Failure strains of anisotropic thin sheet metals: Experimental evaluation and theoretical prediction

Abstract

In the present study, experimental and theoretical analyses were performed to predict the failure strain of EDD and AA5052 sheet metals. In this context, uniaxial tensile tests and stack compression tests were conducted to assess the anisotropic constants of two different yield models viz. Hill48 and Yld2000-2d. Further, the calibrated models were used to estimate the directional properties and yield loci of the investigated sheet metals, and the results were successfully validated with the experimental data. Additionally, the forming limit diagram (FLD) and fracture forming limit diagram (FFLD) were analytically predicted using Marciniak–Kuczyński (MK) model and Bao–Wierzbicki (BW) ductile damage model implementing different yield theories. The estimated failure limits were compared with the experimental necking and fracture strain data obtained from the punch stretching (PS) tests, and found that the theoretical models incorporating Yld2000-2d yield theory best described the experimental limiting strains. Furthermore, the best predicted MK-FLD and BW-FFLD was used for failure strain analyses of three different sheet metal forming experiments such as two-stage forming, single point incremental forming (SPIF) and deep drawing. It was observed that the MK-FLD and BW-FFLD failed to predict the necking and fracture strain of different pre-strained sheets in the principal strain locus. Hence, all the strain data were transposed into polar effective plastic strain (PEPS) locus using Yld2000-2d yield model, and the PEPS based failure limit was developed. It was found that the MK-FLD and BW-FFLD predicted the respective limiting strains in PEPS locus efficiently. Moreover, all the experimental fracture strains were converted into 3D stress triaxiality locus and concluded that the calibrated BW fracture loci had a good predictive efficiency of all the fracture strains obtained from various forming tests.