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ARTICLE INFOArticle ID: 05-11-03-0023Copyright © 2018
SAE Internationaldoi:10.4271/05-11-03-0023
HistoryReceived: 08 Mar 2018Revised: 28 Jun 2018Accepted: 23 Jul
2018e-Available: 17 Sep 2018
KeywordsMetal forming, Abrasive water jet, Blanking, Hole
expansion test, Advanced high-strength steel
CitationBehrens, B., Diaz-Infante, D., Altan, T. Yilkiran, D. et
al., “Improving Hole Expansion Ratio by Parameter Adjustment in
Abrasive Water Jet Operations for DP800,” SAE Int. J. Mater. Manuf.
11(3):2018,doi:10.4271/05-11-03-0023.
ISSN: 1946-3979e-ISSN: 1946-3987
Improving Hole Expansion Ratio by Parameter Adjustment in
Abrasive Water Jet Operations for DP800
Bernd-Arno Behrens, Gottfried Wilhelm Leibniz Universität
HannoverDavid Diaz-Infante and Taylan Altan, The Ohio State
UniversityDeniz Yilkiran, Kai Wölki, and Sven Hübner, Gottfried
Wilhelm Leibniz Universität Hannover
AbstractThe use of Abrasive Water Jet (AWJ) cutting technology
can improve the edge stretchability in sheet metal forming. The
advances in technology have allowed significant increases in
working speeds and pressures, reducing the AWJ operation cost. The
main objective of this work was to determine the effect of selected
AWJ cutting parameters on the Hole Expansion Ratio (HER) for a
DP800 (Dual-Phase) Advanced High-Strength Steel (AHSS) with
s0 = 1.2 mm by using a fractional factorial design
of experiments for the Hole Expansion Tests (HET). Additionally,
the surface rough-ness and residual stresses were measured on the
holes looking for a possible relation between them and the measured
HER. A deep drawing quality steel DC06 with
s0 = 1.0 mm was used for reference. The fracture
occurrence was captured by high-speed cameras and by Acoustic
Emissions (AE) in order to compare both methods. Results indicated
that using, regardless of the material, a small standoff distance,
high water pressure, and slow traverse speed and cutting the sample
underwater will delay the fracture in a hole expansion operation.
Furthermore, the AE have proven to be adequate to measure
cracks when optical methods are not feasible. In conclusion, based
on the impact of the aforementioned parameters, it is possible to
select, appropriately, the AWJ operation parameters to achieve the
edge stretchability required for each forming process.
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1. Introduction
Edge cracking is a common problem in sheet metal forming, mainly
for Advanced High-Strength Steels (AHSS). The noticeable increase
of AHSS in the auto-motive sector, due to the Corporate Average
Fuel Economy (CAFE) regulations, makes this fact an interesting
problem. The relatively low formability of the AHSS and the damage
from previous operations on the edge of the material may lead to
splits starting on the edges, mainly due to the tensile stresses in
these areas during the forming process [1]. It is well known [2, 3,
4, 5, 6, 7] that the edge stretchability strongly depends on the
cutting method (i.e., blanking or laser cutting). However, some
materials are more sensitive to the cutting method than others [8].
A more complex way to avoid edge cracking is by manufacturing a
material with localized strengthened areas [9]; this could help to
reduce the damage at the edge while keeping the rest of the
material as strong as required.
Various researchers evaluated different cutting methods [3, 10,
11], consistently blanking provided samples with the lowest edge
stretchability, while Abrasive Water Jet (AWJ), laser cutting, or
Electrical Discharge Machining (EDM) generated better edges.
However, blanking is the most commonly used due to its relatively
low cost compared to other methods. With advances in cutting
technology, methods such as laser or AWJ cut become more popular
based on an increase in the cutting speed and a reduction in the
mainte-nance costs; therefore, these methods may be used in
low volume production where the cost is justified.
