Summary
A servohydraulic, computer-controlled MTS axial-torsion testing machine with a biaxial clip-on extensometer is employed to test thin-walled tubes of an Al/Mg alloy under nonproportional straining. Digital data acquisition is used to record axial and shear stress as well as axial and shear strain. The influence of three levels of prestrain (0%, 1% and 12%) on the subsequent stress response to a regular, 16-sided, polygonal strain path is investigated. At each corner of the polygon, yield surface probings were performed before straining continued to the next corner. Straining and yield surface probing were done under computer control with no human interaction. The alloy was donated by ALCOA and has a tensile ductility of about 16%.
With no prestrain the stress response to the polygonal strain path is a spiral in the axial-stress —\(\sqrt 3 \) shear stress coordinates. The growth of the radius of the spiral is initially rapid but diminshes as straining continues. If the strain path length is long enough the growth of the spiral subsides at a radius equivalent to the ultimate tensile strength of the material. The effect of prestraining is primarily an increase of the initial radius of the spiral. Ultimately the growth levels off at the same radius as for the specimen without prestrain. It seems that an ultimate surface with a radius equal to the ultimate strength can be postulated. For the present tests at least, this surface is unaffected by prior deformation.
When the effective stresses pertaining to the corners of the polygon are plotted vs. accumulated inelastic strain, an effective stress-strain curve is obtained within a reasonable scatter. A total of eight specimens comprise this plot which includes a tensile and a torsion test. However, not every part of the stress response falls on this curve. The results indicate that a “universal” response can only be obtained after the passage of a transient. It usually lasts about one percent effective strain. This observation is augmented with discussions pertaining to the direction of the initial and the final stress response for a piecewise linear straining.
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Krempl, E., Cheng, S. The experimental determination of the stress responses of an Al/Mg alloy to a polygonal strain path after three levels of prestraining. Acta Mechanica 101, 93–109 (1993). https://doi.org/10.1007/BF01175600
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DOI: https://doi.org/10.1007/BF01175600