Advanced Shipping and Ocean Engineering
Mar. 2013, Vol. 2 Iss. 1, PP. 1-10
Thick Sheet Clinching
Joining up to 20 mm Total Thickness
Markus Israel*1, Reinhard Mauermann2, Julia Schellnock3
Fraunhofer Institute for Machine Tools and Forming Technology IWU, Dresden
Nöthnitzer Str. 44, 01187 Dresden, Germany
*1
[email protected]; [email protected]; [email protected]
Abstract- Clinching as an alternative joining technology to welding today is restricted to thin sheet metal. There is no experience in
clinching thick sheets. Thus the need arises to predict the process conditions by experimental and numerical analysis. This paper
demonstrates investigations in both fields to understand the chances and limits of thick sheet clinching in shipbuilding industry.
Keywords- Joining Technology; Thick Sheet Clinching; Processing Time; Fatigue; Distortion; Accessibility
I. INTRODUCTION
Welding as the main joining technology in shipbuilding industry offers unrivaled flexibility, but there are disadvantages
such as processing time, fatigue weakness or thermal distortion. Contrary clinching is inflexible, but fast, fatigue-potency,
accurate and cheap, in the case of using it in mass production [19, 27] (see also Fig. 1). If the restrictions – mainly accessibility
and ratio of thicknesses – fit, thick sheet clinching is in principle appropriate. Then it can be highly efficient.
Fig. 1 Joining costs of clinching and MIG/MAG welding in comparison for different production scenarios [19], based on the framework segment shown in
chapter IV
Clinching is found in thin sheet applications in the range of 0.5 mm to 4 mm single sheet thickness t 1,2 (Fig. 4). In mass
production (e.g. washer drum) the breakeven point is achieved at the fastest. System suppliers like Eckold, Tox, BTM deliver
the common C-frames and tools on the basis of experience – but only in the common range of thicknesses. Coming from these
applications, mainly automotive industry and white goods, the clinching technology is currently investigated to fit the
requirements of steelwork constructions and railway or commercial vehicles as well. Here significantly higher sheet
thicknesses are to be joined. Also in shipbuilding industry clinching is a perspective joining technology. Based on the current
research focus of total sheet thicknesses up to 20 mm the application for hull plating or huge internal structures is not aspired.
The favored fields for clinching in shipbuilding process will be internal structures and superstructures within the proposed
sheet thickness range. Here is the vision to substitute welding and bolting or riveting processes. Of course, the need of an
overlap limits the field of application.
The clinching process is dependent on knowledge – in engineering and in production process. There are descriptions of
applications [6, 15, 28] and standard procedures [3, 14]. Special clinching variants [1, 17, 18, 12] are described. Besides this majoritarian
academic work the system suppliers have a bright process knowledge using standard thicknesses for clinching technology.
Based on this knowledge the choice of suitable clinching tools is done for thin sheet applications. Thick sheet metal clinching
is discussed recently [7, 13, 19]. However, the system suppliers do not have the knowledge and standardized tools for this special
joining cases. Knowledge-based configuration of clinching tools and process for thick sheet metal is not state of the art.
Because of the large number of variables in the clinching process (e.g. geometry of the tools) FEA is an instrument used
often to fit the tools for the joining task [2, 4, 5, 8, 9, 10, 24, 26, 27, 29]. There are also some investigations focusing the prediction of
suitable process details by mathematical methods [11, 16, 22, 23] or process design methods [20, 21]. The plurality of numerical
process description for clinching shows the tendency of using FEA simulation for process optimization and for getting much
more information about, how the process works in detail. These numerical studies prove that using FE methods for process
development is state of the art. Principal FE models used for clinching processes with thin sheet can be adapted for the thick
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sheets. Possible scale effects are not yet analyzed for clinching and they are not part of this paper. There is just a few
knowledge about useful damage criteria and critical damage values to predict material cracks, which can possibly occur during
the clinching process. None of the named literature uses a damage criterion with this goal. The FEA modeling of clinching in
this paper is also done with the focus on getting well balanced tool sets of punch and die for ensuring a high quality thick sheet
connection with clinching without any damage criterion.
