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© Carl Hanser Verlag Zeitschrift Kunststofftechnik / Journal of Plastics Technology 12 (2016) 3

12 (2016) 3

eingereicht/handed in: 09.02.2016 angenommen/accepted: 09.03.2016

Michael Albrecht, M.Sc.1; Dr.-Ing. Leyu Lin1; Prof. Dr.-Ing. Alois K. Schlarb1,2,3 1Composite Engineering CCe, University of Kaiserslautern, Kaiserslautern, Germany 2INM-Leibniz-Institute for New Materials, Saarbrücken, Germany 3Research Center OPTIMAS, University of Kaiserslautern, Kaiserslautern, Germany

Experimental investigation, modeling and sim-ulation of the deformation behavior of vibration welded nanocomposites

Nano-TiO2 filled polypropylene was compounded on a twin-screw extruder and diluted to a filler con-tent of 1 vol.-%. Nanocomposite and neat matrix material were then injection molded to plates and vibration welded. Thin films of welded specimens were investigated in micro-tensile testing under the light-microscope in order to achieve global and local tensile properties and compared to results of tensile tests for two welding pressures on neat and filled polypropylene. Based on investigations, a model of the welding area has been created with ANSYS, which then was coupled and compared with experimental results based on macroscopic test coupons. Prediction of break position based on stress distribution calculated in the model was in good agreement with experimental results.

Experimentelle Untersuchungen, Modellierung und Simulation des Deformationsverhaltens vibrationsgeschweißter Nanokomposite

In einem Zweischneckenextruder wurden nano-TiO2 und Polypropylen compoundiert und auf 1 vol.-% Füllstoffgehalt verdünnt. Der Nanokomposit und reines Matrixmaterial wurden dann zu Platten spritz-gegossen und vibrationsgeschweißt. Dünnfilme der Schweißungen wurden auf ihre lokalen und glo-balen Eigenschaften hin im Mikrozugversuch unter dem Lichtmikroskop untersucht und mit den Re-sultaten von makroskopischen Normprüfkörpern verglichen. Anhand der Ergebnisse wurde in ANSYS ein Modell der Schweißnaht erstellt, welches anschließend mit den Testergebnissen verknüpft und verglichen wurde. Die anhand der im Modell berechneten Spannungsverteilung getroffene Bruchvor-hersage zeigte gute Übereinstimmung mit den Beobachtungen im Experiment.

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Albrecht, Lin, Schlarb Deformation of vibration welded nanocomposites

Journal of Plastics Technology 12 (2016) 3 185

Experimental investigation, modeling and simula-tion of the deformation behavior of vibration welded nanocomposites

M. Albrecht, L. Lin, A. K. Schlarb

1 INTRODUCTION

In recent decades composite materials have become focus of research due to the effect of fillers on the composite properties. Especially in polymers as a ma-trix material the light weight of the matrix and thus of the entire composite is of special interest for engineering. But also for use in corrosive environments, such as the fuel system in a car or the human body in case of medical applica-tions, polymer based composites are a promising research subject as they are highly resistant or inert to most chemicals. Nano-scale filler materials have also become subject of research recently as they, compared to micro-scale filler ma-terials, provide high functional performance due to their high surface area to volume ratio.

Some geometries can’t be fabricated in direct processing techniques such as injection molding and therefore require an additional joining step. For thermo-plastic polymers welding is often used as the favored joining technique as it has a high potential for transferring forces, ensures a dense joint and due to the ability of thermoplastics to melt and solidify the process is simple and economic. Vibration welding in particular provides benefits in industry as it has short cycle times and has a high degree of automation.

The process of vibration welding and its impact on properties of the welded parts have been widely investigated in different studies [1, 2, 3, 4]. Reaching the quasi-steady stage of vibration welding as well as using low welding pressures is of high importance for maximizing welding strength, as an increase in welding pressure results in lower weld strength [5]. Cooling of the welded part is per-formed under the same pressure as applied during welding. According to Eh-renstein et al. [6] decay time, the delay between the control stopping vibration and actual stopping of the machines vibration, does not have any influence on weld properties of most thermoplastics, which was proven for PP in particular [7].

Factors in weld strength of about one could be shown for different thermoplastic materials including PP for optimized process parameters. For higher welding pressures, factors below one were found to be the result of flow-induced orien-tation of polymer molecules during vibration welding [5].

