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Effect of Setting Velocity on Self-Piercing Riveting Process and Joint Behaviour for Automotive Applications

Authors:
  • Jaguar Land Rover
Article

Effect of Setting Velocity on Self-Piercing Riveting Process and Joint Behaviour for Automotive Applications

Abstract and Figures

The increased application of lightweight materials, such as aluminium has initiated many investigations into new joining techniques for aluminium alloys. As a result, Self-piercing riveting (SPR) was introduced into the automotive industry as the major production process to join aluminium sheet body structures. Although both hydraulic and servo types of SPR equipment are used by the industry, the servo type is most commonly used in a volume production environment. This type uses stored rotational inertia to set the rivet. The initial rotational velocity of the mass dictates the setting force and hence the tool is described as velocity-controlled. A study was therefore conducted to examine the effect of setting velocity on the process including tooling and joint performance. It was found that the setting velocity would have a significant effect on tooling life. Over 80kN force could be introduced into the tooling depending on selection of the setting velocity. The results also suggested that the joint quality and strength are affected by the setting velocity. An increase in the setting velocity would lead to a decrease in the head height and an increase in the interlock. Too low velocity may cause the rivet head to protrude; too high velocity may result in breakthrough failure. Both cases would lead to corrosion concerns. The examination also indicated that the higher the setting velocity the higher the shear strength of the joints.
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10M
-
0249
Effect of setting velocity on self
-
piercing riveting process and joint
L. Han
*
,
D. Li,
M. Thornton
Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7AL, UK
M. Shergold
Jaguar Engineering C
entre, Jaguar Land Rover, Coventry, CV3 4LF, UK
* Corresponding author,
Tel: +44(0)2476575385
, Fax: +44(0)2476575
366
, E
-
mail:
li.han@warwick.ac.uk
Copyright © 2010 SAE International
ABSTRACT
T
he increased applica
tion of lightweight materials, such as aluminium has initiated many investigations into
new joining techniques for aluminium alloys. As a result, Self
-
piercing riveting (SPR) was introduced into the
automotive industry a
s the major production process to jo
in aluminium sheet body structures. Although both
hydraulic and servo types of SPR equipment are used by the industry, the
servo type is most commonly used in
a volume production environment. This type uses stored rotational inertia to set the rivet. The i
nitial rotational
velocity of the mass dictates the setting force and hence the tool is described as velocity
-
controlled. A study was
therefore conducted to examine the effect of setting velocity on the process including tooling and joint
performance. It
was found that the setting velocity would have a significant effect on tooling life. Over 80kN
force could be introduced into the tooling depending on selection of the setting velocity. The results also
suggested that the joint quality and strength are aff
ected by the setting velocity. An increase in the setting
velocity would lead to a decrease in the head height and an increase in the interlock. Too low velocity may
cause the rivet head to protrude; too high velocity may result in breakthrough failure. Bo
th cases would lead to
corrosion concerns. The examination also indicated that the higher the setting velocity the higher the shear
strength of the joints. In addition, photoelastic measurement using Deltavison software was also applied to this
investigati
on. The effect of setting velocity on the residual stress distribution of SPR joints is also discussed.
Key words:
Self
-
piercing riveting, setting velocity, joint behaviour
1.
INTRODUCTION
Today’s automotive industry is a challenging business. It is requ
ired not only to respond to environmental
concerns such as greenhouse gases and fuel economy, but also to meet customer expectations. Therefore, a need
for weight reduction has emerged and this in turn has led to the increased application of lightweight ma
terials,
such as aluminium and polymer composites. The use of aluminium alloys offers an opportunity for vehicle
weight reduction, which can lead to a reduction of fuel consumption and emissions without compromising
performance, comfort and safety [1, 2, 3
]. Aluminium alloys can offer high corrosion resistance, good
formability and good crashworthiness. In addition, the recyclability of aluminium alloys is also a considerable
attraction to manufacturers. However, the use of aluminium requires not only a dif
ferent approach in car design
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but also a different approach to manufacturing technology and in particular joining methods. As a result, self
-
piercing riveting (SPR) was introduced into the automotive industry
to join aluminium sheet body structures.
SPR a
s a
key production process has many advantages, such as no pre
-
drilled hole requirement, capability to
join a wide range of similar or dissimilar materials and combinations of materials, no fume emissions etc.
There
are currently two types of rivet equipme
nt available; the
electric servo
-
motor type and the hydraulic type. The
hydraulic type is force
-
controlled and uses oil pressure to firstly clamp the material and then to insert the rivet.
The pressures used by this process are high. When riveting thick st
ack
-
ups the force required to fully insert the
rivet can equal the amount of force generated by the hydraulic power pack. For automotive mass production,
one of the specifications is that the riveting tools have to be mounted on tool changers so that more
than one
tool can be used on a single robot. This basically rules out the large scale automated use of the hydraulic tool
due to concerns over running high pressure hydraulic hoses through a tool changer, which would be
continuously engaged and dis
-
engaged
. The electric servo
-
tool was therefore developed to solve these
problems for mass application. By switching to electrical power rather than hydraulic force, the tool can be
mounted on a tool changer allowing multiple tools per robot. This type of equipm
ent uses stored rotational
inertia to set the rivet. The initial rotational velocity of the mass dictates the setting force and hence the tool is
described as velocity
-
controlled. Although previous research [
4
,
5
] reported the force characteristics of the
SPR
process for hydraulic equipment, they either aimed at reducing the operational force [
4
], or focused on
developing a process monitoring system [
5
]. Setting velocity, as a key process parameter for servo equipment,
has not been fully investigated. Consi
dering the majority applications of SPR is velocity
-
controlled, it is
important to know the effects of this key parameter on the process and joint behavior. This paper therefore aims
to give a deep insight of the effect of setting velocity on quality of th
e joints, on tooling of the process and on
performance of such joints.
