Explosive Forming

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Explosive forming basics and applications

Text of Explosive Forming

EXPLOSIVES

~ DESTRUCTION

// NOW, THE SAME TECHNOLOGY IS USED FOR CONSTRUCTIVE PURPOSES //

A cockpit fuselage formed using explosive forming

WHY DO WE NEED IT?MAJOR USER THE AEROSPACE SECTOR BIGGER AEROPLANES BIGGER ENGINES BIGGER PARTS (cant be manufactured economically using conventional processes)

In the recent times, explosive forming has developed into a costeffective process for forming a variety of metals and metal alloys. This has resulted in a high degree of reproducibility for complex, large metal structures to tight tolerances.

Afterburner fuel rings

Jet engine diffusers

Missile domes

Heat shields for turbine engines

MILITARY APPLICATIONEXPLOSIVELY FORMED PROJECTILEWW2

Armour penetration at standoff distances

In addition to the previous applications a variety of other forms have been fabricated including: dome shapes beaded panels large shallow reflectors shallow and deep rectangular boxes manhole access covers equipment covers large cylinder parts turbine housings

Spherical vessels of diameters ranging from 300 to 4000 mm have been produced using die-less explosive forming

used as propellants

DYNAMITE RDX

Explosive metalworking exclusively employs secondary explosives such as Dynamite PETN (pentaerythritol tetra nitrate) TNT (trinitrotoluene) RDX (cyclotrimethylene-trinitramine).

PRESSURE RANGE ~ (13.827.6 GN/m2 )

ENERGY COMPARISION 1.5 kg of high explosive ~~~~~ 7.5 MN press

POPULAR IN USAGE:

primacord

Sheet explosive

Deformation is the main tool of explosive forming processes. the aim is to achieve the required deformation in the least number of operations, using the largest permissible weight charge.

During the detonation detonation wave mass of gas Pressures ~ 23104 MPa.

The expansion of this high temperature, highly compressed gas bubble against its surroundings provides the energy for explosive forming.

The volume of gas liberated is approximately 1 litre/gm of explosive.

SHEETMETAL WORKPIECE CONFIGURATION

( 3 mmHg ) x

Best results with Standoff = 2x Most general arrangement Resemblance with deep drawing.

ALTERNATIVE CONFIGURATION

HOW IT WORKS?THE EXPLOSION A primary shock wave travels out from the gas bubble through the surrounding water carrying 50% of the explosive energy. The primary shock wave in the fluid impinging on a blank imparts to it an initial velocity. This lowers the pressure in the water adjacent to the blank until cavitation occurs. reloading phenomenon delivers even more energy to the blank than the primary shock wave ( has been verified experimentally.)

ENERGY TRANSFER MEDIA

Water ( the most common)

Air

Plasticine (deformation of localised areas)

Detonation speeds are typically 22.2 ft/s (6.8 m s1) Metal forming speed 100600 ft/s (30200 m s1).

DIES

Few parts

---

concrete

Small explosive forces

---

glass fibre reinforced epoxy resins

high pressure intensities and frequent use

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ductile cast iron

high quality surface finish and long production runs

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machined tool steel

PROCESS ECONOMY

The capital cost of an explosive forming facility are reported as being less than that of a conventional facility of equal capability by a factor ranging from 10:1 to 50:1. On the other hand, labour costs per part can be appreciably higher for explosive forming.

ACHEIVABLE TOLERANCES0.025 mm ----------------on small explosively formed parts

Final part tolerances behavior: first decreases, almost linearly, with an increase in charge weight finally becomes approximately constant ( hardness and the modulus of elasticity)Dimension Tolerance (mm) Normal Possible

TDiameter Thickness 0.254 0.100 0.128 0.050

C

Tolerances obtainable when explosive forming large domes

As a comparison tolerances of 0.030.2 mm have been reported for the deep drawing of components with diameters of 500 mm .

HARDNESSWorkhardening is less as a result of dynamic deformation ( 10 103 s1 ) than during static deformation to an equivalent strain (IRON AND STEEL)Material Method used to apply static strain Percentag Method of e strain measuring (%) hardness Hardness values Difference in hardness(%)

Statically applied strain Armco iron Mild steel (0.2% C) Mild steel (0.24% C) Aluminium Compression Tension Compression Not reported 2.6 8.0 4.1 35 Vickers Vickers Brinell Vickers 105 155 126 32.3

Dynamically applied strain 95 151 113 33.8 10 4 13 +1.5

STRENGTHMaterial Prestrain (%) Total strain (%) Static flow stress values from samples subjected to static pre-straining (MPa) 224.1 Static flow stress values from samples subjected to dynamic pre-straining (MPa) 206.2 Difference in flow stress: dynamically and statically prestrained samples 2.6 Differen ce in flow stresses (%) 8.0

Armco iron Mild steel (0.025% C) Mild steel (0.2% C) Stainless steel (AISI 304)

2.5

2.7

7.8

8.0

262.0

229.6

4.7

12.4

2.9

4.4

328.9

266.8

9.0

18.9

5.0

5.2

343.4

339.9

0.4

0.8

Material

Prestrain (%)

Total strain (%)

Static flow stress values from samples subjected to static pre-straining (MPa)

Static flow stress values from samples subjected to dynamic pre-straining (MPa)

Difference in flow stress: dynamically and statically prestrained samples

Differen ce in flow stresses (%) 4.6

Aluminium (99.95%) Aluminium (99.99%) Al2.5Mg alloy (5056O)

14.2

15.0

45.2

47.2

0.30

5.5

7.0

48.7

54.1

0.77

10.9

5.0

5.2

241.3

228.2

1.9

5.4

WIN-WIN situation for aluminium

FRACTURE TOUGHNESSFracture toughness is a property which describes the ability of a material containing a crack to resist fracture, one of the important parameters in designing. Explosive forming does not have any appreciable effect upon fracture toughness.

FATIGUE BEHAVIOURnot influenced significantly by the deformation process, irrespective of the process type.

Relative formability of different metals under explosive conditions

MODELLING AND SIMILTITUDE

Small-scale trials are often used before full sized dies are manufacture .

The scaling law requires that the mass of full-scale explosive charge must be n3 times the mass of small-scale charge, where n is the ratio of the full-scale die opening to the corresponding small-scale value. ( USED AS FIRST APPROXIMATION )One-fifth scale model (%) Surface strain Thickness strain 7.08.2 14.0 to 16.5 Full-scale prediction (%) 7.08.2 14.0 to 16.5 Full-scale observation (%) 4.06.3 8 to 12.5

predicated and observed strain for a dome structure formed by explosive forming

CONCLUSIONSSTRENGTS-explosive forming is versatile (complex shapes possible) -requires low capital investment -increased ductility that may be obtained at certain deformation velocities

WEAKNESS-requirement of specialist process knowledge -the need to handle explosives. -adverse effect on work piece surface due to shock waves

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