6. Monotonic and Cyclic Loading Processes

Experimental analysis of 12X18H10T stainless steel specimens subjected to strain-controlled cyclic loading that comprises sequential monotonic and cyclic loading under uniaxial tension-compression and standard temperature is used to identify some features and dissimilarities of isotropic and anisotropic hardening processes that occur during monotonic and cyclic loading. In order to describe these features in terms of the plasticity theory (the Bondar model), which can be classified as a combined-hardening flow theory, plastic-strain redirection criterion and the memory surface concept are introduced in the plastic-strain tensor space so as to separate monotonic and cyclic strain. Evolution equations for isotropic and anisotropic hardening processes are derived to describe the monotonic-to-cyclic and cyclic-to-monotonic evolutions in transients. The basic experiment used to determine the material functions consists of three stages: cyclic loading, monotonic loading, and subsequent cyclic loading until fracture. The results of the basic experiment are fundamental to the proposed method for identifying the material functions. Basic-experiment results and the identification method are used to identify the room-temperature material functions of 12X18H10T stainless steel. The paper compares the computational analysis and the experimental analysis of stainless steel subjected to a strain-controlled fatigue test (loading) in five stages: cyclic, monotonic, cyclic, monotonic, and cyclic loading until fracture. It further compares the computational and experimental kinetics of the stress-strain state throughout the deformation process. Changes in the amplitude and mean cycle stress during the cyclic stress stages are subsequently analyzed. These stages are characterized by hysteresis loop stabilization. Computational and experimental results fit reliably. The theory adequately describes the processes of how the kinetics, the amplitudes, and the mean cycle stress alter when subjecting a specimen to strain-controlled loading, which enables a more adequate description of stress-controlled loading, especially when loading is non-stationary and non-symmetric.