Original Article
Piezoelectric-based non-destructive
monitoring of contact pressures in
fine-blanking process
Proc IMechE Part B:
J Engineering Manufacture
0(0) 1–10
Ó IMechE 2014
Reprints and permissions:
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DOI: 10.1177/0954405413514955
pib.sagepub.com
Ri-xian Ding, Cheng Guo, Zi-cai Zhu, Ming-kai Zhang and Rui-peng Gao
Abstract
A piezoelectric polyvinylidene fluoride film stress sensor was developed to measure the contact pressure on the sheet
near the cutting edge in the fine-blanking process. Two cases with and without cracks in the fine-blanking surface were
studied. The relationship between the contact pressure and crack width was investigated using the Kriging methodology
at different punching speeds. The maximum contact pressure appeared at the beginning of sheet punching, but not at the
end of V-ring indention. For the case with and without cracks in the fine-blanking surface, one and two fluctuations of
contact pressure are observed, respectively. Using the Kriging model, a fine-blanked surface with a reduced crack width
can be obtained using the optimal values of contact pressure and punching speed.
Keywords
Fine-blanking, contact pressure, crack, polyvinylidene fluoride film stress sensor, Kriging model
Date received: 3 May 2013; accepted: 4 November 2013
Introduction
The fine-blanking process produces accurate near-net
shape parts requiring fewer process steps at lower cost
than the conventional blanking process, which are
widely used in automotive and electrical industry.1,2
For the successful operation of the fine-blanking process, the V-ring indenter and blank-holder force play
important roles in establishing high hydrostatic compressive stress in the metal in the shearing zone. The
application of the V-ring indenter increased the hydrostatic compressive stress.3 High hydrostatic compressive stress was important for the prevention of crack
formation.4 The crack width in the fine-blanking surface decreased as a result of using optimal parameters
of the V-ring indenter5,6 and the blank-holder force.7
The blank-holder force is calculated from the
hydraulic pressure in the hydraulic cylinder, which is
the entire effect of the blank-holder on the sheet.8
However, the contact pressure is not uniform at the
contact surface because of the curvature, wrap and elasticity deformation of the metal sheet. According to the
experimental result,9 the strong contacted area was in
the neighbourhood of the blanking contour when using
the blank-holder without the V-ring indenter. Outside
of this contacted area, the blank-holder weakly contacted the blanked sheet. When using the blank-holder
with the V-ring indenter, the contact pressure would
increase significantly on the contact surface, and this
was caused by pressing the V-ring indenter into the
sheet. However, in the case with the V-ring indenter,
limited studies have been carried out to reveal the
change characteristics of the contact pressure near the
cutting edge and its effect on crack width in the fineblanking surface.
In previous studies,10–12 the contact pressure in
metal-forming processes was measured using pin-type
sensors. This conventional pressure sensor requires considerable space to be installed, but there is limited space
to install the sensor near the blanking clearance in the
fine-blanking die. In addition, the hole drilled near
the blanking clearance for the installation reduces the
strength of the die and increases the risk of cracking in
the fine-blanking process. However, film piezoelectric
sensors, which can be pasted directly onto the surface,
offer minimal disturbance to the testing environment
School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, P.R.
China
Corresponding author:
Ri-xian Ding, School of Mechanical Engineering, Xi’an Jiaotong University,
No.28, Xianning West Road, Xi’an, Shaanxi Province 710049, P.R. China.
Email: [email protected]
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Proc IMechE Part B: J Engineering Manufacture 0(0)
due to their mechanical flexibility, high piezoelectric
coefficients, dimensional stability, low weight and
chemical inertness.13,14
This study develops a tailor-made polyvinylidene
fluoride (PVDF) film stress sensor for the measurement
of the pressure on the contact surface of the sheet near
the blanking clearance in the fine-blanking process. The
characteristics of the contact pressure variation were
compared between the two fine-blanking processes that
form the fine-blanking surface with and without cracks.