II. CLINCHING PROCESS
Clinching is bulk metal forming, a kind of extrusion. The material beneath the punch is forced into a radial flow between
punch and die bottom forming the undercut in a closed die (Fig. 2). The shape of the die is a very important issue for ensuring
a high undercut value. Besides the principle of a closed and rigid die many other tool systems are available (e.g. divided dies
with an anvil and two or more moving blades, Fig. 3). Most relevant values of the joint are neck thickness t n , undercut f and
bottom thickness t b . Neck thickness and undercut are fundamental for joint strength, whereas the bottom thickness is important
for non destructive quality control. Both – form fit and force fit – put the inseparable connection into practice. Cold hardening
due to high plastic deformation leads to high local strength, especially in the neck (t n ; Fig. 4) avoiding a weak point.
punch
blank holder
die
Fig. 2 Clinching process with a rigid die concept
Fig. 3 Clinching process with a divided die concept
t t,1,2
tb
thickness (total, blank 1, blank 2)
bottom thickness
tn
neck thickness
f
undercut
h
height of joint
di
inner point diameter
d0
outer point diameter
Fig. 4 Principle geometry of a crosscut
Figure 5 shows a typical curve of stroke vs. force and the cross sections of the point in each of the 4 relevant stages. The
maximum force is reached at the point of the maximum punch stroke. It is also to be seen that the undercut occurs just at the
end of the clinching process.
As can be seen in the process characteristics clinching is a joining technology requiring a two-sided accessibility. This
means a fundamental restriction on the joining devices. A typical single spot application is done by a C-frame. If many joints
should be manufactured all at once, the process is often realized by using a press with a multi spot tool. Another interesting
aspect is the drive technology for the predominantly used C-frame. Hydraulic and pneumo-hydraulic systems are often used. A
relatively new trend is the usage of C-frames with an electromechanical drive.
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joining force F (in kN)
800
600
400
blank holder
force
200
offsetting
flow pressing
die-sinking / upsetting
0
0,0
3,0
6,0
9,0
punch stroke s (in mm)
12,0
15,0
Fig. 5 Steps in the clinching process; S380 / S235 6+4 mm; d o : 30 mm
III. CLINCHING OF THICK SHEET METAL
The transferability of experience in clinching thin sheets to joining thicker sheets is an important issue (Fig. 6). Although
the majority of clinching applications are in the range of 1.0mm to 2.0mm single sheet thickness, the request of using this
technology even for thick sheet metal has risen in recent years.
Fig. 6 Comparison thick (6+4 mm) to thin sheet (1+1 mm) clinched pieces
Independent of the sheet thickness relevant parameters affecting the joint quality (geometry data, as well as strength
properties) is the geometry of the tools, material strength parameters, surface properties of the single plates and the blank
holder parameters. The general recommendations are conferrable from thin sheet to thick sheet applications. The joining
directions “high strength into low strength” and/or “thick sheet into thin sheet” should be favoured for ensuring the best joint
performance. Because of the fact that clinching of thick sheets is a quite new research field there is just a little knowledge of
adapting the know-how from thin sheet applications to thick sheet applications. In [19] there are summarized results of a twoyear research project in the topic of thick sheet metal clinching. One focus in this project was to determine the potential and the
limitations of clinching in this thickness level.
As one result it was pointed out that typical steel grades for steelwork, as S235, S355 or S500, can be clinched in principle.
The limitations, however, are a little bit different, compared to the thin sheet applications. A major limit is the needed force for
joining, which raises progressive with an enlarging joint diameter. Typical clinching forces are between 200 kN and 1.000 kN
for thick sheet metal. So a C-frame application might get difficult by increasing force due to the size, weight and handling of
the frames. Beyond that the accessibility of the frames and tools is getting constricted. So higher distances and greater flange
width is needed for greater clinching joints. In the other direction very small joint diameters lead to low joint strength and to a
greater local material deformation, especially in the neck area. As a worst case scenario there can appear flaws and cracks in
the neck area caused by the clinching process itself.
