3.3. Effect of clearance on load-displacement curve-under robust trimming
The variations of trimming load and total work done during trimming with the upper die penetration for different clearances using the robust trimming process are shown in Fig. 4(a) and (b), respectively. The main outputs as a function of clearance obtained from the load vs. penetration curves are summarized in Table 2. The peak load decreased from 5928.8 N to 5350.8 N as the clearance increases from 10–30%. Upper die penetration at the end of the trimming process increased from 0.780 mm to 0.927 mm as the clearance increased from 10–30%. The clearance showed less effect on trimming work done under robust trimming which slightly increased from 3.14 N.m to 3.75 N.m when clearance increased from 10–30%. The introduction of the support pad under the scrap piece reduced the deflection of the scrap piece induced by the increased clearance, and consequently the slope of the elastic phase (phase II) slightly decreased with the increasing of clearance from 2.93×104 N/mm at 10% clearance to 2.61×104N/mm at 30% clearance.
Table 2
The main outputs in robust trimming (with a scrap support pad) as a function of clearance obtained from the load vs. penetration curve. The values shown are averages of three test results.
Clearance
|
Peak load (N)
|
Upper die penetration at peak load (mm)
|
Upper die penetration at the onset of fracture (mm)
|
Upper die penetration at the end of trimming progress (mm)
|
Trimming work done (Nm)
|
10%
|
5928.8 ± 26
|
0.361 ± 0.003
|
0.529 ± 0.004
|
0.780 ± 0.003
|
3.14 ± 0.15
|
20%
|
5516.3 ± 25
|
0.566 ± 0.003
|
0.685 ± 0.003
|
0.846 ± 0.003
|
3.54 ± 0.14
|
30%
|
5350.8 ± 21
|
0.665 ± 0.002
|
0.788 ± 0.002
|
0.927 ± 0.002
|
3.75 ± 0.11
|
When compared with the conventional trimming process, a more rapid increase of trimming force is noticeable in phase II. The slopes obtained under robust trimming are greater than those for conventional trimming (shown in Fig. 4). In addition, due to the introduction of the support pad under the scrap piece during robust trimming, the stressed spring pad had a continuous contact with the upper die which led to longer displacement under higher trim loads (Fig. 4(a)) compared with conventional trimming (Fig. 3(a)). Consequently, the peak load and the total work done measured at the selected clearances (10%, 20% and 30%) are greater than those in conventional trimming (shown in Figs. 5(a) and (d)). The increasing of the clearance under robust trimming also delayed the onset of fracture: the upper die penetration at the onset of fracture increased from 0.529 mm to 0.788 mm as the clearance increased from 10–30%. It should be noted that the upper die penetration at the onset of fracture under robust trimming (0.685 mm at 20% and 0.788mm at 30% clearance) is lower than that of conventional trimming (0.712 mm at 20% and 0.889 mm at 30% clearance), however, the penetrations at the onset of fracture under 10% clearance are comparable under conventional and robust trimming (Fig. 5). This observation reveals that the introduction of the support pad under the scrap piece expedited the fracture of the sheet at 20% and 30% compared with conventional trimming and this will be further discussed in Section 3.5.
3.4. Effect of clearance and support force on sheared edge quality
Cross sectional profiles of trimmed and scrap pieces after conventional and robust trimming processes for different clearances are shown in Fig. 6(a)-(f). It can be seen that conventional trimming provides a good quality of the sheared edge for 10% clearance. At 20% a small burr was observed on the conventionally trimmed part. For a clearance of 30%, the burr becomes significantly larger. For robust trimming, no burr was observed at 10% clearance, while at 30% a very small burr was visible. Compared to the conventional process, robust trimming has the advantage of producing smaller burrs. It should be noticed that the bending of the scrap (as marked in the red dash box in Fig. 6(a)-(c)) was observed under conventional trimming for each of the selected trimming clearances. The bending of the scrap piece results in additional tension near the upper trim edge and additional compression around the lower trim edge. However, no bending of the scrap piece was observed when the scrap support pad was present, (as marked in the blue dash box in Fig. 6(d)-(f)), and both the trimmed part and the scrap piece showed symmetrical profiles. The absence of a scrap support pad will no doubt affect the crack propagation path and the formation of the burr during conventional trimming.
