Mter an analysis of the crack - source material of the cracked down skin of the right-middle-outer wing in an aircraft by means of the electron scanning microscope , a lot of features known as corrosion , stress corrosion and fatigue damage have heen detected in the photomicrograph. Thus the conchrsion can he drawn that the crack mode of the cracked down skin is of corrosive fatigue cracking caused hy stress corrosion.