IV. EXPERIMENTAL JOINT CHARACTERIZATION
The parameters for joining have to be adjusted according to requirements. If small flanges or joining forces are required,
small spot diameters will be selected. If high strength is required, great tool diameters will be used (Fig. 7). In the following
some properties will be described.
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Outer point diameter: 16mm
Clinching force: 310kN
Static shear strength: 25kN
Outer point diameter: 30mm
Clinching force: 670kN
Static shear strength: 68kN
Fig. 7 Small and large joint dimension in comparison
A. Static Strength
Examples are shown as showcase in Figure 7 and Figure 8. Depending on the outer point diameter (do) there are different
strength levels achievable. Beside the point diameter the direction of assembling the sheets is important. A standard boundary
condition is the ratio of thicknesses: the upper sheet should be thicker than the bottom sheet to achieve best strength values.
Fig. 8 Maximum load forces of different joint dimensions and material combinations in shear tension test
The two typical failure modes are the pull-out and the neck fracture. The reason for pulling out the upper blank of the
bottom blank is either a too less undercut or caused by a high strength ratio of upper and bottom blank. A neck fracture often
occurs if the neck thickness has a very little value or the upper blank has a very low strength. Besides this the failure mode of
clinching joints strongly depends on the type of loading the joint: Loading the joint in the direction of the symmetry axis – as
for example done by cross tension testing – by the majority leads to a pull out failure. In contrast a shear load leads to a neck
fracture in most of the applications.
B. Fatigue Strength
The fatigue tests are carried out by using a load ratio of R=0.1 and a frequency of 15 to 20 Hz using single spot specimens.
One example is shown as showcase in Figure 9. HCF (high cycle fatigue) is on comparable high level with thin sheet clinching.
The ratio of shear load forces HCF / static maximum load is 26.4 / 42.3. Under cyclic load the level is 62% from static strength
level in this special case. The range of this ratio is usual between 50% and 80% depending on the crack characteristic.
Fig. 9 Joint strength, typical fatigue curve; S380 / S235 6+4 mm, point diameter of 22 mm
This ratio is also typical for thin sheet clinching. So one more known positive characteristic of clinching can be adapted to
the thick sheets. A comparison of joint strength to other joining techniques is not yet carried out for thick sheets. The
mentioned ratio of shear load forces HCF / static maximum load is 50% to 60% for self pierce riveting and 15% to 20% for
spot welding for thin sheet applications [25]. This points out the great fatigue characteristics of clinched joints – even for thick
sheets.
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C. Joint Strength vs. Strength of Base Material
Comparing the strength of the base material to the strength of a line of clinched points leads to a surprising result. Figure 7
shows the boundary of the seam strength (y-axis; 0-400 MPa) over the distance between two clinch spots (x-axis; 0-150 mm)
for the outer point diameters d0: 16 mm / 22 mm / 30 mm for the material combination S380 (6 mm) into S235 (4 mm). With
outer point diameter d 0 : 30mm a material utilization of 72% of the tensile strength of the softer material (294 MPa; S325) is
reached. With welding the tensile strength of high strength steel is not exploitable at this level.
Fig. 10 Joint strength vs. strength of base material
D. Deformation
Local radial flow inside a clinch spot is not constant over the thickness of both plates. Local strain vectors inside the spot
are caused by radial and bending load to the structure. Global elastic deformation of the structure is the consequence. Blank
holder force variation can minimize the bending portion of the load. An escalator structure is one of the possible applications
for thick sheet metal clinching, where a prototype was already manufactured. The prototype results show the possibility of
technology substitution considering the connection strength and achieving a highly accurate assembly. A steel frame and the
measured distortion of the structure are shown in Figures 8 and 9. With 3D optical system TRITOP of GOM mbH distortions
of an 40-spot-clinched structure shown in Figure 11 was measured. The black stitch imaginary line in Figure 12 shows the
twist after joining. In Y the structure has a stiffness minimum. Maximum delay in y-direction is 2.95 mm. Maximum distortion
in z- and x-direction is less than 0.3 mm.