The main objective is to reveal the crack formation process that is reflected in the contact pressure variation.
Also, the effect of the contact pressure on the crack
width was also addressed by constructing the Kriging
model.
Experimental methods
PVDF material
PVDF is a semi-crystalline piezoelectric polymer with
approximately 50%–65% crystallinity.15 The chemical
structure is given by (CH2–CF2)n.16 During the manufacturing process, the PVDF resin pellet is made into a
sheet form with a melt extrusion, and the sheet is
stretched. PVDF is an anisotropic material, and its
electrical and mechanical properties thus differ depending on the direction of the external force in the material. The piezoelectric coefficients, dij and gij, which are
the charge and voltage coefficients, respectively, possess
two subscripts. The coefficients are related to the electric field produced by the mechanical stress. The first
Table 1. PVDF properties reported by the manufacturer.
d31
23 pC/N
d32
5–6 pC/N
d33
21 pC/N
subscript refers to the electrical axis, and the second
refers to the mechanical axis. The axes may be 1, 2 or
3, corresponding to the length, width and thickness;
Axis 1 refers to the stretching direction, Axis 2 refers to
the perpendicular planar direction and Axis 3 refers to
the poling axis, which is perpendicular to the material
surface. However, since the electrodes are on the top
and bottom of the film, the electrical axis is always 3.
The mechanical axis may be 1, 2 or 3 since the stress
can be applied to any of these axes. The piezoelectric
coefficients are shown in Table 1. The actual shear
coefficients d15 and d24 are not considered in this study
because their value is negligible.
Design of film stress sensor
The sensor was implemented using commercial PVDF
material. The thickness of the film was chosen to be 50
mm with silver ink metallization. The developed PVDF
sensor consists of two main components: the piezoelectric PVDF film and the insulation films coated with the
circuit made of silver ink metallization. To implement a
sensor element, a 5 mm 3 5 mm piece of material was
cut from a commercial piezoelectric PVDF film. The
flexible printed circuit (FPC) films were prepared. The
circuits were drawn on the polyethylene terephthalate
(PET) films using the screen printing technique with
conductive silver ink. As shown in Figure 1, the piezoelectric PVDF film used as the sensing element was
placed between the two FPC films.17 Double-sided tape
was used to attach the sensor elements together. The
double-sided tape provides a uniform layer between the
sensor elements. The thickness of the sensor was 0.29
mm. The disturbance to the testing environment caused
by the sensor installation is negligible.
The piezoelectric PVDF film can measure the pressure in the fine-blanking process because the charge
generated on the electrodes by straining the foil
Figure 1. PVDF film stress sensor: (a) structure of film stress sensor and (b) film stress sensor.
PET: polyethylene terephthalate; PVDF: polyvinylidene fluoride.
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Ding et al.
3
discharges slowly. The stress changes that have a minimal frequency of about 0.01 Hz can be measured. The
frequency of the fine-blanking process is about 1 Hz,
and the PVDF stress sensor can therefore be used to
detect the pressure change. When a pressure change is
applied to the sensor, the PVDF film is strained, and
due to its piezoelectric properties, an electric charge
and voltage are generated on the electrodes. This electric signal is measured with a simple electric circuit.
The voltage as the output signal was recorded by a
resistor–capacitor (RC) integrator circuit, which was
shown on the screen of the oscilloscope (Tektronix
TDS 2014B). The compressive pressure that is vertical
to the sheet surface was dominant after the V-ring was
pressed into the sheet. The strain of the PVDF film was
mainly in the three directions (Figure 1). Because there
was no heating during assembly of the sensor, the
adverse effects on the accuracy caused by the thermoelectric effect can be ignored.18 The film stress sensor is
suitable for monitoring the contact pressure on the
sheet surface in the fine-blanking process.
