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Journal of Mechanical Science and Technology 28 (6) (2014) 2319~2328

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-014-0522-7

Effect of clearance and inclined angle on sheared edge and

tool failure in trimming of DP980 sheet† Hong-Seok Choi1, Byung-Min Kim2 and Dae-Cheol Ko3,*

1Precision Manufacturing Systems Division, Pusan National University, Busan, 609-735, Korea 2School of Mechanical Engineering, Pusan National University, Busan, 609-735, Korea

3Industrial Liaison Innovation Center, Pusan National University, Busan, 609-735, Korea

(Manuscript Received May 6, 2013; Revised December 10, 2013; Accepted February 2, 2014)

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Abstract Ultra-high-strength steels are widely used in automotive industry for lightweight and high crash performance. In this study, the effects

of process variables such as the clearance and the inclined angle of the die on the sheared edge characteristics of trimmed DP980 have been evaluated in detail. The maximum trimming load decreases with increasing clearance due to a large bending moment leading to a hydrostatic tensile stress in the sheared zone, and tensile typed burr occurs at a trimming clearance above 15.6%t. Also, a negative in-clined angle improves the quality of sheared edge and decreases the trimming load. As a result of the trimming experiment, the burr height gradually increases with an increase in the number of strokes due to tool failure resulted from high contact pressure. Furthermore, the burr height significantly decreases as the localization zone is connected linearly from punch to die edge with the negative inclined angle.

Keywords: Burr; Clearance; DP980; Inclined angle; Trimming; Ultra-high-strength steel ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Owing to higher demands for reduced vehicle weight and improved safety of passengers resulted from excellent crash-worthiness, ultra-high-strength steels (UHSSs) with a tensile strength over 780 MPa are widely used in the automotive in-dustry [1, 2]. Nowadays, DP980 is representatively applied to the automotive structural part requiring crash performance such as B-pillar, B-pillar reinforcement, bumper, rear side member and impact beam. However, there are some obstacles for the implementation of the steel sheet, for example, their limited formability, weldability, dimensional accuracy, wear of the stamping tool, and scratches on the tool and product [3-5].

Most stamped components are trimmed or pierced for the removal of excess materials or for assembly with other com-ponents, respectively. The high strength of the part causes a high contact pressure and stress on the cutting tool. Conse-quently, the service life of the cutting tool is significantly shorter for stronger parts. Some industries are using a laser cutting method that is time-consuming and high-priced for mass production.

Previous researches on the trimming of high-strength steels are in the literature. Golovashchenko and Ilinich reported on

the influence of the trimming conditions on the surface quality of high-strength steels and their ability to stretch. They also suggested that a robust trimming process using elastic pad prevents the occurrence of sliver, burr and splits on the trimmed surface, resulting in a high-quality sheared edge [6]. Chintamani and Sriram investigated the sheared edge charac-teristics of automotive components made with high-strength steel. They found that the roll-over and burr height increased with increasing clearance [7]. Li performed an experimental investigation on the trimmed surface and burr with respect to the clearance, sharpness of the tool, and inclined angle, which demonstrated the micro-mechanisms of deformation and frac-ture in the trimming of aluminum alloys [8]. So et al. investi-gated the blanking process of hot-stamped 22MnB5 and found that the quality of the sheared edge increased as the blanking temperature increased [9]. Mori et al. used local resistance heating to increase the effective sheared zone and to reduce the punching load when trimming UHSSs [10]. Moreover, a local softening method for hot stamping was developed by Choi et al. to prolong the trimming tool life [11]. However, detailed research on the characteristics of the sheared edge from the trimming of UHSSs are still rare.

