Zeitschrift Kunststofftechnik Journal of Plastics Technology

Zeitschrift Kunststofftechnik Journal of Plastics Technology eingereicht/handed in: 15.05.2013 angenommen/accepted: 21.10.2013 Prof. Dr.-Ing. Hans-Peter Heim 1, Prof. Dr.-Ing. Dipl.-Wirt.Ing. Wolfgang Tillmann 2, Dipl.- Ing. Angela Ries 1, Dipl.-Ing. Norman Sievers 2, Dipl.-Ing. Björn Rohde 1, Dr.-Ing. Dipl.- Phys. Reiner Zielke 2 1 Institute of Materials Engineering, University of Kassel, Kassel, Germany 2 Institute of Materials Engineering, Technical University of Dortmund, Dortmund, Germany Visualisation of the degrees of compaction of self-reinforced polypropylene composites by means of ultrasonic testing Self-reinforced fibre composites are solely based on the bonding of matrix and fibre identical thermoplastics and hence do not rely on fibre reinforcements of a foreign material. The self-reinforcement can be transferred to hot-compacted fibre composites by means of stretched textile semifinished products. The two processing conditions temperature and process time, in particular, have a significant influence on the macromolecular orientation and the degree of compaction. Within the framework of these investigations, the effectiveness of the degrees of compaction achieved this way are characterised and discussed on the basis of both non-destructive ultrasonic testing of selected fibre composites and additional quasi-statistically determined tensile properties. Visualisierung der Kompaktiergrade von eigenverstärkten Polypropylen-Verbunden mittels Ultraschallprüfung Wissenschaftlicher Arbeitskreis der Universitäts- Professoren der Kunststofftechnik archivierte, peer-rezensierte Internetzeitschrift des Wissenschaftlichen Arbeitskreises Kunststofftechnik (WAK) archival, peer-reviewed online Journal of the Scientific Alliance of Polymer Technology www.kunststofftech.com; www.plasticseng.com Eigenverstärkte Faserverbunde basieren einzig auf der Verbindung von matrix- und faseridentischen Thermoplasten und greifen somit auf keine materialfremde Faserverstärkung zurück. Die Eigenverstärkung lässt sich über einen Zwischenschritt in Form von verstreckten Halbzeugtextilien auf die heißkompaktierten Faserverbunde übertragen. Insbesondere die beiden Verarbeitungsbedingungen Prozesstemperatur und zeit üben dabei einen starken Einfluss auf die makromolekulare Orientierung und den Kompaktiergrad aus. Im Rahmen dieser Untersuchungen wird die Wirksamkeit der erreichten Kompaktiergrade anhand der zerstörungsfreien Ultraschalluntersuchung und ergänzenden quasistatisch ermittelten Zugeigenschaften an ausgewählten Verbunden charakterisiert und diskutiert. Carl Hanser Verlag Zeitschrift Kunststofftechnik / Journal of Plastics Technology 9 (2013) 6

Visualisation of the degrees of compaction of self-reinforced polypropylene composites by means of ultrasonic testing H.-P. Heim, W. Tillmann, A. Ries, N. Sievers, B. Rohde, R. Zielke 1 INTRODUCTION 1.1 Structure and potential of self-reinforced polypropylene composites Conventional thermoplastic fibre composites consist of a relatively ductile matrix such as polypropylene, polyethylene or polyamide, in which reinforcing fibres, such as glass, carbon, aramid or natural fibres are embedded for load transmission. Self-reinforced polypropylene composites (SRPP) contain highly-stretched thermoplastic reinforcing fibres in the form of tape or filament structures, which in turn, were integrated in an identical thermoplastic matrix. Although no foreignmaterial reinforcement is employed in these Single Polymer Composites (SPC) [1], it is still possible to modify the specified composite properties in order to meet the specific requirements of different stretching or orientation degrees of the reinforcing fibres, the fibre content, the layer configuration and the number of layers [2]. Figure 1 shows a conventional SRPP consisting of 16 fabric layers. The self-reinforcement, however, can only be directly transferred to the fibre composite materials with difficulties, which means that an intermediate step, using thermoplastic textile semi-finished products, incorporating highly-oriented fibres or tapes, are required. These special textile semi-finished products are commercially available and mainly produced on a polypropylene basis. They generally comprise of fairly thick tape structures and are processed into woven fabrics or non-woven fabrics. Alternatively, ultra-fine, highly-oriented fibre filaments as a mat or as filament yarns in woven fabrics or non-woven fabrics (fleeces) can be used as well. [3]. The self-reinforcement can be introduced selectively through molten-phases and solid-phase deformations and is mainly based on the generation of orientated crystalline superstructures as well as on the orientation of macromolecule chains [4, 5]. After hot-compaction to composites, these textures are still visible [6], cf. figure 1. Zeitschrift Kunststofftechnik 9 (2013) 6 276

