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200 Ovidiu Niţă et al. aluminium presents the highest sensibility to lamination direction. Basically, the breaking of aluminium specimen, cut at 90 0 to the lamination direction, occurred in the mid-range elongation obtained for the specimen orientated after rolling direction. a b Fig. 2 – Characteristic diagrams obtained by tensile test: a – aluminium; b – brass. It should be noted that, the steel specimens recorded the lower sensitivity regarding deformation capacity to the lamination direction. This phenomenon is illustrated in Fig. 3, where the characteristic diagram obtained for steel blanks is shown. Fig. 3 – Characteristic diagrams obtained for steel specimens. 3.2. Mathematical Relations between Roughness and Tensile Strain After the accomplish of experimental tests, it was found that the transition from lower to higher levels, of the considered influencing factors, determine a noticeable variation in surface roughness. Both influencing factors consider have a direct influence on the roughness, but only increasing the tensile force leads to a significant increase in roughness. Varying the lamination direction orientation also produces a fluctuation of roughness values but this variation occurs randomly and not after a predictable pattern. The nest step after the acquisition of surface roughness values was to draw the roughness evolution diagrams depending of specimen elongation. For
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 201 exemplification in Fig. 4 are presented the graphics (drawn from point to point) of roughness evolution drawn for brass specimens oriented at 0 0 and 60 0 to the lamination direction. Thus, we can easily see that both graphs shows two zones of inflection, suggesting that at a certain level of specimen’s deformation roughness indicate a pronounced variation. A b Fig. 4 – Roughness evolution depending on elongation: a – brass specimens 0 0 ; b – brass specimens 60 0 . In the first phase of deformation the roughness remain relatively constant then, when the specimen elongation reaches around 35%...40% of maximum, Ra parameter register a rapid growth. After the first inflection point the surface roughness do not show a significant increase but it increases again sharply in the vicinity of the failure zone. According to the results analysis it seems that roughness value increases by 100%, near the area where break occurs, from the initial value read from specimen surface. In all studied cases the mathematical relationship between surface roughness, of the specimens subjected to tensile testing, and elongation can be described by an exponential equation ax y = be . (1) In Fig. 5 a it is presented, for exemplification, the regression function, that describes the phenomenon studied, for steel specimens orientated at 30 0 from rolling direction. It is noted that the coefficient of determination (R 2 ) represents a measure of the quality of predictions about future dependent values and in almost each studied cases this coefficient was above 0.8 which confirms the validity of the model. Also, in Fig. 5 b are presented, for comparative purposes, the roughness evolution graphs for each of the six orientations to the lamination direction, determined for steel specimens. By analysing the graph it can be concluded that the lamination direction can influence the surface roughness but not in a significant way.
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