Calibration of film stress sensor
There is a linear relationship between the normal
applied load and the output voltage under the condition that the friction coefficient is constant.19 The split
Hopkinson pressure bar (SHPB) technique was used
for the calibration of the film stress sensor. The strain
gauge was used as the calibration standard to determine the value of the contact pressure on the sensing
element of the film sensor. The calibration model is
written as follows
Q = 0:1673 + 0:006637s3
ð1Þ
where s3 is the contact pressure (MPa) applied on the
sensing element of the film stress sensor and Q is the
generated charge per square millimetre (mC/mm2) of
the film stress sensor. Figure 2 shows the calibration
results. The analysis of variance (ANOVA) shows that
the contact pressure has a significant linear relationship
with the electric charge (i.e. F0 . F1, 7, 0.99). The value
of R2 for the calibration model is 98.5%, which indicates that 98.5% of the variation of the output can be
explained by the calibration model. Therefore, the calibration model is considered to be the best fit.
The output signal of the film stress sensor was measured by an RC integrator circuit consisting of a capacitor (47 nF) and a resistance (30 O). The electric charge
generated on the film surfaces can be calculated by the
following equation
Q = CU
ð2Þ
where Q is the generated electric charge (C), C is the
capacitance (F) of the electric circuit and U is the output voltage (V) of the film stress sensor. The capacitor
voltage, which is the output voltage of the film stress
sensor, is measured by the oscilloscope (Tektronix TDS
2014B). Then, the value of Q can be determined.
Therefore, the contact pressure can be calculated by
equation (1), according to the value of Q obtained.
Installation of film stress sensor in fine-blanking die
The film stress sensor was placed at the contact surface
of the sheet near the clearance to measure the contact
pressure in the fine-blanking process. Table 2 shows the
mechanical property of the C28000 sheet used in the
experiments. Table 3 shows geometrical parameters of
the fine-blanking die and the model of the press. The
V-ring was first pressed into the sheet, and the sheet
was then punched. Figure 3(a) shows the locations of
the film stress sensors (I–III) on the fine-blanking die.
As shown in Figure 3(a) and (b), the film stress sensor I
on the upper surface of the sheet was used to determine
the time at which the punch contacted the upper surface
in the punching process. The film stress sensor II was
placed over the blanking clearance to determine the
Table 2. Material performance parameters of C28000 sheets
used in the experiments.
Materials
Strength, sb (MPa)
Elongation, dd (%)
Flow curve equation
Thickness of the sheet (t)
UNS C28000
460 MPa
29%
s
= 430 + 325e0:605
1.5 mm
Table 3. Key geometric parameters of the fine-blanking mould
and the press model.
Clearance (C)
Tool cutting edges
Parameters of V-ring
Material of the punch and die
Equipment manufacturer
Type of machine
Figure 2. Calibration of the PVDF film stress sensor.
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0.006 mm
RP = 0.02 mm, RD = 0.4 mm
h = 0.5 mm, d = 1.5 mm
ASTM D2
Feintool AG Lyss
GKP-F-100
4
Proc IMechE Part B: J Engineering Manufacture 0(0)
Figure 3. Measurement of contact stress and time length of sheet punching: (a) locations of the measurement using the film stress
sensors (I–III), (b) installation of film stress sensors (I and II) to determine the time points of contacting upper and lower surfaces
and (c) installation of film stress sensor (III) for measuring contact stress.
time at which the punch reached the lower surface of
the sheet. The time length of the sheet punching process
can be determined according to the two time points.
The punching speed was calculated by dividing the
sheet thickness by the time length under the assumption
that the punch moved at the same speed. In order to
install the film stress sensor II, the sheet did not cover
the cutting contour of the die completely. The area of
the cutting contour uncovered by the sheet was so small
that its influence on the punching speed is considered to
be negligible. Figure 3(a) and (c) shows the installation
of film sensor III for the measurement of the contact
pressure at the contact surface of the blank-holder and
sheet in the fine-blanking process. In Figure 3(a), f represents the plastic film that was laid adjacent to the film
sensor to offset its protrusion caused by the sensor
installation. Each experiment was replicated three
times, and the average value was recorded.
