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Influence of the moisture content on flat-clinch connection of wood materials and aluminium
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Influence of the moisture content on flat-clinch connection of wood materials and aluminium Stephan Lüder ∗ , Sebastian Härtel, Carolin Binotsch, Birgit Awiszus Professorship of Virtual Production Engineering, Institute of Machine Tools and Production Processes, Technische Universität Chemnitz, D-09107 Chemnitz, Germany
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 10 January 2014 Accepted 18 January 2014 Available online xxx Keywords: Flat-clinching Joint strength Aluminium Wood material Moisture content
a b s t r a c t The paper describes the joining of composites made of different types of wood-based materials and aluminium. The wood materials were conditioned in a climate chamber or in a chamber furnace to set specific moisture contents. Afterwards, the conditioned wood materials were joined with aluminium (Al 99.5) by means of flat-clinching. After a relaxation period of 48 h under standard climatic conditions the joint strengths at different moisture contents were quantified by cross tension tests. Based on the well-founded experimental studies the understanding of the flat-clinching technology for this kind of material combination was improved. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The necessity of climate protection and resource conservation implies the need for innovative lightweight construction concepts. Such concepts can be achieved by the use of renewable materials combined with conventional light materials such as pure aluminium or aluminium alloys. Therefore, it is necessary to develop new joining technologies or to adapt existing technologies to the challenges of joining renewable materials with conventional materials. In accordance with DIN 8593-0, joining describes a permanent connection of two or more parts; thereby, a local cohesion is created. For the joining of wood materials, there are several detachable and permanent connections. For the joining of wood materials, screwed, nailed and bolted connections are considered to be detachable, whereas the conglutination of two parts is a permanent connection. According to Wagenführ and Scholz (2008) wood is a natural product and is divided into hardwood and softwood. Wood materials are generally separated into solid wood, plywood, chipboard, fibreboard and composite materials. Fibreboards, such as medium density fibreboard (MDF) are produced by defibration and subsequent assembling of wood fibres (usually by using adhesives).
∗ Corresponding author. Tel.: +49 371 531 33819; fax: +49 371 531 833819. E-mail address: [email protected] (S. Lüder).
Despite the different advantages of conventional wood joining methods, these types of connections all have the same disadvantage. In all of these, auxiliary materials (e.g., adhesives, nails, screws, etc.) are required. In contrast, the mechanical joining of flat-clinching produces a form- and force-closed connection by selective plastic deformation of the joining partners. The forming of wood material represents a special challenge. Because of its orthotropic character, wood has different material properties in all three dimensions and only low material flow properties. Despite the limited ductility of wood, the objective is to produce wood products or hybrid wood components by flat-clinching. 2. Flat-clinching Clinching is, in accordance with DIN 8593-5, assigned to the group ‘joining by forming’. It is used to produce form- and forceclosed connections by plastic deformation using a punch followed by spreading and compression in a die (see Fig. 1). In conventional clinching, a connection occurs by a local forming process, where neither additives nor auxiliary materials are required. This attribute, the single-step process and long tool life characterize this method (Hahn et al., 1999; Kühne, 2000). One disadvantage of conventional clinch connections is the geometrical protrusion of the die-sided sheet metal, which is basically processinduced. Therefore, this kind of connection is not usable for visible and functional surfaces, e.g., for sliding parts and sealing bands. Flat-clinching is a single-step, mechanical joining method that was developed and patented (DE 101 30 726 C 2) at the Technische
0924-0136/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
Please cite this article in press as: Lüder, S., et al., Influence of the moisture content on flat-clinch connection of wood materials and aluminium. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
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Fig. 1. Process of joining a conventional clinch connection (TOX, 2012).
