Citation: Park, J.; Won, C.; Lee, H.-J.;
Yoon, J. Integrated Machine Vision
System for Evaluating Hole
Expansion Ratio of Advanced
High-Strength Steels. Materials 2022,
15, 553. https://doi.org/10.3390/
ma15020553
Academic Editor: Arkadiusz Gola
Received: 10 December 2021
Accepted: 7 January 2022
Published: 12 January 2022
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Article
Integrated Machine Vision System for Evaluating Hole
Expansion Ratio of Advanced High-Strength Steels
Jaehoon Park
1,2
, Chanhee Won
3
, Hye-Jin Lee
3
and Jonghun Yoon
1,2,4,
*
1
Department of Mechanical Design Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu,
Seoul 04763, Korea; jayhoon20@gmail.com
2
BK21 FOUR ERICA-ACE Center, Department of Mechanical Engineering, Hanyang University, 55,
Hanyangdaehak-ro, Sangnok-gu, Ansan 15588, Gyeonggi, Korea
3
Digital Transformation R&D Department, Korea Institute of Industrial Technology, 143, Hanggaul-ro,
Sangrok-gu, Ansan 15588, Gyeonggi, Korea; chan2@kitech.re.kr (C.W.); naltl@kitech.re.kr (H.-J.L.)
4
AIDICOME Inc., 55, Hanyangdaehak-ro, Sangnok-gu, Ansan 15588, Gyeonggi, Korea
* Correspondence: jyoon@hanyang.ac.kr; Tel.: +82-31-400-5255
Abstract:
In this paper, we propose a new method to estimate the hole expansion ratio (HER) using
an integrated analysis system. To precisely measure the HER, three kinds of analysis methods
(computer vision, punch load, and acoustic emission) were utilized to detect edge cracks during
a hole expansion test. Cracks can be recognized by employing both computer vision and a punch
load analysis system to determine the moment of crack initiation. However, the acoustic emission
analysis system has difficulty detecting the instant of crack appearance since the magnitude of the
audio signal is drowned out by noise from the press, which interrupts the differentiation of crack
configuration. To enhance the accuracy for determining the HER, an integrated analysis system that
combines computer vision with punch load analysis, and improves on the shortcomings of each
analysis system, is newly suggested.
Keywords:
advanced high-strength steel (AHSS); edge crack; hole expansion test; hole expansion
ratio (HER); computer vision; punch load; acoustic emission
1. Introduction
Advanced high-strength steels (AHSSs) have been widely applied to body-in-white
(BIW) structures for both weight reduction and crass worthiness because they exhibit
higher tensile strength compared with conventional mild steels [
1
–
3
]. However, unexpected
edge fractures occur during the metal forming process due to low sheared edge cracking
resistance, which prevents their use in automotive body applications [
4
,
5
]. Predicting edge
cracks utilizing numerical simulations with a forming limit diagram is difficult since strain
at the edge is less than that of the forming limit curves, which gives rise to enormous issues
in manufacturing BIW structures with AHSS sheets [6,7].
To predict the edge cracks of AHSS sheets during the stamping and flanging pro-
cess, hole punching and expansion tests (which are common methods to evaluate edge
stretchability) are utilized in industries for estimating the quantitative amounts of edge
fracture resistance [
8
,
9
]. The hole expanding process, which is defined in the ISO 16630-2009
standard, consists of punching a hole on a test specimen and using a conical punch with
60 degrees at the top which moves upward to extend through the thickness of the AHSS
sheets [
10
]. The hole expansion ratio (HER) is an index to determine the flangeability of a
material, which is calculated using the ratio of the initial inner diameter to the final inner
diameter, where the final crack occurs on the sheared edge surface [11].
Conventionally, cracks from the hole expansion test are identified by the human
eye, which leads to inconsistent measurements and increases the inaccuracy of the HER
evaluation. Furthermore, edge cracks of AHSS occur at irregular times for each trial of
Materials 2022, 15, 553. https://doi.org/10.3390/ma15020553 https://www.mdpi.com/journal/materials