The purpose of this study is to evaluate the characteristics of the sheared edge and burr formation for the trimmed part and the damage to the trimming tool with respect to the clearance and the inclined angle of the tool in the trimming of DP980. A

*Corresponding author. Tel.: +82 51 510 3697, Fax.: +82 51 514 3690 E-mail address: [email protected]

† Recommended by Associate Editor Youngseog Lee © KSME & Springer 2014

2320 H.-S. Choi et al. / Journal of Mechanical Science and Technology 28 (6) (2014) 2319~2328

finite element (FE) analysis of the trimming process with a variation of thicknesses is performed according to the clear-ance and inclined angle to observe the roll-over, burnish and burr in detail. Furthermore, a trimming experiment with the sheet thickness of 1.6 mm is performed to investigate the ef-fects of the trimming conditions. On the basis of the FE-analysis and the experimental results, proper trimming condi-tions that are effective to the sheared edge, burr height and tool life are discussed in this paper.

2. FE-analysis and experimental set-up

2.1 FE-modeling of trimming process

The trimming process was analyzed using a two-dimensional plane strain model. The schematic representation of the trimming process is shown in Fig. 1. A commercial FE-code, DEFORM 2D, was used to predict the fracture and de-formation behavior of the material, and to determine the prop-er process parameters with a variation of thickness. In the model, elastic recovery was ignored because of the relatively large plastic deformation in the sheared zone. In the FE-analysis, the type of element used was a four-node rectangular element and the total number of elements was about 10000. The conditions of the analysis are listed in Table 1.

In the study, the effect of the inclined angle of the die on the sheared edge and the burr is examined. Fig. 2 shows a positive and negative inclined angle. For convenience, the inclined angle is defined as the angle of the punch travel direction with

respect to the sheet plane. In conventional trimming processes, the angle is kept as close to 0º as possible, i.e., the punch travels perpendicularly to the sheet plane. As mentioned in the Ref. [12], it is also practical for the punch to penetrate at any angle to the sheet plane. For the trimming analysis, the material properties of DP980, including the relationship be-tween stress and strain obtained from a tensile test, are listed in Table 2.

A ductile fracture criterion is used for predicting whether a fracture occurs during deformation of the steel. The normal-ized Cockroft & Latham criterion that is based on the stress and strain is used in this study [12]. The fracture occurs when the maximum damage value in the deformed material exceeds the critical damage value. To determine the critical damage value of DP980, several simulations were performed with varying values. The simulation results were compared with the experimental results until an agreement was obtained [13]. The critical damage value of DP980 was determined to be 1.2 by comparing analytical and experimental result under the clearance of 0.15 mm using a sheet thickness of 1.6 mm.

2.2 Experimental set up

Trimming experiments were performed to verify the FE-analysis results. The die was placed on a 500 kN single-action mechanical press with a punch speed of 50 mm/s, which ap-proximates the trimming process in the automotive industry. The tool material was flame-hardened cast-tool steel of which the hardness was measured to be HRC 57~58. The radius of the die edge was machined to be approximately 0.03 mm. A stripper force of 15.7 kN was uniformly applied to the sheet metal. Fig. 3 shows the trimming die set used in this study.

The sheared edge was observed using a scanning electron microscope (SEM). The burr height of the trimmed specimen was measured by the tactile stylus method using a Diavite (DH-6), averaging five measured values for each data point. A 20000-stroke trimming experiment was conducted to investi-

Table 1. FE-analysis conditions for trimming process.

Sheet material DP980

Material thickness (mm) 1.2, 1.6, 2.0 (3 cases)

Punch corner radius (mm) 0.03

Die corner radius (mm) 0.03

Punch velocity (mm/s) 50

Stripper force (kN) 15.7

Friction factor (m) 0.12

scrap

sheet(t=1.6mm)

stripper(F=15.7kN) punch

(V=50mm/s)

die clearance

R0.03

R0.03

Fig. 1. Schematic representation of trimming process.

Table 2. Material properties of DP980.

Yield strength (MPa) 730

Tensile strength (MPa) 990

Elongation (%) 14

Flow stress (MPa) 0.111434s e=

Fig. 2. Definition of inclined angle.

H.-S. Choi et al. / Journal of Mechanical Science and Technology 28 (6) (2014) 2319~2328 2321

gate the effect of tool failure on the sheared edge for selected trimming conditions.

