Determination of Unloading Behavior of Uniaxially Strained 6061-T6 Aluminum from Residual Strain Measurements

The zero stress point on the one-dimensional strain, dynamic stress—strain unloading curve of shock loaded 6061-T6 aluminum has been determined by measuring the residual strain in the material after shock loading. As any postulated release path from a given stress should pass through this point, it is possible to deduce certain aspects of the manner in which the material unloads from a shocked state. A comparison of the results of these experiments with the strains predicted on the basis of the simplified elastic—plastic theory establishes the inability of the simplified theory to accurately predict the release path of 6061-T6 aluminum from a stressed state. Two major additions to the simple theory are required to explain the permanent strain data. These are a consideration of the Bauschinger effect, and the incorporation of an increased flow stress greater than that predicted from quasistatic data; the latter effect especially is quite possibly strain-rate dependent. Another somewhat surprising result of the permanent strain experiment is the demonstration of the large variations in permanent strain resulting from seemingly minor changes in material composition.