Issue 35

S. Glodež et alii, Frattura ed Integrità Strutturale, 35 (2016) 152-160; DOI: 10.3221/IGF-ESIS.35.18 154 these results cannot be used for lotus-type porous materials due to different pore shapes and their orientations. Seki et al [10, 14, 15] have been studied the fatigue behaviour of cooper and magnesium lotus-type porous structures where the effect of porosity, anisotropic pore structure, and pore size distribution have been taken into account. Their experimental research 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 because of high stress concentration in this region. Lately, some research works on metal foams have already been done at the University of Maribor [16-18]. However, these materials have not been fully characterized yet, particularly in the way of fatigue life behaviour. In the presented paper, the fatigue process of lotus-type material is divided into the crack initiation and crack propagation period, where the total service life of treated structural element is defined as: i p N N N   (1) where N i is the number of loading cycles required for the fatigue crack initiation and N p is the number of loading cycles required for the crack propagation from initial to the critical crack length when final fracture can be expected to occur. When determining the crack initiation period N i , the strain life approach with consideration of simplified universal slope method [19] has been used 0.832 0.53 0.09 0.155 0.56 m m a i f i 0.623 (2 ) 0.0196 ( ) (2 ) R R N N E E                         (2) where  a is the total strain amplitude, R m is the ultimate tensile strength, E is the modulus of elasticity and  f is the true fracture strain. It is evident from Eq. (2) that the two exponents are fixed for all metals and that only monotonic material properties R m , E and  f control the fatigue behavior. The crack propagation period is in this paper described using simple Paris equation d d m a C K N    (3) where d a /d N is the crack growth rate,  K is the stress intensity factor range (  K = K max  K min ), and C and m are the material parameters which are determined experimentally according to the load ration R = K min / K max (the value R =0.1 has been considered in this study). The number of loading cycles N p required for the crack propagation from initial crack length a i to the critical crack length a c can then be determined with integration of Eq. (3): c i 0 1 d d ( ) a N a a N C K a     (4) C OMPUTATIONAL MODEL n some previous researches [9, 18], the regular models with aligned or for some angle aligned pores have been used when determining the strength behavior of lotus-type porous material. In these studies, the used computational models are built of multiple representative volume elements (RVEs) which are presented by a square block with central cylindrical hole of diameter d . The porosity of such structure is then regulated with change of pore diameters by keeping the size of the RVEs as a constant value [9]. In presented study, the irregular pores distribution of lotus-type material is considered. Pore distribution in transversal and longitudinal direction is assumed, respectively. A special image recognition code was developed, which was used to convert the chosen lotus-type material cross section image into the CAD-model which is then used to create the appropriate numerical model. The transverse computational model by square cross section of treated porous structure with length of 3.3 mm and randomly distributed pores with minimum and maximum diameters d min = 0.084 mm and d max = 0.47 mm, is introduced, respectively (Fig. 3 (a)). For such pores distribution, the porosity is equal to 36 %. The longitudinal computational model (Fig. 3(b)) is obtained as a cut section of I