Universität Chemnitz. Matthes et al. (2000, 2001) found out that the material flow that occurs during the forming process is influenced in such a way that allows the locking of the join partners. This results in a force- and form-closed interlocking within the total sheet thickness. In contrast to conventional clinch connections, a one sided planar connection is created that does not show the diesided protrusion reaching out of the material plane (see Fig. 2). Because of its simple process, the flat-clinching method offers high efficiency. The form- and force-closed interlocking, which is a characteristic feature of clinch connections, is shown in Fig. 3. According to Beyer (2012) the most important variables of the flat-clinching process are the interlocking f, the neck thickness tn , the bottom thickness tb and the punch force FFS . The Interlocking is an almost inseparable form- and force-closed connection between the join partners. Todtermuschke (2006) found out that the mechanical clamping is formed by the punch-sided material flow behind the anvil-sided material. This specific material flow in the bottom area occurs due to the avoidance of a die-sided protrusion by the anvil. In conventional clinching, the formation of the interlocking occurs by a material flow after passing out of the total sheet thickness. Following flat-clinch connections of aluminium and wood materials are investigated. Therefore, it is necessary to characterize the used wood materials first. 3. Wood material Wood is an orthotropic material. This means that, with regard to wood fibre, there are strong distinctions in longitudinal and radial direction. During the flat-clinching process, these different material characteristics influence interlocking in radial direction. The influence of the orthotropic material behaviour on the flat-clinch connection can be illustrated by the example of an aluminium–spruce-composite (see Fig. 4). The irregular shape of the interlocking in the radial direction is caused by the orthotropic character of the wood material (see Fig. 4(a) and (b)). Therefore, the interlocking is insufficient for
representing the quality of the whole connection. Thus, the separation force of cross tension tests is used for characterization of the joint strength. Firstly, flat-clinch connections of plywood and aluminium, as well as fibreboard and aluminium were investigated. Based on the manufacturing of these two wood materials a quasi-isotropic and isotropic material behaviour was observed, respectively. The plywood materials were made of birch and poplar, respectively, which were laminated in multiple orthogonal layers. Despite the identical structure, the two types of wood had completely different mechanical properties. The class of fibreboard was represented by medium density fibreboard (MDF) and hardboard (HB). Based on the manufacturing of MDF and HB from non-orientated wood fibres, nearly isotropic material properties were assumed. 4. Experimental set-up and specimen preparation Fig. 5 shows the experimental flow chart for investigating the influence of moisture content on the joint strength of the wood–aluminium composite. 4.1. Determination of the initial moisture content At the outset the initial moisture content of the wood materials was determined. The measurement of the moisture content was performed with the moisture analyser DLB 160-3A (Kern), which operates according to the method of thermogravimetry. According to DIN EN 13183-1, the moisture ω is defined as the ratio of the water mass mw contained in the wood sample to a kiln-dried wood sample m0 (anhydrous) in mass percentage, Eq. (1). ω (%) =
mw × 100 m0
(1)
Table 1 shows the measured initial moisture content of different wood materials. 4.2. Conditioning Wood is a hygroscopic material that absorbs water from the environment by adsorption; it can also desorb water to the environment. Due to the hygroscopic nature of wood, the mechanical properties of wood-based materials depend on its moisture content. In order to verify the influence of the moisture content on the strength of the clinch connection, the joining was preceded by a conditioning process. Therefore, the KEYLWERTH Chart was used for the conditioning. The chart is based on the hygroscopic isotherm, which shows the dependence of the equilibrium moisture content of the temperature and of the humidity. The chart is Table 1 Initial moisture content of different wood samples.
Fig. 2. Process of joining a flat-clinch connection. (a) Downstroke of blank-holder. (b) Penetration of punch owing to punch force FFS . (c) Joined flat-clinch connection according to Todtermuschke (2006).
Wood material
Poplar plywood
Birch plywood
MDF
HB
Initial moisture content [%]
8.1
6.6
5.3
5.9
Please cite this article in press as: Lüder, S., et al., Influence of the moisture content on flat-clinch connection of wood materials and aluminium. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
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Fig. 3. Form closure and force closure according to Beyer and Awiszus (2010).
Fig. 4. Hybrid flat-clinch connection of spruce (5 mm) and aluminium 99.5 (1.5 mm). (a) Cross section longitudinal to wood fibre without interlocking. (b) Cross section radial to wood fibre with interlocking.
only valid for spruce. Therefore, the sorption behaviour of the wood materials used (poplar plywood, birch plywood, MDF, HB) must be investigated. The conditioning process of the wooden samples (size 75 mm × 30 mm × 3 mm) was performed with the climate chamber HPP 100 (Memmert). Therefore, the specimens were treated under constant climatic conditions (up to 90% humidity) to calibrate the sorption equilibrium by means of adsorption and desorption
processes. Table 2 shows the different approaches for the specimen treatment. The conditioning process is a time-dependent process and due to the behaviour of this process, it is necessary to determine the minimum treatment time needed to reach the sorption equilibrium in the first step. Therefore, the conditioning with 30 ◦ C and 90% humidity was the most critical approach, because of the highest gradient between initial moisture and final moisture in the
Fig. 5. Experimental procedure.
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Fig. 6. Moisture content of different wood materials depending on the conditioning time (RH = 90%, T = 30 ◦ C).
