It is known that Al-Mg alloy sheets produce stretcher strain markings in stretching process. The stretcher strain markings are due to yields of two types; type-A and type-B. The type-A produces random marking and type-B produces parallel marking. In the process of pressing, the random marking appears in the range of very slight strain and bring about a serious defect. The authors chiefly studied the conditions for the generation of the random marking. The results obtained were as follows: (1)

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... e random marking produced during stretching was observed on fine-grained (under 0.04 or 0.05 mm) and recrystallized alloy sheets; but it was scarcely observed on coarse grained (above 0.1 mm) sheets. (2) The relation between the generation of random marking and the structural change during isothermal sheets. However, it was not observed on Al-8% Mg alloy sheets after the start of a subsequent age-hardening and its disappearance was continued until overageing. Whereas, the marking was always observed during 140 days of ageing on alloy sheets containing less than 6% of magnesium. (3) The grain size at which the random marking was observed depended upon the addition of copper 0.05 mm. (4) The age-hardening of Al-8% Mg alloy sheets was accelerated by additional copper as a third element; but, the sheets of containing less than 6% of magnesium were not accelerated by copper.