[en] The demand for lightweight fuel-efficient vehicles and the increasing industrial competitiveness between constructors have motivated the automotive industry to introduce new materials and to reinforce the accuracy of metal forming processes with the aim of reducing costs. Steady advancements in technology, combined with shortened product cycle times and continual demand to reduce costs, have resulted in the dependence on finite element (FE) codes for the simulation of sheet metal forming processes in the automotive industry. It is generally accepted that commercially available FE programs can be successfully utilised to predict formability and the likelihood of splits and wrinkles, even for complex part geometry. However, the prediction of the elastically-driven change of shape after forming a product (springback) remains a challenge. This inaccuracy of springback prediction involves a need to use expensive and time consuming experimental try-outs to determine the proper tool geometry and all other parameters which can lead to the desired final shape. The main objective of the research presented in this thesis has been to improve the current numerical algorithms and material modelling techniques that allow the industry to reach a more accurate springback predictions in sheet metal forming. The literature review shows the relevance of the elastic behaviour on springback predictions. Therefore, first, a material characterisation of the elastic and elastoplastic behaviours has been performed. Aiming at accurately characterising the material, tensile tests, loading-unloading cyclic tests and tension-compression tests have been carried out. In order to improve the elastic behaviour model's accuracy, the classical elastic law has been extended. By means of this extension the elastic model is able to represent both linear and non-linear behaviours. Therefore, it has been possible to introduce the elastic behaviour with different levels of accuracy to predict the springback. Aiming at analysing the improvement introduced by modelling more accurately the elastic behaviour of the sheet, different simulations have been compared with experimental data. V-free bending experimental tests have been conducted in an AHSS sheet steel. Each numerical simulations has been provided with different particularisation of the extended model, representing each one the elastic behaviour of the material with different levels of accuracy. The results show the importance of an accurate modelling of the elastic behaviour for springback prediction.

[eu] Ingurumen legediak errespetatzeko ibilgailuen pisua gutxitze beharra eta merkatuaren lehiakortasuna dela eta, automobil industriak material berriak erabiltzera eta produkzio prozesuak hobetzera behartua izan da. Gaur egun, elementu nitu bidezko kalkuluak ezinbestekoak bihurtu dira automobil industrian prozesuen konplexutasuna ikertzeko eta hauek optimizatzeko, horrela kostuak gutxitzen direlarik. Elementu nituetako softwareen zehaztasuna industriagatik berretsia izan da. Kode hauek xehetasunez iragarri dezakete landutako materialaren forma eta honetan agertuko diren akatsak prozesuaren kon gurazioaren arabera. Hala eta guztiz ere, materiala landu ostean gertatzen den berreskuratze elastikoa (springback) aurresatea oraindik lortugabeko erronka bat da. Zehaztasun falta honek garestia den test-akats teknika bat burutzera behartzen du industria, behar den forma lortzeko fabrikazio prozesua egokitzen delarik. Ikerketa lan honen helburu nagusia springbacka iragartzeko erabiltzen diren material modeloak eta teknika matematikoak hobetzea da. Hobetze honen bitartez, gaur egungo prozesuen optimizazioa hobetzea espero da, iragarpenen zehaztasuna areagotuz, modu honetan kostuak murrizteko. Burututako bibliogra a ikerketaren bitartez, materialaren portaera elastikoaren garrantzia springbacka aurreikusteko ezarri egin izan da. Karakterizazio zehatz bat lortzeko trakzio, trakzio-erlaxazio eta trakzio-konpresio saiakuntzak burutu dira. Portaera elastikoaren zehaztasunaren eragina aztertzeko helburuarekin lege elastiko klasikoaren luzapen bat proposatu da. Luzapen honen bitartez, modelo elastiko berria, portaera lineala zein ez linealak irudikatzeko gai da. Luzapen hau erabiliz zehaztasun desberdineko portaera elastikoak sartu ahal izan dira springback iragarpen simulazioetan. Modelo elastiko zehatz batek springback iragarpenean egindako hobekuntza zenbakiko modelo desberdinak saiakuntza sartu esperimentalekin alderatuz aztertua izan da. Modelo bakoitzak zehaztasun desberdinekin adierazi du portaera elastikoa, lege elastikoaren luzapena erabiliz. Iragarpen hauek alderatu dira erresistentzia handiko altzairu baten burututako V-bending saiakuntza baten datu esperimentalekin. Lan honetatik ondorioztatzen da springback iragarpenentzako portaera elastikoaren irudikapen zehatzaren garrantzia.