3. Results and discussions

3.1 FE-analysis and its verification

First of all, the comparison of sheared edge between FE-analysis and experiment are done before parametric study in trimming process. Fig. 4 illustrates sheared edge after trim-ming under the clearances of 0.05 mm (3.1%t) and 0.15 mm (9.3%t). It can be known that the sheared edge in FE-analysis is similar to that in trimming experiment. Also, it is shown that roll-over and burnish depth increase with increasing clearance. This phenomenon will be discussed in the follow-ing Sec. 3.2. On the other hands, burr heights with increasing

clearance are nearly same due to the same radius at corner edge in initial trimming process.

3.2 Effect of clearance on maximum trimming load

The clearance between the punch and the die is an impor-

Fig. 3. Die set for trimming experiment.

0.110.22

1.27

0.03

StripperPunch

Die

StripperPunch

Die

0.110.22

1.27

0.03

(Roll-over)(Burnish)

(Fracture)

(Burr)

(a)

0.2mm0.2mm

(a) Clearance 3.1%t

0.13

0.27

1.20

0.03

StripperPunch

Die

StripperPunch

Die

0.120.26

1.22

0.03

(Roll-over)

(Burnish)

(Fracture)

(Burr) 0.2mm0.2mm

(b)

(b) Clearance 9.3%t

Fig. 4. Comparison of FE-analysis and experimental results of the sheared edge for a sheet thickness of 1.6 mm.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.018 (1.5%t)C0.037 (3.1%t)C0.074 (6.2%t)C0.112 (9.3%t)C0.150 (12.5%t)C0.187 (15.6%t)C0.262 (21.8%t)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.018 (1.5%t)C0.037 (3.1%t)C0.074 (6.2%t)C0.112 (9.3%t)C0.150 (12.5%t)C0.187 (15.6%t)C0.262 (21.8%t)

C0.018 (1.5%t)C0.037 (3.1%t)C0.074 (6.2%t)C0.112 (9.3%t)C0.150 (12.5%t)C0.187 (15.6%t)C0.262 (21.8%t)

(a) Material thickness 1.2 mm

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.025 (1.5%t)C0.050 (3.1%t)C0.100 (6.2%t)C0.150 (9.3%t)C0.200 (12.5%t)C0.250 (15.6%t)C0.350 (21.8%t)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.025 (1.5%t)C0.050 (3.1%t)C0.100 (6.2%t)C0.150 (9.3%t)C0.200 (12.5%t)C0.250 (15.6%t)C0.350 (21.8%t)

C0.025 (1.5%t)C0.050 (3.1%t)C0.100 (6.2%t)C0.150 (9.3%t)C0.200 (12.5%t)C0.250 (15.6%t)C0.350 (21.8%t)

(b) Material thickness 1.6 mm

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.030 (1.5%t)C0.062 (3.1%t)C0.125 (6.2%t)C0.186 (9.3%t)C0.250 (12.5%t)C0.312 (15.6%t)C0.436 (21.8%t)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

30

35

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

C0.030 (1.5%t)C0.062 (3.1%t)C0.125 (6.2%t)C0.186 (9.3%t)C0.250 (12.5%t)C0.312 (15.6%t)C0.436 (21.8%t)

C0.030 (1.5%t)C0.062 (3.1%t)C0.125 (6.2%t)C0.186 (9.3%t)C0.250 (12.5%t)C0.312 (15.6%t)C0.436 (21.8%t)

(c) Material thickness 2.0 mm

Fig. 5. Trimming load with respect to punch stroke for varying clear-ances with a sheet thickness.


tant process parameter in the trimming process. The clearance strongly affects the shearing load and the quality of the sheared edge [7, 9]. In this study, FE-analyses of the trimming process were performed with clearance between 0.025 mm (1.5%t) and 0.35 mm (21.8%t).

The trimming load according to the punch stroke for vari-ous clearances with considered sheet thickness is shown in Fig. 5. Tendency of load variation with punch stroke are similar according to sheet thicknesses. Initially, the trimming load increases because the material experiences strain hardening until a crack initiates as the punch penetrate into the sheet. The trimming load then sharply decreases because of the separa-tion of the scrap by crack propagation. The maximum load decreases as the clearance increases. An increase of the clear-ance causes a higher bending moment, which results from the increase in the distance between the edge of the punch and the die. In turn, the generation of hydrostatic tensile stress by the bending moment in the deformation zone reduces the maxi-mum trimming load.