2.
EXPERIMENTAL PROCEDU
RE
2.1 Quality Criteria
Based on industry standard practice, a joint quality assessment criterion was created. All joints were assessed
for joint quality agains
t the primary measurements of; head height, interlock and remaining material thickness
of the bottom sheet, together with other secondary considerations as detailed schematically in Figure 1.
Samples were sectioned and then examined/measured using a micr
oscopy equipped with a4i image analysis
software. The head
-
height is the distance between the rivet head and the top riveted sheet, and the interlock is
the distance of the rivet shank flaring into the locked sheet/s. When the head is above the level of th
e top sheet,
a positive value for the head
-
height is given. A negative head
-
height implies that the head is below the level of
the top sheet. Both the head
-
height and the interlock distance are considered to be critical for a SPR joint.
Ideally the rivet h
ead should be flushed with the top sheet. In practice, if the rivet head sticks out too high above
the top sheet, moisture may ingress into the joint leading to corrosion concerns. Alternatively, it may be slightly
below the top sheet but may leave a visib
le dent on the surface leading to visual quality concerns. Therefore, top
and lower limits for head height were defined as +0.3mm to
-
0.5mm for quality assessment. The interlock is a
key feature of a SPR joint holding the riveted sheets together. Sufficien
t interlock is necessary and therefore a
minimum interlock value is required for quality assessment. Considering the different properties between
aluminium and High Strength Steel, different minimum interlock values have been specified; 0.2mm for HSS
and 0
.4mm for aluminium. In addition, fracture of the locked sheet (bottom sheet) must be avoided as it can
lead to severe corrosion [
6
]. A minimum remaining material thickness of 0.2mm is therefore introduced into the
quality assessment criteria.
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Figure
1
: quantitative measurements for a SPR joint
2.2 Materials and stacks
The materials used throughout
this project
were Aluminum AA5754 and HSLA (High Strength Low Alloy) 350
with various thicknesses. The AA5754 alloy sheets were pre
-
t
reated with a chromate free film
, and
coated with
a wax
-
based surface lubricant. The HSLA350 had Zinc plate and passive surface coating.
Table 1 and 2 present
the compositions and mechanical properties of AA5754 and HSLA350 respectively. These two material
s
formed various stacks that are presented in the format of (top/pierced sheet + middle sheet + bottom/locked
sheet). For example, (2.0mm AA5754 + 1.0mm HSLA350) nomenclature indicates that the top sheet is 2.0mm
AA5754 and the bottom sheet is 1.0mm HSLA35
0 with no middle sheet
.
Table
1
Compositions and Mechanical Properties of
aluminium
AA5754
MECHANICAL PROPERTIE
S
Young’s Modulus
(GPa)
Tensile strength (MPa)
Elongation
Hardness (H
V
)
70
250
25%
68
NOMINAL COMPOSITIONS
Si%
Fe%
Cu%
Mn%
Mg%
Al%
0
-
0.40
0
-
0.40
0
-
0.10
0
-
0.50
2.60
-
3.60
Balance
Table
2
Compositions and Mechanical Properties
of HSLA350 (EN 10268
-
H360LA
)
MECHANICAL PROPERTIE
S
Yield strength (MPa)
Max
.
Yield strength (MPa)
Elongation
390
480
26%
Chemical composition
C%Max
Si%Max
Mn%Max
P%M
ax
S%Max
Al%Min
Nb%Max
Ti%Max
0.10
0.50
1.20
0.025
0.025
0.015
0.090
0.15
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2.3 Selection of rivet and die
The selection of rivet and die depends on the sheet material to be joined and is a key factor for achieving good
quality joints. As the purpose of th
is excise is to understand the effect of setting velocity on various aspects of
the SPR joint, the selection process of rivet and die is not discussed in detail. Although the rivets and dies are
designated only by codes, the key parameters relating to the
discussion are given. All the rivets and dies were
supplied by the Henrob Ltd. and Henrob servo SPR gun was also used throughout the project. The velocity
presented through the paper is in nominal units as displayed on the Henrob controller. The velocity i
n mm/min
is proportional to the unit.
2.4
Die load profile
As the setting velocity is a component of setting force, it is understandable that the higher the setting velocity
the higher the setting force. But the setting force is different from the force actin
g on the die. In order to know
the force acting on the die, a KISTLER force transducer with a 120kN capacity was installed directly
underneath the die. A Pico data acquisition system was connected to the transducer to record the load profile
acting on the
die. It should be mentioned here that the die load profile is different from the force
-
displacement
curve, which shows the force against the distance travelled as discussed by previous research [Kim et al, King,].
2.5
Sample preparation and examination
All sa
mples were prepared by using special fixtures designed to maintain consistent dimensions for each
individual purpose. The sample dimensions for metallographic inspection and photoelastic measurement are
showed in Fig
.