The advantage of this method is that the input of
the film stress sensor is related to the contact pressure
distribution on the contact surface. It overcomes the
disadvantage that exists when the input is the force,
which is the required conclusion based on the specific
size of the contact area of the blank-holder and sheet.
The contact pressure can be used as an indicator of the
blank-holder force and V-ring indenter parameters to
evaluate their effect on the crack in the fine-blanking
surface. Therefore, a general and detailed understanding can be obtained.
Meta model–based on Kriging method
The conventional trial-and-error method and empirical
approaches are time-consuming when designing fineblanking dies. These methods are associated with higher
costs, lower efficiency and design inaccuracies. Because
of these deficiencies, there is a need for a more reliable
and better design approach. Application of a metamodel is proposed to reduce the cognitive load on the
designer. The proposed methodology helps to select the
set of optimal parameters to be used. There are several
methods, including the response surface methodology
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Ding et al.
5
(RSM), Taguchi method, neural networks (NNs),
inductive learning and Kriging, which can be used to
construct a meta-model for the relationships between
the inputs and outputs over the range of interest.
The polynomial based RSM is the most widely used
meta-model.20 RSM, which is based on experimental programming, normally requires the assumption of the order
of the approximated base function. This will sometimes
be difficult because it requires an understanding of the
qualitative tendency of the entire design space.
Instabilities may arise when higher order response surfaces are used to model a non-linear design space. In
addition, the amount of data used to estimate all of the
coefficients determines the order of the polynomial.21
However, it may not be possible to obtain enough samples to estimate all of the polynomial coefficients in practice, particularly in high dimensions. The Taguchi
method is easy to perform to obtain the optimal condition. However, the optimal condition is the best combination of given levels of the factors and is not the optimal
solution in the design space.22 The model based on NNs
requires many sample points for the training, which is a
practical difficulty.23 The other problem when using NNs
is the need for the operator to be skilled or experienced.24
The Kriging method25 is a method of spatial prediction that is based on minimizing the mean error of the
weighting sum of the sampling values. There is no need
for an assumption related to the order of the base function for approximation during the process of model
building, which is superior to that of RSMs. The
Kriging method involves only parameter optimization
for three variables to determine a model and results in
lower computational costs for the model generation. It
is also superior to NN approximation, which requires a
high computational cost for learning.
The Kriging method was originally developed in geostatistics as a statistical interpolation method to estimate
mineral concentrations over an area of interest.26 The
mathematics was further developed as an interpolative
approximation based on an exponentially weighted sum
of the sample data.27 Kriging models are very flexible
because of the wide range of correlation functions that
can be chosen to build the model. The Kriging model
was applied early to model the mechanical behaviour
and phenomena of the shape memory alloys.28,29 It is a
semi-parametric interpolation technique, which estimates
the unknown information at one point according to the
known information.30 A flow chart of the design process
is shown in Figure 4. Sample points were selected using
the uniform design (UD) as the experimental design. The
average value of the three repeated experiments was used
in constructing the Kriging model. The approximation
of the original model can be expressed as the following
formula under the conditions of multiple inputs and a
single output30,31
^
y(X) =
k
X
j=1
bj fj (X) + Z(X) = bf(X) + Z(X)
ð3Þ
Figure 4. Steps to construct approximate model.
UD: uniform design; PSO: particle swarm optimization.
where X = ½x1 x2 xn is the input, fj (X) is the known
polynomial function chosen by the user, bj is the coefficient to be determined, k is the dimension number of
polynomial functions that is also chosen by the user
and Z(X) is a stochastic function, which is generally
considered to be a normal distribution (0, s2).