Morphological investigations have also been performed to show the develop-ment of different morphological layers along the welding direction [5]. All welds

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were reported to show a layer of highly oriented spherulites at the transition of bulk material to welding area as well as a non-structured zone in the center of the weld. Towards the edges of the material, a delta-shaped increase in welding area thickness was shown, which was longer for higher welding pressure ap-plied. For low pressures an additional recrystallized area with larger spherulites could be seen. Molten-film thickness, defined as the thickness of all zones be-tween both layers of deformed spherulites, decreased with an increase in weld-ing pressure, which was also reported by Lin [8].

Most investigations reported in literature have been performed on polymers or polymer matrix materials with micro-scale fillers, such as glass fiber reinforced Polyamides [9].

Few researches of vibration welded nanocomposites have been reported so far. Bates et al. [10] showed a reduction of tensile strength of PP, filled with 3 and 6 wt.-% of organoclay and vibration welded at 4 MPa, by approximately 30% and 50% compared to neat PP. Their investigation showed high orientation and ac-cumulation of nano-platelets within the weld. One of the reasons for the weak-ening effect of nanofillers is the interface between matrix and filler material, which could be improved by the incorporation of compatibilizer [11].

Latest works, such as [8], have provided further insight in the behavior of nano-particle filled composites. Investigations of the same material used as in this work, PP filled with nano-TiO2 with an aspect ratio of 3, showed small reinforc-ing effects for low filler contents. After welding, however, tensile properties in welding direction as well as impact strength along the welding plane showed a significant drop for all welding pressures and filler contents investigated. The tensile test results and part of the micro-tensile images of Lin [8] have also been used in this work to compare tensile and micro-tensile properties. In order to get further insight, this work addresses the morphology-dependent material proper-ties inside the weld by investigating local deformations as has been shown by Gehde et al. for hot-tool welded PP [12].

2 EXPERIMENTAL

2.1 Materials and nanocomposite preparation

Isotactic polypropylene (HD120MO, Borealis, Vienna, Austria) has been used for this investigation neat and filled with 1 vol.-% of nano-rutile particles with an aspect ratio of 3 (Hombitec RM130, Huntsman, Duisburg, Germany). Degrees of filler content higher than 1% have been shown to reduce impact strength due to agglomerization, while at 1% a maximum reinforcement effect of about 7% at tensile strengths almost independent of filler content [8]. During manufacturing, first a masterbatch of 25 wt.-% of TiO2 has been extruded in a co-rotating twin-screw extruder (TSK-N 030, Theysohn Extrusionstechnik GmbH, Salzgitter,

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Germany) at barrel temperatures of 190°C near the hopper and 210°C at the die and a screw speed of 160 rpm. The masterbatch has been dried for 4 hours at 80°C and then diluted to 1 vol.-% in a second extrusion step at the same conditions. Double-step extrusion has been performed as it provides a high dis-persion quality of nanoparticles [13].

After another drying step the nanocomposites and neat PP were then injection molded to sheets of a length and width of 50 mm each and a thickness of 4 mm in an injection molding machine (Arburg Allrounder 420 C, Arburg GmbH & Co.KG, Loßburg, Germany). In correlation to their filler volume content, neat PP is called PP-V0, while TiO2 filled PP is called PP-T-V1 in this work.

2.2 Vibration welding

Vibration welding was carried out on a Branson Ultraschall Lab. vibration weld-ing machine (M-112H, Branson Ultraschall, Dietzenbach, Germany). The gate was removed by sawing the plates beforehand and two plates were mounted in the vibration welding machine, surfaces opposite of the gate in contact. Vibra-tion welding was then carried out by pneumatic application of a normal force on the upper clamp, bringing both plates in contact, and a linear vibration of the lower clamp, causing friction between the plates and thus heating up to the melting point. Vibration was started and performed until a defined welding pene-tration was reached. For this investigation, an amplitude of 0.7 mm and welding pressures of 0.4 MPa and 2.0 MPa were chosen with a machine-set vibration-frequency of 240 Hz, respectively. Axial force, meltdown and amplitude were recorded as a function of time. According to Schlarb [1], welding penetrations were chosen to reach the third phase, the quasi-stationary melt generation, which is mandatory to guarantee best performance of the welded parts.

2.3 Sample preparation

First, welded parts were cut to a length of 50 mm with welding area in the cen-ter. 20 mm long, 20 mm wide and 4 mm thick samples were sawed off the welded part for microscopy and micro-tensile testing, while the remaining part was milled to tensile specimen according to DIN EN ISO 527-2/1BB as shown in Figure 1. Micro-tensile samples were then cut into thin slices of 20 µm thick-ness, using a microtome (HYRAX M25, Carl Zeiss, Göttingen, Germany). Spec-imens were fixed on paper frames to improve handling. Force-bearing thickness and width of the samples were then measured to calculate applied stresses.

