HARDENING LAWS, SURFACE-ROUGHNESS AND BIAXIAL TENSILE LIMIT STRAINS OF SHEET ALUMINUM-ALLOYS

The roles of hardening laws and surface roughness have been assessed i n the prediction of biaxial tensile limit strains of two Al alloy shee t materials with different strain hardening characteristics namely AA6 111-T4 and AA5754-O, utilizing the surface roughness model proposed by Parmar, Mellor and Chakrabarty [6]. In the work of Parmar et al., the predictions of limit strains were based on the Marciniak-Kuczynski in homogeneity analysis and utilized the commonly used power hardening la w (Hollomon equation) to describe the stress-strain behavior of the ma terial. In the present work, (i) the suitability of a Voce hardening l aw, (ii) the effect of surface roughness parameters and (iii) the effe ct of grain size parameters on the prediction of biaxial limit strains has been studied. The biaxial limit strains based on Voce equation we re obtained by modifying the set of equations of Parmar et al, and uti lizing the experimentally measured surface roughness in 3-D, grain siz e parameters and stress-strain curves from uniaxial tensile and hydrau lic bulge tests for the two Al sheet materials. The predictions from V oce and Hollomon equations have been compared with the experimental fo rming limits determined by hemispherical punch stretching of gridded b lanks. The discrepancy between predictions from Holloman equation and experiments is small for the low strain hardening AA6111-T4 material b ut is quite significant for the high strain hardening AA5754-O materia l. Further, the predictions are also strongly dependent upon the measu re of surface roughness and the grain size utilized in the calculation s. The results indicate good predictions of limit strains for the two alloys when (i) stress-strain data from tensile or hydraulic bulge tes ts are fitted to a Voce equation and (ii) half of the maximum peaks-to -valley height and grain thickness are utilized as a measure of surfac e roughness and grain size respectively. The results are discussed in the context of the characteristics of the hardening laws, assumptions of surface roughness model and surface and grain characteristics of th e alloys studied.