2.
For metallographic inspection, a sp
ecially designed fixture was also used to ensure samples were sectioned at
the centre of the joints consistently. A Buehler Delta AbrasiMet cutting machine was used with very low
cutting speed in order to make the section surface smooth and even. The sect
ioned samples were then examined
microscopically and measured using a4i Analysis software.
The residual stress distribution around the rivet head and tail is not fully understood due to the complexity of
the process. It is assumed that the residual stress
generated during SPR process will affect the behaviour of such
joints, similar to the effect of residual stresses on the properties of a formed component. Therefore, this project
has for the first time obtained the residual stress distribution using photo
-
elastic techniques.
The photoelastic
technique requires the test sample to have
a special coating material, PS
-
1D, bonded to the surface. This is an
excellent high
-
sensitivity plastic, manufactured by the Photolastic Division of Vishay Measurements Group,
Inc, for accurate analysis in the elastic and elastic
-
plastic ranges of most metals [9]. The PS
-
1D sheet was cut to
40x40mm with a clearance hole punched in the potential rivet area. Then it was adhered to degreased aluminum
specimen at the overlap area a
nd left in a dry clean environment for 24 hours.
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Figure
2
: Sample dimensions for sectioning and Photoelastic Analysis
The dimensions for lap shear samples are shown in Fig. 3. At least five samples w
ere made and tested for each
setting velocity selected. A standard Instron tensile test machine with 30kN load capacity and a cross head
speed 10mm/min was used for all lap shear test.
Figure
3
: Sample dimensions for shear test
2.
RESULTS AND DISCUSSI
ON
3.1 Effect of setting velocity on joint quality
Fig
.
4
shows cross sections of the
joints in a (2.0mm AA5754 +1.0mm HSLA350)
stack,
made using C50d42
rivets and a
DZ0902050
die
at different velocities.
As the velocity was increa
sed incrementally
from 120 to 180
units, the characteristics of the sections
changed
in terms of
interlock, remaining material thickness, head
-
height
and gap existence. Comparing the two extremes of 120 and 180; at the minimum velocity 120, there is a visi
ble
gap at the faying surfaces between the two riveted sheets and an interlock of only 0.25mm. At the maximum
setting velocity of 180 the gap has been eliminated and the interlock increased to 0.33mm. The headheight is
40mm
40mm
15mm
20mm
R=12mm
100mm
PS
-
1D
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0.52mm at 120 velocity with a remaini
ng material thickness of 0.25mm. At the higher 180 velocity the head
height was reduced to 0.03mm with an associated reduction in remaining material thickness to 0.21mm.
For the sections (a) and (b), the head heights were both greater than 0.3mm and there
fore the joints failed the
quality criteria. The relationship between the velocity and head
-
height, interlock and remaining thickness is
shown in Fig. 5. In summary; t
he increase in the setting velocity led to a decrease in the head height and the
remainin
g thickness; but an increase in the interlock. The head
-
height had the highest variation rate.
(a) 120 (b) 140
(c) 160
(d) 180
Figure
4
:
Cross sections of the joints C50d42DZ0902050 (2+1) at different velocities
As the setting velocity reflects the force applied to the rivets, it follows that at high velocity the rivet
has more
dynamic energy available to pierce and deform the top sheet as well as to flare the rivet legs into the bottom
sheet. Again comparing the extreme velocities of 120 and 180 shown in Figures 4(a) and (d); at 120 velocity the
2.0 mm top sheet was red
uced to 1.58mm when measuring from underneath the rivet head to the bottom sheet,
whilst at 180 velocity this distance was 1.28mm. A further effect of high velocity is that the rivet would have
more energy to deform the bottom sheet by flaring into it. The
se factors contributed to the reduction in the head
-
height and remaining thickness, as well as the increase in the interlock that were obtained with the higher 180
velocity. It is worth mentioning that for this material combination, the bottom sheet is 1.0
mm HSLA350, it is
not easy to deform further following initial deformation which would inevitably lead to hardening of the
HSLA350. This may explain why the reduction in remaining material thickness is not as significant as in the
head
-
height, and the incr
ease in the interlock is minimal as the setting velocity increased.
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Figure
5
:
Effect of setting velocity on the joint head height
When the rivet and die combination was changed, the effect of setting velocity was exaggerated. Fi
g. 6 shows
two cross sections of the joints that have same
(2.0mm AA5754 +1.0mm HSLA350)
stack and C50d42 rivet as
shown in Fig. 5, but used a different die profile from the sections shown in Fig. 4. At 120 velocity, the joint
achieved reasonable head
-
heig
ht and interlock with no breakthrough. However, a relatively small change in
velocity from 120 to 140 produced a joint with too low head
-
height and with part of the bottom material
missing leading to a breakthrough failure. This suggested that even with t
he same rivet and die combination,
proper selection of setting velocity is required in order to achieve acceptable joints.
