Effect of punch geometry on hole expansion ratio

Abstract

Stretch-flangeability of sheet metal is normally quantified by hole expansion ratio. Researchers have determined hole expansion ratio with various punch geometries such as conical, flat-bottom and hemispherical, and reported different hole expansion ratio values for identical test condition. Finite element investigation confirms that alteration of deformation path with punch geometries consequence different hole expansion ratio values. Necking and failure take place slightly away from the central hole edge for flat-bottom and hemispherical punches, while at the central hole edge for conical punch. Approximately plane strain tensile deformation prevails at the failure location for flat-bottom and hemispherical punches, while pure uniaxial tensile deformation prevails for conical punch.

Effect of Different Conical Punch Angle Geometries and the Initial Hole Diameters on the Hole Expansion Ratio of DP steels

“…During hole expansion test with a conical punch angle, uniaxial strain condition occurs at the hole edge, regardless of its punch angle [27]. However, the difference in conical punch angle causes different stress gradients on the hole edge during hole expansion tests.…”

“…Along with the effects of properties of material and edge condition on the HER, it has been observed that the punch geometry also takes an important factor for the edge-stretching ability of the same material [25], [26], [27]. The reason of the variation of HER value for different punch geometries result mainly from the creation of different strain path on the sheet specimen for different punch geometries.…”

“…They have shown that while the initial crack occurs at the hole edge for conical punch geometry, it does at a location away from the hole edge for flatbottom punch, therefore, flat-bottom punch geometry has been insensitive to initial edge conditions for the hole expansion test. Paul [27], has utilized finite elements analysis (FEA) to describe the reasons for the different HER values and the positions of the initial crack formation for different punch geometries such as flat-bottom, hemispherical and conical punches. The author has shown that while uniaxial type strain occurs at the hole edge for conical type punch, for flat-bottom and hemispherical punches plane-strain, which is the most critical type strain, occurs at a location slightly away from the hole edge, therefore, the author has concluded that the initiation of cracks for flat-bottom and hemispherical punches at a distance from the hole edge occurs due to the plane-strain condition, which also result in significantly lower HER, while the cracks initiate at the hole edge for conical type punch due to uniaxial type strain deformation.…”

Measurement of edge fracture strain of dual-phase steels by in-plane bending test

“…This punch geometry produces in-plane deformation without contact and friction between the edge of the hole and the punch [14]. While it seems that a cylindrical punch may provide edge material properties independent of test parameters [15], it also has been reported that fracture can occur far from the edge due to different stress state compared to the the material edge [16]. The material can endure less deformation in the plane-strain condition (lowest point on FLC) than uniaxial stress state.…”

Does friction contribute to formability improvement using servo press?

“…This trend was attributed to relatively very few defects with hole edge preparation from W-EDM process compared to those of other machining processes. It has been shown that in addition to the hole edge, strain path also plays a significant role in HER [10]. The influence of strain path on edge formability can be tested by varying the punch geometry [10,11].…”