Several studies [12, 13, 14] deal with the optimization of the
blanking process evaluated by means of the Hole Expansion Test
(HET). Some of them [1, 15, 16] state the importance of the tool
wear and the uniform punch/die clear-ance in this operation,
between 10% and 20% for AHSS [4, 17]. Regarding the laser cutting
technology, some researchers [18, 19] have analyzed the effect of
gas pressure, pulse width and frequency, power, focus position, and
cutting speed on the kerf characteristics, similar to the ones
observed in AWJ operations, such as kerf width, angle, burr height,
or produced surface roughness (Figure 1). Thomas [20] used the HET
rather than the kerf characteristics to evaluate the effect of the
laser cutting parameters on the edge stretchability. Moreover,
these results were compared with the ones obtained from the HET for
mechanically blanked holes. The compar-ison showed that depending
on the input parameters for the laser cutting, it is also possible
to produce an edge with lower Hole Expansion Ratio (HER) than the
one for a blanked hole.
It has been observed that the use of AWJ technology may lead, in
some cases, to better edge stretchability than laser cuts [21].
Furthermore, the AWJ eliminates the problem of thermal distortion
or reflectivity due to the material coatings. Nevertheless, there
are limitations to the AWJ, mainly because of the blank length that
can be cut/placed on the work table.
Just as for the laser cutting technology, there are some studies
about the parametric optimization in an AWJ opera-tion. Several
researchers have determined that parameters
such as water pressure, traverse speed, standoff distance,
material thickness, or nozzle diameter have an impact on the
quality of the edge [21, 22, 23, 24]. The most common param-eters
involved in AWJ were listed by Kechagias et al. [23] as shown
in Figure 2. Table 1 shows the results obtained from Wang
et al. [21] who used hot-dipped aluminum/zinc alloy-coated
structural steel, s0 = 1 mm thick, in their
experiments. Focusing on AHSS, Kechagias et al. [23] applied
the AWJ to TRIP700 with s0 = 0.9 mm and TRIP800 with
s0 = 1.25 mm steels. With similar findings, TRIP
steels were also used by Vaxevanidis et al. [24]. Hascalik
et al. [25] only observed the effect of the traverse speed on
AWJ of Ti-6Al-4V alloy with s0 = 4.87 mm.
However, these investigations, for the AWJ method, evaluate the
edge quality by means of the kerf characteristics rather than by
its capability to be stretched without cracks. Therefore, the
relation between the parameters used in AWJ and the edge cracking,
in sheet metal forming, is missing.
FIGURE 1 Schematic of kerf characteristics in AWJ cutting
according to Wang et al. [21].
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et al. [23].
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The present study aims to determine, in a water jet opera-tion,
the effect of the cutting parameters on the edge stretch-ability
evaluated by means of an HET (Figure 3).
Additionally, a possible relation between the surface roughness
of the cut hole and the edge stretchability was i nvestigated. The
same action was taken for the measured residual stresses. It has
been observed that increasing the compressive residual stresses
while shearing the material delays the fracture on the blank edge
[26], while the tension stresses promote the microcrack generation
leading to earlier fracture. For this study, it was hypothesized
that the edge stretchability increases for low surface roughness
and high compressive residual stresses.
Parallel to the aforementioned objectives, and aiming for the
trends of the so-called Industry 4.0, the feasibility of the
Acoustic Emissions (AE) to measure cracks in the material is tested
and compared with the results obtained by high-speed cameras. [27,
28, 29, 30] have investigated the AE applied to material process
monitoring. Other optical measurement methods are also available
such as a fiberscopic fringe projec-tion system used by [31].
The authors are aware of the, initially, low practicality of
using AWJ cutting technology in mass production. Nevertheless, it
must be mentioned that, for specific applica-tions where a
high edge stretchability must be assured to produce
successfully a part, operations such as the laser or AWJ cut may
be the most feasible solution.
2. Experimental Procedure of the HET with Conical Punch
2.1. Input and Output Parameters
Based on the reviewed literature and technical experience, water
pressure, traverse speed, standoff distance, abrasive flow ratio,
and sample location were the parameters selected for this study
(Table 2).
The effects of the five parameters on the HER, the residual
stresses, and the surface roughness (Ra) were analyzed by means of
a Design of Experiments. The param-eters are varied in two levels,
each of them making possible 32 combinations. In the interest of
only main effects of the input parameters, a half factorial design
was considered adequate for this purpose. Therefore, the 16
experimental combinations shown in Table 3, with 5 replicates for
each of them, were analyzed for each material. From the data
collected the highest and lowest value from each case were
disregarded in order to avoid possible outliers in the
experi-ments. It means that only three samples were effectively
taken into account for each parameter combination.