(a) 120 (b) 140
Figure
6
:
Cross sections
of the joints C50d42DZ11000 (2+1) at different velocities
3.2
Effect of setting velocity on die force profile
It is important to know the force acting on the die as die failure is a major concern in production. Figure 7
shows the die load profiles generated f
rom riveting a (2.0mmNG5754+2.0mmNG5754) stack with a
combination of rivet and die C50D42/DZ0902050 at various setting velocities. The peak load varied from about
40kN to over 80kN as the velocity changed from 80 to 180, confirming that increasing the sett
ing velocity leads
to an increase in the peak load value. In addition, the pattern of the load profile also changed when the setting
velocity changed. As the setting velocity reflects the setting force, it is understandable that the higher the setting
velo
city the higher the impact force on the die. The decaying wave pattern may be caused by matching the
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resonance of the particular C
-
frame. In summary for a given stack, the higher the setting force the higher the
force action on the die.
Figure
7
: Effect of setting velocity on die load profile
(2.0+2.0mm) AA5754
Figure
8
: Die load profile for different stacks withC50D42DZ09050 at velocity 140
Figure 8 shows the load profiles obtained from two stacks w
ith the only difference being that one has a bottom
layer of HSLA350 instead of AA5754 for the other. Although identical combination of the rivet and die as well
as setting velocity was applied to make the joints, the resulting peak force and force pattern
acting on the die are
different. As the setting velocity is the same for both stacks, the force acting on the rivets should be the same.
However, as the HSLA350 high strength steel requires more energy to deform than the lower strength AA5754
during the r
iveting process, the peak load on the die was reduced and a different load pattern was observed. It
follows that the same velocity applied to different material combinations leads to different impact force on the
die.
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3.3
Effect of setting velocity on joint l
ap shear behaviour
Figure 9 shows lap shear test results for the joints corresponding to the micro sections shown in Fig. 4. At
velocity 120, the average peak load was 4.01kN; whilst at velocity 180, the shear strength was 5.21kN. As the
setting velocity i
ncreased, the shear strength of the joints also increased. For this group of joints, pull
-
out of the
rivet from the bottom sheet, as shown in Fig. 10(a), was the only failure mode.
The shear strength, for a SPR
joint, depends on a combination of the sheet
material tensile strength, the bearing resistance and the interlock
strength of the joint [li].
The
pull
-
out
failure mode indicated that the interlock and bearing resistance of the
bottom sheet joints governed the joints shear strength. The increase in the
setting velocity led to an increase in
the interlock, and consequently to an increase in the shear strength of the joints.
F
rom
velocity
120 to 140, the
amount of free
-
play in the joint was the greatest and thereafter reduced.
The free
-
play is attributed
to the
existence of the gap between the two riveted sheets, which causes loosening of the joints and over protruded
rivet head at velocity 120. Both the gap and over protruded head facilitated pull
-
out of the rivet during lap shear
test leading to much low
er strength of the joints.
Figure
9
:
E
ffect
of setting v
elocity on
shear
strength of (2+1) joint
s
(a)
Pull
-
out failure
(b) Tearing of the bottom sheet
Figure
10
: Failure modes occurr
ed during lap shear test
Figure 11 shows peel and lap shear test results for the joints shown in Fig. 6. The joints made at velocity 120
had over 40% lower peel strength and slightly lower shear strength than the joints made at velocity 140.
T
he
load versu
s velocity trend shows the higher velocity the higher strength, but this increase in strength is actually
attributed to different failure mechanisms. For lap shear test,
the joints made at 120 velocity failed by pulling
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the rivet out of the bottom sheet,
a
s shown in Fig. 10a, indicating that the interlock and the bearing resistance of
the bottom sheet contributed to the shear strength of the joints; whilst the joint made at 140 velocity failed by
tearing of the bottom sheet (Fig. 10b) suggesting that the te
aring strength of the bottom sheet was sole
contributor of the shear strength of the joints. Similar to shear test geometry, the peel strength for a SPR joints
relies on the interlock. The failure modes that occurred during peel testing are presented in Fi
g.12. Although all
joints failed by pull
-
out of the rivet from the bottom sheets, the joints made at 140 velocity left an empty hole in
the locked sheet (Fig. 12a) indicating the full contact between the rivet and the locked sheet or a possible buckle
at t
he locked sheet, whilst the button in the locked sheet for the joints made at velocity 120 remained intact
(Fig.12b). This explained the higher peel strength of the joints made at velocity 140.
The small change in
velocity which altered the quality of the
joints led to a significant change in failure mechanism of the joints.
0
1000
2000
3000
4000
5000
6000
Mean Load (N)
120 velocity
140 velocity
Peel
Shear
Figure
11
: Peel and shear test results corresponding to the joints shown in Fig. 6
(a)
Pull
-
out failure
(b) Button intact
Figu
re
12
: Failure modes occurred during peel test
3.4
Effect of setting velocity on residual stress distribution
Fig
ure 13
shows residual stress distribution images obtained from the joints shown in Fig. 4 using photo
-
elastic
measurement
with Deltavision software. It can be seen that the fringe pattern differed as setting velocity varies
indicating the variation of residual stress distribution on the riveted sheets. Although multi
-
fringes occurred due
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1
1
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13
to the effect of coating, the results
still showed that the increase in setting velocity led to an increase in
deformation of the transparency coatings, which reflected the deformation of the riveted sheet.