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Figure 1: Scheme of tensile and micro-tensile testing sample extraction with x being the direction of amplitude, y the melt flow direction and z the di-rection of welding penetration

2.4 Microtensile tests and microscopy

Micro-tensile tests were performed on a micro-tensile test machine (Zugmodul 5kN, Kammrath & Weiss, Dortmund, Germany) equipped with a 10 N load measuring cell. 20 mm long and 4 mm wide – their thickness after injection molding - samples were used for investigations with a measuring length of 9 mm. The micro-tensile test machine was set up on a microscopy table with the sample fixed in horizontal direction and a polarization microscope (Nikon Eclipse LV100, Nikon GmbH, Düsseldorf, Germany) taking videos in vertical perspective in transmitting light mode as shown in Figure 2. Prior to testing the sides of the paper frame were cut in order to limit tensile load to the film.

2.5 Simulation

The model has been created in ANSYS based on geometry and knowledge of the investigations done in this work using 3-D volume element SOLID 186 with 20 mesh points per element. This element uses a shift law with the power of two and has three translational degrees of freedom per mesh point, and is used for viscoelastic, incompressible and hyperelastic materials [14]. Layer thickness has been set according to sizes measured in experiment. After a convergence analysis an edge length of 0.04 mm at maximum was chosen for the bulk mate-rial and finer elements inside the weld in order to define layer-specific proper-ties. The size of the model was reduced to the area inside and close around the welding area of the delta-shaped area and the area of constant molten-film thickness 1 mm away from the edge. The material was treated as isotropic-

z

y x

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multilinear in respect of the unidirectional tensile tests performed and of the vis-coelastic behavior of the polymer. As an input, tensile force measured in z-direction was used to calculate the entire stress state, which was then calculat-ed and shown as von-Mises-stresses.

Figure 2: Micro-tensile testing setup with transmitted light microscope, micro-tensile testing machine controller and PC (top) and samples fixated on paper (bottom)

3 RESULTS

3.1 Morphology

The morphology, as shown in Figure 3, shows big differences between varia-tions of welding pressure and filler content. Each of the samples shows the fol-lowing layers: bulk material (B), deformed spherulites (D), inner area (I) and the welding line (W).

Non-filled PP welded at 0.4 MPa, the lower performed welding pressure, shows the biggest molten-film thickness of 100 µm and developed a recrystallized area

≈ 9 mm

v v

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in addition to the other four areas. A thin recrystallized area (R) can be seen in welds of neat PP welded at 2.0 MPa and PP-T-V1 welded at 0.4 MPa with 84 and 72 µm molten-film thickness, while it is not visible for PP-T-V1 welded at 2.0 MPa (48 µm molten-film). The disappearance of recrystallized area and de-crease of molten-film thickness with an increase in welding pressure and with addition of nano-TiO2 is in agreement with Lin [15].

Figure 3: Morphology of PP-V0 and PP-T-V1 welded at 0.4 MPa and 2.0 MPa with marked layers and spherulites-transition angle α

Deformed spherulites show differences not only in their thickness, but also in their angle α between the transition of bulk material and deformed spherulites. The layer thickness of deformed spherulites is about 20 µm with an angle of 31° for neat PP welded at 0.4 MPa. An increase in welding pressure lowers the thickness significantly to 5 µm as well as the angle to 23°. At 0.4 MPa, incorpo-ration of nanoparticles leads to a slightly decreased layer thickness of 14 µm, while showing an even smaller angle of 20°. For 2.0 MPa the layer size of 2 µm was too small to evaluate the spherulite angle.

2

.0 M

Pa

0

.4 M

Pa

PP-V0 PP-T-V1

α B

D

R

I

W

50 µm

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3.2 Micro-tensile properties

Figure 4 shows the results of all tensile and micro-tensile tests. As widely acknowledged in literature (e.g. in [5]), measurements of PP welded at low pressure of 0.4 MPa show tensile strengths about equal to the bulk material while higher pressure, 2.0 MPa, leads to a decrease in tensile strength of about 11% in this case. Incorporation of nanoparticles reduces the tensile strength of the bulk material by about 4%. Welding nanocomposites results in a further weakening effect of 16% and 23% for low and high pressures, respectively. So the incorporation of nano-TiO2 increases the weakening effect of vibration weld-ing.