Let k = 1 and f1 (x) = 1, equation (3) becomes the
Kriging model
b
b + r(x)T R1 (Y I b)
^y(X) = b
ð4Þ
b = I R Y=(I R I) and I denote an n-vector
where b
of ones. r(x) = ½R(x, x1 )R(x, x2 ) R(x, xn )T , where
R(x, xi ) is the correlation between an unknown point
and the known sample points.31–34
The mean squared error (MSE) of this predictor is
minimized with unbiased estimation. The quality of the
Kriging model can be assessed according to the accuracy of the prediction of the original model at unobserved locations.
New sampling points were required if the obtained
Kriging model is not acceptable. The particle swarm
optimization (PSO) is a population-based heuristic global optimization technology.35 The idea of the PSO
algorithm is based on the simulation of simplified animal social behaviours. The PSO algorithm became
popular36 due to its effectiveness in difficult optimization problems, as well as because of its ease of implementation and its ability to quickly converge to the
good solution.
T
1
T
1
Experimental results and discussion
Characteristics of the curves of contact pressure
Figure 5 shows the contact pressure measured at the
contact surface of the blank-holder and sheet near
the blanking clearance in the fine-blanking process.
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Proc IMechE Part B: J Engineering Manufacture 0(0)
Figure 5. Output of the film stress sensors in fine-blanking
process.
Figure 6. Clean-cut surface and the corresponding contact
stress.
The moment at which the punch contacted the upper
and lower surfaces of the sheet can also be determined.
Figure 6 shows the curve of the contact pressure versus
the time in the fine-blanking process and the corresponding clean-cut surface. The punching speed is
about 4 mm/s. The first fluctuation of the contact pressure was caused by pressing the V-ring indenter into
the sheet. The second fluctuation occurred during the
punching process. The maximum contact pressure
occurs at the beginning of punching. Then, the value of
the contact pressure shows a decreasing trend with an
increase in time. The maximum contact pressure in the
punching process was about five times higher than that
caused by pressing the V-ring indenter into the sheet.
This indicates that the direct contribution of the blankholder force is limited when increasing the contact pressure during the punching process. The increase in the
contact pressure is mainly and directly from squeezing
the metal by punching in the shearing zone. However,
Figure 7. Relationship between crack generation and the
contact stress variation.
applying a high blank-holder force can minimize the
increase in the sheet thickness near the clearance during
the punch penetration. These two factors lead to an
increased hydrostatic compressive stress in the metal
between the cutting edges of the die and the punch,
which prevents crack formation during the punching
process.
Figure 7 shows the fine-blanking surface with cracks
and the corresponding curve of contact pressure versus
the time. The burnish zone is on the left side, while the
crack zone is on the right. The process of generating a
new fine-blanking surface is in the positive time direction. This fine-blanking surface with cracks was
obtained using a lower blank-holder force and faster
punching speed than in the case, which generated the
clean-cut surface. The punching speed is approximately
11 mm/s. The maximum contact pressure in the punching process (Figure 7) is smaller than that in the fineblanking process, which obtained the clean-cut surface
(Figure 6). In the process that formed the clean-cut surface (Figure 6), the contact pressure decreased smoothly
and monotonically until the punch reached the lower
surface of the sheet. However, two troughs are observed
in the process, which formed the fine-blanking surface
with cracks (Figure 7). The punching speed is assumed
to be constant during the punching process. The sheet
thickness corresponds to the time lengths of the sheet
punching. The crack ratio is the ratio of the crack width
to the width of the cut surface. The time at which the
crack occurred was the crack ratio multiplied by the
time duration of sheet punching. It was found that
the second trough occurs almost simultaneously with
the crack generation (Figure 7). Before the appearance
of the crack, the contact pressure first decreased and
then increased. After the occurrence of the crack, the
contact pressure increased until the end. The second
trough can be attributed to the crack generation in the
cut surface. The variation of the contact pressure
reflects the change in the stress condition in the metal
near the clearance in the punching process. The change
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Ding et al.