2.2. MaterialsThe materials used in this set of experiments
were DP800 AHSS with s0 = 1.2 mm and a deep drawing
quality steel DC06 with s0 = 1.0 mm. Their chemical
and mechanical properties are listed in Tables 4 and 5,
respectively.
2.3. Equipment and SamplesAn Erichsen machine was used to
conduct the experiments. In accordance to the ISO16630 standard
[34], a 60° conical punch, with a 59.7 mm diameter, was used.
The punch moved upwards at 1 mm/s to expand the water jet cut
hole. Nevertheless, the 10 mm hole described in the ISO16630
was modified intentionally to 20 mm in order to be able
to see larger differences between the different cutting parameters
[35]. A larger hole size was discarded because, for DC06 with
s0 = 1.0 mm, there was a high possibility that the
expanded hole would not crack for the given hole/punch ratio
[36].
TABLE 1 Effect of AWJ parameters on kerf characteristics
according to Wang et al. [21].
Water pressure
Standoff distance
Abrasive flow rate
Traverse speed
Kerf width Increase Increase Not significant
Decrease
Kerf taper Not significant
Increase Not significant
Increase
Surface roughness
With a minimum
Increase Decrease Increase
Burr height Decrease Increase Not significant
Increase
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FIGURE 3 Schematic of the HET.
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TABLE 2 Input parameters to the AWJ operation.
Input parameter “Low value” “High value”Water pressure
200 MPa 400 MPa
Traverse speed 11.1 mm/s 5.55 mm/s
Standoff distance 4.0 mm 2.0 mm
Abrasive flow ratio 0.15 kg/min 0.3 kg/min
Sample location Above water Underwater© SA
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The 20 mm holes cut by water jet were centered in
square 130 mm × 130 mm blanks created by shearing. The
experi-mental equipment is shown in Figure 4. All the samples were
placed burr side up; it means avoiding contact between the burr and
the punch.
A 20 mm diameter centering device, located on the top of
the punch, was tightly fitted into the holes to assure the
concentricity during the experiments, as shown in Figure 5. This
device was only removed after the blanks were fully clamped using a
300 kN force, therefore eliminating any off-centering
possibility.
High-speed cameras were installed in order to detect the
fracture as it is explained later. Additionally, in order to
determine the feasibility of the use of AE to detect fractures,
two sensors were fixed on top of the die to measure the AE during
the experiments.
2.4. Crack DetectionA significant scattering has been observed
by Atzema et al. [37] in HET when using the ISO16630. One of
the multiple possible causes for this scatter is the method used to
detect the crack occurrence. According to the ISO16630 the crack
must go through the thickness of the material before stopping the
punch movement. However, nowadays, it is well known that depending
on the skills of the technician running the experiment and the
method used to detect the crack, there are several delays between
the crack occurrence, the crack detection, and the press stop.
2.4.1. Crack Detection by Camera Following the recent trends in
the field, the authors decided to use two
TABLE 3 Half factorial design for five parameters and two
levels.
CaseStandoff distance
Traverse speed
Abrasive flow ratio
Water pressure
Sample location
1 2.0 mm 11.1 mm/s 0.15 kg/min 200 MPa Above
water
2 4.0 mm 11.1 mm/s 0.15 kg/min 200 MPa
Underwater
3 2.0 mm 5.55 mm/s 0.15 kg/min 200 MPa
Underwater
4 4.0 mm 5.55 mm/s 0.15 kg/min 200 MPa Above
water
5 2.0 mm 11.1 mm/s 0.30 kg/min 200 MPa
Underwater
6 4.0 mm 11.1 mm/s 0.30 kg/min 200 MPa Above
water
7 2.0 mm 5.55 mm/s 0.30 kg/min 200 MPa Above
water
8 4.0 mm 5.55 mm/s 0.30 kg/min 200 MPa
Underwater
9 2.0 mm 11.1 mm/s 0.15 kg/min 400 MPa
Underwater
10 4.0 mm 11.1 mm/s 0.15 kg/min 400 MPa
Above water
11 2.0 mm 5.55 mm/s 0.15 kg/min 400 MPa
Above water
12 4.0 mm 5.55 mm/s 0.15 kg/min 400 MPa
Underwater
13 2.0 mm 11.1 mm/s 0.30 kg/min
400 MPa Above water
14 4.0 mm 11.1 mm/s 0.30 kg/min
400 MPa Underwater
15 2.0 mm 5.55 mm/s 0.30 kg/min
400 MPa Underwater
16 4.0 mm 5.55 mm/s 0.30 kg/min
400 MPa Above water ©
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TABLE 4 Chemical composition for the examined steel materials.