Figure 4: Ultimate tensile strength UTS of PP-V0 and PP-T-V1 before welding, after welding at 0.4 MPa and 2.0 MPa in macro-tensile test [8] and thin-film micro-tensile test

Micro-tensile tests, compared to tensile tests according to DIN EN ISO 527-2/1BB, show tensile strengths about 34% below the value of tensile tests, as the correlation in Figure 4 shows. The primary reason is the difference in geometry, as states of stress differ from flat to volume bodies. Furthermore, tensile tests were performed on 30 mm long specimens (gap length) at a test speed of 50 mm/min, whereas micro-tensile tests are performed on 9 mm long samples at only 1.2 mm/min. Due to the viscoelastic and thus time dependent behavior of polymers, lower elongation rates result in lower strengths, yet higher elonga-tions. Another reason is the degree of surface properties compared to specimen thickness, as defects, such as scratches and inhomogeneity of material and geometry, have a much higher relative impact on sample properties in thin specimens than they have in thick ones.

0

5

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35

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45

0 5 10 15 20 25 30 35 40 45

UT

S (

mic

ro-t

en

sil

e t

es

t),

MP

a

UTS (macro-tensile test), MPa

PP-V0

PP-T-V1

y = 0.6645x

UTSmacro tt, MPa

UTSmicro-tt, MPa

0/n 39.5 ± 2.5 29.1 ± 0.7

0/04 39.7 ± 0.5 27.5 ± 2.2

0/20 35 ± 0.3 19.5 ± 2.4

1/n 37.9 ± 0.7 27.3 ± 1.4

1/04 31.7 ± 1.7 21.3 ± 2.4

1/20 29.2 ± 0.7 15.5 ± 2.1

Legend: 0: PP-V0 1: PP-T-V1 n: non welded 04: welding pressure p = 0.4 MPa 20: welding pressure p = 2.0 MPa

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By using optical investigation of local deformation for given stresses, properties specific to the area of deformed spherulites and inner area can be separated. Stress-strain behavior is characteristic for every individual layer as shown in Figure 5 due to local tensile properties and local break development as can be seen in the top microscopy image. With microscope images, layer thickness can be determined at different times in order to calculate local strain by comparing length of a layer n ln at certain times t’ to original layer length as shown in equa-tion 1, which then can be correlated with micro-tensile measurement data.

Figure 5: Exemplary comparison of global and local stress-strain-behavior of PP-V0 welded at 0.4 MPa with primary break propagation in red. Lo-cal deformation was calculated based on layer thickness measure-ments in constant thickness area (1 mm away from the bead)

𝜀𝑛(𝑡′) =𝑙𝑛,𝑡′

𝑙𝑛,𝑡=0− 1 (1)

As individual stress-strain-curves of non-welded (bulk properties) and welded (full sample and layer-wise separated properties) samples in Figure 6 show, elongation at break of welded PP (red) is much lower than that of the bulk mate-rial (blue). In agreement with Schlarb [1] for non-filled material welded at a low welding pressure of 0.4 MPa (a) no impact on ultimate tensile strength of non-filled material is shown, whereas an increase in welding pressure to 2.0 MPa (c) results in a 34% reduction of ultimate tensile strength. Incorporation of nanopar-ticles (b and d) shows a very low elongation at break of about 8% compared to 35% in neat PP as well as an 8% reduction of ultimate tensile strength. Welding weakens the material even further, especially for high welding pressures. This has also been shown for bigger scale specimens by Lin [8].

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Te

ns

ile

str

es

s, M

Pa

Strain, %

global

local (D)

local (I+W)

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Figure 6: Average stress-strain-curves of all investigated combinations

b)

a)

c)

d)

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Results of welded samples in comparison with those of non-welded samples are shown in Table 1. The expectation of filled PP having a higher Young’s Modulus can be carefully confirmed by the measuring results showing slightly lower results, but big standard deviations. Both, non-filled and filled material, show an overall and local Young’s modulus below the bulk material after weld-ing, especially within the area of deformed spherulites. Young’s Modulus also shows a similar impact of geometry as has been found for ultimate tensile strength with a decrease of 40% compared to tensile samples with a modulus of about 1500 MPa. For small deformations, Young’s modulus can be calculat-ed by global change of length being the sum of local changes and solved for Young’s Modulus as shown in equation 2 with l being the layers length fraction of the entire model for each zone and stress being equal in all zones. As length of the weld layers compared to total sample test length is below 2%, deviations between global and bulk properties are dominating, leaving the method too in-sensitive to verify layer properties.

