Numerical Biaxial Tensile Test of Aluminum Alloy Sheets Using Crystal Plasticity Model Implemented in Commercial FEM Software

To improve the accuracy of forming simulations for sheet metal, the use of material models calibrated by multiaxial material tests is essential. Adequate material models can be calibrated on the basis of the contours of equal plastic work obtained by multiaxial material tests. However, because the tests often require special experimental equipment, they are not widely used by the industry. This paper proposes a methodology for a numerical biaxial tensile test that uses ABAQUS, a popular commercial software package for finite element analysis. In numerical tests, an open-source user-defined material model (UMAT) is used to implement crystal plasticity models. In order to validate our methodology, we performed a numerical biaxial tensile test on a 6000-series aluminum alloy sheet, and the results were compared with those of biaxial tensile tests with a cruciform specimen. The results demonstrated that the proposed numerical biaxial tensile test provides a reasonable prediction of stress-strain curves and the contours of equal plastic work.

Time to Fracture and Fracture Strain are Negatively Related in Sweet Cherry Fruit Skin

Rain cracking of sweet cherry fruit (Prunus avium L.) is said to occur when the volume increase associated with water uptake, extends the fruit skin beyond its upper mechanical limits. Biaxial tensile tests recorded fracture strains (εfracture) in the range 0.17 to 0.22 mm2·mm−2 (equivalent to 17% to 22%). In these tests, an excised skin segment is pressurized from its inner surface and the resulting two-dimensional strain is quantified. In contrast, the skins of fruit incubated in water in classical immersion assays are fractured at εfracture values in the range 0.003 to 0.01 mm2·mm−2 (equivalent to 0.3% to 1%)—these values are one to two orders of magnitude lower than those recorded in the biaxial tensile tests. The markedly lower time to fracture (tfracture) in the biaxial tensile test may account for this discrepancy. The objective of our study was to quantify the effect of tfracture on the mechanical properties of excised fruit skins. The tfracture was varied by changing the rate of increase in pressure (prate) and hence, the rate of strain (εrate) in biaxial tensile tests. A longer tfracture resulted in a lower pressure at fracture (pfracture) and a lower εfracture indicating weaker skins. However, a 5-fold difference in εfracture remained between the biaxial tensile test of excised fruit skin and an immersion assay with intact fruit. Also, the percentage of epidermal cells fracturing along their anticlinal cell walls differed. It was highest in the immersion assay (94.1% ± 0.6%) followed by the long tfracture (75.3% ± 4.7%) and the short tfracture (57.3% ± 5.5%) in the biaxial tensile test. This indicates that the effect of water uptake on cracking extends beyond a mere increase in fruit skin strain resulting from a fruit volume increase. Instead, the much lower εfracture in the immersion assay indicates a much weaker skin—some other unidentified factor(s) are at work.