Figure 1: Self-reinforced polypropylene composite left: right: Hot-compacted composite (consisting of 16 layers) with a thickness of 2 mm Confocal laser-light image of a self-reinforced co-extruded pure composite structure with tapes in machine direction (MD) and cross direction (CD) [7] 1.2 Hot-compaction of SRPP To manufacture self-reinforced thermoplastic composites, polypropylene-based fabrics or fleeces are layered and hot-compacted to form composites by means of thermal and pressure coupling. The term "hot-compaction" exclusively refers to the consolidation of self-reinforced thermoplastic composites, with reinforcing fibres and a matrix phase only made of one single type of polymer [8]. In this way, composites are produced on the basis of oriented, thermoplastic fibres without the need to employ classical processes of resin impregnation [8]. A consolidation process has to be performed under thermo-mechanically coupled processing conditions in order to embed the orientated fibre and tape structures in a matrix of identical material by means of selective incipient melting [3]. The previously layered textile semi-finished products are interlaminary bonded, turning them into an integral composite in the classical sense [9]. Nevertheless, a severe thermal stress or temperatures which exceed significantly the actual melting point to long will trigger relaxation processes in the reinforcing fibres that will irreversibly weaken the macromolecular orientation or even eliminate it [10]. The self-reinforcement will then be reduced, having a direct impact on the mechanical properties of the layer composites and, combined with a high level of thermal stresses; the composites can even adopt the structure of a non-reinforced compact material. 1.3 Motivation Structurally, the self-reinforced polypropylene composites possess a particularly high sensitivity to thermo-mechanically coupled processing conditions. Already minor changes in the temperature and pressure control, or an increase in the Zeitschrift Kunststofftechnik 9 (2013) 6 277 MD CD

processing time, lead to detectable differences of the composite properties. Triggered relaxation processes influence the self-reinforcement, leading to significantly impaired characteristic values. This result does not necessarily have to be regarded as a disadvantage. The local insertion of different temperature and pressure conditions can be utilised to realise a functional grading. In this way, the property distributions in the laminate can be aligned to match their latter functions in the component without using mixed material compounds or including additional downstream process stages [11]. The main aim of this study is to produce thermo-mechanically graded components with local property characteristics that result from according local changes of the microstructure [12]. Non-destructive testing methods are especially effective to prove the gradation as well as the gradation quality to examine the local compacting ratio. Commonly used methods such as the destructive tensile strength test or the measurement of the impact behaviour cannot provide very detailed information about the graded characteristics of such components [13], however, they are suitable for an excessive assessment of the composite properties [14-16]. A testing method is needed that visualizes the material behaviour precisely. In this study, the authors applied the ultrasonic testing method on SRPP in order to visualize the compaction degree of a total of three settings. The ultrasonic testing, conducted within the scope of these investigations, allow to draw conclusions on the influence of settings respecttively process conditions on the composite properties, which can be correlated by means of mechanical testing methods, which were conducted as well. Compared to the acoustic emission testing, which is a passive method that needs an acoustic event such as a fracture in the observed material during the measurement [17, 18], the active ultrasonic testing allows analysing the volume of the entire component. Therefore, three different homogenous composite series were produced with a highly dissimilar degree of compaction (low, medium, high) and three different base materials were selected. In this regard, the specimens were further characterised regarding their mechanical tensile modulus to comprehend the results of the non-destructive ultrasonic testing. 2 EXPERIMENTAL 2.1 Textile semi-finished products employed In order to examine the different mechanisms of the textile semi-finished products on the subsequent properties of the composite, this paper introduces three different textile systems out of two different material classes. Therefore, it is necessary to distinguish between mono-extrudates and co-extrudates in the case of thermoplastic textile semi-finished products that are used to produce self-reinforced polypropylene composites: Zeitschrift Kunststofftechnik 9 (2013) 6 278

