Civil Engineering Reference
In-Depth Information
σ
TRUE
=
(
FORCE
)(
L
)
A
0
L
0
(7.2)
where Force is the instantaneous measured force,
L
is the instantaneous gauge
length,
A
0
is the initial cross-sectional area, and
L
0
is the initial gauge length.
Thus, the presented stress represents a mean stress state for the material. Due to
diffuse deformation during the process (as a result of specimen geometry and ther-
mal conditions), regions in the test sample will experience both lower and higher
stress states. Also of note is that the overall achievable elongation is near equiva-
lent to the room temperature behavior of the material. It was expected that form-
ing under a constant load would improve the amount of deformation the material
could withstand before fracture. However, this did not occur as a result of interac-
tion of thermal gradients generating localized areas of deformation which lead to
regions in a much high stress state, ultimately causing fracture.
The current applied during the process is summarized in Fig.
7.7
where a maxi-
mum allowable current was set (300 A).The results show the filtered response, but
the current actually increases to 300 A and then shortly decreases as the forming
force is reduced. After this initial spike, the current is modulated by the controller
such that a constant force is maintained. As seen, lower forming force set points
require a greater current input to maintain the desired force level. To compare this
profile to constant force forming under a compressive loading, it is expected that
the current will again have an initial spike to reduce the forming load to the desired
force set point. Once the force set point is reached, the current would presuma-
bly increase during the forming process due to the increasing cross-sectional area
and material strain hardening. Also, for compressive operations, this may require
higher maximum allowable current levels to achieve the desired force set point.
Fig. 7.7
Current application during constant force forming. Absolute current decreases with
elongation due to reduction in area [
1
]