3.3 Effect of clearance on quality of sheared edge

Fig. 7 shows the characteristics of the sheared edge of DP980 with respect to the clearance. The roll-over increases with increasing clearance because of the increase of the bend-ing moment in the sheet on the trimming edge, especially on the punch side. The burnish also increases as the clearance increases. In general sheet materials, the burnish for small clearances is higher than that for large clearances because the higher hydrostatic pressure in the sheared zone increases the ductility of the material, as indicated by Yukawa et al. [14]. This would be effective in relatively ductile materials. Al-though hydrostatic pressure is generated in the sheared zone

when trimming the DP980, it is insufficient to provide ductil-ity. Therefore, burnish increases for large clearance because the strength of the material is extremely higher level than the hydrostatic pressure.

As the clearance increases from 1.5%t to 12.5%t, the burr height does not change remarkably. However, the burr height increases significantly above a clearance of 15.6%t. The me-chanism of burr generation is different from the conventional mechanism for large clearances. Generally, the cracks started near flank edge of the punch and the die and then propagated into the scrap side. Subsequently, they meet in the scrap and

(a) Hydrostatic stress distribution

(b) Damage distribution

Fig. 6. Hydrostatic stress and damage distributions of DP980 with a thickness of 1.6 mm for different clearances with punch stroke of 0.16 mm (10%t).

(a) Material thickness 1.2 mm

(b) Material thickness 1.6 mm

(c) Material thickness 2.0 mm

Fig. 7. Characteristics of sheared edge with respect to clearance.


the material is sheared. In the trimming process, the bending of the scrap alters the overall symmetry of the trimming proc-ess. This bending creates a tensile stress in the punch flank and a compressive stress in the die flank. The ductility of ma-terials increases under compressive stress, i.e., higher hydro-static pressure. This leads to the occurrence of a tensile-type burr for clearances over 0.25 mm (15.6%t). Fig. 8 shows the hydrostatic stress distribution at a punch penetration depth of 0.56 mm (35%t) for two clearances, 0.05 mm (3.1%t) and 0.15 mm (9.3%t) with a thickness of 1.6 mm.

When the scrap is bent by the penetration of the punch, the sheet material is in contact with the die edge and flank. As the penetration of the punch increases, the die materials undergo compressive stress, which can prevent the crack initiation at the die edge. The crack initiated at the edge of the punch un-der tensile stress is propagated into not the die edge, but the die flank. Fig. 9 illustrates sheared edge that appears tensile typed burr resulted from high compressive stress at the die edge by large clearance. Therefore, it is recommended in Figs. 7 and 9 that the clearance should be within 12.5%t when trimming DP980.

3.4 Effect of inclined angle on maximum trimming load

The effect of the inclined angle on the trimming load was investigated with the sheet thickness of 1.6 mm. Fig. 10 shows the trimming loads versus punch stroke curves for different inclined angles at a clearance of 9.3%t. The maximum trim-ming load decreases with the increase of the negative inclined

angle. Fig. 11 shows the distribution of hydrostatic stress at a punch stroke of 0.16 mm (10%t) for different negative in-clined angles.

The distribution indicates that the hydrostatic tensile stress increases as the negative inclined angle increases. Negative hydrostatic stress acting on sheared area increases ductility of the material which results in high burnish depth. When the inclined angle is 0º, bending moment occurs before the punch penetrates into material. This bending moment tends to in-crease tensile stress towards perpendicular direction to shear-ing zone rather than shear stress. This phenomenon results in decreased hydrostatic stress and subsequent lower burnish depth. As negative inclined angle increases, bending moment decreases before punch penetrates sheet. This gives locally high compressive hydrostatic stress in the material at punch side at same stroke as shown in Fig. 11. This stress state in the deformation zone reduces the maximum trimming load, as shown in Fig. 10.

The decrease in the increasing rate of the trimming load at the initial trimming stage in Fig. 11 can be explained by the penetration of the punch at the same stroke. Fig. 12 shows that the punch penetrates earlier as the inclined angle decreases. This phenomenon provides the reduction of roll-over for the trimmed part.