As mono-extrudates consist of an identical polymer, they display an inherently homogeneous structure. One disadvantage of this cost-effective textile system is the highly temperature-sensitive processing operation involved. On the one hand, a sufficiently large quantity of melt is required to generate the necessary interlaminar adhesion and, on the other hand, it is essential to prevent any loss of self-reinforcement or any reduction of the mechanical properties of the composite due to high temperatures. In order to ensure a temperature-reduced processing with mono-extrudate types so that the self-reinforcement is maintained, they can be combined with low-melting matrix additives in the form of powder, film or melt on an alternating basis [19, 20]. For this investigation, a mono-extruded standard woven tape SG 30/30 (Bonar Technical Fabrics) was utilized. As a further textile variant, a mono-extruded needle-punched staple fibre fleece with filament diameters of approx. 20 µm with the type designation SNW 17, (Bonar Technical Fabrics) was used as well. With mono-extruded textile semi-finished products SG 30/30 and SNW 17, the matrix is, as already mentioned above, controlled by the continuous fusing of the tapes and fibres. On the contrary, co-extrudates possess the following multilayer structure: The inner highly-oriented core area with a higher melting point is enclosed by two surface layers which, as a result of prior melting, generate the matrix phase. A temperature difference of at least 10 C is frequently achieved between the core layer and the outer layer in this process. The ratio between the surface (S) and core (C) is S/C/S = 5/90/5 [3]. Hence, the highly-stretched core areas remain more or less thermally unchanged during hot-compaction. Thus, recrystallization and relaxation processes are only triggered to a limited extent, and the advantageous mechanical properties can be retained beyond the processing operation. On this account, the compacting behaviour of co-extrudates is in general different from the compacting behaviour of the mono-extrudated textile types. Within the framework of this investigation a co-extruded PP woven tape with the trade name Pure of the Lankhorst Company was utilized. In both cases, the thermoplastic textile semi-finished products are layered and consolidated into SRPP by means of thermal and pressure coupling. All textile semi-finished products are thermoplastics and are based on polypropylene. The classification of the textile semi-finished products employed, together with their layer numbers and a comparison of the basic textile properties, are listed in table 1. Zeitschrift Kunststofftechnik 9 (2013) 6 279

Type SG 30/30 SNW 17 Pure Table 1: Class Monoextruded PP woven tape 2/1 twill weave Needlepunched PP staple fibre mat, monoextrudate Co-extruded PP woven tape, plain weave Tensile strength of tape (longitudinal /transverse) Tensile strength (longitudinal direction) Tensile strength (transverse direction) Density Weight per unit area No. of layers [N/mm²] [kn/m] [kn/m] [g/cm³] [g/m²] [-] DIN EN ISO DIN EN ISO DIN EN ISO DIN EN - - 527 10319 10319 ISO 1183 30 30 0.909 124 16 9.8 10.5 0.909 150 12 500 0.760 105 16 Classification and basic properties of used textile semi-finished products 2.2 Process stages of hot-compaction The fully automated hot-compaction process essentially comprises of two stages: hot-compaction and cooling. Tailored textile layers are first layered in a clamping frame and fixed to prevent any expansion. The fixing is necessary in order to avoid temperature-related shrinking processes in the stretched textiles. The melting and compaction of the textile stack for the formation of a homogeneous layer composite is performed on an 800 kn laboratory press (LAP80, Gottfried Joos Maschinenfabrik GmbH & CO. KG) under the influence of heat and pressure. During this step, the stacked textile layers are consolidated for the specific pressing time in the pre-heated sheet mould, with a basic dimension of 500x500 mm², applying a compaction pressure of 2 MPa. In order to fix the smooth sheet geometry, the subsequent cooling sequence is carried out isochorically with a mean cooling rate of 40 C/min. After the automatic opening of the press unit, the clamping frame can be removed and the cooled laminate can be remoulded. Figure 3 shows exemplary pressure and temperature profiles during the production of a high compaction ratio (setting III). Regardless of the textile type employed, uniform laminate thicknesses of approx. 2 mm are achieved by the simple adaptation of the number of layers employed. Zeitschrift Kunststofftechnik 9 (2013) 6 280

