Apparatus for evaluating plane strain stretch formability, and method therefor

An apparatus for evaluating a plane strain stretch formability of thin steel sheet material, comprising:

• a mating lower die and an upper die having a central opening therein and having complementary grooved and raised surface areas defining a lock bead circumscribing the central opening for clamping a test specimen of said sheet material therebetween;
• a punch, movable within the central opening of the dies, said punch having a saddle-like, semi-cylindrical shape for engaging said test specimen, said punch having a length l defined as a distance between opposed semi-cylindrical ends of said punch and having rounded edges of a certain curvature and wherein said test specimen is rectangular in shape having a width which is greater than the length of said punch;
• whereby, in use, said punch is moved to apply a stretch force on said test specimen, wherein said punch moves a distance to stretch and subsequently fracture said test specimen under a plane strain mode and the stretch formability of said sheet material is evaluated based on a fracture limit punch height defined by the distance travelled by said punch from first contacting said test specimen until fracture thereof.

An apparatus for evaluating a plane strain stretch formability of thin steel sheet material, comprising:

• a mating lower die and an upper die having a central opening therein and having complementary grooved and raised surface areas defining a lock bead circumscribing the central opening for clamping a test specimen of said sheet material therebetween;
• a punch, movable within the central opening of the dies, said punch having a saddle-like, semi-cylindrical shape for engaging said test specimen, said punch having a length defined as a distance between opposed semi-cylindrical ends of said punch and having l rounded edges at said ends, said rounded edges having a radius of curvature equivalent to 5-9% of the length e, and wherein said test specimen is rectangular in shape having a width which is greater than the length of said punch;
• whereby, in use, said punch is moved to apply a stretch force on said test specimen, wherein said punch moves a distance to stretch and subsequently fracture said test specimen under a plane strain mode and the stretch formability of said sheet material is evaluated based on a fracture limit punch height defined by the distance travelled by said punch from first contacting said test specimen until fracture thereof.

An apparatus for evaluating a plane strain stretch formability of thin steel sheet material, comprising:

• a mating lower die and an upper die having a central opening therein and having complementary grooved and raised surface areas defining a lock bead circumscribing the central opening for clamping a test specimen of said sheet material therebetween;
• a punch, movable within the central opening of the dies, said punch having a saddle-like, semi-cylindrical shape for engaging said test specimen, said punch having a diameter of 70 mm and a length l of 70 mm defined as a distance between opposed semi-cylindrical ends of said punch and having rounded edges at said ends, said rounded edges having a radius of curvature R of 5 mm, and wherein said test specimen is rectangular in shape having a width which is greater than the length l of said punch;
• whereby, in use, said punch is moved to apply a stretch force on said test specimen, wherein said punch moves a distance to stretch and subsequently fracture said test specimen under a plane strain mode and the stretch formability of said sheet material is evaluated based on a fracture limit punch height defined by the distance travelled by said punch from first contacting said test specimen until fracture thereof.