Issue 35

S. Glodež et alii, Frattura ed Integrità Strutturale, 35 (2016) 152-160; DOI: 10.3221/IGF-ESIS.35.18 152 Focussed on Crack Paths Fatigue crack initiation and propagation in lotus-type porous material S. Glodež, S. Dervarič, J. Kramberger University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia srecko.glodez@um.si, saso.dervaric@student.um.si , janez.kramberger@um.si M. Šraml University of Maribor, Faculty of Civil Engineering, Smetanova 17, 2000 Maribor, Slovenia matjaz.sraml@um.si A BSTRACT . The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation ( N i ) and crack propagation ( N p ) period where the total fatigue life ( N ) is defined as: N = N i + N p . The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, N i , required for formation of initial cracks. The number of stress cycles, N p , required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction. K EYWORDS . Lotus-type porous structures; Fatigue crack initiation; Fatigue crack propagation; Numerical analysis. I NTRODUCTION enerally, metal foams are relatively new class of materials with low densities and novel physical, mechanical, thermal, electrical and acoustic properties [1-5]. These materials offer potential for light-weight structures, energy absorption, sound absorption, etc. Therefore, metal foam materials present a unique opportunity for adoption in engineering applications. The base metals (aluminium, steel, etc.) are well understood and in many cases readily modelled with a high degree of accuracy. Foaming the metal, i.e. introducing voids in the microstructure, decreases the density and increases the apparent thickness. A number of distinct process-routes have been developed to make metal foams. Some of them produce open-cell foams and others produce foams in which majority of cells are closed. G