Figure 3: Schematic diagram of the process parameters for producing a highly compacted SRPP (setting III: 185 C/180s/2MPa) It is the aim of this investigation to illustrate not only the different textile systems but also the entire spectrum of compaction ratios and to evaluate them with respect to their influence on the mechanics of the material. Three distinctive processing series were selected to this end from the full range of composites produced. They cover the different degrees of compaction from low (setting I) via medium (setting II) up to a high level of compaction (setting III) for the interlaminar bonding state. The depiction of extreme layer-composite conditions, such as a complete melting of the textile structure, or a just sufficient melting of the still flaccid textile semi-finished product were not in the centre of attention. Instead, optically uniform layer composites displaying virtually identical laminate thicknesses while still showing differences in their mechanical properties were selected. The ideal, selected processing parameters (settings) are given in [3], ensuring that the pressing temperature and time were increased step by step (table 2) in order to obtain the settings for different degrees of compaction. Table 2: Setting Degree of Temperature Time Pressure compaction [ C] [s] [MPa] I Low 175 60 2 II Medium 180 120 2 III High 185 180 2 Processing parameters with varying pressing temperatures and times for the hot-compaction of self-reinforced polypropylene composites, displaying different compaction ratios Zeitschrift Kunststofftechnik 9 (2013) 6 281

2.3 Ultrasonic testing Ultrasonic testing is a non-destructive method to analyse components with acoustic waves [21]. Thus, this testing method is particularly suitable to detect volume defects such as pores and cracks. But it is also possible to examine the material properties of polypropylene polymers on the basis of appropriate evaluation methods [22]. Hence, the ultrasonic testing method was selected for the visualisation of the material properties of self-reinforced polypropylene composites. Due to a high positioning accuracy and a highly reproducible sound coupling, the examination was conducted using the immersion technique. The experimental setup is visualised in figure 4. Sensor Composite Sample holder Figure 4: Experimental setup of ultrasonic testing in immersion technique used for SRPP in this study The focusing sensor (IAP-F50) works with a frequency of 50 MHz and has a focus diameter of 0.3 mm. The generated ultrasonic impulse is transferred to the composite using water as a coupling agent. During the test in a water basin, the composite is fixed in a sample holder to prevent floating. Fixing the material further ensures that there is only water and no solid substrate behind the composite, since the surface echo of the background would overpower the back wall echo of the material sample. The main challenge within this process is to define uniform measuring parameters. These are essential in order to be able to compare all material samples with respect to both the set degrees of compaction and the material systems used. Depending on the degree of compaction and the material type, the ultrasonic waves are scattered or reflected. For this, a comparison of the A- Scan of a material specimen with a high level of compaction (left) and a material specimen with a low level of compaction (right) is shown in figure 5. In an A- Scan, the amplitude of the acoustic wave is registered by a sensor head during the ultrasonic test and is plotted over the transit time of the acoustic wave. Zeitschrift Kunststofftechnik 9 (2013) 6 282