When the inclined angle is +10º, the load versus stroke curve is different from the other curves. The load increases

C 3.1%t C 9.3%t

Fig. 8. Hydrostatic stress distribution at punch penetration depth of 0.56 mm (35%t) for two clearances (3.1%t, 9.3%t).

Punch stroke 0.8mm (50%t) Punch stroke 0.9mm (56%t) Punch stroke 0.9mm (63%t)

Crackpropagation Tensile typed burr

High compressive stress(Delay crack initiation)

Crack initiation bytensile stress

Fig. 9. Burr formation at large clearance of 12.5%t (material thickness: 1.6 mm).

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

25

Trim

min

g lo

ad (k

N)

Punch stroke (mm)

a=+10o

a=-20o

a=-10o

a=0o

StripperPunch

Die

StripperPunch

Die

StripperPunch

Die

StripperPunch

Die

stripperpunch

die

α

Fig. 10. Trimming load versus punch stroke curves for different in-clined angles at a clearance of 9.3%t.

Fig. 11. Hydrostatic stress distribution at punch stroke of 10%t for negative inclined angle.


slowly until the punch stroke is 0.8 mm. In this stage of the process, the material is bent by the bottom of the punch during the initial trimming stage. The bending of the material before the penetration of the punch causes strain hardening, which results in the increase of the maximum trimming load for posi-tive inclined angles.

3.5 Effect of inclined angle on quality of sheared edge

The characteristics of the sheared edge with respect to the inclined angle are shown in Fig. 12. As expected from Figs. 10 and 11, roll-over is reduced with the decrease in the in-clined angle. In the meantime, excessive roll-over of approxi-mately 20%t appears at the positive angle, +10º, owing to a high bending moment until the punch penetrates into the ma-terial. Furthermore, the burnish decreases for increasing posi-tive inclined angles. This can be explained by the hydrostatic pressure and the plastic strain concentrations within the sheet metal at the edge of the die.

Fig. 13 shows the distributions of hydrostatic stress and ef-fective strain for inclined angles between +10º and -10º at the same punch penetration. For trimming with a positive inclined angle, the level of hydrostatic tensile stress, which promotes the initiation and propagation of a crack, is higher as com-pared to the stress with a negative inclined angle. Also, the effective strain is more concentrated at the edge of the die for positive inclined angles, causing rapid propagation of the crack into the material. Additionally, the burnish at the sheared edge increases with increasing negative inclined angle magnitude.

The burr height decreases as the inclined angle decreases. The burr height for the inclined angle of -20º is significantly lower than the burr of 0º, as shown in Fig. 13. Because the burnish length is an indicator of the onset of cracking, the burr height is expected to decrease as the burnish increases, as mentioned by Atkins [15].

From the FE-analysis results, it can be known that the trim-ming load decreased with increasing clearance and increasing negative inclined angle. From the aspect of sheared edge, burr height decreased with decreasing clearance while burnish depth decreased slightly. Also, it is obvious that the negative inclined angle gives lower burr height. In order to reduce burr height, practical range of negative inclined angle between -10º and -20º is recommended. As can be seen in Fig. 12, there is little difference in the burr height between inclined angle of -10º and -20º. Consequently, proper inclined angle can be se-lected as -10º considering the shape accuracy because inclined angle in trimming tool occurs inclined sheared edge at the part.

3.6 Effect of clearance and inclined angle on tool failure

To investigate the effect of clearance on the sheared edge in detail, a trimming experiment of 20000 strokes was carried out for the above clearances for a sheet thickness of 1.6 mm. Fig. 14 illustrates the sheared surface under the clearance of 0.05 mm (3.1%t) and 0.15 mm (9.3%t) after 20000 strokes. It is observed that significantly unstable sheared edge with lo-cally high burr height appears in the trimmed specimen of 20,000 strokes under the clearance of 0.05 mm (3.1%t). This is due to local wear and chipping on the trimming tool edge. On the other hands, uniform sheared edge can be obtained with strokes when trimmed under the 9.3%t clearance.

