Abstract—The key role in phenomenological modelling of cyclic plasticity is good understanding of stress-strain behaviour of given material. There are many models describing behaviour of materials using numerous parameters and constants. Combination of individual parameters in those material models significantly determines whether observed and predicted results are in compliance. Parameter identification techniques such as random gradient, genetic algorithm and sensitivity analysis are used for identification of parameters using numerical modelling and simulation. In this paper genetic algorithm and sensitivity analysis are used to study effect of 4 parameters of modified AbdelKarim-Ohno cyclic plasticity model. Results predicted by Finite Element (FE) simulation are compared with experimental data from biaxial ratcheting test with semi-elliptical loading path. Keywords—Genetic algorithm, sensitivity analysis, inverse approach, finite element method, cyclic plasticity, ratcheting. I. INTRODUCTION XPERIMENTAL measurement of any kind always bears certain level of uncertainty. The more complex the measured phenomena is the higher level of uncertainty brings to the measured values. Huge number of experiments would have to be performed to obtain good knowledge and understanding how different material parameters and their combination affect final result. Numerical modelling and simulation could help to reduce number of those experiments and also could give us better insight into this problematic. In this paper, we focus on FE modelling of the phenomenon called ratcheting (cyclic creep). It can be described as the accumulation of plastic deformation in a component or specimen under cyclic loading. The ratcheting may occur in practice for instance in the rolling/sliding contact. One of the first plasticity models, which can qualitatively capture ratcheting in numerical calculations, is Chaboche model [1]. Cyclic plasticity models have been extensively developed over the past three decades. The most popular kinematic hardening rules introduced into new constitutive theories are Ohno-Wang model II [2] and AbdelKarim-Ohno model [3]. The main aim of this contribution is comparison of various approaches to cyclic plasticity model calibration with emphasize on ratcheting. Two algorithms have been applied M. Cermak and T. Karasek are with the IT4Innovations, VSB - Technical University of Ostrava, 17. listopadu 15/2172, Ostrava, Czech Republic (e- mail: martin.cermak@vsb.cz, tomas.karasek@vsb.cz). J Rojicek is with the Faculty of Mechanical Engineering, VSB - Technical University of Ostrava, 17. listopadu 15/2172, Ostrava, Czech Republic (e- mail: jaroslav.rojicek@vsb.cz). genetic algorithm and a sensitivity analysis to estimate 4 parameters of modified AbdelKarim-Ohno model [4] using a fatigue test conducted under non-proportional loading. II. EXPERIMENTAL DATA To compare effectiveness of two different algorithms for material parameters estimation a multi-axial cyclic test was realized on reconstructed electro-servo-hydraulic system INOVA 100kN/1000Nm at the VSB-Technical University of Ostrava [5]. The semi-elliptical loading path (Fig. 1) was gained by the symmetric tension/compression and simultaneous repeated torsion applied as harmonic function of time with 90 of phase shift. The frequency of loading was 0.1 Hz. The test was proposed by McDowell [6] and simulates the stress-strain history in a point on a semi-infinite elastic-plastic half plane loaded by repeated Herzian pressure with Coulomb friction assumption. The case with axial stress magnitude of 625MPa and shear stress magnitude of 328 MPa was realized. For experiment a tubular specimen made of Class C wheel steel was produced. The outer diameter was 12.5mm, while the inner diameter was 10 millimetres. The shear strain and axial strain were measured simultaneously by extensometer EPSILON 3550 with the gauge length of 25 millimetres. The stress-strain hysteresis loops evaluated for 20 cycles are presented at Figs. 1 and 2. It could be mentioned from Figs. 1 and 2 that the shear strain accumulation occurs cycle by cycle in the same direction as the torque is applied, because of a non-zero value of mean torque. Fig. 1 Axial stress-strain hysteresis loops from biaxial fatigue test [7] Material Parameter Identification of Modified AbdelKarim-Ohno Model M. Cermak, T. Karasek, J. Rojicek E ‐800 ‐600 ‐400 ‐200 0 200 400 600 800 ‐0.01 ‐0.005 0 0.005 0.01 Axial stress [MPa] Axial strain World Academy of Science, Engineering and Technology International Journal of Aerospace and Mechanical Engineering Vol:9, No:4, 2015 306 International Scholarly and Scientific Research & Innovation 9(4) 2015 scholar.waset.org/1307-6892/10001051 International Science Index, Aerospace and Mechanical Engineering Vol:9, No:4, 2015 waset.org/Publication/10001051