Amplitude [%] Surface Back wall Gate High compaction (SNW 17-III) Transit time of the acoustic wave [ms] Surface Gate Low compaction (SG 30/30-I) Noise due to an inhomogeneous material structure Transit time of the acoustic wave [ms] Figure 5: Sample A-Scans of the ultrasonic test with differing levels of acoustic wave absorption due to the varying compaction degrees of PP composites The acoustic wave is coupled into the material at the surface. Due to the high reflection factor at the interface between the water and the sample, the A-Scan initially shows the overmodulated amplitude of the surface echo. Depending on the degree of compaction, a homogenisation of the material composite takes place so that the ultrasonic wave is scattered to a different extent. In the case of high compaction, the echo of the back wall can be clearly identified in the A- Scan (left), while, with low compaction, the acoustic wave is weakened or scattered by the inhomogeneous material structure to such an extent that the back wall can no longer be identified (right). The tapes or fleece filaments that have not yet been dissolved by hotcompaction are inside the noise. The resulting multiple reflections complicate the delimitation of the individual interfaces. Hence, utilizing the amplitude of the back wall echo is suitable to visualise the degree of compaction. In preceding researches, it was also proven that the acoustic velocity is highly dependent on the degree of compaction [23]. However, in order to compare different degrees of compaction with each other in the amplitude image, the sound velocity was specified at the average value of 2800 m/s for this investigation. The position of the gate at a depth of 1.5 to 2.0 mm ensured that the amplitude of the back wall echo would be detected in any case. For each material series and processing batch, the ultrasonic tests are conducted on test specimens that were removed from a central point in order to be able to determine the thermal grading obtained with the process parameters. The sensor scans an area of 50x10 mm² and registers the highest amplitude within the gate in the area of the back wall for each testing position. The increment of 0.15 mm is lower than the focus diameter of the sensor head, ensuring that the entire area is covered. The 22,000 measured values for the amplitude in the region of the back wall are visualised in C-Scans. In a C-Scan, the highest amplitude within the gate is registered for each testing position and displayed in a pixel as a grey-scale value. Scanning the entire testing area, this Zeitschrift Kunststofftechnik 9 (2013) 6 283

gives a grey-scale image of the specimen and provides an overview of the achieved compaction ratio, figure 6. I II III Pure SG 30/30 SNW 17 50 mm 0% Amplitude 100% low Compaction ratio high Figure 6: Ultrasonic C-Scans to visualise the process-dependent (I-III) degrees of compaction by evaluating the back wall amplitude for the Pure SG 30/30 and SNW 17 composites The higher the amplitude, or the darker a pixel in the C-Scan, the higher the attained compaction ratio of the material, since the ultrasonic wave is weakened to a lesser extent in front of the back wall than in the case of a low degree of compaction (light pixels), figure 6. The varying process setting I to III were compared for each composite. The scans prove that there is no homogeneous material property distribution over the individual test surfaces to generate a uniform ultrasonic image. Instead, the high variation in the registered amplitudes indicates that the homogenisation of the material in the individual test areas, due to hot-compaction, varies to a great extent. Whereas for the coextruded tape fabric-based composites (made of Pure) scarcely any sound can be registered at the depth of the back wall for I and II, while a clear shift in the amplitude is detected for III. In this processing stage, particularly intensive temperature and time exposures are employed, which means that the materialspecific, low-melting (co-extruded) outer surface layers, as well as the inside core layer are also molten, thereby contributing to a high compaction ratio. This is reflected in distinct back wall amplitudes. The contrast of the C-Scan makes it even possible to identify individual tapes in the fabric. The C-Scans for the mono-extruded composite system (made of SG 30/30) demonstrate such an advanced level of compaction that the ultrasonic image of the back wall is clearly blackened, which means that a homogenisation has already taken place. Here, the contrast in the images for the compaction levels II and III allows identifying individual tapes as well. In the case of the composites manufactured from the needle-punched staple fibre fleece system SNW 17, individual parts of the areas of the C-Scan are blackened already at the first compaction level. An increase of the degree of compaction leads to a continuous extension of the dark areas across the scanned area. The structure of the fleece system generates a cloud pattern in the images. Thus, it can be ascertained that the images generated by the ultrasonic immersion technique can be used for all Zeitschrift Kunststofftechnik 9 (2013) 6 284

