Issue 42

R. Pawliczek et alii, Frattura ed Integrità Strutturale, 42 (2017) 30-39; DOI: 10.3221/IGF-ESIS.42.04 30 Focused on Mechanical Fatigue of Metals Modeling of the stress-strain relationship for specimens made of S355J0 steel subjected to bending block loading with mean load R.Pawliczek, C.T Lachowicz Opole University of Technology, Mikołajczyka 5, 45-271 Opole, Poland r.pawliczek@po.opole.pl , c.lachowicz@po.opole.pl A BSTRACT . The paper presents results of calculation for modelling of the stress- strain relationship in the case of block loads with mean load value. A model, based on the stable hysteresis loops, was assumed and modified to use for block loading analysis. For stress history calculation, the proposed model and two other constitutive models were used. Results of fatigue test of specimens made of S355J0 steel subjected to bending block loading with mean load value are presented and used to verify the proposed model. In the tests, the mean load was increased and de- creased in subsequent blocks. The changes of strain recorded during the tests shown in the paper indicate a different behavior of the material. Damage accumulation degree for block sequence was used to compare the results of calculations. It was shown, that stress history parameters (stress amplitude and mean stress value in this case) are similar for all investigated models. K EYWORDS . Stress-strain relationship; Block loading; Mean load. Citation: Pawliczek R., Lachowicz C.T., Modeling of the stress-strain relationship for specimens made of S355J0 steel subjected to bending block loading with mean load, Frattura ed Integrità Strutturale, 42 (2017) 30- 39. Received: 31.05.2017 Accepted: 08.06.2017 Published: 01.10.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION bservation of the relation between strains of the structural materials and loading applied to the structure allow assessing the phenomena which occurs during fatigue of materials [1, 2]. For given material the value of load is a parameter which decides about strain-stress relationship. The area of the stress-strain hysteresis loop is identified with the plastic strain energy density. This energy parameter is widely used in so called energy based criteria to describe and predict the fatigue life of material, eg. [3, 4] as well for unnotched and notched components eg. [5]. In the case of complex load, during operation of the structures, some changes of the shape and location of the hysteresis loop are observed. It has to be considered in the algorithm of the fatigue life calculation, where the influence of the different load parameters on the strain-stress relationship is important. Using a proper combination of cyclic loading blocks it is possible to realize complex load histories as laboratory tests. Also the mean stress influence can be investigated – in this case, a constant component should be added to the generated blocks of variable amplitude loading. The load sequence can change the final fatigue life of materials and components, because damage accumulation is changed. The most important load sequences are these, which are strongly different with their parameters – amplitude and mean stress. Usually the Hi-Lo or Lo-Hi sequences are used [1]. Studying papers on that problem, we can find that there is no standard procedure for fatigue life calculation and some researchers obtained completely opposite results [6]. Most of those considerations were O