Figure
13
:
Residual stress distributio
n
images of the joints made at different velocities
Further analysis, using a line profile function in the Deltavision software, showed the maximum residual
stresses in a line extending from the edge of the rivet into the sheet at different setting velocit
ies, as shown in
Figure 14
. Although, the curves fluctuate, due to multi
-
fringe effects, the results are still predictable. For the
joints made at velocity 120, the maximum residual stress was about 12 camera units, which are proportional to
the stress val
ue, and occurred at about 3.2mm from the edge of the rivet head. In comparison, the maximum
residual stress for the joint made at velocity 180 was over 15 units and occurred at about 5.0mm from the edge
of the rivet head. In summary; the residual stress v
alue and the area of distribution increased as the velocity
increased.
120
1
4
0
1
6
0
1
8
0
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(a)
Velocity
120
(b)
Velocity
180
Figure
14
:
Stress distributions of the joints made at different velocities
4
C
ONCLUSIONS
As a key process parameter, setting vel
ocity reflects the riveting force and therefore controls the level of
deformation that occurs for both rivet and sheets during the riveting process. The experimental work reported
above showed noticeable effects of setting velocity on various aspects. Thes
e effects can be summarised as
below:
Setting velocity affects SPR joint quality. As the setting velocity increases, the head height and remaining
material thickness decreases; whilst the interlock increases. For s given stack, there is a range of minimum
and maximum velocity. Any velocity outside this range will lead to the head height, remaining material
thickness and interlock outside the quality criteria, causing failure of the joints.
Setting velocity influences SPR joint strength but this can vary de
pending on joint quality and consequently
failure mode. For rivet pull
-
out failure, the joint strength increases as the setting velocity increases. For
mixed failure modes of rivet pull
-
out and sheet failure, the joint strength may be determined by the she
et
material strength and therefore does not necessarily follow a consistent trend with the setting velocity.
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13
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Setting velocity affects die force profile. For a given stack the higher the setting velocity, the higher the
force acting on the die. However, t
his effect can vary depending on the material properties and combinations
to be joined.
Setting velocity affects residual stress distribution. As the setting velocity increases, the peak value of
residual stress and the area of residual stress distribute
d increases.
Acknowledgments
We would like to thank Advantage West Midlands, Jaguar and Land Rover, Novelis for their support of the
advanced body joining research program.
References
1.
Polmear
.
I
.
J, “Light alloys”, Third edition, Edward Arnold, 1995.
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Lei
termann
,
W and Christlein
,
J
, “The 2
nd
generation Audi space frame of the A2: A trendsetting all
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aluminium car body concept in a compact class car”, Seoul 2000 FISITA World automotive congress,
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15, 2000, Seoul, Korea.
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Komatsu
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Y
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K
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and Shlokawa
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, “Application of all
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Kim
,
Dae
-
Wook,
Xu
,
J,
Li
,
W, and
Blake
,
D,
Force characteristics of self
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piercing riveting
”,
JEM525
,
IMechE 2006
,
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5.
King
.
R. P
, “
Analysis and quality monitoring self
-
pierce riveting process
”, PhD dissertati
on, University
of Hertfordshire, 1997
.
6.
Han
,
L
,
Young
,
K
,
Hewitt
,
R and
Chrysanthou
,
A
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-
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HSLA joints”, Transactions of Society of Automotive Engineers of
Japan, Vol.36 No.5 pp181
-
186, 2005.
... The H 1 directly influences the cosmetic appearance and corrosion resistance of the connected structure. According to the study of [20], the H 1 also determines the final position of the rivet inserted into the substrates and therefore affects the final magnitudes of the interlock and T min . The I is very critical for the mechanical strengths [8] and failure behaviours of SPR joints. ...
... The interlock should be greater than 0.4mm for joints with aluminium alloy bottom sheet and greater than 0.2mm with a steel bottom sheet. The T min should be always greater than 0.2mm and fracture of the bottom sheet should be avoided [20]. Therefore, the accuracy of the captured joint cross-sectional profile is very important for the joint quality evaluation. ...
... Therefore, it is relatively easy to correct the interlock error induced by the improper cutting position. According to the geometrical relationships shown in Fig. 19, the I true can be expressed as a function of six dimensions measured on the sectioned joint specimen, as shown in Eq. (20). The similar strategy was also utilized by Gerstmann and Awiszus [25] to compensate the interlock error caused by the improper cutting position of flat-clinch joints, but only the offset distance was considered in their study. ...
Article
Full-text available
This study systematically investigated the influences of cutting positions on the measurement accuracy of the self-piercing riveted joint quality indicators. Evaluation and correction methods were proposed for the first time to estimate and compensate the interlock error caused by improper cutting positions. It was found that the measurement accuracy of the rivet head height was not influenced, but the accuracy of the interlock and the remaining bottom sheet thickness were affected by the joint cutting position. A pure offset distance could lead to an overestimated interlock while a solo rotation angle could result in an underestimated interlock. For the studied joint configurations, the relative interlock error was found in the range of −5%–5% with the offset distance smaller than 1.0 mm and the rotation angle less than 10°. The offset distance and rotation angle can cause larger errors on the remaining bottom sheet thickness around the joint central area than around the rivet tip. Moreover, the proposed correction strategy for the interlock has been proved effective and the relative interlock error could be reduced to 1%–3%.
... The rivet head height directly affects the cosmetic appearance of the connected structure and the joint corrosion resistance. Meanwhile, it also influences the final rivet position in the substrates and therefore affects the magnitudes of the interlock and T min [32]. The interlock is very important for the joint mechanical strengths and failure behaviors. ...
