Evaluating the statistical significance of a fatigue-life reduction due to macro-porosity/Ocena statisticne znacilnosti vpliva makro poroznosti na padec zdrzljivosti

This study focuses on an evaluation of the significance of the fatigue-life reduction due to macro-porosity present in pressure-die-casted aluminium specimens. Three statistical models, i.e., univariate analysis of variance, multivariate analysis of variance and linear regression with dummy variables, were applied to test the statistical significance of the fatigue-life reduction. The three statistical models were applied for the case of experimentally determined fatigue-life data for an AlSi9Cu3 alloy with different levels of macro-porosity. Cylindrical specimens according to ASTM E606 were manufactured by pressure die casting using different manufacturing parameters (die pressure, die temperature) to artificially introduce detectable macro-pores into the specimens. The manufactured specimens were classified into three groups, representing their levels of porosity, which were identified based on x-ray images of the specimens. For each group, strain-controlled fatigue tests were performed at different strain levels. Of these approaches, linear regression with dummy variables proved to be the most appropriate, due to its ability to robustly identity the differences between the fatigue lives for different porosity levels. Keywords: AlSi9Cu3 alloy, porosity, fatigue life, ANOVA, MANOVA, linear regression with dummy variables V strojni industriji je teznja po zmanjsevanju teze konstrukcij odprla pot uporabi ulitkov iz aluminijevih zlitin kot alternativo jeklu. Medtem ko so dobre mehanske lastnosti, cenovno ugodne proizvodnje tehnologije in moznost recikliranja utijevale polozaj aluminijevih so vedno bolj prihajale do izraza tudi njihove slabe lastnosti. Ulitki so namrec podvrzeni pojavu nehomogenosti v strukturi. Kljub temu, da je njihova prisotnost navadno razlog za padec zdrzljivostnih sposobnosti ulitka, pa se nehomogenostim v celoti ne moremo izogniti. Pri ulitkih so najbolj kriticne tiste nehomogenosti, ki so posledica tehnoloskega procesa, saj so navadno vecje od ostalih, mikro strukturnih znacilnosti ulitka (hrapavost povrsine, mikrostruktura, mikro-poroznost, itn.). Najbolj nevarna tipa nehomogenosti pri ulitkih sta poroznost in vkljucki. Raziskava se osredotoca na statisticni popis vpliva makro poroznosti na padec zdrzljivostnih lastnosti tlacno litih preskusancev iz AlSi9Cu3 zlitine. Statisticna znacilnost vpliva je bila raziskana s tremi statisticnimi modeli in sicer z: univariantno analizo variance, multivariantno analizo variance in linearno regre