Effect of geometric factors and processing parameters on plastic damage of SUS304 stainless steel by small punch test

The effects of friction coefficient, specimen thickness, punching rate and the diameter of center hole in lower die on the plastic damage of SUS304 stainless steel have been studied by small punch test (SPT). The finite element model (FEM) was established based upon Gurson–Tvergaard–Needleman (GTN) ductile damage constitutive equations. The results showed that the load–displacement (L–D) curves and final fracture locations of the broken specimens produced by experiment and FEM had a good agreement. The growth of plastic damage had a stage from the constant speed to the accelerated speed. The initial crack and necking region were away from the center when the friction coefficient increased. The initial crack occurred at 0.74 mm away from the center with the friction coefficient of 0.3. The increase of specimen thickness retarded the fracture time and it made the position of the crack initiation moving toward the center of specimen. When the punching rate changed from 0.5 mm/min to 10 mm/min, the size of dimples reduced and the number of dimples increased. There were some brittle fracture features in the final fracture surface, and the fracture time will delay with increasing the diameter of center hole in lower die.

► The damage has a stage from the constant speed to the accelerated speed.
► The initial crack moves toward the center with increasing specimen thickness.
► The high punching velocity impacts slightly on the distribution of micro-voids.
► The damage positions are similar when the lower die diameter is more than 5 mm.