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198 Ovidiu Niţă et al. rejection of the piece. Therefore, it is particularly important that, in addition to plotting FLD, to establish a relationship of interdependence between maximum stamped parts deformation values and the parameters imposed by rejection criteria (Banabic et al., 2005; Altmeyer et al., 2012). In the literature are presented several experimental methods used to determine the deformation limits (Marciniak et al., 2002; Ramir et al., 2012). One of the methods that deserve a special attention is the method called after Kobayashi (Banabic et al., 1992). Together with his colleagues, Kobayashi found, after analyzing the influence of deformation of work piece on surface roughness, that, the appearance of necking coincides with a sharp increase in surface roughness. This sudden increase of surface roughness is used, by Kobayashi, as a criterion for the occurrence of localized necking. To define this moment, in the deformation processes, is necessary to measure surface roughness at different stages of deformation and develop a chart with roughness variation depen<strong>din</strong>g on major strain ε1. Based on the method developed by Kobayashi and his team, we wanted to establish, using the tensile method, a mathematical relationship between thin sheets surface roughness and the deformation of analyzed material. 2. Preparing an Conducting the Experiments For this approach we have conducted a series a tensile test which were aimed to obtain a different numbers of deformation stages for each material analysed. For the tensile experiments we took into account three of the most common used metallic sheets produced in Romania. Thus, in the experimental research we used rectangular specimens cut from thin sheets of the following materials: steel (wide strip steel B2), brass (CuZn37) and aluminium (Al 99.5%). In order to determine the number of experimental tests and, of course, to determine the number of samples necessary to meet the proposed approach, we resorted to a factorial plan. Given that the ultimate goal of the entire process is to make a chart that will reflect the changes of specimen surface roughness depen<strong>din</strong>g by deformation, the tensile force is considered the main parameter to be change during the test. Thus, we used five different levels of the force generated by the universal test machine (Table 1). Nr. Crt. 1 2 Factor Cod Table 1 Factors of influence Factor level 1 2 3 4 5 6 Lamination direction A 6 0 0 30 0 45 0 60 0 75 0 Tensile force (FkN) B 6 F1 F2 F3 F4 F5 90 0 F6
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 199 Another aspect that was taken into account for evaluating the behaviour of material is the lamination direction of blanks. Thus, six different directions to the direction of rolling were taking into account for the tests: 0 0 , 30 0 , 45 0 , 60 0 , 75 0 and 90 0 (Table 1). In conclusion, for the specific experimental tensile tests of the study two factors were considered the main influential factors of the system. Should be noted that, for conducting the experimental research we used the universal test machine WDW-50E and the strain rate was considered constant at 10mm/min. Also, the measurements of the surface roughness were undertaken using TAYLOR HOBSON-Surtronic25. Both experimental planning and data processing was made in Minitab v.15 (trail version). In Fig. 1 are captured two specimens in various stages of deformation. From the images we can, easily, see the direction of localized necking or how the rupture is propagate. a b Fig. 1 – Specimens subjected to tensile test: a – aluminium specimen; b – brass specimen. Since the first phase of the research was found that, from all of the three types of material studied, steel and brass specimens showed the most pronounced elongation compare with tensile force applied to the sample. In aluminium specimens case, necking and material failure occurred very early and elongation value stood at less than a third of the elongation recorded by brass or steel. 3. Experimental Results 3.1. Thin Sheets Deformation Capability in Relation to Lamination Direction The aspect that stood out even during the first phase of experimental tests was that the lamination direction has a significant influence on the deformation capability of specimens. For example, in Fig. 2 are illustrated the characteristic diagrams, obtained from the tensile test, for specimens orientated at 0 0 and 90 0 degrees orientation to main rolling direction. As can be seen from Fig. 2,
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