objective of this thesis work is to determine structure and shell thickness for maximum fragmentation and to analyze mechanical stress due to different loads acting on 120mm mortar shell. The objective includes in general terms the following. • To determine the basic structure of the mortar shell • To determine the fragmentation, internal ballistic, range and stability of the mortar shell • To determine the variation of the mechanical stress and deformation across the shell at three positions at the launching caused by propellant gas pressure and temperature impact at the instant of hitting the target due to developed explosion pressure • To select the appropriate material and dimension in order to withstand the mechanical stresses before bursting pressure • To determine and analyze the structural performance of the mortar shell using finite element analysis (FEA) These objectives will be achieved by employing the following procedures: • Designing the structural configurations based on the gun design empirical and experimental standards • Warhead fragmentation analysis • Creating a 3D modeling of the shell • Develop finite element solution/ANSYS analysis for the problem. • Analyzing of FEM results in reference of failure criterion. • Suggesting appropriate design and method of manufacturing. xiv Limitation and Scope of the work Projectile design is a process that entails many complicated procedures which involve many aspects of knowledge, experience and interrelationships between disciplines. These decisions were typically made sequentially by individuals or teams with expertise in various areas of the design process. The design process utilizes a combination of hand estimations, predictions from software codes, and physical testing at each phase of the design process, iteratively, in order to arrive at an optimum configuration. Each discipline involved in the design process has over-time developed its own set of automated tools. Traditionally, projectile designers used a combination of formulas, charts, and rules of thumb and verified their predictions with experiments. These experiments included mechanical testing, wind tunnel testing, flight testing and target effects testing of pit fall and arena fragmentation. Measurements of warhead performances require very complex measuring equipment and measuring process itself is expensive as well. Out of many years of such experimentation, useful references evolved, such as the Army Design Handbook and Pamphlet series, specialized notes, and textbooks. These are not completely ignored in contemporary procedures and codes and will be included in this environment. These basic methods and models will be part of the initial (or rough cut) phase estimates in this system. Due to unavailability of datas, softwares and test facility, the detail of exterior and intermediate ballistic parts, physical and chemical behaviour of explosives and the effect of fragment with the body of human tissue aren’t taking into account in this work. It has been shown that the development of projectile consists of four stages. In the first stage the basic structural shape dimension are determined based on the required effect of fragment and range causes the internal mechanical response of the mortar shell inside and outside the barrel structures. The second stage consists of the development of fragmentation analysis. This will be done using warhead fragmentation analysis. The present work focuses on the first stage: the determination of the internal and external mechanical response of mortar casing using finite element models. There have been some efforts made to integrate the various areas of projectile design; the software code ANSYS comes closet to achieving this. Important aspects of these models are: geometry, loading that mimic the actual loading conditions and interface conditions with the barrel structures. The focus in this study will be on designing , anlysing at each design phases and modeling of mortar shell based on gun design method. xv Outline of the Thesis In Chapter 1, we give an overview of mortar projectile, system description and literature review. Chapter 2, Different gun design equations and empirical relations of high explosives are used and a suitable modification to determine the basic structural configuration of the shell at a desired load level is incorporated. Material properties of fabrics is described. In Chapter 3, the thickness of the shell is determined from the fragmentation point of view. Intensive Mott’s equation computations have been performed to search for the best fragment velocity and optimum fragment weight. In Chapter 4, here, the primarily concern of basic internal ballistics are studied to determine muzzle velocity, pressure and charge consequence on range. In Chapter 5, Aerodynamic moments and forces for two dimensional domains are presented. A method to compute the static and dynamic stability of a spinning and finned type projectiles is validated. In Chapter 6, modeling and finite element solutions are presented for three conditions of the projectile along its trajectory from launching up to its end effect. In Chapter 7, we draw conclusions from this thesis and suggest possible avenues for future work.

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Applied Mechanics Stream

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