Values provided in mass percentages [32, 33].
Material DC06, s0 = 1.0 mm DP800,
s0 = 1.2 mmC 0.02 0.15
Si - 0.42
Mn 0.25 2.06
P 0.02 0.008
S 0.02 0.002
Ti + Nb 0.3 -
Cr + Mo - 0.408
Al - 0.57 © SA
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TABLE 5 Mechanical properties obtained from the tensile test for
the examined steel materials [32, 33].
MaterialDC06, s0 = 1.0 mm
DP800, s0 = 1.2 mm
Tensile strength [MPa] 270-350 450-550
Minimum yield strength [MPa] 170-180 780-900
Min total elongation [%] 41 18 © SA
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FIGURE 4 Experimental setup. Top, equipment for the HET.
Bottom, equipment for Acoustic Emission detection.
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high-speed cameras from an ARAMIS system, eight pictures/
second, to detect the fracture within an accuracy of
±0.125 mm. The cameras were not located exactly on the top of
the center of the experiment but on the right and left side;
nevertheless, using this configuration, it is also possible to
observe the cracks as well as to measure the hole diameter from the
pictures. An example of such optical crack detection is shown in
Figure 6.
2.4.2. Crack Detection by AE The AE were recorded
threshold-based and evaluated with the AE measuring system AMSY-6
from the manufacturer Vallen Systeme GmbH. During the experiments
the following settings were used: threshold 30 dB (sensor 1),
threshold 45 dB (sensor 2), rearm time 100 μs, and
duration discrimination time 100 μs. Two AE sensors, presented
in Figure 7, with different characteris-tics were placed on the
tool. Sensor 1 was a broadband sensor mounted with modelling clay
with a frequency range of 200-2500 kHz and sensor 2 was a
resonant sensor (peak frequency
of about 375 kHz) fastened using glue with a frequency
range of 250-700 kHz. To eliminate noise signals
sensor-matched digital filters were applied to the sensors:
230-2200 kHz (sen-sor 1) and 95-800 kHz (sensor 2).
The AE were measured with a sampling rate of 5 MHz and the
stroke was measured synchronously with a sampling rate of
20 kHz. Furthermore, preamplifiers (type AEP4) which amplify
small input signals with 34 dB were used. An example of crack
detection using AE is shown in Figure 8.
FIGURE 5 Schematic of the HET tooling.
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FIGURE 6 Crack detection by high-speed camera.
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FIGURE 8 Example of crack detection by acoustic sensors.
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2.5. Evaluation of Edge Stretchability
The high-speed cameras as well as the acoustic sensors were
manually synchronized, each one of them individually, with the
punch stroke sensor. It is proposed to evaluate the edge
stretchability by the punch stroke at crack as well as the HER, two
methods which are directly related by geometrical conditions.
In order to evaluate the HER, finite element (FE) simu-lations
were conducted using PAM-STAMP. The hole diameter obtained in
simulation for the stroke at crack measured in experiments was used
to calculate the HER. Additionally, the hole diameter at crack was
verified by pictures. Since the real diameter is known for the
initial and final stage of the expanded hole, as well as the number
of pixels in the pictures of the initial, final, and crack
occur-rence stage, a linear interpolation was used to calculate the
hole diameter at crack. The sequence of pictures aforemen-tioned is
shown in Figure 9. The comparison between both methods showed an
error of about ±0.3 mm in the calcu-lated hole expanded
diameter. Therefore, calculating the hole diameter at crack by
pictures is a feasible method when FE simulations are not
available.
For each sample, the location where the water jet path completes
the circumference was marked as a reference since it was assumed
that this might be the weakest point of the hole.