... The rivet diameter, rivet hardness and die pip height were also fixed at 5.3 mm, H0 and 0.0 mm respectively. The rivet head height directly links with the final position of the rivet inserted into the sheets and thus affects the final values of the interlock and T min [32]. For consistency, a uniform rivet head height (i.e. ...
Article
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In this study, artificial neural network (ANN) was adopted to predict the quality of SPR joints. Three ANN models were developed respectively for the key joint quality indicators: the interlock, the remaining bottom sheet thickness at the joint center (Tcen) and under the rivet tip (Ttip). Experimental SPR tests were performed and the results verified the high prediction accuracy of the ANN models. The mean absolute errors (MAE) between the experimental and prediction results for the interlock, Tcen and Ttip reached 0.058mm, 0.075mm and 0.059mm respectively, and the corresponding mean absolute percentage errors (MAPE) were 14.2 %, 22.4 % and 10.9 %. Moreover, two innovative approaches were proposed to simplify the selection of rivet and die for new joint designs. One was realized by combining the genetic algorithm (GA) with the ANN models, and can generate optimal rivet and die combinations for different joint quality standards. The second was achieved by plotting application range maps of different rivet and die combinations with the help of ANN models, and can quickly select the suitable and accessible rivet and die. Furthermore, interaction effects between different joining parameters on the joint quality were also discussed by analyzing the contour graphs plotted with the ANN models.
... The SPR joint quality is usually assessed by three quality indicators measured on the joint cross-sectional profile [9,10] as shown in Fig. 2: the rivet head height (H), the interlock (I) and the minimum remaining bottom sheet thickness (T min ). The H directly affects the cosmetic appearance of the connected structure and the joint corrosion resistance [11]. The I is very important for the joint mechanical strengths and failure behaviors. ...
Article
By combining experimental SPR tests with a finite element (FE) model, the influences of top sheet thickness (Tt), bottom sheet thickness (Tb) and rivet length (L) on the joint formation mechanisms were systematically investigated in this study. Single factor experiments were firstly conducted to uncover the three joining parameters' effects on the joining results. Interrupted SPR tests were also performed to confirm the prediction accuracy of the FE model on the joint formation process. Then, the joint formation mechanisms were analyzed by numerically inspecting the events during the riveting process, including the sheet deformation behaviors, rivet shank flare behavior and formation of quality indicators. The results revealed that the joining parameters affected the interlock formation by directly altering the positions of two interlock boundaries. The rivet shank flare behavior was apparently affected by the Tt and Tb, but less influenced by the L. The formation of remaining bottom sheet thickness at the joint center (Tc) was significantly affected by the Tt and Tb. Rapid reduction of the Tc occurred before the top sheet was penetrated and after the rivet cavity was fully filled. Meanwhile, it is highlighted that the FE model is an excellent tool to analyze SPR joint formation.
... During the design phase of any new joint, the rivet, material and die combination is chosen such that the produced joint would satisfy the quality specifications when the head height is in the range 20.5 and 0.3 mm, as defined by industry standard practice. 7 Five samples are made at each of the three head height levels: 20.5, 0 and 0.3 mm, to verify that the interlock and t-min remain within tolerance for each associated head height level, before confirming a particular joint configuration as valid. Therefore, in subsequent reproductions of the joint, if the head height is within tolerance then it implies that the overall joint quality is acceptable. ...
Article
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Self-pierce riveting (SPR) is a complex joining process where multiple layers of material are joined by creating a mechanical interlock via the simultaneous deformation of the inserted rivet and surrounding material. Due to the large number of variables which influence the resulting joint, finding the optimum process parameters has traditionally posed a challenge in the design of the process. Furthermore, there is a gap in knowledge regarding how changes made to the system may affect the produced joint. In this paper, a new system-level model of an inertia-based SPR system is proposed, consisting of a physics-based model of the riveting machine and an empirically-derived model of the joint. Model predictions are validated against extensive experimental data for multiple sets of input conditions, defined by the setting velocity, motor current limit and support frame type. The dynamics of the system and resulting head height of the joint are predicted to a high level of accuracy. Via a model-based case study, changes to the system are identified, which enable either the cycle time or energy consumption to be substantially reduced without compromising the overall quality of the produced joint. The predictive capabilities of the model may be leveraged to reduce the costs involved in the design and validation of SPR systems and processes.
... A protruded rivet head usually causes gaps between the rivet and the connected sheets and thus increases the moisture or water invasion problem. The rivet head height also directly links with the final position of the rivet inserted into the sheets and thus affects the final values of the interlock and T min [11]. The assessment criteria for these three indicators are generally determined by the application requirements of each company and may vary in different industry sectors. ...
Article
Full-text available
Self-pierce riveting (SPR) is a major joining method used in the automotive industry. However, there still lacks a fast and easy-to-use joint quality prediction tool available for the automotive engineers. In this study, the simple but effective regression analysis method was applied to quickly predict the SPR joint quality. Two regression models were developed for the prediction of the interlock and the minimum remaining bottom sheet thickness (Tmin). The prediction accuracy of the developed regression models was validated by comparing with the experimental results. Under the studied joint configurations, the mean absolute errors (MAE) of the interlock and Tmin were 0.047 mm and 0.053 mm, respectively, and the corresponding mean absolute percentage errors (MAPE) were 10.4% and 12.3%. With the developed models, the interaction effects between rivet and die parameters on the joint interlock and Tmin were also systematically analysed. The results revealed that the rivet and die parameters demonstrated significant influences on the interlock but not on the Tmin. These interaction effects were further examined by analysing the deformations of the rivet and substrate materials. Moreover, the die-to-rivet volume ratio (R) was found to be critical for the formation of interlock, and a larger interlock is more likely achieved when the R is close to 1.0.
