Some observations on the shear fracture mode in AIMg6.5 alloy sheet 
Miljana Popovi¢, Endre Romhanji, Vojin Milenkovié 
Department of Metal Forming, Faculty of Technology and Metallurgy, University of Belgrade, 
Yugoslavia 
Abstract. AIMg6.5 alloy sheet samples with different width to thickness ratio (w/t=2.5+8.3) were 
tested in uniaxial tension test. SEM observations revealed transgranular ductile dimple type of 
failure occured by shear process. It was found that the orientation of fracture surface depends on the 
w/t ratio (due to the transition from plane stress (w//=6.3+8.3) to plane strain (w/=2.5+5) condition) 
and also on the grain size (through the grain size effect on the intensity of PLC bands appearance). 
At high w/f ratio, w/t=6.3+8.3, fracture occurs at ~60-70° to the tensile axis, parallel to the PLC 
bands, which are very intensive in that case. For low w/t ratio, w/=2.5+5, PLC bands are less 
evident, and fracture occurs at ~55° through the specimen thickness and at ~90° to the tensile axis. 
Introduction 
Non-heat-treatable Al-Mg alloys have a wide range of application especially in automotive industry 
due to their high strength-to-weight ratio, good formability, corrosion resistance and good 
weldability [1,2]. It is well known that Al-Mg alloys exhibit serrated flow curves (PLC effect) and 
shear fracture, when they are deformed at conventional strain rates in room temperature tensile tests 
[3-5]. These effects are induced by dynamic strain aging (DSA) process, which also causes negative 
strain rate sensitivity (SRS) of flow stress and highly localized deformation. The strain localization 
appeared as formation of shears bands in the microstructure. At the sheet surface, Liiders and PLC 
bands can develop during the yield stress elongation (Liiders elongation) and serrated flow (due to 
DSA), respectively. Finally, the deformation is usually terminated by the appearance of ductile shear 
failure [6-9]. The purpose of this paper is to present some experimental observations on the fracture 
surface features in room temperature tensile testing of high strength AIMg6.5 type alloy sheet, using 
specimens with different width to thickness ratio (w/r). 
Experimental 
The as-received Al-Mg alloy sheet was 2.5mm thick and in the fully annealed condition. The 
chemical composition of the alloy used in this study is given in Table 1. 
Table 1: Chemical composition of Al-Mg alloy, wt.% ] _ 
Me | Mn [re | si [za] m |cu[w]m]eTa 
6.5 | 064 | 0.2 0.1 | 0.03 | 0.054 | 0.001 | 0.005 | 0.002 | 0.001 | rest 
The as-received AIMg6.5 alloy sheet was cold rolled with reductions ranging between 5% and 70%, 
producing thicknesses of 2.25mm to 0.75mm, and then annealed at 320°C/3h. Room temperature 
tensile testing was performed at an initial true strain rate of 6.7x107 s”', using “Zwick” universal 
tensile testing machine. Rectangular tensile specimens used with 25mm gauge length, 6.25mm 
width, and the width to thickness ratios were ranged to w/t=2.5+8.3. The fracture surfaces at the 
tensile specimens were examined by “Philips” (515 type) scanning electron microscope (SEM). 
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