Fig. 11 Clinched steel frame
z
y
x
Fig. 12 Measured distortion in Y of the steel frame, dimensions of the frame: x: 3.0 m; z: 1.0 m
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Up to now there has not carried out any comparison between clinching and welding or bolting regarding the global
deformation of joined thick sheet assemblies. A direct comparison between welding and clinching is hindered because of the
fact that design and construction would be different for both techniques in the majority of cases. But it is known from thin
sheet applications in the automotive industry that global deformation caused by clinching is much smaller than thermal
deformation during welding. Of course this very low global deformation can only be achieved by using suitable parameters,
especially the blank holder force and its geometry.
V. NUMERICAL PROCESS DESIGN
Numerical based design of process (Fig. 10) and tool dimensioning is effective, if the experimental tests are timeconsuming and expensive in tool machining. Focusing the state of the art thin sheet connections FEA is not efficient for laying
out the connection, because there are a lot of tools and a bright know-how on experimental base. Having no standardized tools
for thick sheet applications, the simulation is a very powerful instrument for designing the tools and for quantifying the
properties and the robustness of clinched connections.
A. FEA Model of Clinching and Verification by Experiment
The FEA system Deform of the Scientific Forming Technology Corporation is used for the numerical simulation. Fig. 13
shows the basic model structure. Since a clinching system is under observation that has a rigid die (Tox round point), it is
possible to use an axially symmetrical FEA model with the numerical description. The sheet metal is modeled as elasto-plastic
where the corresponding material characteristic values and functions are ascertained in the tension test (E module and apparent
yielding point) as well as in the upsetting experiment (flow curve).
Fig. 13 FEA model for clinching
Verification by FEA and experiment is usually done by comparison of force-stroke diagram (Fig. 14) or achieved geometry
(Fig. 15). Force-stroke comparison shows differences less than 10%. Geometry comparison shows differences about 20% in
calculated undercut f. The accuracy of the simulation forecast depends strongly on the used material parameters (e.g. flow
curve), the accuracy of the tool geometries and the friction coefficients. In summery a sufficient forecast is possible.
Fig. 14 Comparison of force-stroke diagram; S380MC 6+4 mm
Fig. 15 Comparison of geometry in experiment and FEA; S380MC 6+4 mm; die diameter: 22 mm
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VI. SENSITIVITY STUDY
Variation of input parameters, like sheet thickness or sheet strength, leads to different output parameters, such as neck
thickness, undercut and process force. These variations of the input parameters are a result of sheet production tolerances. To
indicate the sensitivity of the clinching process simulations are carried out with varying parameters. Based on sensitivity
studies the most important varying parameters and the process robustness can be determined. The case of fluctuation of sheet
thicknesses is shown in Figure 16. Default values:
- Thickness tolerance punch-side: 5.67-6.33 mm
- Thickness tolerance die-side: 3.7-4.3 mm
-
punch-side plus tolerated
-
die-side plus tolerated
Fig. 16 Sensitivity study
The red marked case (diagram neck thickness) is clinching a minus tolerated thin punch-side to a plus tolerated die-side
sheet. This case can be critical for the joint strength, which is dependent on the requirements. Also the thinning of the neck is
critical regarding a possible material damage (cracks), occurring just in the joining process. Looking at the second diagram in
Figure 16 the critical edge is, when using on both sides minus tolerated sheets. Current research activities at Fraunhofer IWU
focus on coupled analysis with FEA tool and a statistical and DOE tool. In this combination a great space of parameter
variations can be investigated with a minimum number of parameter designs.
VII. CONCLUSION
For getting the clinching technology a part of the production process in shipbuilding, a lot of research is still to do. Based
on the confirmation of the principle clinchability and the joint strength of typical steel grades and thicknesses, which is the
main part of this paper, following research activities are required:
• Tool lifetime investigation: There are high normal pressures appearing in the clinching process. After the clinching step
the punch must to be pulled out of the connection, whereby tensile stresses may occur. Tests up to 1.000 joining operations for
thick sheets show no negative effect on the tools surface. Whether the tools have a durability of ca. 200.000 joining operations,
as typical for thin sheet clinching, has yet to be studied.