Figure 7(a) shows the characteristics of the sheared edge of DP980 for different clearances under conventional trimming. The rollover increases with increasing clearance due to the increase of the bending moment in the sheet at the trim edge. The burnish also increases as the clearance increases. In general, the height of the burnish zone has been observed to decrease with increasing clearances, since the higher hydrostatic pressure generated within the SAZ improves the ductility of the sheet material, as indicated by Yukawa et al. [21]. It was also observed that the length of the fracture zone decreased with increasing clearances. For robust trimming (as shown in Fig. 7(b)), similar trends were observed compared with the conventional trimming, however, the introduction of a support pad reduced the burr height by 4.0, 4.6 and 16.3 times at 10%, 20% and 30% clearance, respectively. It should be noticed that the height of the burnish zone at 20% and 30% clearances was less than that under conventional trimming, which revealed that the upper die penetration at the onset of fracture decreased with the presence of the support pad, as indicated by Fig. 5(c).
3.5. Effect of clearance and support force on crack initiation and propagation during trimming of DP980 sheet
In order to investigate the effect of clearance on the fracture and burr formation mechanisms during conventional and robust trimming, partial trimming tests were conducted. Figure 8 shows the partial shearing in conventional trimming with a 10% clearance; the trimming was interrupted when the upper trim edge reached a penetration of 20% of the sheet thickness. At this stage of upper die penetration, cracks (identified in the red and blue dashed boxes in Fig. 8(a) and shown at higher magnification in Fig. 8(b) and Fig. 8(c)) are seen to have initiated from the top and bottom surfaces of the sheet adjacent to the locations of the upper and lower trim edges, respectively. Due to the shearing induced plastic deformation in the SAZ, some horizontal cracks branching off the main crack can be observed in the region close to the upper surface (Fig. 8(b)). It can also be seen that a band of martensite islands, which was originally parallel to the rolling plane (normal to the shear plane), became displaced in the trimming direction and small horizontal cracks are observed around the martensite islands, as shown in Fig. 8(b). The shearing forces in the trimming direction caused the sheet material to plastically deform. And in order to accommodate the plastic deformation between the upper and lower trim edges, decohesion occurs at the interface between the hard martensite and the soft ferrite matrix and the martensite islands tend to rotate. By using the sample preparation methodology proposed in Section 2.3, the cross-section surface profile indicated within the white dash box in Fig. 8(a) was scanned with a 3D profilometer and is shown in Fig. 8(d). It is evident that the sheet material near the corner indentation left by the sharp trim edge was subjected to large plastic deformations. Moreover, the damage distribution is practically symmetrical and exhibits similar levels of plastic deformation in the vicinity of the upper- and lower trim edges.
Figure 9 shows the partial shearing at a penetration of 30% in conventional trimming with a 10% clearance. At this further stage of upper die penetration, the cracks that had previously initiated from the top and bottom surfaces of the sheet (identified by the red and blue dashed boxes in Fig. 9(a) and shown at higher magnification in Fig. 9(b) and Fig. 9(c), respectively) are seen to continue to propagate with increasing penetration. And the deformed martensite band experienced further displacement accompanied with more horizontal cracks around the martensite, as shown in Fig. 9(b). Again, the damage distribution (Fig. 9(d)) remains practically symmetrical, showing similar levels of plastic deformation in the vicinity of the upper and lower trim edges. Assuming that accumulated plastic deformation is the only factor driving the development of a fracture, the mechanism of shearing for 10% clearance without support pad is based on cracks propagating from the upper and lower surfaces toward each other and meeting somewhere in the middle of the sheet. Finally, it can be pointed out that the upper crack is more significant than the lower crack, which may indicate that the first crack to initiate is that which occurs near the upper trim die.
Figure 10 shows the partial shearing to a penetration of 38% in conventional trimming with a 30% clearance. At this level of die penetration and with the greater clearance, a crack (identified in the red dashed box in Fig. 10(a) and shown at larger magnification in Fig. 10(b)) initiated from the top surface of the sheet near to the location of the upper trim edge, but no crack was initiated from the lower shearing edge (Fig. 10(c)). The cross-section surface profile indicated within the white dashed box in Fig. 10(a) was examined by 3D profilometer and shown in Fig. 10(d). It should be noticed that the area around this sharp corner indentation left by the upper trim die was subjected to larger plastic deformation than that near the lower corner indentation, the damage distribution is evidently asymmetrical, showing different levels of plastic deformation in the areas near the upper and lower trim edges.