In trimming with the inclined angle of +10º, burnish depth is lower than that with the angle of 0º, and also localized burr

Fig. 12. Characteristics of sheared edge with respect to inclined angle under the clearance of 0.15 mm (9.3%t).

(a)

(b)

(a) Hydrostatic stress distribution

(a)

(b)

(b) Effective strain distribution

Fig. 13. Hydrostatic stress and plastic effective strain distributions of DP980 with respect to inclined angle at same punch penetration.


can be founded in the strokes of 5000 as illustrated in Fig. 15. This reveals that premature wear and fracture occurs on the edge of trimming tool resulted from the stress concentration. Moreover, significantly higher burr over the depth of 0.2 mm is observed in the trimmed specimen of 20000 strokes. This indicates that the premature regrinding or replacement of tool is needed for the quality of sheared edge. In the mean time, relatively higher burnish depth is obtained when trimmed under the inclined angle of -10º and also uniform sheared sur-face appears with stokes.

The sheared edge characteristics with respect to the number of strokes are shown in Fig. 16. Both the burnish and burr heights increase with more strokes for both clearances. Ac-cording to Husson et al. [16], the burnish and burr both in-crease owing to the wearing of the tool. A tool with a dull

edge resulted from wear and micro-chipping causes material flow instead of shearing deformation. Fig. 17 shows an SEM image of the tool edge for a clearance of 9.3%t after 20000 strokes. Micro-chipping as well as wear is observed along the width of the die edge owing to high contact pressure during the trimming of DP980. The burr height increases statistically along the width direction by the tool failure owing to the local micro-crack at the edge of tool.

(a) Clearance 0.05 mm (3.1%t)

(b) Clearance 0.15 mm (9.3%t)

Fig. 14. Sheared surface of the trimmed specimen under the different clearances at inclined angle of 0º after 20000 strokes trimming.

(a) Inclined angle +10º (b) Inclined angle -10º Fig. 15. Sheared surface of the trimmed specimen under the inclined angle of +10º and -10º after 5000 and 20000 strokes trimming.

(a) Clearance 3.1%t

(b) Clearance 9.3%t

Fig. 16. Length of sheared edge with respect to number of strokes. The number in the bracket indicates burr height.

Fig. 17. SEM image of trimming die edge after 20,000 strokes under the clearance of 9.3%t.


Generally, the burr height increases with increasing clear-ance in most materials such as mild steel and aluminum. However, for DP980, the lower burr height results from the clearance of 9.3%t as opposed to 3.1%t throughout the life-time of the tool. Fig. 18 shows the effective stress distribution of the trimming die for the clearances of 3.1%t and 9.3%t. The effective stress of the tool edge with a 3.1%t clearance is slightly higher than that with a 9.3%t clearance. The values are nearly critical because the compressive yield strength of a case tool material with a hardness of HRC 57~58 is in the range between 1800 MPa and 2500 MPa [17]. Therefore, the tool stress analysis revealed that the failure at the tool edge is more frequent when trimmed with extremely narrow clear-ances. The subsequent burr height would increase when trim-ming UHSSs with a narrow clearance.

Fig. 19 shows the sheared edge characteristics with respect to the number of strokes for both angles. A similar tendency appears wherein the burnish increases with the stroke number. Furthermore, the burr height for the inclined angle of +10º is greater than that for -10º. Fig. 20 illustrates SEM image of trimming die with different inclined angles for a clearance of 9.3%t after 20000 strokes. It is obvious that tool failure of trimming die with the angle of +10º is severer than the other trimming angles. This phenomenon corresponds to the in-crease of burr height when trimmed with positive inclined angle as shown in Fig. 19.

On the other hands, it is found that the localization zone is curved for parallel trimming and does not extend from the punch edge to the die edge as explained by Li [12]. However, the localization zone appears to be straight for negative in-clined angles. Fig. 21 shows the localization zone produced by inclined angles of +10º and -10º. The white dotted line indi-cates the crack path observed from a perfectly trimmed spe-cimen. The crack initiates and propagates through the localiza-tion zone. The localization zone appears to be straight for the inclined angle of -10º, where the zone is connected from the punch edge to the die edge. This is the reason that the burr is smaller for negative inclined angles.

