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

(can’t be manufactured economically using conventional processes)

In the recent times, explosive forming has developed into a cost-effective 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 PROJECTILE

Armour penetration at standoff distances

WW2WW2

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

DynamitePETN (pentaerythritol tetra nitrate)TNT (trinitrotoluene)RDX (cyclotrimethylene-trinitramine).

PRESSURE RANGE ~ (13.8–27.6 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 ~ 2–3×104 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

Most general arrangement

Resemblance with deep drawing.

( 3 mmHg )

x

Best results with

Standoff = 2x

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 s−1)

Metal forming speed 100–600 ft/s (30–200 m s−1).

DIES

Few parts --- concrete

Small explosive forces --- glass fibre reinforced epoxy resins

high pressure intensities --- ductile cast iron

and frequent use

high quality surface finish --- machined tool steel

and long production runs

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.

PROCESS ECONOMY

ACHEIVABLE TOLERANCES

±0.025 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)

0.0500.100Thickness

0.1280.254Diameter

PossibleNormal

Tolerance (mm)Dimension

As a comparison tolerances of 0.03–0.2 mm have been reported for the deep drawing of components with diameters of 500 mm .

Tolerances obtainable when explosive forming large domes

T

C

HARDNESS

Workhardening is less as a result of dynamic deformation ( 10 –103 s−1 )

than during static deformation to an equivalent strain (IRON AND STEEL)

+1.533.832.3Vickers35Not reportedAluminium

−13113126Brinell4.1CompressionMild steel (0.24% C)

−4151155Vickers8.0TensionMild steel (0.2% C)

−1095105Vickers2.6CompressionArmco iron

Dynamically applied strain

Statically applied strain

Difference in hardness(%)

Hardness valuesMethod of measuring hardness

Percentage strain (%)

Method used to apply static strain

Material

STRENGTH

0.80.4339.9343.45.25.0Stainless steel (AISI 304)

−18.9−9.0266.8328.94.42.9Mild steel (0.2% C)

−12.4−4.7229.6262.08.07.8Mild steel (0.025% C)

−8.0−2.6206.2224.12.72.5Armco iron

Difference in flow stresses (%)

Difference in flow stress: dynamically and statically pre-strained samples

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

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

Total strain (%)

Pre-strain (%)

Material

−5.4−1.9228.2241.35.25.0Al–2.5Mg alloy (5056-O)

10.90.7754.148.77.05.5Aluminium (99.99%)

4.60.3047.245.215.014.2Aluminium (99.95%)

Difference in flow stresses (%)

Difference in flow stress: dynamically and statically pre-strained samples

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

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

Total strain (%)

Pre-strain (%)

Material

WIN-WIN situation for aluminium

FRACTURE TOUGHNESS

Fracture 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 BEHAVIOUR

not 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 chargemust 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 )

−8 to −12.5−14.0 to −16.5−14.0 to −16.5Thickness strain

4.0–6.37.0–8.27.0–8.2Surface strain

Full-scale observation (%)

Full-scale prediction (%)

One-fifth scale model (%)

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

CONCLUSIONS

STRENGTS

-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