Aerospace & Defense Technology - February 2021 - 22

Aerospace Manufacturing

Steel

HRC

YS ksi

UTS ksi

El %

RA %

CVN, ft-lb
@ 70°F

@ -40°F

Military

50-51

195-205

255-265

14-15

48-54

38-42

28-32

Eglin

48-50

195-205

250-260

12-14

44-48

26-32

11.5-15.5

USAF-96

48

187

245

13

-*

-*

30

*unknown

Typical non-penetrating munitions include aerial laser-guided bombs (top) and the laser-guided Mk 84 GP
bomb (bottom). Although effective against surface targets, neither will penetrate reinforced bunkers.
(Photo courtesy of author)

gets [2] . A newly developed highstrength steel ( " Military-Steel " ) exhibits
superior strength to Eglin steel at the
same level of ductility and toughness[3].
Concentrations of expensive elements
and the cost of raw materials of Military-Steel are significantly lower than
Eglin steel, while the cost of melting,
hot forging, and heat treatment are
comparable.
The table above shows the typical
room temperature quasi-static tensile
test results of the air-melted, quenched
and low-tempered Military-Steel, Eglin
steel, and USAF-96 steel [4] , wherein
HRC, YS, UTS, El, RA, and CVN are
Rockwell hardness scale C, a yield
strength at 0.2% offset, an ultimate tensile strength, an elongation at break, a
reduction of area, and Charpy v-notch
impact toughness energy respectively.
Eglin steel exhibits ~10% increase in
yield and tensile strength when strainrates are increased from quasi-static to

200 s -1, Military-Steel shows approximately the same sensitivity to increasing strain-rates. The tensile and yield
strengths of the steels at high strainrates will be referred to as the dynamic
tensile and yield strength.
Military-Steel is superior to the Eglin
steel and USAF-96 steel due to:
* Higher hardness and strength at the
same ductility and toughness;
* Better formability at hot working;
* Higher hardness and strength at the
same ductility and toughness;
* Reduction in total raw material cost by
50% or more through lower concentrations of Mo and elimination of W;
* Better formability at hot working and
better machinability.
New designs of the monolithic cases
for deep penetrating bombs BLU-122,
BLU-109 and its improvement, the BLU137, have been developed to increase
their penetration distances. The new designs are based on the concept that the

22

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Cov

ToC

material and nose of a penetrator play
crucial roles in the strength and durability of their cases. If the material of the
penetrator does not have enough
strength, large deformation of the nose
and wall occurs, and as a result, the
nose is flattened, and the wall is
warped. On the other hand, if the highstrength material of the penetrator does
not have enough impact toughness, a
fracture of the nose and wall occurs.
Both scenarios lead to a reduction of
penetration distance and do not meet
the strict requirements for the penetrators. Therefore, the main parameters
that affect penetration distance are:
strength and toughness of the case material; shape and length of nose; thickness of the wall; length and diameter of
the case. The new monolithic cases
have the same weight as the standard
cases and the same capacities of explosive materials. Projected penetration
distance in 5,000 psi (34.5 MPa)
strength reinforced concrete of the BLU122 penetrator with the new monolithic case made from Military-Steel is
40% higher than the penetration distance of the standard case made from
Eglin steel[5-6].
Recently developed ultra-high-performance concrete (UHPC) can withstand
bombs and bunker penetrators, making
it a unique new material that can be utilized for bunkers[7]. UHPC is made by
combining pure powdered quartz with a
mixture of metals and nanofibers and
has compression strength of 30,000 psi
(207 MPa) compared to traditional concrete at just 4,000 psi (27.6 MPa).
To reach and destroy well-protected
bunkers, new weapon systems such as
the 30,000 lb (13,600 kg) Massive Ordnance Penetrator (MOP) and the High
Velocity Penetrating Weapons (HVPW)
have been designed. However, neither
BLU-122 bomb nor MOP or HVPW
bombs can penetrate bunkers made
from UHPC with compression strength
of >10,000 psi (69 MPa) and thickness
of >10 ft (3 m). Penetrators with monolithic cases made from materials such
as 250-260 ksi (1725-1800 MPa)
strength Eglin steel or 280-300 ksi
(1930-1970 MPa) strength MilitarySteel cannot penetrate bunkers made
from UHPC.

Aerospace & Defense Technology, February 2021
http://www.aerodefensetech.com http://www.abpi.net/ntbpdfclicks/l.php?202102ADTNAV
Aerospace & Defense Technology - February 2021

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