2.6. Surface Roughness and Residual Stress
Measurements
Using a 3D microscope (Keyence VR-3200), the surface rough-ness
(Ra) of a sample was determined as the average value of three
consistent measurements in the same 1 mm2 spot of the cut edge
(Section A in Figure 10). Three samples were measured per case
tested.
Following a similar calculation method, the residual stresses on
the cut holes were measured using X-ray diffraction method. Tables
with the findings are presented in the following section.
3. Results
3.1. Crack LocationWhile selecting the water jet path, it was
intended to avoid the initial jet on a point over the circumference
to avoid a direct damage to the edge of interest; therefore, it was
decided to start the cutting process from the center of the hole.
Nevertheless, as expected, a slightly visible notch appears where
the water jet path ends. This notch is more visible for the AWJ
parameter combinations that match with the worst edge
stretchability (Figure 11). This point is considered as the weakest
point of the hole. The edge fracture occurred at that location for
158 parts out of 160 tested. This suggests that either this weak
point should be intentionally located where no high tensile
stresses are expected unless a “smoother” water jet path is
selected.
FIGURE 9 Pictures from the blank before, at, and after crack
occurrence. Initial diameter was 20 mm.
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FIGURE 10 Surface roughness measurements using a 3D microscope
Keyence VR-3200.
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FIGURE 11 Crack location at the weakest point of the cut
hole.
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3.2. Edge Stretchability, Extrusion Height and HER
From Figures 12 and 13 below, it can be seen that,
regardless of the material used, the parameters that produce a
better edge stretchability are consistent; the same is observed on
the left side, worst cases, of the tables. These figures also show
that there was a difference of about 10 mm in the punch stroke
between the best and the worst case for the DC06 with
s0 = 1.0 mm steel. This difference was about
5 mm for the DP800 with s0 = 1.2 mm AHSS. These
results are translated to HER by using FE simula-tion, illustrated
in Figures 14 and 15, to have a better under-standing of the hole
diameters that can be achieved for the aforementioned punch
strokes. It clearly shows the big differ-ence between the HER for a
mild steel and an AHSS.
For DC06 with s0 = 1.0 mm, the difference in the
HER between the worst and the best case was about 50%. An increase
of about 15% of the HER was observed when using the best AWJ
parameter combination for DP800 with s0 = 1.2 mm.
Therefore, it is seen that the parameters used in
the process must be also specified when evaluating the HER
of AWJ cut samples.
In Figures 14 and 15, Case 15, which is formed by a short
standoff distance, a slow traverse speed, a high water pressure,
and a sample cut underwater, lead to the highest edge
stretch-ability, regardless of the amount of abrasive material
used. Case 6 was in the opposite side; as it can be inferred,
high standoff distance, fast traverse speed, low water pressure,
and sample cut out of the water lead to the lowest edge
stretch-ability (Table 6).
3.3. Effect of Input AWJ Parameters on the HER
The data collected from the HET was analyzed using Minitab. Only
the main effects were analyzed in this study. Regardless of the
material, the parameters resulted ranked in the same order. The
traverse speed resulted to be the parameter with the highest
impact on the edge stretchability; the lower the speed the better
the edge stretchability. Unfortunately, this is a weakness of the
AWJ method, since a fast speed is required for mass production.
FIGURE 12 Effect of AWJ parameters on stroke at crack for DC06
with s0 = 1.0 mm. See also Table 3.
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FIGURE 13 Effect of AWJ parameters on stroke at crack for
DP800 with s0 = 1.2 mm. See also Table 3.
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l FIGURE 14 Effect of AWJ parameters on the HER for DC06 with
s0 = 1.0 mm. See also Table 3.
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FIGURE 15 Effect of AWJ parameters on the HER for DP800 with
s0 = 1.2 mm. See also Table 3.
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Using a mechanical press, very often, more than 25 holes a
minute can be punched; a maximum of 10 holes in a minute would
be cut using the largest speed considered in this study.
However, as mentioned earlier, the AWJ is more focused on
prototyping, for example, or special operations which do not
require a large number of parts but high edge stretchability.