𝐸𝑡𝑜𝑡𝑎𝑙 = 1

𝑙𝐵/𝐸𝐵+ 𝑙𝐷/𝐸𝐷+ 𝑙𝐼+𝑊/𝐸𝐼+𝑊 (2)

Bulk material

B

Deformed spherulites

D

Inner area and weld line

I+W

PP-V0/0.4 MPa σU, MPa 29.1 ± 0.6 26.8 ± 2.5 27.6 ± 0.8

P, % 44 12 44

E, MPa 899 ± 160 236 ± 59 683 ± 324

PP-T-V1/0.4 MPa σU, MPa 26.6 ± 1.4 19.7 ± 2.6 20.4 ± 2.0

P, % 29 6.5 64.5

E, MPa 849 ± 248 350 ± 138 425 ± 32

PP-V0/2.0 MPa σU, MPa 29.1 ± 0.6 18.2 ± 2.2 20.1 ± 0.1

P, % 0 57 43

E, MPa 899 ± 160 629 ± 96 402 ± 62

PP-T-V1/2.0 MPa σU, MPa 26.6 ± 1.4 15.1 ± 2.0 15.5 ± 2.0

P, % 0 34 66

E, MPa 849 ± 248 573 ± 159 573 ± 159

Table 1: Ultimate tensile strength (σU), portion of initial crack development (P) and Young’s modulus (E) per layer in experiment

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Ultimate tensile strength of all weld areas has been taken from the von Mises stress results of simulation by reading maximum stresses at yield conditions as shown later, while bulk material ultimate tensile stress was taken from bulk sample test directly. Neat PP welded at 0.4 MPa shows ultimate tensile stress-es within a very small range as expected by the statistical distribution of break positions. All other welds have ultimate tensile strengths way below the bulk material with deformed spherulites showing lower values than inner zone and weld line.

Measured local elongation at break is particularly high for both, deformed spherulites and inner area/weld line as local cracks occur for high stresses and increase the apparent elongation measured (as shown in Figure 5). Due to the small layer thickness, deformed spherulites could not be investigated separately for PP-T-V1 welded at 2.0 MPa and were given properties equal to the inner area for simulation. At 2.0 MPa welded PP-V0 has a modulus of deformed spherulites higher than the bulk material, as the evaluation of the small layers is very sensitive regarding their thickness measured and provides big standard deviations.

A correlation of different layer properties and layer geometries is shown in Fig-ure 7. As has also been reported in [8] before, a) shows an increase in ultimate tensile strength with increasing molten-film thickness, as it is seems to improve the stress distribution of the inner layer in favor of overall higher tensile strength. With the exception of PP-V0 welded at 2.0 MPa, which breaks primari-ly in deformed spherulites, ultimate tensile strength global and local inside the inner area are approximately equal.

For neat PP ultimate tensile strength increases from 20.9 to 27.7 MPa by 32.5%, for filled PP it increases from 15.5 to 21.0 MPa by 35.4%. In [8] an in-crease of tensile strength in tensile tests of 35.0 to 39.7 MPa (13.4%) for neat PP and of 29.2 to 31.7 MPa (8.6%) for PP-T-V1 has been reported. Impact strength of the same materials shows a much higher sensitivity to molten-film thickness, as it rises from 16.9 to 27.0 kJ/m² (59.7%) in neat PP and from 9.6 to 17.9 kJ/m² (86.5%) in PP-T-V1 as reported in [16]. While the overall Young’s modulus shows no clear correlation, deformed spherulites have a lower Young’s modulus for higher deformation angles.

Figure 7 b) shows an increase in ultimate tensile strength and, inside the layer of deformed spherulites, a decrease of Young’s modulus with increasing angle of deformed spherulites. The increase of tensile strength can be explained by a less weakened molecular interphase at lower shear induced deformation and less orientation of filler particles. A similar result was reported for impact strength with a much higher sensitivity [8]. A decrease of modulus is in contrary to the expectation of a reinforcing effect of molecules oriented in tensile direc-tion. As little data is provided, this effect will be investigated further in future works.

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Figure 7: Correlation of global and inner areas ultimate tensile strength with inner layer thickness (a) and deformed spherulite layers ultimate ten-sile strength and Young’s modulus with deformed spherulites angle α (b)

Crack investigations during micro-tensile tests also give an additional insight to the deformation behavior. Table 1 also shows the portions of the samples inves-tigated under the same conditions breaking in particular layers.

