Issue 48

O. Plekhov et alii, Frattura ed Integrità Strutturale, 48 (2019) 451-458; DOI: 10.3221/IGF-ESIS.48.43 457 Figure 5: Energy dissipation histories during the uniaxial test carried out at constant stress amplitude and at constant stress intensity factor (Solid line – experimental results, dotted line –approximation (14)). The process of heat dissipation is determined by the plastic work. This relationship could be complex due to the peculiarities of energy storage in material structure but taking into account the simplicity of Eqn. (13) we will use the linear dependence of heat dissipation versus energy of plastic deformation for the first approximation of experimental results. The Eqns. (2)-(4) are used for the qualitative analysis of plastic deformation into one loading cycle. Qusistatic loading and unloading of materials near crack tip are considered to approximate dissipation caused one cycle. The analysis allows us to divide the dissipations caused by monotonous and revers loadings and propose both explanation and approximation for energy dissipation process. The qualitative calculation of energy dissipation based on this model will request more precise analysis and determination and correct exponent in Ramberg-Osgood model. Figs. 4, 5 present the comparison of approximation (14) and results of the contact heat flux sensor. The Eqn. (14) gives a good qualitative description of peculiarities of heat dissipation in both regimes with the constant stress amplitude and constant stress intensity factor. In case of uniaxial loading for constant stress amplitude the plastic work and, as a consequence, energy dissipation at the crack tip is determined by crack rate as is shown in [7,8] but for constant crack rate we can observe the regimes with the decrease of the heat dissipation caused by the decrease of the applied stress amplitude. For biaxial loading there is good agreement between experimental and theoretical results. C ONCLUSIONS n this work, experimental and theoretical studies of energy dissipated at fatigue crack tip in AISE 304 steel were carried out. Two types of experiments were performed: uniaxial loading tests and biaxial loading tests with constant stress amplitude for different values of the biaxial coefficient. For samples under uniaxial loading, two regimes of crack propagation were realized: constant stress amplitude and constant stress intensity factor. The results of the experiments have shown that at constant stress amplitude there is an increase in energy dissipation correlated with the fatigue crack rate. Crack propagation at constant stress intensity factor is accompanied by a decrease in energy dissipation. This effect was observed independently in two different experimental programs. To provide an adequate explanation for the observed results, a theoretical analysis was performed in order to study the plastic work at the fatigue crack tip taking into account the evolution of both monotonic and cyclic plastic zones. This analysis allowed us to propose a simple approximation for energy dissipation at the fatigue crack tip. The obtained theoretical results give a good qualitative description of the specific features of energy dissipation in both regimes: loading with constant stress amplitude and loading with constant stress intensity factor. For constant stress amplitude, plastic work and, as a consequence, energy dissipation at the crack tip were determined by the crack rate, but for constant crack rate regime the scenario with a drop in energy dissipation takes place. In the case of biaxial loading there were two stages of energy dissipation: a constant heat flux accompanied by crack initiation and a sharp increase in energy dissipation associated with the occurrence of a large crack. The theoretical approach proposed for calculating energy dissipation correlates well with the experimental results. I