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METALS
Key factors of stretch-flangeability of sheet materials
Jae Ik Yoon
1
, Jaimyun Jung
1
, Jung Gi Kim
1
, Seok Su Sohn
2
, Sunghak Lee
1,2
, and Hyoung Seop Kim
1,2,3,
*
1
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
2
Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
3
Center for High Entropy Alloys, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
Received: 19 December 2016
Accepted: 15 March 2017
Published online:
20 March 2017
ÓSpringer Science+Business
Media New York 2017
ABSTRACT
Stretch-flangeability evaluated using hole-expansion testing represents the
ability of sheet materials to resist edge fracture during complex shape forming.
Despite a property imperative for automotive part applications of advanced
high-strength steels, factors governing stretch-flangeability are not yet well
understood. In this study, the mechanical properties of a selected group of
materials with different microstructures were investigated using tensile, fracture
toughness, and hole-expansion tests to find the factor governing the stretch-
flangeability that is universally applicable to a variety of metallic materials. It
was found that the fracture toughness of materials, measured using the fracture
initiation energy, is a universal factor governing stretch-flangeability. We veri-
fied that fracture toughness is the key factor governing stretch-flangeability,
showing that the hole-expansion ratio could be well predicted using finite ele-
ment analysis associated with a simple ductile damage model, without explicitly
taking into account the microstructural complexity of each specimen. This val-
idates the use of the fracture toughness as a key factor of stretch-flangeability.
Introduction
Currently, automotive industries are subjected to
strong pressure to reduce exhaust emission of their
automobiles, which is an important cause of air pol-
lution. To address this imposition, automotive
industries have focused on the development of a
lightweight body-in-white using high-strength sheet
steel. The development of lightweight automobile
bodies has a variety of other goals as well, such as
increasing fuel efficiency, increasing strength and
safety, and improving durability [1–4]. Advanced
high-strength steels (AHSS) with excellent tensile
properties such as dual phase (DP), transformation
induced plasticity (TRIP), twinning induced plastic-
ity (TWIP), quenching and partitioning (Q & P), and
lightweight steels have been developed to meet these
goals [5–11].
However, formability, which is the ability to be
shaped into a desirable structure without fracture,
becomes a problem in high-strength grade steels.
Generally, the formability of these steels is
Address correspondence to E-mail: hskim@postech.ac.kr
DOI 10.1007/s10853-017-1012-y
J Mater Sci (2017) 52:7808–7823
Metals
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