Table 2 Parameters for the conditioning process. Name
Relative humidity [%]
Temperature [◦ C]
Climate 1 Climate 2 Climate 3 Climate 4 Climate 5 Climate 6
30 60 70 80 85 90
35 35 30 35 30 30
specimen. The increase of the moisture content as a function of treatment time is shown in Fig. 6. The saturation point of birch plywood, poplar plywood and MDF was reached after 18 h and the moisture content did not significantly increase further. Only HB showed a slight increase of the moisture content during the specified time range. The difference between 24 h and 48 h was only 2.2%. Therefore, all specimens were treated for 24 h to ensure the comparability of the moisture content as well as the experiments. The results of the conditioning based on the approaches of Table 2 are shown in Table 3. Furthermore, the additional approach climate 7 was added, which represents a kiln-dried condition (160 ◦ C, 15 min) for classifying the gained values. The removal of the specimens from the climate chamber took place shortly before the flat-clinching process, so that no modification of the moisture content was possible. 4.3. Joining The punch-sided work piece was a 1.5 mm aluminium sheet (Al 99.5) and the anvil-sided work piece was a 3 mm piece of wood (poplar plywood, birch plywood, MDF, HB). This condition
reflects the joining rule of flat-clinching processes – thin in thick (work piece thickness) and hard in soft (based on yield strength) (Todtermuschke, 2006). For the comparability of the experimental results, all material combinations were performed with the same process conditions. The active tools were the cylindrical punch (D = 5 mm) and the blank holder (D = 6.1 mm) with an edge radius of R = 2 mm. The punch velocity was 60 mm/s and the blank holder load was 10 kN. The resulting punch force varied in dependency of the materials between 25.8 kN (MDF) and 30.6 kN (HB). Following the flat-clinching process the joined specimens were relaxed under standard climatic conditions (20 ◦ C, 65% humidity). After a relaxation period of 48 h the wood material reached the initial moisture content. 4.4. Testing In accordance with DIN EN ISO 14272, cross tension tests were performed to analyze the strength of the flat-clinch connection. Therefore, the specimens were produced, conditioned and joined by means of flat-clinching technology (see Fig. 7) and relaxed under standard climatic conditions. The analyses were carried out with five repetitions to ensure statistical reliability. The tests were performed with the universal testing machine Quasar 50 kN (Galdabini SpA) with a pull rate of 10 mm/min. During the cross tension test the load was set vertical to the flatclinch connection, thereby the force was mainly transmitted by the form closure. If the load was too high, the flat-clinch connection failed because the interlocking lost its form closure, followed by a loss of the force closure.
Table 3 Moisture values depending on conditioning process. Name
Initial moisture content Climate 1 Climate 2 Climate 3 Climate 4 Climate 5 Climate 6 Climate 7 (kiln dry)
Moisture content [%] Poplar plywood
Birch plywood
MDF
HB
8.1 7.0 9.4 11.5 12.3 15.8 18.9 0.8
6.6 5.7 8.1 10.0 11.4 14.9 17.5 0.2
5.3 4.7 7.4 8.0 10.7 13.8 17.6 0.8
5.9 5.1 7.6 8.6 11.0 12.0 14.7 0.2
Fig. 7. Specimen for cross tension test; loading direction.
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Fig. 8. Separation force due the cross tension test.
5. Experimental evaluation Fig. 8 shows the load–moisture curves of different wood–aluminium flat-clinch connections obtained by cross tension tests. It can clearly be seen that the joint strengths differ between the examined wooden joining partners. Birch plywood joined with aluminium achieved the highest joint strength with about 201 N (at ω = 5.7%), followed by MDF–aluminium (Fmax = 153 N at ω = 7.4%), HB–aluminium (Fmax = 106 N at ω = 11.0%) and poplar plywood–aluminium (Fmax = 52 N at ω = 9.4%). qualitative strength curve of the individual The wood–aluminium composites shows a similar progress. For all material combinations, the required separation force increased, starting from the kiln-dry condition, and decreased after reaching a maximum value. Because of the diverse types of wood and their structure, each wood material shows a characteristic sorption behaviour. This behaviour influences the swelling and shrinkage processes, which have an effect on the joint strength. Hence, regarding the moisture content of the wood-based material, a working range for clinching exists, where joint strength leads to an optimum joining result.
Table 4 Optimal joint strength of wood–aluminium-connections depending on moisture content of wood materials. Wood material
Initial moisture content [%]
Birch plywood MDF HB Poplar plywood
6.6 5.3 5.8 8.1
Optimal working range moisture content [%] 3.7–7.7 6.4–10.1 8.6–11.2 9.0–9.8
Separation force (cross tension test) [N] 181–201 138–153 96–106 47–52
In this paper, the working range is defined in such a way that the maximum joint strength of the cross tension load can be undercut by 10%. This leads to the following optimal ranges for moisture during the flat-clinching process (see Table 4). Furthermore, Table 4 shows that the joining of birch plywood with aluminium can be done without additional conditioning of the samples. The initial moisture (ω = 6.6%) was already located within the optimal range (ω = 3.7–7.7%). For the other tested materials, conditioning was necessary in order to optimize the composites in terms of strength. On the basis of cross-sectional images of the flat-clinch connections, the cause of the initial increase and the subsequent
Fig. 9. Interlocking depending on moisture content (hybrid flat-clinch connection of HB and Al 99.5).