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• Dimensioning of mobile tools for application directly in the shipbuilding yard: For thick sheet clinching with joining
forces up to 1.000 kN, it is natural to use hydraulic systems. The typical application in automotive is done by a robot controlled
C-frame, but also a multipoint application in a press brake for instance is possible. The design and dimension of a C-frame
strongly depends on the accessibility of the joining areas. Current research shows that designing a mobile C-frame for an
application with a joining force of 700kN is feasible.
• Extended knowledge about the effect of corrosion on the joint performance (included strategies to avoid corrosion by
using zinc coated steel and/or sealing): In automotive production clinching is often combined with adhesive bonding,
especially when having multi-material designs. Clinching is principle possible, when having precoated sheets. Though the
quality of the coating layer after the clinching process strongly depends on the kind and quality of the coating. Especially
investigations on joining of hot zinc coated sheets have to be carried out.
• Water-tightness of the assemblies: The field of application for clinching in shipbuilding are internal structures, which are
not directly in contact with the seawater. It is supposed, that overlapping connections come along with the risk of crevice
corrosion for instance. Together with the above mentioned point, this issue is to be analyzed in combination with corrosion
prevention methods like adhesive bonding, sealing or special coatings. If there are positive results in this field, an application
in seawater contact structures additional can be discussed.
Summary:
• Clinching of thick sheet is possible
• Nearly the same performance is scalable from state-of-the-art clinched joints in thin sheet
• Base material up to 700MPa tensile strength is clinchable, higher strength steel must be tested (best “hard to soft”
clinching direction)
• Good numerical forecast
• Decision for “optimal joint dimension” depends on several parameters
• Advantages:
• Process Safety
• Best ratio price / strength (€/kN), low process costs (no elaborate preparation and finishing of sheets, no protective
measures are needed; e.g. gas extraction; bolts)
• High fatigue level compared to welding
• Less part distortion; comparable with lock bolting
• Best energy efficiency – energy / strength ratio (J/kN)
Disadvantages:
• Low accessibility with C-frames
• Applications are restricted to overlap joints with restricted thickness ratio
• Only a few know-how regarding the corrosion resistance for thick sheet applications
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Markus Israel
Research fellow
Fraunhofer Institute for Machine Tools and Forming Technology
From 2001 until 2005 Markus Israel has been a student at HTW Dresden, University of Applied Science in Production Technology
where he received his diploma. Afterwards he attended the Dresden University of Technology part-time with a degree in Constructive
Mechanical Engineering in 2012.
Since June 2006 he is working as a Development Employee at the Joining Technology Division of the Fraunhofer IWU in Dresden.
Reinhard Mauermann
Division director
Fraunhofer Institute for Machine Tools and Forming Technology
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Reinhard Mauermann was born in Zittau on the 5 th January in 1956. After finishing school in 1974 Reinhard Mauermann became a
student at the Dresden University of Technology where he received his diploma in 1978. He was specialized in Metal Forming and Surface
Treatment during the Extrusion. From 1978 until 1985 he was a scientific assistant of professor Voelkner at the TU Dresden where he got his
doctorate in 1985.
18 months from 1979 to 1980 he served in the army. In 1984 he became a Research Engineer at a bearing industry and then a
Manufacturing Engineer at Robotron. In the years of 1990 to 1996 he first worked as a Project Engineer at Raskin SA Lausanne until its
takeover of Beyeler GmbH Gotha and afterwards he joined, as a Sales Engineer, the Carea Treuenbritzen Company. In 1997 he decided to
work with his former Professor Voelkner as a Development Engineer at the TU Dresden, where he specialized in forming and joining. Since
April 2000 he has been a Development Engineer at the Fraunhofer IWU Dresden.
Julia Schellnock
Research assistant
Fraunhofer Institute for Machine Tools and Forming Technology
Since November 2011 she is working as research assistant at the Joining Technology Division of the Fraunhofer IWU in Dresden.
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