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Research Papers

Development of an Empirical Model to Characterize Fracture Behavior During Forming of Advanced High Strength Steels Under Bending Dominated Conditions

[+] Author and Article Information
S. Sriram1

ArcelorMittal Global R&D – E. Chicago Center, 3001 E. Columbus Drive, E. Chicago, IN 46312sriram.sadagopan@arcelormittal.com

H. Yao, N. Ramisetti

ArcelorMittal Global R&D – E. Chicago Center, 3001 E. Columbus Drive, E. Chicago, IN 46312

1

Corresponding author.

J. Manuf. Sci. Eng 134(3), 031003 (Apr 25, 2012) (18 pages) doi:10.1115/1.4006092 History: Received February 01, 2011; Revised January 17, 2012; Published April 24, 2012; Online April 25, 2012

Higher strength advanced high-strength steels (AHSS) such as DP780 and DP980 are more susceptible to fractures at bend radii during press stampings in comparison with more ductile low carbon sheet steels used by the automotive industry. Most research work to develop predictive guidelines for preventing failures at bend radii have centered on determining critical R/t ratios to avoid failures caused by bending. In this paper, results from bending tests with and without applied tension conducted on a number of AHSS steel lots to generate different conditions for fracture are presented. For bending tests with applied tension, measures of overall formability as a function of R/t ratio of the punch are presented. Consistent with other studies reported in literature, the overall formability was found to increase with increasing R/t ratio reaching saturation for higher R/t ratios. In addition, local formability was determined for all the bending tests by measuring the thickness strains at failure using an optical microscope. It was observed that the thickness strain at failure was dependent on the R/t ratio and the loading mode. Examination of fracture surfaces from the different tests using an SEM reveals that fracture initiation occurs primarily at the ferrite/martensite interphase boundary. To analyze the local loading conditions leading to fracture, 2D finite element analyses (FEA) of the different bending tests using ABAQUS standard were conducted. Results of the FEA were analyzed, and a parameter describing bending dominance in a stamping process was isolated. An empirical fracture criterion relating the thickness strain at fracture as a function of this parameter was developed. Implications of the generated results and their applications for part design and evaluation of stamping feasibility are also discussed.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 9

Schematic of the stretch bend test showing the determination of the increase in length of line

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Figure 10

Overall measures of formability given by (a) % increase in length of line and (b) normalized sidewall stress for 2 mm DP590 as a function of R/t ratio

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Figure 11

Overall measures of formability given by (a) % increase in length of line and (b) normalized sidewall stress for 1.8 mm DP780 as a function of R/t ratio

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Figure 12

Overall measures of formability given by (a) % increase in length of line and (b) normalized sidewall stress for 1.6 mm DP980 as a function of R/t ratio

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Figure 13

Comparison of the % increase in length of line as a function of R/t ratio for 2 mm DP590, 1.8 mm DP780, and 1.6 mm DP980 for the (a) stretch bend test and (b) draw stretch test

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Figure 1

Optical micrographs of (a) 2 mm DP590, (b) 1.8 mm DP780, and (c) 1.6 mm DP980 in the transverse direction

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Figure 2

Schematic of the test methods used in the study: (a) V bend test, (b) stretch bend test, and (c) draw stretch test

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Figure 3

Photographs of the draw stretch and the stretch bend samples for 1.6 mm DP980

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Figure 4

(a) Specimen photograph and (b) cross section of the failed region showing shear fracture (“Sh”) at the location of the punch radius for 1.6 mm DP980

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Figure 5

(a) Specimen photograph and (b) cross section of the failed sample showing localized necking at the punch radius (“NR”) for 1.6 mm DP980

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Figure 6

Specimen photograph showing shear bands and localization in the unsupported sidewall (“SW”) for 1.6 mm DP980

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Figure 7

Load displacement history as recorded by the instrumented press for (a) stretch bend test and (b) draw stretch test for 2 mm DP590, 1.8 mm DP780, and 1.6 mm DP980

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Figure 8

Height at failure as determined by (a) stretch bend test and (b) draw stretch test for 2 mm DP590, 1.8 mm DP780, and 1.6 mm DP980

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Figure 17

Thinning strain at failure as a function of R/t ratio for the different tests for 1.8 mm DP780

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Figure 18

Thinning strain at failure as a function of R/t ratio for the different tests for 1.6 mm DP980

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Figure 19

SEM images of areas close to the failure region for a sample of 2 mm DP590 formed using a 2 mm radius punch by the stretch bend test. The voids are indicated by arrows.