Please cite this article in press as: Lüder, S., et al., Influence of the moisture content on flat-clinch connection of wood materials and aluminium. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
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decreasing of the joint strength with increased moisture were analyzed. Therefore, the clinch connection needed to be separated in the middle and analyzed in terms of interlocking. A hardboard–aluminium composite was the most suitable specimen for the visual comparison, because considerable differences were visible (see Fig. 9). The analyses showed that flat-clinching of wood-based materials in a kiln-dry condition (ω = 0.2%) does not achieve an interlocking. The reason for this can be found in the lack of ductility of the wood-based material in the kiln-dry condition. The penetration of the aluminium sheet in the wood leads to the destruction of the wood fibres, which resembles a cutting process and no interlocking can be formed. Therefore, the clinching of kiln-dry wood-based materials results in a low joint strength. By increasing the moisture to 8.7% or to 14.7%, an almost identical interlocking was determined. The reason for this was that both ways of conditioning lead to an increase of the moisture content of the wood fibres and thus to a higher ductility. Nevertheless, a significantly lower joint strength was achieved at a moisture content of 14.7%. This effect can be explained by the swelling processes due to the adsorption and by shrinkage processes as a result of desorption. While relaxing under standard climatic conditions, the shrinkage of the wood material led to a decrease in the volume. The wood became porous (see highlighted area in Fig. 9), which caused a weakening of the flat-clinch connection. 6. Summary Intelligent lightweight construction concepts are necessary to increase the use of renewable raw materials. Flat-clinching technology offers the opportunity to manufacture composites, made of wood materials and metallic materials, without using any auxiliary materials like adhesives, nails or screws. With flat-clinching, a form- and force-closed connection can be produced by selective plastic forming of the joining partners. The result is a one-sided planar composite made of wood materials (poplar plywood, birch plywood, MDF or HB) and aluminium (Al 99.5). By means of cross tension tests the fundamental influence of the moisture content of wood materials on the joint strength of flatclinch connections was proven. By conditioning the wood materials in a constant climate chamber, specific moisture contents can be set to improve their ductility. Thus, an optimal working range for the moisture content was detected for each wood material, thereby the maximum joint strength was achieved. The flat-clinch connection of birch plywood and aluminium achieved the highest
joint strength with a separation force of 201 N at a moisture content of ω = 5.7%. The lowest joint strength was achieved for poplar plywood–aluminium composites. The maximum separation force was 52 N at a moisture content of ω = 9.4%. For further investigations, the optimal working range should also be specified for other types of wood materials like solid wood or chipboard. Furthermore, the process parameters should be optimized to improve the joint strength of the flat-clinch connection. Furthermore, by means of numerical simulations, an optimization of this innovative technology and the flat-clinch connection itself will be possible. Acknowledgement The authors thank the German Research Foundation (DFG) for the financial support of this work. References Beyer, U., Awiszus, B., 2010. Flat-clinching – a new possibility for joining different kinds of components in a flexible and effective way to a planar material compound. In: Steel Research International 81. Special Edition Metal Forming 2010, pp. 1124–1127. Beyer, U., (Dissertation) 2012. Multi Material Fügen mittels Flach-ClinchTechnologie. Technische Universität Chemnitz. DIN EN 13183-1, 2002. Moisture content of a piece of sawn timber. Part 1. Determination by oven dry method. DIN 8593-0, 2003. Manufacturing processes joining. Part 0. General: classification, subdivision, terms and definitions. DIN 8593-5, 2003. Manufacturing processes joining. Part 5. Joining by forming processes: classification, subdivision, terms and definitions. DIN EN ISO 14272, 2002. Specimen dimensions and procedure for cross tension testing resistance spot and embossed projection welds. Hahn, O., Kurzok, J., Meschut, G., Schulte, V., 1999. Fügen durch Umformen. Stand der Technik und Entwicklungstendenzen. In: Forschungskolleg Stahlanwendung 99. Innovative Fügetechniken für Stahl., pp. 1–11. Kühne, T., 2000. Mechanisches Fügen – von der Alternative zum Favoriten. EckoldClinchen im Karosserie-Rohbau. Blech, Rohre, Profile 47, 24. Matthes, K.-J., Riedel, F., Todtermuschke, M., 2000. Entwicklung einer einseitig ebenen, einstufig mechanisch gefügten Verbindung. In: Tagungsband 7. Paderborner Symposium Fügetechnik, pp. 47–52. Matthes, K.-J., Riedel, F., Todtermuschke, M., 2001. Entwicklung eines einstufig mechanisch gefügten Flachpunktes. AiF-Abschlussbericht 12153B. Matthes, K.-J., Riedel, F., Todtermuschke, M.(Erfinder), 2003. Verfahren und Einrichtung zur Herstellung einer Verbindung zwischen sich plattenförmigen Bauteilen. DE 101 30 726 C 2. Todtermuschke, M., (Dissertation) 2006. Verfahrensoptimierung zur Herstellung einer punktförmigen, mechanisch gefügten, einseitig ebenen Verbindung ohne Verbindungselement. Technische Universität Chemnitz, Fakultät für Maschinenbau. 2012. TOX Produktbeschreibung: TOX Rundpunkt, www.tox-de.com Wagenführ, A., Scholz, F., 2008. Taschenbuch der Holztechnik. Hanser-Verlag, Leipzig.
