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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
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 209 material failure occurs not because the material has reached its deformation limits. In conclusion, the use of elliptical or square active shapes can affect the accuracy of results. In the following lines is presented, through a series of comparative charts, the variation of FLD depending on active plate shape and the thickness of specimens. e1 e1 FLD variation for brass 0.5mm 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 e2 0.25 0.20 0.15 0.10 0.05 0.00 0.00 0.05 Variable Circular Eliptic Square e1 0.4 0.3 0.2 0.1 0.0 0.0 FLD variation for brass 1mm 0.1 0.2 e2 a b Fig. 5 – FLD variation depending on active shape (brass) a – 0.5mm thickness specimens; b – 1mm thickness specimens. FLD variation for aluminum 1mm 0.10 0.15 e2 0.20 0.25 Variable Circular Eliptic Eliptic e1 0.25 0.20 0.15 0.10 0.05 0.00 0.00 0.05 0.10 0.15 e2 0.3 0.20 0.4 FLD variation for aluminum 1.5mm a b Fig. 6 – FLD variation depending on active shape (aluminium) a – 1mm thickness specimens; b – 1.5mm thickness specimens. 0.25 Variable Circular Eliptic Square Variable Circular Eliptic Square Looking at Fig. 6 it can be easily seen that, for aluminium, the use of elliptical and square active plates causes very small deformations of the material comparative to circular plate. This leads to the conclusion that the aluminium chosen for the experimental tests it is not suitable for stamping processes. The analysis of Fig. 7, which depicts the variation of FLC for steel specimens, leads to the idea that the use of elliptical active plate shape provides distorted information on material deformability under analysis. As stated earlier this phenomenon is due, on the one hand, to the big difference between the surfaces exposed to deforming pressure and blank flange, and on the other hand, to additional tangential efforts located on the radius edge what limits the active plate form.
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BULETINUL INSTITUTULUI POLITEHNIC D
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