PSI - Issue 21

O. Berk Aytuna et al. / Procedia Structural Integrity 21 (2019) 120–129 O. Berk Aytuna et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Portevin-Le Cha telier bands are unique plastic instabilities observed mostly in a lloys having solute a toms. Al-Mg a lloys are common examples showing deforma tion behavior based on the PLC bands. Presence of solute Mg a toms and their interaction (breakaway a nd pinning) with the mobile (gliding) disloca tions or other obstacles causes dynamic stra in aging in micro-sca le (Cottrell (1953), Hertzberg and Hauser (2010), McCormick (1971); Mulford and Kocks (1979), Sleeswyk (1958)). At macro sca le, the PLC bands appear as angled and narrow stra in loca lized lines during plastic deforma tion (Shibkov et a l. (2018), Yilmaz (2011)). PLC bands cause heterogenous deforma tion as a result of stress and stra in jumps (serra tions in flow curves) and the consequence is undesired v isua l sharp surface irregu larities (shear bands) that can act as a stress concentra tor for the ca tastrophic fa ilures (Beaudoin et a l. (2005), Shuklinov et a l (2011), Yuzbekova et a l. (2017)). Since 5xxx Al a lloys are used in the automotive industry due to its sufficient strength, good formability and good corrosion characteristics (Hirsch and Al-Samman, (2013)), irregu larities tha t affect these properties i.e. poor surface finish after shaping opera tions should not be desirable. On the other hand, dynamic stra in aging phenomena leads to nega tive stra in ra te sensitivity (Antolovich and Armstrong (2014)) which affects the mechanica l response of Al-Mg a lloys nega tively during shaping and complicates the optimiza tion of shaping parameters. PLC band characteristics basica lly depend on two factors; stra in rate (Jiang et a l. (2007)) and tempera ture (Cha tterjee et a l. (2011)). Varia tion of these two factors results in different sequence of nuclea tion and propaga tion of PLC bands such as continuous (type A), discontinuous (type B) or random (type C) (Ait-Amokhtar and Fressengeas (2010)). Factors apart from stra in ra te and temperature; gra in size (Yuzbekova et a l. (2017)), precipita tes (Wen et a l. (2005), hea t trea tment (Tian et a l. (2018)), surface irregularit ies (Shuklinov et a l, (2011)) or specimen shapes (Clausen et a l. (2004)) a lso affects the PLC forma tion in Al-Mg a lloys. Nonetheless, most of the previous studies used uniaxia l loading (Ait-Amokhtar and Fressengeas (2010), Cha tterjee et a l. (2011), Clausen et a l. (2004), Jiang et a l. (2006, 2007), Saad et a l. (2010); Tian et a l. (2018), Wen et a l. (2005)) to understand the deformation characteristics. Few studies in the litera ture focused on the PLC bands under different stra in pa ths. These studies include observa tion of PLC band formation region with hydraulic bu lge testing (Min et a l. (2015)), yield loci o r yie ld surface of Al-Mg a lloys (Hamasaki and Tamashiro (2018), Iadicola et a l. (2011)), PLC band detection with therma l observa tion techniques instead of digita l image corre la tion (Le Cam et a l. (2017)), forming and fracture stra ins of Al-Mg a lloys (Song et a l. (2016)) or hardening behavior and serra tion types under uniaxia l, pla in stra in and biaxia l condition (Houet a l. (2018)). However, a systema tic study under various stra in pa ths is needed for the deta iled characteriza tion of the shape and contribution of individua l PLC bands and their effects on the fracture and deforma tion mode. It is we ll known tha t under uniaxia l loading, fracture occurs from loca lized shear bands (Motsi et a l. (2014), Shibkov et a l. (2018), Shuklinov et a l. (2011), Zhemchuzhnikova et a l. (2018)). The shear stress sta te under uniaxia l tension is responsible from this and it may a lso affect the PLC band forma tion. There is no shear stress under biaxia l tension, on the other hand, and the PLC forma tion may be different than the uniaxia l. In this work, the behaviour of PLC bands in Al-Mg a lloy was investiga ted under uniaxia l and equibiaxia l condition in order to test the effect of stra in pa th on their formation. An in-plane biaxia l testing appara tus with st ra in ana lysis capability was utilized for this purpose. First ly, shape of the bands and stra in amount in the bands were ana lyzed with the DIC technique. Digita l image correla tion technique as an optica l method could ana lyse spa tio tempora l characteristics of these bands by displaying stra in field of focused region (commonly gage length). When the PLC band nuclea tes a t any area in that region, the stra in distribution map has angled stra in loca lized band(s) (genera lly 50 ᵒ -60 ᵒ to the tensile axis) (Ca i et a l. (2016), Sene et a l. (2018)). This certa in bands could provide informa tion about nucleation or propaga tion characteristics of PLC bands depending on adjusted fps of CCD cameras. In other words, the type of propagation (A, B or C) could be described . After observing connections of PLC bands and the deforma tion mode characteristics of ma teria l under different loading conditions, PLC band contribution to the fa ilure was established. Fina lly, texture evolution during deforma tion was investiga ted under the different stra in pa ths and PLC conditions to investiga te their combined effects on the fina l texture, which is a critica l fea ture controlling the overa ll formability of Al sheets.