7
Figure 9. Difference between the measurement of blankholder force (F) and contact pressure (C).
Figure 8. Blank-holder force in the fine-blanking processes.
in the contact pressure is caused by the combined effect
of extruding the metal by punching, strain hardening
and crack formation. The crack initiation and propagation also has a similar effect on the punch force during
the punching process.8,9,37 As shown in Figures 6 and 7,
a similar characteristic was found in the curve of the
contact pressure. The maximum contact pressure
occurs at the beginning of the punching sheet. The
value of the contact pressure shows an overall decreasing trend as time increased until the punch reaches the
lower surface of the sheet.
The advantage in measuring the contact pressure
can be found when compared with the measurement of
the blank-holder force. In the experiments mentioned
above, the hydraulic pressure in the blank-holder cylinder was also measured by an oil pressure gauge as the
method used in the previous studies.8,9 The blankholder force was calculated according to the hydraulic
pressure using the active piston area of the corresponding cylinder. The measurement result of the blankholder force in the fine-blanking process was shown in
Figure 8. Curves of Forces 1 and 2 represent the blankholder force in the fine-blanking processes, which made
the cut surface without and with cracks, as shown in
Figures 6 and 7, respectively. The value of the blankholder force was not significantly changed and was
nearly constant in the fine-blanking process. The crack
generation in the punching process was not reflected in
the curve of the blank-holder force. The details of the
changes in blank-holder force cannot be obtained by
measuring the hydraulic pressure in the cylinder. The
loss of the signal may have been caused by the redundant transmission links in the blank-holder force measurement (Figure 9).
The findings enable us to understand the fineblanking process. The relationship between crack formation and contact pressure provides the basis for the
sensing method when monitoring the fine-blanking
process, leading to the idea that the formation of
cracks is related to the variation of the contact
pressure at the contact surface of the sheet near the
clearance. The information pertaining to the contact
pressure can be utilized for process diagnosis and process optimization.
Optimization of crack ratio based on the Kriging
model
In this article, the Kriging model was built for the prediction of the crack ratio (the ratio of the crack width
to the width of the fine-blanking surface) and to facilitate the design optimization of the contact pressure
under different punching speeds. The contact pressure
changes over time during the fine-blanking process
(Figures 6 and 7). The contact pressure as a function of
time is continuous for the closed interval [t0, t1]. The
average value of ss(t) from t = t0 to t = t1 is the
integral
ðt1
1
ss ðtÞdt
t1 t0
ð5Þ
t0
where ss(t) is the contact pressure (MPa) on the contact surface of the blank-holder and sheet during the
punching process, and t0 and t1 are the times at which
the punch makes contact with the upper and lower surfaces of the sheet, respectively. According to the meanvalue theorem for integrals, there exists c in the closed
interval [t0, t1], such that
ss ðcÞðt1 t0 Þ =
ðt1
ss ðtÞdt
ð6Þ
t0
In the Kriging model, both the average value (i.e.
ss(c)) and punching speed (mm/s) are taken as the
input variables to predict the crack ratio. The contact
pressure is adjusted by varying the blank-holder force.
The range of the average value (i.e. ss(c)) is from 0.1 to
0.2, while the range of the punching speed is from 3 to
15 mm/s.
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Proc IMechE Part B: J Engineering Manufacture 0(0)
Table 4. Levels of the variables and crack ratio.
Number
Mean integral
value
Punching
speed (mm/s)
Crack ratio
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.1428
0.1900
0.1300
0.1771
0.1171
0.1642
0.1042
0.1514
0.1985
0.1386
0.1857
0.1257
0.1728
0.1128
0.1600
0.1000
0.1471
0.1943
0.1343
0.1814
0.1214
0.1686
0.1086
0.1557
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
0.550
0.250
0.413
0.43
0.395
0.26
0.387
0.280
0.406
0.471
0.393
0.462
0.422
0.470
0.455
0.548
0.407
0.502
0.479
0.488
0.541
0.580
0.395
0.553
Figure 10. Contour of the Kriging model.
