ON THE DESCRIPTION OF TRANSIENT CYCLIC HARDENING BEHAVIOR OF MILD-STEEL CK-15

A series of strain-controlled experiments have been performed on thin-walled tubes made of mild steel CK 15. The experimental program includes uniaxial standard tests such as cyclic straining between fixed strain limits until cyclic stress amplitude saturation, as well as non-standard tests that include prestrain and cyclic mean stress relaxation. In addition, a tension-torsion test program has been designed. Nonradial strain paths of circular, rectangular, and triangular patterns have been examined, and significant extra hardening compared to the uniaxial case occurs. Specifically, the triangular pattern causes peak stresses exceeding the equivalent uniaxial stress amplitude by more than 50%. Furthermore, the v. Mises kinematic and isotropic parts of hardening are quantified by a reliable procedure involving nonlinear regression with the help of which the yield point is determined. On the basis of the experimental knowledge, a plasticity model has been developed that contains a new feature. This is represented by an isotropic memory surface that either expands or shrinks during plastic loading and thus allows for simulating hardening and softening due to high-low loading sequences.