Please cite this article in press as: Lüder, S., et al., Influence of the moisture content on flat-clinch connection of wood materials and aluminium. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
ARTICLE IN PRESS
PROTEC-13879; No. of Pages 6
Journal of Materials Processing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Influence of the moisture content on flat-clinch connection of wood materials and aluminium Stephan Lüder ∗ , Sebastian Härtel, Carolin Binotsch, Birgit Awiszus Professorship of Virtual Production Engineering, Institute of Machine Tools and Production Processes, Technische Universität Chemnitz, D-09107 Chemnitz, Germany
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 10 January 2014 Accepted 18 January 2014 Available online xxx Keywords: Flat-clinching Joint strength Aluminium Wood material Moisture content
a b s t r a c t The paper describes the joining of composites made of different types of wood-based materials and aluminium. The wood materials were conditioned in a climate chamber or in a chamber furnace to set specific moisture contents. Afterwards, the conditioned wood materials were joined with aluminium (Al 99.5) by means of flat-clinching. After a relaxation period of 48 h under standard climatic conditions the joint strengths at different moisture contents were quantified by cross tension tests. Based on the well-founded experimental studies the understanding of the flat-clinching technology for this kind of material combination was improved. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The necessity of climate protection and resource conservation implies the need for innovative lightweight construction concepts. Such concepts can be achieved by the use of renewable materials combined with conventional light materials such as pure aluminium or aluminium alloys. Therefore, it is necessary to develop new joining technologies or to adapt existing technologies to the challenges of joining renewable materials with conventional materials. In accordance with DIN 8593-0, joining describes a permanent connection of two or more parts; thereby, a local cohesion is created. For the joining of wood materials, there are several detachable and permanent connections. For the joining of wood materials, screwed, nailed and bolted connections are considered to be detachable, whereas the conglutination of two parts is a permanent connection. According to Wagenführ and Scholz (2008) wood is a natural product and is divided into hardwood and softwood. Wood materials are generally separated into solid wood, plywood, chipboard, fibreboard and composite materials. Fibreboards, such as medium density fibreboard (MDF) are produced by defibration and subsequent assembling of wood fibres (usually by using adhesives).
∗ Corresponding author. Tel.: +49 371 531 33819; fax: +49 371 531 833819. E-mail address: [email protected] (S. Lüder).
Despite the different advantages of conventional wood joining methods, these types of connections all have the same disadvantage. In all of these, auxiliary materials (e.g., adhesives, nails, screws, etc.) are required. In contrast, the mechanical joining of flat-clinching produces a form- and force-closed connection by selective plastic deformation of the joining partners. The forming of wood material represents a special challenge. Because of its orthotropic character, wood has different material properties in all three dimensions and only low material flow properties. Despite the limited ductility of wood, the objective is to produce wood products or hybrid wood components by flat-clinching. 2. Flat-clinching Clinching is, in accordance with DIN 8593-5, assigned to the group ‘joining by forming’. It is used to produce form- and forceclosed connections by plastic deformation using a punch followed by spreading and compression in a die (see Fig. 1). In conventional clinching, a connection occurs by a local forming process, where neither additives nor auxiliary materials are required. This attribute, the single-step process and long tool life characterize this method (Hahn et al., 1999; Kühne, 2000). One disadvantage of conventional clinch connections is the geometrical protrusion of the die-sided sheet metal, which is basically processinduced. Therefore, this kind of connection is not usable for visible and functional surfaces, e.g., for sliding parts and sealing bands. Flat-clinching is a single-step, mechanical joining method that was developed and patented (DE 101 30 726 C 2) at the Technische
0924-0136/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
Please cite this article in press as: Lüder, S., et al., Influence of the moisture content on flat-clinch connection of wood materials and aluminium. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.01.010
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Fig. 1. Process of joining a conventional clinch connection (TOX, 2012).
