Practical dividing line between a ductile and brittle materials is suggested, when the ultimate elongation is about 5%. Generally, larger the knuckle radius, stronger is the corner torus section of a head. The knuckle radius provided should be less than __________ of the head.

• 6% of I.D.
• 3 times the thickness
• either (a) or (b), whichever is larger
• either (a) or (b), whichever is smaller