The localization zone is curved for the inclined angle of +10º, which may be the reason for the larger burr at positive

Fig. 18. Effective stress acting on trimming tool for two clearances.

(a) Inclined angle +10º

(b) Inclined angle -10º

Fig. 19. Characteristics of sheared edge with respect to inclined angle at clearance of 9.3%t. The number in the bracket indicates burr height.

(a) α = 0º

(b) α = +10º

(c) α = -10º

Fig. 20. SEM image of trimming die edge with difference inclined angle after 20000 strokes under the clearance of 9.3%t.


inclined angles. As a result of the tool stress analysis, the ef-fective stress acting on the tool edge for the inclined angle of +10º was 1950 MPa, which is high enough to accelerate tool failure. Once tool failure occurs at the edge, the edge is locally dull. The tool failure, especially wear, causes a highly curved localization zone, and the consequent burr height increases with increasing strokes for the positive angle. Therefore, a negative inclined angle is recommended for trimming UHSSs to obtain a sheared edge with high quality.

4. Conclusions

In this study, the characteristics of the sheared edge of DP980, which is used in the automotive industry, was investi-gated with respect to the clearance and the inclined angle. On the basis of FE-analysis and experiments, the following con-clusions can be drawn.

(1) The maximum trimming load decreases as the clearance increases. In trimming using 1.6 mm thickness sheet, maxi-mum trimming load is shown as 24 kN for a clearance of 1.5%t while 22 kN for a clearance of 21.8%t. This is because of the generation of hydrostatic tensile stress, which results from the bending moment that reduces the trimming load in the deformation zone. Also, the hydrostatic pressure increases at the sheet on the edge of the die with decreasing clearance, which increases the trimming load.

(2) The burnish increases slightly with an increase of clear-ance when trimming DP980 for the range of clearance be-tween 1.5%t and 21.8%t. This is because the punch stroke for reaching critical damage that generates crack initiation and propagation increases with increasing clearance.

(3) Tensile typed large burr of 0.25 mm occurs for large clearance of 15.6%t. As the clearance increases, the crack initiated from the edge of the punch under tensile stress prop-

agated into not the die edge, but the die side. This is because the material at the edge of the die is under compressive stress, preventing crack propagation. For this region, the clearance should be controlled within 12.5%t for the minimization of burrs in the trimming of DP980.

(4) A negative inclined angle within -20º results in a more desirable trimming load and better sheared edge characteris-tics. The roll-over was reduced by the early penetration of the punch without bending. The burnish increased owing to the generation of hydrostatic pressure at the tool edge.

(5) Based on the experimental results, the burr height sig-nificantly decreased when trimmed with a negative angle of -10º and a clearance of 9.3%t. This is because the localization zone is linear, and the zone is connected from the punch edge to the die edge. Tool failure that leads to burr formation is reduced owing to the prevention of stress concentration at the edge of the die.

(6) Practically, inclined angle can be provided by control-ling the position or location of stamped part on trimming tool using transferring robot system for improving sheared edge and reducing burr height. Although every circumference of the part cannot be trimmed by negative inclined angle, proper design of trimming tool considering weak position from the aspect of tool life or sheared edge will guarantee higher tool maintenance as well as minimize burr height.

Acknowledgment

This work was supported by the National Research Founda-tion of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012R1A5A1048294).

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(a) Inclined angle +10º

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Fig. 21. SEM image of localization zones at different inclined angles.


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Hong-Seok Choi received his Ph.D. degree from Pusan National University, Korea, in 2014. He is currently studying on mechanical trimming of hot stamped part having ultra-high-strength of 1500 MPa and evaluation of scratch related tool life in cold and hot stamping process.

Dae-Cheol Ko received his Ph.D. degree from Pusan National University, Korea, in 1998. He is currently a professor at Pusan National University. His current research interests are mechanical trim-ming of hot stamped part, hot press forming of CFRP and tool life evaluation.

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