Explosive Forming Process

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Presentation on the topic Mechanics for Explosive Forming Process

Gagandeep Singh 820982008 ME(P&I Engg.)

INTRODUCTION The explosive forming of metals has been known for about

100 years. Explosive forming was first proposed at the Kharkov Aviation Institute in the 1940s and was widely used in the mid-1950s in the production of oversize parts for rockets and airplanes. Some suggestions to apply explosives were made at end of the 18th century, but they were not implemented as industrial applications. Practical research on explosive forming of pipe-shaped specimens has been conducted since 1970, in the G. D. R. (German Democratic Republic) , at the Ludwigsfelde truck company and at the Magdeburg Technical University.

WHAT IS EXPLOSIVE FORMING PROCESS The metal forming involves the detonation of an explosive

charge, causing a shock wave and a large momentum in the water, the expansion of detonation gases, and the subsequent interaction with the metal plate. A metal plate is clamped into a mould, and then immersed in a water basin. The energy releasing upon explosion of high explosive substance acts directly or through a conductive medium on a metal plate and deform it to fit the die profile. Usually, explosive forming is done with the explosive charge in water, at a certain stand-off distance of the part to be formed

E F P SET UP

The charge used is very small, but is capable of exerting

tremendous forces on the work piece. In Explosive Forming chemical energy from the explosives is used to generate shock waves through a medium (mostly water), which are directed to deform the work piece at very high velocities. The shock waves act as a punch. Explosive Forming is also known as HERF (High Energy Rate Forming)

METHODS OF EXPLOSIVE FORMINGExplosive Forming Operations can be divided into two groups, depending on the position of the explosive charge relative to the work piece.

Standoff Method In this method, the explosive charge is located at some

predetermined distance from the work piece and the energy is transmitted through an intervening medium like air, oil, or water. Peak pressure at the work piece may range from a few thousand psi (pounds/inch2) to several hundred thousand psi depending on the parameters of the operation.

Schematic Diagram of Stand off Method

Contact Method In this method, the explosive charge is held in direct contact

with the work piece while the detonation is initiated. The detonation produces interface pressures on the surface of the metal up to several million psi (35000 MPa).

WORKING OF E.F.P The system used for Standoff operation consists of

following parts: 1) An explosive charge 2) An energy transmitted medium 3) A die assembly 4) The work piece.

E F P SCHEMATIC DIAGRAM

Figure 1 shows an arrangement of Standoff Explosive forming

operation. The die assembly is put together on the bottom of the tank. Work piece is placed on the die and blank holder placed above. A vacuum is then created in the die cavity. The explosive charge is placed in position over the centre of the work piece. The explosive charge is suspended over the blank at a predetermined distance (known as standoff distance). The complete assembly is immersed in a tank of water. After the detonation of explosive, a pressure pulse of high intensity is produced. A gas bubble is also produced which expands spherically and then collapses until it vents at the surface of the water. When the pressure pulse impinges against the work piece, the metal is displaced into the die cavity.

EXPLOSIVE FORMING EQUIPMENT

EXPLOSIVES USEDExplosives are substances that undergo rapid chemical reaction during which heat and large quantities of gaseous products are evolved. Explosives can be Solid (TNT-trinitro toluene), Liquid (Nitroglycerine), or Gaseous (oxygen and acetylene mixtures). The type of charge, its location, method and place of its initiating determine the pressure area of a sheet metal being deformed. Most distributed types of charges are: concentrated charge (generally spherical), linear (detonating cable), adjustable charge.