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0 20 40 60 80 100 120

Ult

ima

te t

en

sil

e s

tren

gth

, M

Pa

Molten-film thickness, µm

global

local (I+W)

a) PP-V0 PP-V02.0 MPa 0.4 MPa

2.0 MPa 0.4 MPaPP-T-V1 PP-T-V1

0

200

400

600

800

1000

0

10

20

30

40

0 10 20 30 40 50

Yo

un

g's

mo

du

lus

, M

Pa

Ult

ima

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, M

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α, °

PP-V0 PP-V02.0 MPa 0.4 MPa

PP-T-V10.4 MPa

local (D)

b)

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PP-V0 welded with small welding pressures, as can be seen in Figure 8, shows development of micro-cracks along the entire specimen, inside and outside the welding area, and fractures statistically at any of these points. In the sample shown, break occurs inside the bulk material outside the welding area, while crack propagation often starts at the transition between bulk material and weld-ing bead at a local stress maximum. High welding pressures, however, result in the development of most micro-cracks and final break inside the welding area. This development of micro-cracks is also responsible for characteristic stress-strain curves as shown in Figure 5 before, leading to higher local elongations.

Figure 8: Crack propagation of welding area without load, during load and after failure: PP-V0 welded at 0.4 MPa and PP-T-V1 at 2.0 MPa welding pressure (markers indicate crack development) [8]

The cracks always appear in direction perpendicular to the direction of welding penetration and load. This is in agreement with the investigations of Schlarb [5], showing the primary orientation of molecules in direction perpendicular to the welding direction due to elongation and shear induced orientation during melt flow, as the bonding between oriented molecules is weaker in direction perpen-dicular to their orientation. Filler particles, which also show similar orientation effects [8], increase this weakening effect. Inside the layer of the deformed spherulites micro-cracks appear parallel to the alignment of lamellas as it can be seen in Figure 5.

PP

-T-V

1, 2

.0 M

Pa

P

P-V

0,

0.4

MP

a

prior to load during load at break

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3.3 Simulation

Based on experimental data a 3D-model as shown in Figure 9 was created. The model contains the layers of bulk material, deformed spherulites as well as the inner area. Recrystallized area, due to its occurrence only in one of the welds investigated, has been treated as part of the inner area. Due to the limited reso-lution in light microscopy the welding line has not been given individual material properties and will be addressed in future works. As the molten-film thickness along the center of the specimen is constant, simulation of the delta-shaped weld at the edge provides more information.

The model was used to calculate the stress distribution along the geometry. Von-Mises stresses were calculated to resemble total stresses created within the specimen as transverse deformation leads to stresses in x- and y-direction, as load was applied in z-direction. Local stresses in simulation were then com-pared with experimental results of tensile properties of certain layers in order to predict the initial position of failure once global strain at break was reached. The calculated stress distribution at strain at break is shown for PP welded at 0.4 MPa in Figure 11 with color and scaling set to represent maximum stresses for each layer. Stress distribution for all other parameters was achieved according-ly.

Stresses in z-direction calculated were compared with the force-input in z-direction to confirm stresses are calculated properly, as total force has to be equal in every cut in the x-y-plane.

Local von Mises stresses were then compared to ultimate tensile stresses of each area (Table 1) as shown in Figure 11 by correlating

𝜎𝑙𝑜𝑐𝑎𝑙,𝑙𝑎𝑦𝑒𝑟

𝜎𝑈,𝑙𝑎𝑦𝑒𝑟

Therefore, red sections show spots, at which the local ultimate tensile strength was reached within a 1% gap, matching the break condition within standard de-viation. While absolute values show the highest stresses at the transition of bulk material and weld bead, Figure 11 shows this stress only being critical for mate-rial welded at low pressures and for neat PP in particular. Only in neat PP weld-ed at 0.4 MPa ultimate tensile strength was also reached by local stresses in the bulk layer and outside bulk-bead-transition. For parts welded at high pres-sure, local stresses do not exceed 90% at the transition and 80% in the remain-ing part of the bulk. Thus the probability of material failure inside the bulk mate-rial can be confirmed not to happen for high, and to happen for low welding pressure. Stresses close to ultimate tensile strength occur primarily along the layer transition between deformed spherulites and inner zone as well as in the center of the inner zone.

