Fatigue analysis of porous structures using XFEM method

Predmet magistrskega dela je analiza utrujanja lotus porozne strukture z numeričnimi simulacijami v programu Abaqus. Celoten proces utrujanja analizirane lotus porozne strukture je razdeljen na dobo iniciranja in dobo širjenja razpoke. Doba iniciranja razpoke se izračunana po deformacijski metodi, doba širjenja razpoke pa se izračuna s pomočjo Parisovega zakona širjenja utrujenostnih razpok. Za izračun teh dob trajanja smo uporabili materialne parametre poroznega gradiva in veličine, ki smo jih izračunali s pomočjo numeričnih simulacij. Izvedene numerične simulacije so vsebovale enak geometrijski 2D model lotos porozne strukture, kateri je bil obremenjen za dva različna obremenitvena primera. Pri 1-osni obremenitvi modela so bile numerične simulacije narejene po klasični metodi končnih elementov (pri kateri smo v model ročno vstavljali in širili razpokepri tem smo upoštevali kriterij maksimalne tangencialne napetosti za širjenje razpok) in z uporabo razširjene metode končnih elementov (XFEM metoda). Pri XFEM metodi smo podali kriterije za iniciacijo in kriterije za širjenje razpok, na podlagi katerih je potem program sam vstavil razpoko v model in jo samodejno širil. Pri 2-osni obremenitvi modela pa so bile izvedene numerične simulacije samo po XFEM metodi. Pri obeh metodah numeričnih simulacij smo pričakovali podobne rezultate za dosego dobe trajanja lotus porozne strukture pri 1-osni obremenitvi, vendar do tega ni prišlo. Največje razlike so bili pri številu nihajev potrebnih za iniciacijo razpoke, to pa zaradi tega, ker smo pri XFEM metodi upoštevali samo elastični del materiala. Primerjane so bile analize utrujanja lotus porozne strukture pri dveh različnih obremenitvenih primerih. Kot je bilo pričakovano se pri 2-osno obremenjenem modelu hitreje inicirajo in širijo razpoke do kritične dolžine ac.In this thesis a study of the investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using numerical simulations has been performed. The complete fatigue process of analysed porous structure was divided into the crack initiation and crack propagation period. The crack initiation period was determined using strain life approach and the number of stress cycles required for crack propagation from initial to the critical crack length was determined with integration of Paris equation. For the calculation of these periods we used the material properties of porous material and quantities that were determined using numerical simulations in Abaqus. Performed numerical simulations contained the same geometric 2D model of lotus-type structure which was loaded in two different load cases. In the model which was loaded only in 1-axle we made numerical simulations using the finite element method (by this method we manually put and propagated the cracks using the MTS criterion) and the Extended Finite Element Method (XFEM). With the XFEM method we specified crack initiation and crack propagation criterion after which then the Abaqus determined the location of initial crack and propagated the crack to a critical length. In the model which was loaded in 2-axle only XFEM method was used in numerical simulations. We expected the same computational results of total fatigue life in the model which was loaded in 1-axle with both methodshowever this did not happen since the biggest differences were in crack initiation period with XFEM method where we considered only elastic material properties in numerical simulations. Compared analyses of models in the two different load cases were as we expected. In the 2-axial loaded model crack initiated and propagated faster to a critical length compared to the 1-axial model