The standoff distance should be kept as short as possible
and the water pressure in a high value in order to improve the cut
edge. The standoff distance is a simple parameter and it is
costless to manipulate it, but it can significantly affect the
process. On the other hand, the water pressure is directly related
to the electric energy consumption; therefore, this parameter
should be adequate according to the user edge requirements. It
was observed that when the sample is cut underwater, not only the
edge stretchability increases but also the cleanliness of the work
place while the noise reduces signifi-cantly creating a less
stressful environment. It is hypothesized that cutting underwater
may reduce slightly the pressure of the water jet; however, it also
focuses better the abrasive parti-cles creating a better cut while
avoiding these particles that make the workplace dirty. A very
interesting finding was that the abrasive flow ratio did not make a
significant difference in the edge stretchability. This may lead to
a significant reduction of the cost of the process when the minimum
amount of abrasive material is used. A graph with the individual
effects for each parameter tested is shown in Figure 16. The larger
the
slope of the lines presented, the bigger the effect of the
param-eter on the edge stretchability.
3.4. Effect of Surface Roughness on HER
The surface roughness measurements show a very small differ-ence
of about 1 micron, for both materials, between the “smoothest” and
the “roughest” surface as shown in Figures 17 and 18. These results
indicated that the surface roughness is not strongly related to the
edge stretchability in AWJ opera-tions. This idea backs up the
result which indicated that the abrasive flow ratio has not a
significant impact on the edge stretchability, since it is well
known that a higher amount of abrasive material will decrease the
surface roughness.
3.5. Effect of Residual Stresses on HER
The measured residual stresses, as described in Section 2.6,
were in compression and slightly higher, about 20 MPa, for the
best than for the worst case, 15 and 6, respectively, when measured
for the DC06 with s0 = 1.0 mm material. Due to the
sensitivity of the X-ray diffraction method, a lot of scatter was
observed when measuring the residual stresses for the DP800 with
s0 = 1.2 mm. It is hypothesized that these
observations are mainly due to the two phases of the material; it
is possible that the same phase was not consistently measured
during each attempt. Two samples were measured per case, best and
worst, without any success for DP800. The measurements for the same
sample, at apparently the same point, using the same machine
configuration delivered totally different results. Since the X-ray
diffraction was the only measuring method available at the moment
of the study, it was not possible to obtain reliable measurements
of the residual stresses of DP800; however, the same trend is
expected, higher compression stresses could be related to
higher edge stretchability. The residual stresses of DC06 with
s0 = 1.0 mm are shown in Figure 19.
TABLE 6 Best and worst AWJ parameter combination tested.
CaseStandoff distance
Traverse speed
Abrasive flow ratio
Water pressure
Sample location
6 4.0 mm 11.1 mm/s 0.30 kg/min
200 MPa Above water
15 2.0 mm 5.55 mm/s 0.30 kg/min
400 MPa Underwater
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FIGURE 16 Individual effects of AWJ parameters on edge
stretchability.
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FIGURE 17 Effect of AWJ parameters on surface roughness for
DC06 with s0 = 1.0 mm before forming. See also Table
3.
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3.6. Crack Detection Using AE
The acoustic signals were acquired for all the HET conducted. In
most of the cases the signals were in good correlation with the
fracture observed by the cameras. The two more significant cases, 6
and 15, for both materials are illustrated in this article.
As it can be seen in Figure 20, both methods, camera and
AE, evaluated Case 15 as the one with the largest edge
stretch-ability. The results indicated about the same values at
crack for DC06. However, for DP800 the results are off by about
2 mm. In both cases, the AE were delayed in comparison with
the visual measurements. It was expected from the previous author’s
experience that the AE could recognize a fracture faster than a
camera. Therefore, this delay is attributed to an offset error
during the synchronization with the punch stroke for one of the
crack detection methods since they were set individually. For
future experiments, it is suggested that the systems have to
be coupled/synchronized. Another possibility is that the
points determined as a fracture occurrence using
the AE are not the crack but the crack propagation. In this
case, further analysis should be conducted.
It should be mentioned that using AE requires a lot of user
expertise to interpret the signals and isolate them from the noise
in the surroundings. Extensive work is being conducted at the
Institute of Forming Technology and Machines (IFUM) in Germany
about the use of AE not only to detect cracks but also to prevent
them by finding microcracks.