Table 5. Sample points for cross-validation.
Number
Mean integral
value
Punching
speed (mm/s)
Crack ratio
1
2
3
4
5
6
0.1428
0.1857
0.1285
0.1714
0.1143
0.1571
13.2857
11.5714
9.8571
8.1428
6.4286
4.7142
0.50
0.43
0.44
0.36
0.42
0.33
In this study, Uniform Design (UN) is used as the
sampling method to obtain the training group to construct the Kriging model. Table 4 shows the levels of
the factors and the crack ratio. Other experiments with
six sample points in the design space were performed as
a holdout group for the fivefold cross-validation, as
shown in Table 5. In this study, both leave-one-out
cross-validation and fivefold cross-validation were
applied to select the best Kriging model. Figure 10
shows the contour of the established Kriging model. In
the design space, the sample points marked by ‘’
belonged to the training group. The sample points
marked by ‘ + ’ were the holdout group for the fivefold
cross-validation. In the Kriging mode, the optimized u
is 0.9742 and 0.4515 for the punching speed and average
value of the integral of the contact pressure, respectively. Figure 11 is the MSE of the Kriging model. The
value of MSE is small in the design space. The Kriging
model can be used to predict the crack ratio on the fineblanking surface. When the average value of the integral
Figure 11. Distribution of the MSE of the Kriging model.
is 0.15 and the punching speed is 8.72 mm/s, the crack
ratio is 42.7%. This predicted value was validated by
carrying out the experiment under the same condition.
The crack ratio obtained in the experiment was 42.5%,
as shown in Figure 12. In the previous studies,38–40 it
was found that the elongation at failure was less at a
high strain rate than that at a low strain rate. In the
fine-blanking process, the strain rate increased with the
increase of the punching speed. Figure 10 shows that in
most cases, the crack ratio is usually greater at higher
punching speeds than at lower punching speeds. A small
crack ratio can be obtained using an appropriate value
of the punching speed and the average value of the integral. The minimum crack ratio appeared in the condition with the largest average value of the integral and
the smallest punching speed. The result indicated that
the average value of the integral of the contact pressure
can be considered to be a factor when predicting the
crack ratio for the fine-blanking process.
Conclusion
In this article, a PVDF film stress sensor was developed
to measure the contact pressure on the contact surface
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Ding et al.
9
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit
sectors.
References
Figure 12. Fine-blanking surface with crack and the
corresponding contact pressure curve.
of the sheet with a blank-holder near the blanking
clearance. The characteristics of contact pressure when
forming the fine-blanking surface with and without
cracks were determined. Based on the results, the following conclusions can be drawn:
The maximum value of the contact pressure
occurred at the beginning of the punching sheet,
which was about five times higher than that caused
by pressing the V-ring indenter into the sheet. It
indicated that the punch squeezing the metal in the
shear zone is the direct and main reason for increasing the contact pressure in the fine-blanking
process. In addition, the application of a high blankholder force prevented the increase in the sheet thickness during the punch penetration. The following two
factors increased the hydrostatic compressive stress in
the metal near the blanking clearance:
contact pressure decreased smoothly and
8 The
monotonically during the sheet punching process, which forms the clean fine-blanking surface. In the punching process, which forms the
fine-blanking surface with cracks, two troughs
were found in the fluctuation curve of contact
pressure. The second trough appeared at the
same time that the cracks occurred in the fineblanking surface.
non-linear relationship between the process
8 The
parameters (i.e. contact pressure and the punching speed) and crack ratio was approximated
using the Kriging method. It has been found that
a high punching velocity usually leads to a large
crack ratio. A fine-blanking surface with a small
crack ratio can be obtained using the optimal
value of the contact pressure and punching speed.
Declaration of conflicting interests
The authors claim that none of the material in this article has been published or is under consideration for
publication elsewhere.
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