Universität Chemnitz. Matthes et al. (2000, 2001) found out that the material flow that occurs during the forming process is influenced in such a way that allows the locking of the join partners. This results in a force- and form-closed interlocking within the total sheet thickness. In contrast to conventional clinch connections, a one sided planar connection is created that does not show the diesided protrusion reaching out of the material plane (see Fig. 2). Because of its simple process, the flat-clinching method offers high efficiency. The form- and force-closed interlocking, which is a characteristic feature of clinch connections, is shown in Fig. 3. According to Beyer (2012) the most important variables of the flat-clinching process are the interlocking f, the neck thickness tn , the bottom thickness tb and the punch force FFS . The Interlocking is an almost inseparable form- and force-closed connection between the join partners. Todtermuschke (2006) found out that the mechanical clamping is formed by the punch-sided material flow behind the anvil-sided material. This specific material flow in the bottom area occurs due to the avoidance of a die-sided protrusion by the anvil. In conventional clinching, the formation of the interlocking occurs by a material flow after passing out of the total sheet thickness. Following flat-clinch connections of aluminium and wood materials are investigated. Therefore, it is necessary to characterize the used wood materials first. 3. Wood material Wood is an orthotropic material. This means that, with regard to wood fibre, there are strong distinctions in longitudinal and radial direction. During the flat-clinching process, these different material characteristics influence interlocking in radial direction. The influence of the orthotropic material behaviour on the flat-clinch connection can be illustrated by the example of an aluminium–spruce-composite (see Fig. 4). The irregular shape of the interlocking in the radial direction is caused by the orthotropic character of the wood material (see Fig. 4(a) and (b)). Therefore, the interlocking is insufficient for
representing the quality of the whole connection. Thus, the separation force of cross tension tests is used for characterization of the joint strength. Firstly, flat-clinch connections of plywood and aluminium, as well as fibreboard and aluminium were investigated. Based on the manufacturing of these two wood materials a quasi-isotropic and isotropic material behaviour was observed, respectively. The plywood materials were made of birch and poplar, respectively, which were laminated in multiple orthogonal layers. Despite the identical structure, the two types of wood had completely different mechanical properties. The class of fibreboard was represented by medium density fibreboard (MDF) and hardboard (HB). Based on the manufacturing of MDF and HB from non-orientated wood fibres, nearly isotropic material properties were assumed. 4. Experimental set-up and specimen preparation Fig. 5 shows the experimental flow chart for investigating the influence of moisture content on the joint strength of the wood–aluminium composite. 4.1. Determination of the initial moisture content At the outset the initial moisture content of the wood materials was determined. The measurement of the moisture content was performed with the moisture analyser DLB 160-3A (Kern), which operates according to the method of thermogravimetry. According to DIN EN 13183-1, the moisture ω is defined as the ratio of the water mass mw contained in the wood sample to a kiln-dried wood sample m0 (anhydrous) in mass percentage, Eq. (1). ω (%) =
mw × 100 m0
(1)
Table 1 shows the measured initial moisture content of different wood materials. 4.2. Conditioning Wood is a hygroscopic material that absorbs water from the environment by adsorption; it can also desorb water to the environment. Due to the hygroscopic nature of wood, the mechanical properties of wood-based materials depend on its moisture content. In order to verify the influence of the moisture content on the strength of the clinch connection, the joining was preceded by a conditioning process. Therefore, the KEYLWERTH Chart was used for the conditioning. The chart is based on the hygroscopic isotherm, which shows the dependence of the equilibrium moisture content of the temperature and of the humidity. The chart is Table 1 Initial moisture content of different wood samples.
Fig. 2. Process of joining a flat-clinch connection. (a) Downstroke of blank-holder. (b) Penetration of punch owing to punch force FFS . (c) Joined flat-clinch connection according to Todtermuschke (2006).
Wood material
Poplar plywood
Birch plywood
MDF
HB
Initial moisture content [%]
8.1
6.6
5.3
5.9
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Fig. 3. Form closure and force closure according to Beyer and Awiszus (2010).
Fig. 4. Hybrid flat-clinch connection of spruce (5 mm) and aluminium 99.5 (1.5 mm). (a) Cross section longitudinal to wood fibre without interlocking. (b) Cross section radial to wood fibre with interlocking.
only valid for spruce. Therefore, the sorption behaviour of the wood materials used (poplar plywood, birch plywood, MDF, HB) must be investigated. The conditioning process of the wooden samples (size 75 mm × 30 mm × 3 mm) was performed with the climate chamber HPP 100 (Memmert). Therefore, the specimens were treated under constant climatic conditions (up to 90% humidity) to calibrate the sorption equilibrium by means of adsorption and desorption
processes. Table 2 shows the different approaches for the specimen treatment. The conditioning process is a time-dependent process and due to the behaviour of this process, it is necessary to determine the minimum treatment time needed to reach the sorption equilibrium in the first step. Therefore, the conditioning with 30 ◦ C and 90% humidity was the most critical approach, because of the highest gradient between initial moisture and final moisture in the
Fig. 5. Experimental procedure.
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Fig. 6. Moisture content of different wood materials depending on the conditioning time (RH = 90%, T = 30 ◦ C).