4. ConclusionsIn this study, the effect of selected AWJ cutting
parameters on the edge stretchability, evaluated by means of an
HET, was determined. Additionally, the effect on the HET of the
surface roughness and residual stresses produced by these cutting
parameters was analyzed. The principal conclusions of this study
are listed as follows:
• In order to avoid a large scattering in the results, sensors
to measure the punch stroke and high-speed cameras to detect the
crack start should be used. The stroke at crack may substitute
the HER as an edge stretchability parameter due to the savings on
measurement time. In any case it is possible to estimate
approximately the HER using FE simulations or pictures from the
hole within a reasonable error as shown previously.
• The HER can be varied within a certain range by adjusting
the AWJ parameters. This can help to optimize the costs of the
operation by cutting edges with the quality required for the
forming process.
• The crack at the edge tends to occur where the water jet
finishes its path. It is recommended to select this point where low
tensile stresses are expected in the forming operation.
• The stroke at crack, used as an edge stretchability parameter,
increases when increasing the water jet
FIGURE 18 Effect of AWJ parameters on surface roughness for
DP800 with s0 = 1.2 mm before forming. See also
Table 3.
© S
AE
Inte
rnat
iona
l FIGURE 20 Crack detection using AE vs high-speed camera (1
picture/0.125 mm) for DC06 with s0 = 1.0 mm
(left) and DP800 with s0 = 1.2 mm (right).
© S
AE
Inte
rnat
iona
l
FIGURE 19 Residual stresses in compression for DC06 with
s0 = 1.0 mm.
© S
AE
Inte
rnat
iona
l
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10 Behrens et al. / SAE Int. J. Mater. Manuf. / Volume 11, Issue
3, 2018
© 2018 SAE International. All Rights Reserved.
pressure and decreasing the traverse speed and the standoff
distance.
• The abrasive flow ratio is not cost effective; therefore, it
can be minimized to improve operational costs.
• The location of the sample, under or above water, has a small
effect. Nevertheless, the noise is reduced and the edge
stretchability and the cleanliness of the work space are improved
by cutting underwater.
• The traverse speed has the biggest impact. The slower the
motion the better the edge stretchability. This represents a
disadvantage of the AWJ operation when compared with mechanical
punching.
• The surface roughness (Ra) has not a significant relation to
the edge stretchability within the parameters tested.
• The residual stresses measured in DC06 with
s0 = 1.0 mm suggest that higher compressive stresses
help to delay the crack start. This is in agreement with findings
from other researchers. Due to the large scatter on the
measurements for DP800 with s0 = 1.2 mm no
conclusion is obtained for this material.
• The AE seem to be able to detect macrocracks. However,
the signal evaluation requires a lot of user expertise. A criterion
for crack detection using AE is desirable. Further analysis must
be done in this regard.
AcknowledgmentsThe presented work is a result of the project
“Acoustic emission analysis for online monitoring in sheet metal
forming,” project number BE 1691/183-1, granted by the German
Research Foundation (DFG). The authors are thankful for the
financial support. Additionally, the authors would like to thank to
the Institute of Forming Technology and Machines (IFUM), Leibniz
Universität Hannover, for hosting a guest researcher and allowing
him to conduct the experiments at their facilities during a
scientific exchange as an international collaboration with the
Center for Precision Forming at The Ohio State University.
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10.4271/05-11-03-0023: Improving Hole Expansion Ratio by
Parameter Adjustment in Abrasive Water Jet Operations for
DP80010.4271/05-11-03-0023: Abstract10.4271/05-11-03-0023:
Keywords1 Introduction2 Experimental Procedure of the HET with
Conical Punch2.1 Input and Output Parameters2.2 Materials2.3
Equipment and Samples2.4 Crack Detection2.4.1 Crack Detection by
Camera2.4.2 Crack Detection by AE2.5 Evaluation of Edge
Stretchability2.6 Surface Roughness and Residual Stress
Measurements
3 Results3.1 Crack Location3.2 Edge Stretchability, Extrusion
Height and HER3.3 Effect of Input AWJ Parameters on the HER3.4
Effect of Surface Roughness on HER3.5 Effect of Residual Stresses
on HER3.6 Crack Detection Using AE
4 Conclusions
AcknowledgmentsReferences