Civil Engineering Reference
In-Depth Information
Table 11.1
EAB testing parameters [
1
,
2
]
Parameters
Bend diameter (mm)
Current density
(A/mm
2
)
Pulse duration (s)
Pulse period (s)
Values
38.1
20
2
15
20
30
3
30
60
50.8
density (Amps/mm
2
or A/mm
2
)]. Specifically, a tolerance of
±
0.127 mm was
held while shearing. Four uniaxial tensile tests were performed, and the material
parameters were determined as being
K
=
1,275 MPa and
n
=
0.51.
To fully examine the effects of electricity on an air bending process, many dif-
ferent parameters were explored, while holding others constant (Table
11.1
). For
all tests, a single displacement speed of 6.35 mm/min and a constant displacement
depth of 15 mm were used. A variable bending fixture was used to test two dif-
ferent bending diameters (38.1 and 50.8 mm). Additionally, two current densities
of 20 and 30 A/mm
2
were tested. Preliminary testing resulted in the selections of
these current densities since they were above the “electrical threshold” (i.e., the
current density in which formability enhancement occurs). Pulse durations of 2
and 3 s were tested in this work, since preliminary testing revealed that these dura-
tions were most beneficial. Pulse period (i.e., time in between electrical pulses)
was also varied, and pulse periods of 60, 30, 20, and 15 s were tested.
11.1.6.1 Experimental Setup
The EAB experimental testing setup can be seen in Fig.
11.6
. In the figure, a
manually adjustable bending fixture is utilized to produce a variety of die widths.
An insulated punch with a diameter of 15.9 mm was fabricated. Electrical pulses
were supplied to the bending fixture by a Lincoln R3R-500 constant current source
welding unit. Beneath the bending fixture, a Sensotec 22,250 N capacity load cell
with a signal conditioning box was used to record instantaneous bending force
data. The resolution of this system was
±
4.45 N, which was able to read the low
bending forces as well as help illustrate the flow stress drop-offs due to application
of the electricity. This instrument was used because the resolution of the load cell
on the testing machine was too low to recognize such low bending forces.
Insulation was used in the construction of the bending fixture for two main rea-
sons: (i) to ensure that all electricity flows through the sheet metal test specimens
and (ii) to prevent the high electrical current from harming any sensitive electronic
devices. Fiberglass-reinforced plastic was used for the insulating material, due to its
strong/rigid compressive properties. This material was used in the construction of
the punch (to prevent electricity from traveling into the Instron testing machine), was
placed between the contact points and the bottom of the bending fixture, and was
inserted between the bottom of the bending fixture and the load cell to ensure all
components were completely isolated from electricity.
