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Study presented in this paper is concerned with fatigue crack initiation detection, crack propagation observation and measurement as well as with numerical simulation of damage accumulation and propagation in the nodular cast iron grade EN-GJS-400-18-LT. Material properties of nodular cast iron are well elaborated in previous authors' papers. Crack initiation and its propagation observation as well as crack length measurement is performed on standardized specimens using ARAMIS 4M optical system. Based on the experimental results, a new three-dimensional constitutive model is proposed to simulate the low-cycle fatigue behaviour of considered material. An efficient algorithm for modelling cyclic plasticity is used for performing numerical simulation of crack initiation and growth on standardized specimens made from nodular cast iron. The computational procedure accuracy is verified by comparing the computed results with the real experimental data.

An experimental and numerical study of the cyclic deformation and low-cycle fatigue behaviour of the aluminium alloy AlCu5BiPb–T8 is presented. The experimental program included monotonic tensile tests, symmetric and unsymmetric strain-controlled fatigue tests, fracture toughness tests, as well as fatigue crack initiation and propagation tests. Within the framework of numerical investigations an efficient algorithm for modelling of cyclic plasticity is proposed. This algorithm is implemented into the finite element program ABAQUS and applied to the analysis of a crack growth near the notch. The accuracy of the computational procedure is tested by comparing the computed results with the real experimental data.

Većina komponenti modernih konstrukcija koje su izložene dinamičkom opterećenju i posljedično zamoru izrađene su od nodularnog lijeva normirane oznake EN-GJS-400-18-LT. Razlog tome je što se ovaj materijal odlikuje visokom istezljivošću i dinamičkom izdržljivošću čemu doprinose grafitne nodule u feritnoj mikrostrukturnoj matrici. Navedeni materijal korišten je za izradu glavčine postojećeg KONČAR-vjetroagregata snage 1MW, ali i kod drugih proizvodnih komponenata tvrtke Končar, kao što su vlakovi, tramvaji, energetska oprema itd. Iz tog razloga istraživanje procesa deformiranja navedenog materijala predstavlja značajan interes od strane hrvatske industrije. Kako utjecaj mikrostrukture na mehaničko ponašanje nodularnog lijeva EN-GJS-400-18-LT nije dovoljno istražen, cilj rada bio je eksperimentalno istražiti utjecaj mikrostrukture na proces iniciranja i rasta oštećenja u materijalu te na temelju eksperimentalnih rezultata predložiti konstitutivni model s pripadnim numeričkim algoritmom za simuliranje procesa deformiranja materijala. Kako bi se mehaničko ponašanje materijala dovelo u vezu s mikrostrukturom materijala, u radu su analizirane četiri različite serije materijala dobivene različitim tehnologijama lijevanja. Karakterizacija grafitne morfologije kao i mikrostrukture matrice materijala obavljena je na pedesetak uzoraka za svaku seriju nodularnog lijeva. Provedeni eksperimenti obuhvaćali su statičko vlačno ispitivanje, simetrična i asimetrična ciklička ispitivanja s petljama histereze, ispitivanje lomne žilavosti, te ispitivanja u kojima se istražuje rast zamornih pukotina, za svaku od četiri serije nodularnog lijeva. Pritom je za detekciju nastanka i mjerenje rasta zamornih pukotina na površini epruvete razvijena nova optička metoda primjenom mjernog sustava Aramis. Eksperimentalni rezultati pokazuju da veličina, oblik i raspodjela grafitnih nodula nema značajan utjecaj na cikličko očvršćenje materijala, ali igra veliku ulogu u procesima nastanka i rasta pukotina. Zaključeno je da veće nodule nepravilnijeg oblika, nepovoljno utječu na zamorno ponašanje materijala u usporedbi s većim brojem manjih nodula pravilnog oblika. Nadalje, rezultati pokazuju da perlitna mikrostrukturna faza ne utječe značajno na zamorni životni vijek ukoliko njen udjel ne prelazi 10%. U okviru numeričkog modeliranja, postojeći algoritam za analizu problema cikličke plastičnosti ljuskastih konstrukcija, prilagođen je za trodimenzijski konstitutivni model te je implementiran u programski paket ABAQUS. Točnost numeričkog algoritma provjerena je usporedbom s dobivenim eksperimentalnim rezultatima na epruvetama sa i bez zareza pomoću kojih je definiran konstitutivni model. To je omogućilo točnije opisivanje stanja naprezanja i deformacija, kao i procesa nastanka i rasta pukotine u materijalu za slučaj cikličkih opterećenja.

The present paper evaluates the fatigue life of ductile nodular cast iron EN-GJS-400-18LT with four different microstructures. Characterisation of the graphite morphology and the matrix microstructure is performed on 50 samples for every material type. Tensile stress–strain curves, symmetrical and unsymmetrical stress–strain hysteresis loops, cyclic stress–strain curves, crack resistance curves and fatigue life curves of these four microstructures are obtained. Experimental results show that size, shape and distribution of the graphite nodules has no significant influence on cyclic hardening of the material but they play a great role in the crack initiation and propagation process. It is shown that the larger irregularly shaped nodules reduce fracture toughness and fatigue strength. Furthermore, the results demonstrated that pearlitic phase does not strongly affect fatigue life if its proportion does not exceed 10%. The monitoring of crack length during the tests is performed by an optical method developed in the present work.

This paper presents an experimental and numerical study on two different types of the nodular cast iron EN-GJS-400-18-LT. The experimental procedure includes symmetrical and unsymmetrical strain controlled tests on the cylindrical specimens as well as crack initiation and propagation tests on the compact tension and single edged notched specimens. Different loading regimes are applied, and monitoring of the crack length during the tests is performed by an optical system. Within the framework of numerical investigations an efficient algorithm for modelling of cyclic plasticity is examined. Experimental results show that two material types have significantly different the crack behaviour.