Figure 11 shows the partial shearing at a penetration of 45% in conventional trimming with a 30% clearance. At this more advanced stage of the trimming cycle, the crack (located in red dashed box in Fig. 11(a) and shown at higher magnification in Fig. 11(b)) that initiated from the top surface of the sheet continued to propagate, while no crack has formed from the lower surface. Due to the increased penetration, the deformed martensite band experienced further displacement accompanied with more horizontal cracks around the martensite islands, as shown in Fig. 11(b). The distribution of the damage (Fig. 11(d)) within the shearing zone became increasingly asymmetrical as the upper die penetration increased. At higher clearance (30%), bending of the scrap piece changes the overall symmetry of the shearing process and creates additional tension near the upper die edge and additional compression near the lower die edge. It is known that most materials have greater ductility in compression than in tension [22], and this may provide a qualitative explanation for the preferential development of cracks from the upper trim edge rather than from the compressed area near the lower trim edge.
The constraining force from the support pad was shown to minimize the bending of the scrap piece during trimming, as pointed out in Section 3.4 and this no doubt results in different fracture and burr formation mechanisms. Fig. 12 shows the evolution of the cross-sectional profile of DP980 sheet at three different stages during a trimming stroke with a support pad and for a 30% clearance. At 20% upper die penetration, a crack initiated from the bottom surface of the sheet near the lower trim edge as indicated by the blue box in Fig. 12(a), whereas no crack was observed at the top surface near the upper trim edge. When the penetration was increased to 30%, it can be seen that the lower crack continued to propagate upward through the sheet, as indicated by the blue box in Fig. 12(b). Meanwhile, a microcrack was initiated from the top surface of the sheet near the upper trim edge as identified by the red box in Fig. 12(b). When the penetration of the upper die increased to 38%, the upper and lower cracks continued to propagate toward each other and intend to meet in the middle of the SAZ. The damage (Figs. 12(d)-(f)) within the shearing zone has an almost symmetrical distribution except that the area near the lower corner indentation is subjected to a slightly greater plastic deformation than that of area near the upper corner, and this observation is consistent with the crack initiating from the lower trim edge earlier than that from the upper trim edge.
The different trimming clearances and the presence or absence of a support pad resulted in different fracture mechanisms within the SAZ when using sharp trim edges. Consequently, the burr formation was also affected by these parameters, as shown in Fig. 13. Figure 13(a) is an optical microscopy image of the profile of the DP980 sheet at the onset of the separation of the scrap piece from the strip in conventional trimming with a 10% clearance. Since the crack initiated from the lower surface of the sheet, the burr is negligible. Figure 13(b) shows the optical microscopy image of DP980 sheet at the onset of separation in conventional trimming with a 30% clearance. The mechanism of burr generation at 30% clearance is different from that at 10% clearance: the cracks started near upper trim edge and then propagated downward into the sheet metal until complete separation occurred. The bending of the scrap during this conventional trimming process alters the overall symmetry of the deformation and damage distribution. This bending moment creates a tensile stress near the upper surface and a compressive stress near the lower surface resulting in the formation of a tensile-type burr. Figure 13(c) shows the optical microscopy image of the DP980 sheet profile at the onset of separation in robust trimming with a 30% clearance. The mechanism of burr formation in this trimming configuration is different from that in conventional trimming. Since the bending of the scrap is prevented, the overall trimming process remains symmetrical. The cracks started from both the lower and upper trim edges and then meet in the middle of the SAZ. And once the shearing is complete, practically no burr can be seen during the robust trimming of DP980 sheet with a 30% clearance.
The partial shearing tests with the presence of a scrap support pad confirmed that a first crack is initiated from the bottom surface of the sheet near the lower trim edge, and then this is followed by the initiation of a crack from the upper trim edge. This leads to practically no burr at the sheared edge. In conventional trimming, the bending moment has a very significant effect on the deformation, damage distribution and crack initiation, and the bending moment increases with increasing clearances. The significant bending of the scrap piece in the absence of a support pad applies additional stretching near the upper surface and additional compression near the lower surface, and therefore the first crack initiates from the upper surface.