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Figure 9: Layer-wise model of the weld with (B) bulk material, (D) deformed spherulites, (I) inner area and (W) weld line

0.5 mm

4.0 mm

0.5 mm

0.44 mm

I I

B B D D W

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Figure 10: Von Mises stress distribution at global strain at break simulated for micro-tensile tests performed on PP-V0 welded at 0.4 MPa with stress-scale according to layer-wise ultimate tensile strength

Figure 11: Percentage of local von Mises stresses to layers ultimate tensile strength at global strain at break simulated for micro-tensile tests performed on PP-V0 welded at 0.4 MPa (a), PP-V0 welded at 2.0 MPa (b), PP-T-V1 welded at 0.4 MPa (c) and PP-T-V1 welded at 2.0 MPa (d)

a) b)

c) d)

0-1% 1-50% 50-80% 80-90% 90-95% 95-98% 98-99% 99-100%

σ / σU, layer

15.0 23.0 26.1 26.2 27.4 27.6 28.0 29.1

σ, MPa

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4 CONCLUSIONS

Micro-tensile properties of PP and PP-TiO2-nanocomposite thin-films prior and after vibration welding were investigated and compared to tensile testing re-sults. Equivalent weakening effects could be observed dependent on the pro-cessing conditions: neat PP vibration welded at low welding pressure showed strength similar to the bulk material, higher pressure and incorporation of nano-particles lower the strength. Due to different testing speeds and different sizes leading to differences in stress distribution and stronger scaling of defects, strengths reported were about 30% lower while showing higher elongations at break.

Tensile properties also can be correlated to their morphology and layer geome-try, as micro-tensile investigations show the same positive effect of increasing layer thickness on tensile properties as has been reported for tensile tests by Lin [8]. An impact of the layer of deformed spherulites was shown, but requires further investigation.

Local deformations and tensile force applied to an ANSYS-model built in re-spect to light microscope images were used to create a map of local stresses and therefore local limits of ultimate tensile strength. A comparison of stresses with measured bulk properties predicted primary positions of break successfully.

The match of the simulation with experimental results also allows using the model for further studies, such as an optimization of geometry by calculating stress distribution for different geometrical models in order to reduce or distrib-ute stresses. Combining the geometric model with individual morphology based layer properties, induced by processing conditions like melt flow and cooling, the model can help improving the design of welds and weld properties in order to exploit the mechanical properties of filled thermoplastics in welded compo-nents.

5 ACKNOWLEDGEMENTS

The authors thank the German Research Foundation (DFG) for the financial support according to the DFG-project SCHL 280/19-1. The authors are grateful to Mr. V. Demchuk, Polymer Engineering Bayreuth, for the helpful cooperation. They also gratefully acknowledge Borealis Group and Huntsman for the kind donation of the materials used.

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Literature

[1] Schlarb A.K. Ehrenstein G.W.

Parameterwahl beim Vibrationsschweißen von Po-lypropylen

Plastverarbeiter 40 (1989) 4, S. 110-115

[2] Schlarb A.K. Ehrenstein G.W.

The impact strength of butt welded Vibration welds related to the microstructure and welding history

Polymer engineering and science 29 (1989), S. 677-682; DOI: 10.1002/pen.760292309

[3] Varga J. Ehrenstein G.W. Schlarb A.K.

Vibration welding of alpha and beta isotactic poly-propylenes: mechanical properties and structure

eXPRESS Polymer Letters 2 (2008), S. 148-156

DOI: 10.3144/expresspolymlett.2008.20

[4] Stokes V.K. Vibration welding of thermoplastics, part IV: Strengths of Poly-Butylene Terephthalate, Polyeth-erimide and Modified Polyprophylene Oxide Butt Welds

Polymer Engineering and Science 28 (1988), S. 998-1008; DOI: 10.1002/pen.760281509

[5] Schlarb A.K. Zum Vibrationsschweißen von Polymerwerkstoffen

Dissertation, Universität Kassel (1989)

[6] Ehrenstein G.W. Künkel R. Gehde M.

Bemessungskennwerte für die Verbindungsausle-gung und werkstoff-/prozessabhängige Nahteigen-schaften beim Vibrationsschweißen verstärkter Thermoplaste

Abschlussbericht: AiF 13.512 N/DVS 11.003 (2005)

[7] Lin L.Y. Schlarb A.K.

Vibration welding of polypropylene-based nanocom-posites – The crucial stage for the weld quality

Composites: Part B 68 (2015), S. 193-199

DOI: 10.1016/j.compositesb.2014.08.052

[8] Lin L.Y. Processing Controlled Properties of Thermoplastic-based Nanocomposites

Dissertation, Technische Universität Kaiserslautern (2013)

[9] Bates P.J. Macdonald J.J. Wang C.Y. et al.

