| Abstract | Automobilska industrija jedna je od najvećih svjetskih industrija koja godišnje proizvodi otprilike 50 milijuna vozila. U novije vrijeme se, zbog ograničenja emisija ispušnih plinova te zahtjeva za povećanjem sigurnosti cestovnog prometa, ali i zahtjeva kupaca, suvremena autoindustrija suočava s izazovima smanjenja mase vozila, poboljšanja ekonomičnosti potrošnje goriva te optimiranja u svrhu povećanja kompatibilnosti tijekom sudara. Elektrootporno točkasto zavarivanje je najzastupljeniji postupak spajanja metalnih limova u autoindustriji, a zavar nastaje uslijed topline koja se razvija prolaskom struje kroz elektrode te otpora materijala koji se zavaruje. Dakle, točkasti zavar nastaje kao rezultat kompleksnog međudjelovanja topline, pritiska elektroda i naglih faznih pretvorba u materijalu. Budući da točkasti zavar predstavlja geometrijski i materijalni diskontinuitet, oko zavarenog spoja javlja se koncentracija naprezanja, što ima negativan utjecaj na pojavu pukotina uslijed cikličkih opterećenja te na integritet konstrukcije tijekom dinamičkih opterećenja, tj. sudara. Nadalje, numerička analiza pomoću metode konačnih elemenata te razvoj metoda za proračun zavara na temelju sila omogučuju brz i jednostavan proračun točkasto zavarenih spojeva uslijed cikličkih i dinamičkih opterećenja, čime se smanjuju troškovi i vrijeme eksperimentalnih ispitivanja u fazi izrade prototipa i konstruiranja. Stoga je cilj ovog diplomskog rada razviti numeričke procedure za proračun točkasto zavarenih spojeva uslijed cikličkih opterećenja s konstantnim amplitudama i stohastičkih uzbuda te uslijed dinamičkog šoka. |
| Abstract (english) | Occupant safety, weight reduction, fuel efficiency and vehicle crashworthiness remain the most challenging design objectives of the modern automotive industry. Recent improvements in sheet metal joining processes and the use of advanced high strength steels have increased fuel-efficiency and enhanced vehicle durability, as well as the integrity of passenger compartment, simultaneously increasing weight reduction. Resistance spot welding is the most widely used metal joining process in automotive industry intended for joining of light gauge overlapping metal sheets. Although resistance spot welding includes complex interaction between mechanical loading, heating generated by the electric current and rapid microstructural transformations, compared to other joining processes, resistance spot welding is fast, easily automated and does not require the additional filler material. Since the material heterogeneity and geometric discontinuity around the weld nugget circumference cause stress concentration, spot-welded components are prone to premature failure under fatigue and crash loading conditions. Hence, it is crucial to understand the welded region microstructure and the mechanical behavior of spot-welded joints to balance competing design objectives and thus enhance vehicle durability and crashworthiness. In recent times numerical analyses have had a major impact on design optimization and cost reduction of full-scale experiments during prototype testing. Moreover, various numerical methods have been proposed to evaluate the fatigue strength of spot welds under constant amplitude loading. Generally, the fatigue analysis of spot-welded joints is classified into stress-based and force-based approaches. In comparison with the stress-based method, the force-based approach offers a quick solution and an accurate fatigue life estimation. Therefore, one of the objectives of the conducted research was to evaluate the existing methods for the fatigue analysis of spot-welded structures under constant amplitude loading and to study the effect of geometric characteristics on the fatigue behavior of spot-welded specimens. However, vehicle components are seldom subjected to the constant amplitude cyclic loading, thus the method of random vibration fatigue life prediction for spot-welded structures is evaluated. Finally, quasi-static damage analysis of spot-welded specimens is performed to evaluate the coupled force-based damage initiation criterion, which captures the complex behavior of spot-welded joints under general loading conditions, yet greatly simplifying the analysis. |
