Development of Backstress Under Different Strain Paths in an Aluminum Alloy: Stress Dip Testing and Modeling

It is generally accepted that backstress/internal stress development plays an important role in the response to deformation of many metals. Techniques for quantifying such stresses, at both a macroscopic and a microscopic level, are vital to ongoing efforts to model and predict such deformation behavior. The stress dip test has been used mostly for the study of creep, including the development and calibration of related models; however, it has also been noted that the quantification of macroscopic backstress is another potential application. In this research, the stress dip test is used to assess backstress evolution during tensile deformation in AA6016-T4, including the extraction of backstress from the observed data. The results are compared with those for samples prestrained under biaxial and plane strain modes. It is found that backstress accumulates with epsilon 0.61 under uniaxial tension, indicating an approximately linear relationship between geometrically necessary dislocation and backstress development in this material for this strain path. Furthermore, samples prestrained using different strain paths have lower backstress than those strained entirely under uniaxial tension. A crystal plasticity finite element model, with an additional backstress law, captures the stress dip behavior well. After calibration to the stress dip tests, application to cyclical testing is also accurate. However, the model may require a more sophisticated backstress saturation constraint to accurately mirror the strain rate response during stress dip tests at higher deformation levels.