Table 2 Parameters for the conditioning process. Name
Relative humidity [%]
Temperature [◦ C]
Climate 1 Climate 2 Climate 3 Climate 4 Climate 5 Climate 6
30 60 70 80 85 90
35 35 30 35 30 30
specimen. The increase of the moisture content as a function of treatment time is shown in Fig. 6. The saturation point of birch plywood, poplar plywood and MDF was reached after 18 h and the moisture content did not significantly increase further. Only HB showed a slight increase of the moisture content during the specified time range. The difference between 24 h and 48 h was only 2.2%. Therefore, all specimens were treated for 24 h to ensure the comparability of the moisture content as well as the experiments. The results of the conditioning based on the approaches of Table 2 are shown in Table 3. Furthermore, the additional approach climate 7 was added, which represents a kiln-dried condition (160 ◦ C, 15 min) for classifying the gained values. The removal of the specimens from the climate chamber took place shortly before the flat-clinching process, so that no modification of the moisture content was possible. 4.3. Joining The punch-sided work piece was a 1.5 mm aluminium sheet (Al 99.5) and the anvil-sided work piece was a 3 mm piece of wood (poplar plywood, birch plywood, MDF, HB). This condition
reflects the joining rule of flat-clinching processes – thin in thick (work piece thickness) and hard in soft (based on yield strength) (Todtermuschke, 2006). For the comparability of the experimental results, all material combinations were performed with the same process conditions. The active tools were the cylindrical punch (D = 5 mm) and the blank holder (D = 6.1 mm) with an edge radius of R = 2 mm. The punch velocity was 60 mm/s and the blank holder load was 10 kN. The resulting punch force varied in dependency of the materials between 25.8 kN (MDF) and 30.6 kN (HB). Following the flat-clinching process the joined specimens were relaxed under standard climatic conditions (20 ◦ C, 65% humidity). After a relaxation period of 48 h the wood material reached the initial moisture content. 4.4. Testing In accordance with DIN EN ISO 14272, cross tension tests were performed to analyze the strength of the flat-clinch connection. Therefore, the specimens were produced, conditioned and joined by means of flat-clinching technology (see Fig. 7) and relaxed under standard climatic conditions. The analyses were carried out with five repetitions to ensure statistical reliability. The tests were performed with the universal testing machine Quasar 50 kN (Galdabini SpA) with a pull rate of 10 mm/min. During the cross tension test the load was set vertical to the flatclinch connection, thereby the force was mainly transmitted by the form closure. If the load was too high, the flat-clinch connection failed because the interlocking lost its form closure, followed by a loss of the force closure.
Table 3 Moisture values depending on conditioning process. Name
Initial moisture content Climate 1 Climate 2 Climate 3 Climate 4 Climate 5 Climate 6 Climate 7 (kiln dry)
Moisture content [%] Poplar plywood
Birch plywood
MDF
HB
8.1 7.0 9.4 11.5 12.3 15.8 18.9 0.8
6.6 5.7 8.1 10.0 11.4 14.9 17.5 0.2
5.3 4.7 7.4 8.0 10.7 13.8 17.6 0.8
5.9 5.1 7.6 8.6 11.0 12.0 14.7 0.2
Fig. 7. Specimen for cross tension test; loading direction.
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Fig. 8. Separation force due the cross tension test.
5. Experimental evaluation Fig. 8 shows the load–moisture curves of different wood–aluminium flat-clinch connections obtained by cross tension tests. It can clearly be seen that the joint strengths differ between the examined wooden joining partners. Birch plywood joined with aluminium achieved the highest joint strength with about 201 N (at ω = 5.7%), followed by MDF–aluminium (Fmax = 153 N at ω = 7.4%), HB–aluminium (Fmax = 106 N at ω = 11.0%) and poplar plywood–aluminium (Fmax = 52 N at ω = 9.4%). qualitative strength curve of the individual The wood–aluminium composites shows a similar progress. For all material combinations, the required separation force increased, starting from the kiln-dry condition, and decreased after reaching a maximum value. Because of the diverse types of wood and their structure, each wood material shows a characteristic sorption behaviour. This behaviour influences the swelling and shrinkage processes, which have an effect on the joint strength. Hence, regarding the moisture content of the wood-based material, a working range for clinching exists, where joint strength leads to an optimum joining result.
Table 4 Optimal joint strength of wood–aluminium-connections depending on moisture content of wood materials. Wood material
Initial moisture content [%]
Birch plywood MDF HB Poplar plywood
6.6 5.3 5.8 8.1
Optimal working range moisture content [%] 3.7–7.7 6.4–10.1 8.6–11.2 9.0–9.8
Separation force (cross tension test) [N] 181–201 138–153 96–106 47–52
In this paper, the working range is defined in such a way that the maximum joint strength of the cross tension load can be undercut by 10%. This leads to the following optimal ranges for moisture during the flat-clinching process (see Table 4). Furthermore, Table 4 shows that the joining of birch plywood with aluminium can be done without additional conditioning of the samples. The initial moisture (ω = 6.6%) was already located within the optimal range (ω = 3.7–7.7%). For the other tested materials, conditioning was necessary in order to optimize the composites in terms of strength. On the basis of cross-sectional images of the flat-clinch connections, the cause of the initial increase and the subsequent
Fig. 9. Interlocking depending on moisture content (hybrid flat-clinch connection of HB and Al 99.5).
