Issue 37

J. Kramberger et alii, Frattura ed Integrità Strutturale, 37 (2016) 153-159; DOI: 10.3221/IGF-ESIS.37.21 154 When porous material is used for a structural component, the fatigue behavior should be taken into account. Some investigations presented in [6-8] cannot be used to describe the fatigue behavior of lotus-type porous materials due to different pore shapes and orientations. The experimental research of the fatigue behavior of copper and magnesium lotus- type porous structures presented by Seki et al [9, 10] has shown that, for the fatigue loading parallel to the longitudinal axis of pores, the stress field in the matrix is homogeneous, and slip bands appear all over the specimen surface. This is not the case for transverse loading, where stress field is inhomogeneous, and slip bands are formed only around pores due to of high stress concentration in this region. Some existing researches show that the most common method to determine structural properties of porous materials is the experimental investigation [11-14] Determined mechanical property is usually only compressive yield strength or plateau strength while fatigue properties have less frequently been published in details, especially for lotus-type porous materials. This paper discusses the low-cycle fatigue (LCF) behavior of lotus-type porous material, and evaluates the fatigue damage initiation and propagation through computational simulations using the direct cyclic analysis procedure in the framework of Abaqus/Standard software [15]. It is shown that in displacement controlled regime, cracks are easily detectable via reaction forces. M ETHODOLOGY omputational simulations allow us a better insight into analyzed structure behavior, and can provide information, which is sometimes very difficult or even impossible to determine with experimental measurements. The direct cyclic analysis procedure, implemented in Abaqus/Standard [15], is used in this work to compute the stabilized response of the structure directly, without having to compute a number of sequential cycles that would lead to such a stabilized cycle in the traditional approach. Abaqus/Standard offers a general capability for modelling the progressive damage and failure of ductile materials due to stress reversals and the accumulation of inelastic strain energy when the material is subjected to sub-critical cyclic loadings. Damage initiation and evolution criteria are adopted to determine the low-cycle fatigue damage. These two criteria are based on the stabilized accumulated inelastic hysteresis strain energy per cycle, Δw, as is illustrated in Fig. 1. Figure 1 : Inelastic hysteresis energy for the stabilized stress cycle Material failures refer to the complete loss of load-carrying capacity which results from progressive degradation of the material stiffness. The stiffness degradation process is modelled using a damage mechanics theory, which takes into account the process of material degradation due to the initiation, growth and coalescence of micro-cracks/voids in a material element under applied fatigue loading. For low-cycle fatigue analysis, the direct cyclic procedure can be used to directly obtain the stabilized cyclic response of the model. The direct cyclic procedure combines a Fourier series approximation with time integration of the nonlinear material behaviour in order to obtain the stabilized solution iteratively using the modified σ ε 3 2 1 1 2 Δ w Δε = const C