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Figure 20

SEM images of areas close to the failure region for a sample of 2 mm DP590 formed using a 2 mm radius punch by the draw stretch test. The voids are indicated by arrows.

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Figure 21

SEM images of areas close to the failure region for a sample of 1.8 mm DP780 formed using a 2 mm radius punch by the stretch bend test. The voids are indicated by arrows.

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Figure 22

SEM images of areas close to the failure region for a sample of 1.8 mm DP780 formed using a 2 mm radius punch by the draw stretch test. The voids are indicated by arrows.

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Figure 23

SEM images of areas close to the failure region for a sample of 1.6 mm DP980 formed using a 2 mm radius punch by the stretch bend test. The voids are indicated by arrows.

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Figure 24

SEM images of areas close to the failure region for a sample of 1.6 mm DP980 formed using a 2 mm radius punch by the draw stretch test. The voids are indicated by arrows

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Figure 25

Fracture surfaces for samples formed using the stretch bend test for (a) 2 mm DP590, (b) 1.8 mm DP780, and (c) 1.6 mm DP980

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Figure 14

Comparison of the normalized sidewall stress as a function of R/t ratio for 2 mm DP590, 1.8 mm DP780, and 1.6 mm DP980 for the (a) stretch bend test and (b) draw stretch test

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Figure 15

Sections of failed regions used for thickness measurements for (a) specimens exhibiting shear fracture and (b) specimens showing highly localized necking in the region of contact with the punch

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Figure 16

Thinning strain at failure as a function of R/t ratio for the different tests for 2 mm DP590

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Figure 26

Comparison of experimental and predicted load deflection curves for 1.6 mm DP980 formed using a 3 mm punch (R/t = 1.875) for (a) stretch bend and (b) draw stretch tests

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Figure 27

Comparison of experimental and predicted load deflection curves for 1.6 mm DP980 formed using a 8 mm punch (R/t = 5) for (a) stretch bend and (b) draw stretch tests

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Figure 28

FEA calculations showing (a) the contour of the maximum principal strain and (b) the distribution of circumferential strain through the thickness of the sheet for the 1.6 mm DP980 at failure as determined experimentally for the different R/t ratios for the stretch bend test

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Figure 29

FEA calculations showing (a) the contour of the maximum principal strain and (b) the distribution of circumferential strain through the thickness of the sheet for the 1.6 mm DP980 at failure as determined experimentally for the different R/t ratios for the draw stretch test

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Figure 30

FEA calculations showing (a) the contour of the maximum principal strain and (b) the distribution of circumferential strain through the thickness of the sheet for the 1.6 mm DP980 at failure as determined experimentally different bending angles for the V bend test

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Figure 31

Trend of the thinning strain at failure as a function of γ in a stamping process for 1.6 mm DP980. γ can be considered to be a measure of bending dominance in a stamping process

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Figure 32

Trend of the thinning strain at failure as a function of γ in a stamping process for 1 mm DP980, 1.6 mm DP980, and 2 mm DP980

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Figure 33

Comparison of the curve fit in Eq. 3 with the experimental results for the thinning strain at failure for the 1 mm DP980, 1.6 mm DP980, and 2 mm DP980

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Figure 34

Comparison of the curve fit in Eq. 3 with the experimental results for the thinning strain at failure for the 1.2 mm DP780 and 1.8 mm DP780

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Figure 35

Comparison of the curve fit in Eq. 3 with the experimental results for the thinning strain at failure for the 1 mm DP590, 1.4 mm DP590, and 2 mm DP590

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Figure 36

Comparison of the curve fit in Eq. 3 for DP590, DP780, and DP980. The dotted line for DP590 is an extrapolation of the behavior based on the fit parameters determined using Eq. 3 and presented in Table 2.

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Figure 37

Comparison of the results obtained for the thinning strain at failure as a function of γ and the thinning strain at the point of incipient necking as determined by the FLD0 for the 1.6 mm DP980

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