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decreasing of the joint strength with increased moisture were analyzed. Therefore, the clinch connection needed to be separated in the middle and analyzed in terms of interlocking. A hardboard–aluminium composite was the most suitable specimen for the visual comparison, because considerable differences were visible (see Fig. 9). The analyses showed that flat-clinching of wood-based materials in a kiln-dry condition (ω = 0.2%) does not achieve an interlocking. The reason for this can be found in the lack of ductility of the wood-based material in the kiln-dry condition. The penetration of the aluminium sheet in the wood leads to the destruction of the wood fibres, which resembles a cutting process and no interlocking can be formed. Therefore, the clinching of kiln-dry wood-based materials results in a low joint strength. By increasing the moisture to 8.7% or to 14.7%, an almost identical interlocking was determined. The reason for this was that both ways of conditioning lead to an increase of the moisture content of the wood fibres and thus to a higher ductility. Nevertheless, a significantly lower joint strength was achieved at a moisture content of 14.7%. This effect can be explained by the swelling processes due to the adsorption and by shrinkage processes as a result of desorption. While relaxing under standard climatic conditions, the shrinkage of the wood material led to a decrease in the volume. The wood became porous (see highlighted area in Fig. 9), which caused a weakening of the flat-clinch connection. 6. Summary Intelligent lightweight construction concepts are necessary to increase the use of renewable raw materials. Flat-clinching technology offers the opportunity to manufacture composites, made of wood materials and metallic materials, without using any auxiliary materials like adhesives, nails or screws. With flat-clinching, a form- and force-closed connection can be produced by selective plastic forming of the joining partners. The result is a one-sided planar composite made of wood materials (poplar plywood, birch plywood, MDF or HB) and aluminium (Al 99.5). By means of cross tension tests the fundamental influence of the moisture content of wood materials on the joint strength of flatclinch connections was proven. By conditioning the wood materials in a constant climate chamber, specific moisture contents can be set to improve their ductility. Thus, an optimal working range for the moisture content was detected for each wood material, thereby the maximum joint strength was achieved. The flat-clinch connection of birch plywood and aluminium achieved the highest
joint strength with a separation force of 201 N at a moisture content of ω = 5.7%. The lowest joint strength was achieved for poplar plywood–aluminium composites. The maximum separation force was 52 N at a moisture content of ω = 9.4%. For further investigations, the optimal working range should also be specified for other types of wood materials like solid wood or chipboard. Furthermore, the process parameters should be optimized to improve the joint strength of the flat-clinch connection. Furthermore, by means of numerical simulations, an optimization of this innovative technology and the flat-clinch connection itself will be possible. Acknowledgement The authors thank the German Research Foundation (DFG) for the financial support of this work. References Beyer, U., Awiszus, B., 2010. Flat-clinching – a new possibility for joining different kinds of components in a flexible and effective way to a planar material compound. In: Steel Research International 81. Special Edition Metal Forming 2010, pp. 1124–1127. Beyer, U., (Dissertation) 2012. Multi Material Fügen mittels Flach-ClinchTechnologie. Technische Universität Chemnitz. DIN EN 13183-1, 2002. Moisture content of a piece of sawn timber. Part 1. Determination by oven dry method. DIN 8593-0, 2003. Manufacturing processes joining. Part 0. General: classification, subdivision, terms and definitions. DIN 8593-5, 2003. Manufacturing processes joining. Part 5. Joining by forming processes: classification, subdivision, terms and definitions. DIN EN ISO 14272, 2002. Specimen dimensions and procedure for cross tension testing resistance spot and embossed projection welds. Hahn, O., Kurzok, J., Meschut, G., Schulte, V., 1999. Fügen durch Umformen. Stand der Technik und Entwicklungstendenzen. In: Forschungskolleg Stahlanwendung 99. Innovative Fügetechniken für Stahl., pp. 1–11. Kühne, T., 2000. Mechanisches Fügen – von der Alternative zum Favoriten. EckoldClinchen im Karosserie-Rohbau. Blech, Rohre, Profile 47, 24. Matthes, K.-J., Riedel, F., Todtermuschke, M., 2000. Entwicklung einer einseitig ebenen, einstufig mechanisch gefügten Verbindung. In: Tagungsband 7. Paderborner Symposium Fügetechnik, pp. 47–52. Matthes, K.-J., Riedel, F., Todtermuschke, M., 2001. Entwicklung eines einstufig mechanisch gefügten Flachpunktes. AiF-Abschlussbericht 12153B. Matthes, K.-J., Riedel, F., Todtermuschke, M.(Erfinder), 2003. Verfahren und Einrichtung zur Herstellung einer Verbindung zwischen sich plattenförmigen Bauteilen. DE 101 30 726 C 2. Todtermuschke, M., (Dissertation) 2006. Verfahrensoptimierung zur Herstellung einer punktförmigen, mechanisch gefügten, einseitig ebenen Verbindung ohne Verbindungselement. Technische Universität Chemnitz, Fakultät für Maschinenbau. 2012. TOX Produktbeschreibung: TOX Rundpunkt, www.tox-de.com Wagenführ, A., Scholz, F., 2008. Taschenbuch der Holztechnik. Hanser-Verlag, Leipzig.
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