... There is no single minimum value for the interlock distance, as it is highly dependent on the joint stacking sequence, sheet material properties and thickness. For example, the minimum required interlock distance for aluminium joints is 0.4 mm while for steel is 0.2 mm based on industry standard practice [51]. On the die side, there should be a symmetrical button of the correct diameter and shape (determined by the die). ...
Article
Self-piercing riveting (SPR) is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole. The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications. However, SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process. In this paper, SPR joints of advanced materials and their corresponding failure mechanisms are discussed, aiming to provide the foundation for future improvement of SPR joint quality. This paper is divided into three major sections: 1) joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions, 2) joint corrosion issues, and 3) joint optimisation via process parameters and advanced techniques.
Article
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Self-piercing riveting is a promising method to join thin-wall structures in the automobile industry, especially for the connection of different materials. Due to their excellent strength-to-weight ratios and vibration/noise reduction characteristics, foam metal sandwich composite aluminum plates are the best choices for modern automobiles. In this paper, the self-piercing riveting forming qualities and joint strengths of foam iron-nickel/copper sandwich composite aluminum plates with AA5052 aluminum alloys are investigated, and the fracture morphologies of tensile failure samples are characterized. The results showed that: the foam metal sandwich composite aluminum plates can increase the interlock width and improve the self-locking performance of the joints, and the bottom thicknesses are significantly increased when the foam metal sandwich composite aluminum plates are riveted as the bottom plates. In the tension-shear tests, the foam sandwiches reduce the maximum failure loads and increase the maximum failure displacements of the joints. Moreover, the macro/micro-structure characteristics of the foam metal sandwich composite aluminum plates affect the failure modes of the joints. When the foam metal sandwich composite aluminum plates are used as the top plates, the failure mode is that the composite aluminum plates break down in the direction perpendicular to the loading of the plates. When they are riveted as the bottom plates, the failure mode is that the rivets are entirely pulled out from the bottom base plates of the composite plates, partially separated from the foam metal sandwich composite aluminum plates, and the top base plates and the sandwich metals are torn apart at the same time.
Article
The mechanical response of aluminium–steelself-piercing riveted (SPR) joints is investigated under quasi-static and dynamic loadings using experimental and numerical methods separately. First, a 2D axisymmetric numerical model is constructed based on r-adaptivity method to simulate the SPR process and is subsequently validated by tests. Second, a novel generation method of 3D finite element (FE) model of SPR joints, of which the stress–strain field is mapped from the 2D axisymmetric model, is proposed to simulate the mechanical response of aluminium–steel SPR joints. The mechanical response of SPR joints under quasistatic and dynamic loadings are compared in detail by experimental tests and numerical simulation. The results reveal that the established 3D FE model can accurately simulate the quasi-static and dynamic strength of SPR joints. It is observed that the peak force of the SPR joints is larger under dynamic loading than under quasi-static loading; while the energy absorption by peeling and cross-tension SPR joints is lower under dynamic loading than quasistatic loading. Finally, the relationships between process parameters, process quality indexes and mechanical response of SPR joints are parametrically investigated under quasi-static and dynamic loadings, in which (1) increasing the rivet length reduces the minimum thickness, increases undercut, and increases energy absorption; (2) increasing the rivet length increases the peak force within a limited range for shear joints and cross-tension joints under dynamic loading; (3) the total effect of process parameters on the mechanical response of SPR joints under dynamic loading are similar to the effect observed under quasi-static loading.
Article
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Cost-efficient multi-material design requires suitable joining techniques, ideally with low investment cost by re-using existing assembling lines. The recently developed resistance rivet spot welding (RRSW) technique combines mechanical joining with spot welding and enables cost-efficient joining of aluminum (Al) to steel for multi-material body-in-white structures. Here, the static and fatigue strengths of different hybrid Al-steel specimens made by RRSW were measured and compared to other state-of-the-art joining techniques, such as self-piercing riveting (SPR) and RSW. The static strength of RRSW matched or exceeded that of SPR regardless of the sheet thickness, whereas the fatigue strength of the RRSW joints showed a strong dependency on the thickness of the steel sheets. For thinner steel sheets, the fatigue of the RRSW-joined metal sheets was lower in comparison with SPR. Fatigue cracks were initiated in thin steel sheets around the weld nugget. By contrast, for thicker steel sheets, the fatigue strength of RRSW matched or exceeded that of SPR. With a thicker material combination of 1.5 mm steel and 1.0 mm Al, fatigue cracks occurred only in the Al sheet in both SPR and RRSW. For suitable steel sheet thickness, RRSW is thus a durable technique to join steel and Al.
