buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

208 Ovidiu Niţă and Vasile Braha
3. Experimental Results
As was stated in the introduction Forming limit diagram (FLD) is
determined experimentally through the points of coordinates (ε1, ε2), where ε1
and ε2 define the appropriate limit deformations of a given mode of loading of
the specimen. The mathematical relationship that characterizes the addiction
between ε1 and ε2 is, in fact, the equation which outlines the forming limit
curve.
After conducting the experimental research it was found that, in all
studied situations, the mathematical equation that fits the best on our case is the
second-degree polynomial relation
2
y = ax + bx + c . (1)
Fig. 4 shows, for exemplification, the forming limit diagrams determined
for 0.5mm and 1.5mm, brass and aluminium specimens, using circular active
plate.
e1
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.00
0.02
FLD brass 0.5mm
0.04
0.06 0.08
e2
0.10
0.12
0.14
e1
0.25
0.20
0.15
0.10
0.05
0.00
0.00
FLD aluminum 1.5mm
0.05
0.10
e2
a b
Fig. 4 – FLD obtained with circular active plate:
a – brass specimen 0.5mm thickness; b – aluminium specimen 1mm thickness.
The most important observation that lies in forming limit curves analysis
is that, the method chosen to obtain strains does not give sufficient information
regarding the negative deformation. Basically, the charts obtain by hydraulic
bulging test shows the limit deformation curve only for the positive principal
strains.
If we analyze the variation of forming limit curves positions depending
on active plate form we can easily see that the only valid curve is obtained only
for the circular shape of the active mold. When using elliptical or square forms
the specimens do not reach their full potential deformation and tearing
prematurely install. It was also found that material fracture does not occur in the
maximum deformation stage, as would have been natural, but along the edge
radius that limits matrix form. This can only lead to the hypothesis that, when
using elliptical and square plates, a series of additional tangential efforts
appears, exceeding the maximum allowable material capacity. Thus, the
0.15
0.20
0.25