Vibration Welding Nylon 66 – Part I Experimental Study

Journal of Thermoplastic Composite Materials 16 (2003), S. 101-119

DOI: 10.1177/0892705703016002869

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- un

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http://onlinelibrary.wiley.com/doi/10.1002/pen.760292309/abstract

http://www.expresspolymlett.com/articles/EPL-0000572_article.pdf


http://www.sciencedirect.com/science/article/pii/S1359836814003904

http://jtc.sagepub.com/content/16/2/101



[10] Bates J. Braybrook T. Kisway T. et al.

Vibration Weld Strength of Polypropylene-Based Nanocomposites

Journal of Thermoplastic Composite Materials 20 (2007), S. 5-16; DOI: 10.1177/0892705707067919

[11] Das C.K. Rajasekar R. Friedrich S. Gehde M.

Effect of nanoclay on vibration welding of LLDPE nanocomposites in presence and absence of com-patibilizer

Science and Technology of Welding & Joining 16 (2011), S. 199-203

DOI: 10.1179/1362171810Y.0000000017

[12] Gehde M. Bevan L. Ehrenstein G.W.

Analysis of the Deformation of Polypropylene Hot-Tool Butt Welds

Polymer Engineering and Science 32,9 (1992), S. 586-592; DOI: 10.1002/pen.760320904

[13] Schlarb A.K. An approach for successful compounding of nano-composites in large-scale production

Masterbatch Asia, Singapore, 26th of March 2012

[14] Ensslen F. Zum Tragverhalten von Verbund-Sicherheitsglas un-ter Berücksichtigung der Alterung der Polyvinylbuty-ral-Folie.

Dissertation, Ruhr-Universität Bochum (2005)


Processing controlled properties of vibration welded thermoplastic-based nanocomposites

Journal of Plastics Technology 9,3 (2013), S. 130-137


Vibration Welding of Nano-TiO2 Filled Polypropylene

Polymer engineering and science 55 (2015) 2, S. 243-250; DOI: 10.1002/pen.23891

Bibliography DOI 10.3139/O999.03032016 Zeitschrift Kunststofftechnik / Journal of Plastics Technology 12 (2016) 3; page 184–204 © Carl Hanser Verlag GmbH & Co. KG ISSN 1864 – 2217

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http://jtc.sagepub.com/content/20/1/5

http://www.tandfonline.com/doi/abs/10.1179/1362171810Y.0000000017?journalCode=ystw20





Keywords: vibration welding, micro-tensile, nanocomposites, simulation

Stichworte: Vibrationsschweißen, Mikrozugversuch, Nanokomposit, Simulation

Autor / author:

Michael Albrecht, M.Sc. Dr.-Ing. Leyu Lin Prof. Dr.-Ing. Alois K. Schlarb Lehrstuhl für Verbundwerkstoffe Technische Universität Kaiserslautern Gottlieb-Daimler-Straße, Gebäude 44 67663 Kaiserslautern

E-Mail: [email protected] Webseite: www.mv.uni-kl.de/cce/home Tel.: +49 (0)631/205-5116 Fax: +49 (0)631/205-5141

Herausgeber / Editors:

Editor-in-Chief Prof. em. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein Lehrstuhl für Kunststofftechnik Universität Erlangen-Nürnberg Am Weichselgarten 9 91058 Erlangen Deutschland Tel.: +49 (0)9131/85 - 29703 Fax: +49 (0)9131/85 - 29709 E-Mail: [email protected] Europa / Europe Prof. Dr.-Ing. Dietmar Drummer, responsible Lehrstuhl für Kunststofftechnik Universität Erlangen-Nürnberg Am Weichselgarten 9 91058 Erlangen Deutschland Tel.: +49 (0)9131/85 - 29700 Fax: +49 (0)9131/85 - 29709 E-Mail: [email protected]

Amerika / The Americas Prof. Prof. hon. Dr. Tim A. Osswald, responsible Polymer Engineering Center, Director University of Wisconsin-Madison 1513 University Avenue Madison, WI 53706 USA Tel.: +1 608/263 9538 Fax: +1 608/265 2316 E-Mail: [email protected]

Verlag / Publisher:

Carl-Hanser-Verlag GmbH & Co. KG Wolfgang Beisler Geschäftsführer Kolbergerstraße 22 D-81679 München Tel.: +49 (0)89/99830-0 Fax: +49 (0)89/98480-9 E-Mail: [email protected]

Redaktion / Editorial Office:

Dr.-Ing. Eva Bittmann Christopher Fischer, M.Sc. E-Mail: [email protected] Beirat / Advisory Board:

38 experts from science and industry listed at www.plasticseng.com

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