Explosives are divide into two classes; Low Explosives in which the ammunition burns rapidly rather than exploding, hence pressure build up is not large, High Explosive which have a high rate of reaction with a large pressure build up. Features of Low and High ExplosivesPROPERTY HIGH EXPLOSIVE LOW EXPLOSIVE

METHOD OF INITIATION

Primary HE-ignition, spark, flame or impactSecondary HE-detonator , or detonator and booster combination

Ignition

CONVERSION TIME PRESSURE

Microseconds Upto 4,000,000 psi

Milliseconds Upto 40,000 psi

PROPERTIES OF SOME EXPLOSIVESExplosives Relative Form of power charge (% TNT) 170 Pressed granules Detonatio n Velocity, m/s 8380 Energy, KJ/kg 1270 Maximum pressure, GPa 23.4 RDX (Cyclotrimethylene trinitramine, C3H6N6O6)

TNT (Trinitrotoluene, C7H5N3O6PETN (Pentaerythritol tetranitrate, C5H8N12O4) Tetryl (Trinitrophenylmethylini tramine, C7H5O8N5) Blasting gelatin

100170

CastPressed granules Pressed granules

70108290

7801300

16.522.1

129

7832

-----

-----

99

Cartridg e plastic

7985

1220

17.9

DIE MATERIAL Different materials are used for the manufacture of dies for

explosive working, for instance high strength tool steels, plastics, concrete. Relatively low strength dies are used for short run items and for parts where close tolerances are not critical, while for longer runs higher strength die materials are required. Kirksite (An alloy of Al-Zn, can be casted at low temperature) and plastic faced dies are employed for light forming operations; Tool steels, cast steels, and ductile iron for medium requirements.

APPLICATION AREAS FOR DIESMaterial of die Kirksite Fiberglass and Kirksite Application area Low pressure and few parts Low pressure and few parts

Fiberglass and ConcreteEpoxy and Concrete Ductile Iron Concrete

Low pressure and few partsLow pressure and few parts High pressure and many parts Medium pressure and large parts

CONDUCTING MEDIUM Gaseous, liquid, elastic and loose medium is used in practice

as a conductive medium. The use of water as a universal conductive medium of good characteristics for transmission of energy impulse has gained most spread. It has been observed that energetic efficiency in explosive forming by using conducting medium as conveying medium is 4 to 5 times more than the energy efficiency of same process using air as medium

PARAMETERS The process typically involves placing the metal over a die

and then using the concussive force of high explosives to press it into shape. Considering the important influence of the shock wave produced by the explosion and also the effect of medium (in which explosion takes place) on transition of shock wave to work piece during explosive forming, analysis of shock wave transition and its effective parameters such as characteristics of conveying medium, have crucial role in understanding the explosive forming process and also help to have control over it.

Many researches and experiments have been conducted in

such concern. One of such research is being tried to explained in upcoming slides. Explosion is important step in this process. The explosion in two different condition is performed that is in air and under water. The effect of the same is observed. Now an Explosion is a phenomenon which results in the sudden release of energy from an explosive charge as a sort of environmental source of energy. The explosive charge is in fact defined to be an unstable compound that the sudden release of energy causes several chemical changes in it and so becomes a more stable product. The very first mechanical effect of explosive blast is a forceful blow from the instantaneous pressure jump, in its shock front.

EXPLOSION IN AIR The sudden change of the condition in the surrounding

environment is in fact the first step of the complicated explosion phenomena. The explosive blast travels faster than the speed of sound and hence an explosion does not give its target any advance warning of impending destruction Analyzing the pressure of a definite point, right after the explosion in air shows that it suddenly increases in some micro seconds and reaches its maximum value and then starts decreasing (the positive phase) and goes under the pressure of the environment (the negative phase). Impulse is an important aspect of damage causing ability of the blast

The impulse of unit area of the shock wave front up to the

time t after the arrival is given by: (1) There are several equations in different references to

calculate impulse of an explosion. For example Goodman presented expression to calculate the impulse of unit area for in air explosion as given below: (2) 0.04337

z 9.1020

In which the scaled distance Z is defined as:

(3)

In the above expression, R is the stand-off in meter and W is the equivalent TNT mass of the used explosive charge in Kg.

EXPLOSION UNDER WATER Regarding the under water explosion, the propagation velocity is

many time more than the speed of sound but within the distance of 10 times of the charge radius from the centre of the explosion, it rapidly drops to the sound velocity (approximately 1440 m/s for sea water). The sudden energy release associated with the explosion of a high explosive leads to the formation of a superheated, highly compressed gas bubble and the generation of a shock wave in the surrounding water. The underwater shock wave generated by the e