Experimental Observations of Evolving Yield Loci in Biaxially Strained AA 5754-O
Mark Iadicola, Timothy J. Foecke, Stephen W. Banovic
Experimental measurement of the plastic biaxial mechanical response for an aluminum alloy (AA 5754-O) sheet metal is presented. Traditional methods of multiaxial sheet metal testing require the use of finite element analysis (FEA) or other assumptions of material response to determine the true stress versus true strain behavior of the as-received sheet material. The method used here strives to produce less ambiguous measurements of data for a larger strain range than previously possible through a combination of the Marciniak flat bottom ram test and an x-ray diffraction technique for stress measurement. The study is performed in conjunction with a study of the microstructural changes that occur during deformation, and these microstructural results are briefly mentioned in this work. Issues of calibration and applicability are discussed, and results are presented for uniaxial (U), plane strain (PS), and balanced biaxial (BB) extension. The results show repeatable behavior (within quantified uncertainties) for U to 20%, PS to almost 15%, and BB to above 20% in-plane strains. The results are compared with a basic yield locus, and show some unexpected results in the shape change of the yield locus at high strain levels (>5% strain).