THE EXPERIMENTAL-DETERMINATION OF THE STRESS RESPONSES OF AN AL MG ALLOY TO A POLYGONAL STRAIN PATH AFTER 3 LEVELS OF PRESTRAINING/

A servohydraulic, computer-controlled MTS axial-torsion testing machin e 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% a nd 12%) on the subsequent stress response to a regular, 16-sided, poly gonal strain path is investigated. At each corner of the polygon, yiel d surface probings were performed before straining continued to the ne xt corner. Straining and yield surface probing were done under compute r control with no human interaction. The alloy was. donated by ALCOA a nd 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 - square-root 3 shear stress coordinates. The growth of the radius of the spiral is initially rapid but diminishes as straining continues. I f the strain path length is long enough the growth of the spiral subsi des at a radius equivalent to the ultimate tensile strength of the mat erial. The effect of prestraining is primarily an increase of the init ial radius of the spiral. Ultimately the growth levels off at the same radius as for the specimen without prestrain. It seems that an ultima te surface with a radius equal to the ultimate strength can be postula ted. For the present tests at least, this surface is unaffected by pri or deformation. When the effective stresses pertaining to the corners of the polygon are plotted vs. accumulated inelastic strain, an effect ive stress-strain curve is obtained within a reasonable scatter. A tot al of eight specimens comprise this plot which includes a tensile and a torsion test. However, not every part of the stress response falls o n this curve. The results indicate that a ''universal'' response can o nly be obtained after the passage of a transient. It usually lasts abo ut one percent effective strain. This observation is augmented with di scussions pertaining to the direction of the initial and the final str ess response for a piecewise linear straining.