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Research Papers

The Draw-Bend Fracture Test and Its Application to Dual-Phase and Transformation Induced Plasticity Steels

[+] Author and Article Information
Ji Hyun Sung

Green Transformation Technology Center, Korea Institute of Industrial Technology, 711 Hosan-dong, Dalseo-gu, Daegu, 704-230, South Koreajsung@kitech.re.kr

Ji Hoon Kim

Materials Deformation Group,  Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam 642-831, South Koreakimjh@kims.re.kr

R. H. Wagoner1

Department of Materials Science and Engineering,  The Ohio State University, 2041 College Road, Columbus, OH 43210wagoner.2@osu.edu

The UTS is measured with a standard tensile test at the maximum strain rate attainable in our laboratory, 0.7/s. Thus, depending on the pulling rate of the DBF test and the strain-rate sensitivity of the material, F′ can be different from 1 even for tensile failures. However, in view of the estimated maximum strain rates in bending for the DBF tests performed here for most R/t ratios, and in view of the low strain-rate sensitivity of these alloys, F′ for tensile failures approximates 1.

This suggests that when type 2 failure occurs, it is the type 2/3 transition that is most similar to the type 1/3 transition when type 2 is suppressed.

The true maximum temperatures are much higher than those recorded at the onset of localizations, but they occur in such a local area and for such short times that resolution is difficult. Nonetheless, one empirical observation is of a puff of smoke arising from the fracture area. This vaporized lubricant (mineral oil) probably indicates a local temperature of 250–450 °C, consistent with the plastic work per volume done locally at the fracture surface.

1

Corresponding author.

J. Eng. Mater. Technol 134(4), 041015 (Sep 06, 2012) (15 pages) doi:10.1115/1.4007261 History: Received March 06, 2012; Revised July 02, 2012; Published September 06, 2012; Online September 06, 2012

Unpredicted sheet forming failures of dual-phase (DP) steels can occur in regions of high curvature and with little apparent necking. Such failures are often referred to as “shear fractures”. In order to reproduce such fractures in a laboratory setting, and to understand their origin and the inability to predict them, a novel draw-bend formability (DBF) test was devised using dual displacement rate control. DP steels from several suppliers, with tensile strengths ranging from 590 to 980 MPa, were tested over a range of rates and bend ratios (R/t) along with a TRIP (transformation induced plasticity) steel for comparison. The new test reliably reproduced three kinds of failures identified as types 1, 2, and 3, corresponding to tensile failure, transitional failure, and shear fracture, respectively. The type of failure depends on R/t and strain rate, and presumably on the initial specimen width, which was constant in this study. Two critical factors influencing the lack of accurate failure prediction were identified. The dominant one is deformation-induced heating, which is particularly significant for advanced high strength steels because of their high energy product. Temperature rises of up to 100 deg. C were observed. This factor reduces formability at higher strain rates, and promotes a transition from types 1 to 3. The second factor is related to microstructural features. It was significant in only one material in one test direction (of 11 tested) and only for this case was the local fracture strain different from that in a tensile failure. Alternate measures for assessing draw-bend formability were introduced and compared. They can be used to rank the formability of competing materials and to detect processing problems that lead to unsuitable microstructures.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Shear fracture of a front rail with DP780 [20]

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Figure 2

The draw-bend test and its relationship to drawing over a die radius in sheet forming: (a) the DBF test equipment, specimen, and principal parts, (b) the mechanics of draw over a die radius

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Figure 3

Engineering stress–strain curves for seven experimental steels

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Figure 4

Schematic of DBF test and the boundary conditions for the original back force controlled test (V1 , F2 ) and the new dual-displacement controlled test (V1 , V2 , V2 /V1  = α, a constant)

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Figure 5

Fracture at back side of the original draw-bend fracture test (with back force control): (a) a fractured sample and (b) stress-displacement curve

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Figure 6

Stress and displacement curves of front and back grips using dual displacement rate control

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Figure 7

Simulated and measured force–displacement curves for various roller conditions. Lines and symbols represent simulations and experiments, respectively (solid lines: front force; dot lines: back force; squares: rolling tools (open-front force, closed-back force); circles (open-front force, closed-back force); triangles: rolling tools (open-front force, closed-back force)).

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Figure 8

Setup of FLIR-A40 infrared camera over a draw bend tester

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Figure 9

Effect of edge condition

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Figure 10

Measurement of area from perspective view of section of a DBF fracture using a Clemax L13C image analysis camera and an Olympus SZH10 stereo microscope at 3.5 × 

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Figure 11

Examples of three types of fracture produced by the DBF test

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Figure 12

Two kinds of fracture appearance for type 3 (shear) fractures: (a) apparent plastic localization at the unbending/tangent point, (b) “cracking” without apparent strain localization

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Figure 13

Normalized maximum force versus R/t (DP780(D), V1  = 51 mm/s): (a) α = 0 and (b) α = 0.3

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Figure 14

Comparison of normalized maximum forces for DP steels and critical bend ratios R/t* with V1  = 51 mm/s: (a) α = 0 and (b) α = 0.3

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Figure 15

Comparison of normalized maximum forces and critical bend ratios R/t* for DP780(D) and TRIP780(D): (a) α = 0 and (b) α = 0.3

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Figure 16

The role of strain rate of reduction of peak draw force and the type of fracture, DP590(B)

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Figure 17

Fracture type maps, DP590(B): (a) α = 0 and (b) α = 0.3

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Figure 18

Fracture type maps comparing tested materials: (a) α = 0, R/t13*, (b) α = 0.3, R/t23*, and (c) α = 0.3, R/t12*

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Figure 19

Effect of friction on fracture type maps, DBF results with fixed and free rolling tools

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Figure 20

Effect of bend ratio on peak draw force (F′), net draw distance to fracture (ΔUf ), and fracture type, DP980(D)

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Figure 21

Comparison of formability (net draw distance to fracture, ΔUf ) for various AHSS tested: (a) α = 0 and (b) α = 0.3

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Figure 22

Comparison of formability (net draw distance to fracture, ΔUf ) for DP980 steels from four suppliers: (a) α = 0 and (b) α = 0.3

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Figure 23

Comparison of ΔUf for four DP980 steels and a DP780 steel at R/t = 3: (a) α = 0 and (b) α = 0.3

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Figure 24

Comparison of true fracture strain (εf ) for different R/t: (a) DP780(D) for α = 0 and 0.3 and (b) RD and TD for DP980(D)

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Figure 25

Change of leff based on FE simulations of the DBF test: (a) for different α, (b) for different V1 , and (c) for different R/t

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Figure 26

Comparison of stress–strain curves of a tensile test and DBF tests based on effective deforming length (leff ) at various R/t

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Figure 27

Comparison of equivalent total elongation (ef ) with R/t: (a) α = 0 and (b) α = 0.3

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