Article
Self-piercing riveting (SPR) process has been widely applied in different industrial fields to join similar or dissimilar materials. This paper aims to investigate the SPR joinability of aluminium–steelhybrid sheets and obtain the design range of material strength and thickness distribution. A 2D axisymmetric numerical model is constructed based on r-adaptivity method to simulate the SPR process. The accuracy of the numerical model is validated by comparing the cross-sectional shape of SPR joint between test and simulation Parametric study is performed to explore the effects of material strength and sheet thickness on the SPR joinability and process quality indexes. The results show that (i) SPR joinability of aluminium–steel hybrid sheets can be improved by setting the softer and thicker sheet as the lower sheet when the flow stress of upper sheet is equal from 183MPa to 517MPa; (ii) Undercut decreases with an increase of sheet flow stress ratio, while the opposite trend is observed for minimum thickness. Undercut and minimum thickness decrease with an increase of sheet thickness ratio. (iii) a qualified SPR joint with good connection strength can be obtained when sheet flow stress ratio is equal from 0.27 to 1.84 and sheet thickness ratio is equal from 0.77 to 1.78. This study provides an available reference for determining the material strength and thickness distribution in the application of SPR of aluminium–steel hybrid sheets.
Article
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Self-piercing riveting (SPR) is one of the main joining methods for lightweight aluminium automotive body structures due to its advantages. In order to further optimise the structure design and reduce the weight but without compromising strength, reduction of redundant materials in the joint flange area can be considered. For this reason, the influence of rivet to sheet edge distance on the fatigue strengths of self-piercing riveted joints was studied. Five edge distances, 5 mm, 6 mm, 8 mm, 11.5 mm and 14.5 mm, were considered. The results showed that the SPR joints studied in this research had high fatigue resistance and all specimens failed in sheet material along joint buttons or next to rivet heads. For lap shear fatigue tests, specimens failed in the bottom sheet at low load amplitudes and in the top sheet at high load amplitudes except for specimens with very short edge distance of 5 and 6 mm; whereas, for coach-peel fatigue tests, all specimens failed in the top sheet. For both lap shear and coach-peel fatigue tests, specimens with an edge distance of 11.5 mm had the best fatigue resistance. It was found that for coach-peel fatigue, length of crack developing path before specimens lost their strengths was the main factor that determined the fatigue life of different specimens; for lap shear fatigue, the level of stress concentration and subsequent crack initiation time was the main factor that determined the fatigue life.
Article
Self-piercing riveting (SPR) is a high-speed mechanical fastening technique for point joining of sheet-material components. SPR is becoming important in automotive applications for aluminium vehicle body assembly. However, compared with current sheet-metal joining processes in the automotive industry, SPR requires a substantially higher operating force. Reduction of the operating force is one of the key objectives for enhancing the mobility and applicability of the SPR system. In this research, the force characteristics of the SPR process were studied with the aim of reducing the operating force. The effects of various workpiece temperatures and thicknesses and the mechanical properties of the SPR joints were examined. It was found that the operating force of SPR was determined by the rivet deformation force. Under certain process conditions, an increase in temperature in workpiece material could help decrease the operating force as it permits the use of softer rivets.
Article
The fretting behaviour of self-piercing riveted aluminium alloy joints with three different interfacial conditions has been investigated in this study. The fatigue life of the joints was observed to be dependent on the fretting behaviour under different interfacial conditions. The presence of a wax-based solid surface lubricant on the surface of the aluminium alloy sheet could delay the onset of fretting damage leading to longer fatigue life. Inserting PTFE tape at the interface between the two riveted sheets led to the reduction and even elimination of fretting damage in a self-piercing riveted joint. However, the presence of PTFE tape at the interface resulted in a significant reduction in the fatigue life and led to a change in the failure mode. The effect of the frictional force during the fretting fatigue process of a self-piercing riveted joint is also discussed.
Effect of breakthrough on the behaviour of self-piercing riveted aluminium 5754 -HSLA joints. Transactions of Society of Automotive Engineers of Japan
  • L Han
Han, L., et al., Effect of breakthrough on the behaviour of self-piercing riveted aluminium 5754 -HSLA joints. Transactions of Society of Automotive Engineers of Japan, 2005. 36(5): p. 181-186.
Application of all aluminium automotive body for Honda NSX Detroit, USA: 910548. 2. Leitermann, W. and J. Christlein. The 2 nd generation Audi space frame of the A2: A trendsetting all-aluminium car body concept in a compact class car
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Komatsu, Y., et al. Application of all aluminium automotive body for Honda NSX. in SAE World Congress. 1991. Detroit, USA: 910548. 2. Leitermann, W. and J. Christlein. The 2 nd generation Audi space frame of the A2: A trendsetting all-aluminium car body concept in a compact class car. in FISITA World automotive congress. 2000. Seoul, South Korea.
Effect of breakthrough on the behaviour of selfpiercing riveted aluminium 5754 -HSLA joints
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Han, L, Young, K, Hewitt, R and Chrysanthou, A, "Effect of breakthrough on the behaviour of selfpiercing riveted aluminium 5754 -HSLA joints", Transactions of Society of Automotive Engineers of Japan, Vol.36 No.5 pp181-186, 2005.
The 2 nd generation Audi space frame of the A2: A trendsetting allaluminium car body concept in a compact class car
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Leitermann, W and Christlein, J, "The 2 nd generation Audi space frame of the A2: A trendsetting allaluminium car body concept in a compact class car", Seoul 2000 FISITA World automotive congress, June 12-15, 2000, Seoul, Korea.
Analysis and quality monitoring self-pierce riveting process
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