Issue 35

J. Kramberger et alii, Frattura ed Integrità Strutturale, 35 (2016) 142-151; DOI: 10.3221/IGF-ESIS.35.17 148 Figure 8 : Number of cycles to initialize the damage (CYCLEINI) at the material points for sinusoidal cyclic load: a) aligned, b) 45° rotated, c) shifted and d) irregular pore structure. Once damage was initiated, the damage parameter D for damage evolution was quantified. In Fig. 9, the contour plot of damage parameter (SDEG) distribution at 100 loading cycles is given. Finite elements, in which damage parameters exceeded value 1.0, were removed from the mesh and are not shown in the Fig. 9. As seen in Fig. 9a, b and c, the damage in regular structures was still localized locally around pores. It is shown in the model with irregular structure (Fig. 9 d) that narrow areas between pores were intensively damaged. The small pores were linked and wider local areas were already cracked. In order to further quantify the damage evolution during fatigue of porous structure, the damage evolution can be monitored. Figs. 11 through Figs. 13 show damage evolution until approximate prior final fracture. The model with regular aligned (Fig. 10) and irregular (Fig. 13) pore topology demonstrated lower cycle fatigue strength in comparison with the model with regular rotated (Fig. 11) and shifted pores (Fig. 12). Different patterns (crack paths) of damage propagation is also observed. As expected, the cyclic damage evolves gradually, beginning at pore corners and than propagate thru the shortest distance between pores. Porous material exhibits the highest resistance for 45° rotated pore topology (Fig. 11). Some qualitative modeling results of damage evolution was confirmed by experimental worked, available in the literature [7]. C ONCLUSIONS n this study computational modelling was used to study behavior of the lotus-type porous material in low-cycle regime of tensile loading. Numerical simulations have shown the possibility to follow the evolution of damage in lotus-type porous material under fatigue load. Since classical computational approach with simulating the whole load history is computationally intensive, we obtained the stabilized response of the structure subjected to low-cyclic mechanical load using faster direct cyclic algorithm. The efficiency of direct cyclic algorithm was demonstrated on 2-D models. Four types of computational models with simplified regular (aligned, rotated and shifted) and more realistic irregular pore topologies, were considered. It should be noted that the heterogeneous distribution of pores plays a significant role on fatigue behavior of lotus-type material. With the cyclic loading conditions fatigue cracks are formed I a) b) c) d)