measured materials to identify a texture that corresponds with the structure of the textile semi-finished product used in each case. 3 RESULTS 3.1 Ultrasonic testing The A- and C-Scans provide evidence that statements can be made about selfreinforced PP composites on the basis of the ultrasonic testing. While the A- Scan only shows a spot measurement, entire areas can be tested with the C- Scan, so as to visualise defects in the tested specimens, for example. To facilitate the comparability of the results from the ultrasonic test with those from the tensile tests, the mean values of the amplitudes of a C-Scan were calculated and set against the three different process stages for each material in the diagrams shown in figure 7. The C-Scans of figure 6 contain in each tested area zones with high-amplitudes as well as low-amplitudes. For this reason, the lowest and highest amplitude of an area is registered as an error bar in the diagrams in order to characterise the scattering of the amplitude within the test area. In general, the averaged amplitude increases with the degree of compaction for all composites. The Pure-based co-extruded composite system only shows an increase in the averaged amplitude of approximately 4% between the compaction level I and II. Even if the average value scarcely changes, the increase in the upper error bar from 26 % (I) to 41 % (II) suggests that a higher degree of compaction has already commenced in small areas. From setting II to III, a clear increase of almost 50 %, can ultimately be recognized in the mean amplitude. For the processing conditions employed in setting III (185 C/180s), this obviously melts the higher-melting inside core layer of the woven tape essentially, thus increasing the degree of compaction, and reducing the extent to which the amplitude is weakened significantly. The mono-extruded composite systems made of SG 30/30 and SNW 17 display an identical behaviour. With higher process settings, there is also an increase in the averaged amplitude. While the average amplitude for SNW 17 composite increases from 23% (I) to 85% (III), a clear jump from 16% (l) to 70% (II) can be observed for SG 30/30 composites. This behaviour is related to the monoextruded structure of the material concept. While the coarser woven tapes of the SG 30/30 composite require a minimum level of process conditions to show a sharp increase of the degree of compaction, the finest filaments of the fleecebased SNW 17 composite react much more sensitively to heat and compaction time and generates a linear increase in the amplitude above the depicted process conditions. Zeitschrift Kunststofftechnik 9 (2013) 6 285

Amplitude in % Amplitude in % Amplitude in % 100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0 Pure SG-30/30 SNW17 I II III Process Prozess I II III Prozess Process I II III Process Prozess Figure 7: Results of the ultrasonic testing for the composite systems of Pure, SG 30/30 and SNW 17; the average amplitude of one C-Scan per measuring point with the highest and lowest amplitude as an error bar Zeitschrift Kunststofftechnik 9 (2013) 6 286

3.2 Mechanical properties The mechanical characterisation of the composite properties was conducted in comprehensive investigations in quasi-static tensile tests according to DIN EN ISO 527-4. Due to the exceptionally high ductility of self-reinforced PP composites, only the analyses of the stiffness are taken into account within the framework of this study. A clear delamination of the textile outer layers in the clamping device and between the individual layers occurs with an increased material stretching. It is not possible to evaluate the measured data up to the point of fracture, since the tests were not performed up to the moment of complete failure within the context of these investigations. From the laminates measuring 500x500 mm², 15 test specimens were taken respectively diagonally from the composite in order to measure the Young's modulus. The preparation of the flat test specimens with dimensions of 160x15x2 mm³ was performed in a multi-stage process to achieve a consistent laminate quality, especially in the outer zone of the test specimens. First, the irrelevant areas of the laminates are sawn off by rough machining, using a band saw. Subsequently, the specimens are pre-cut on a thermoplastic circular saw and finally a smooth outer contour is achieved by a finishing on an NC milling unit. This final step reduces the crack initiation, triggered by notch effects in the outer area of the laminate, during the tensile test. In general, the delamination phenomena at the edge of the specimen are prevented by increasingly precise multi-stage machining. Additionally, it is ensured that the lowest possible thermal stress is imposed on the specimens through machining. The quasi-static tensile tests were conducted on a universal testing machine of type 1446 from Zwick. The test is conducted taking a constant strain rate of 5 mm/min into consideration. The Young's modulus is determined at a strain between 0.05 and 0.25% at 1 mm/min, according to the norm. Figure 8 shows the tensile stiffness of the composites produced, which will be explained in the following. Co-extrudate Pure The mechanical values of the Pure layer composites clearly stand out from the SNW 17 and SG 30/30 composites on account of their high Young's moduli and display stiffness values between 5000 and 5500 MPa. This is due to the fact that the tapes in the Pure textiles have co-extruded, low-melting outer layers. Therefore, the self-reinforced core layer is mainly retained when the heat is introduced during hot-compaction as long as the critical material-specific processing temperature is not reached. With an increasing degree of compaction (a higher temperature over a longer period), a reduction of the Young's modulus is evident in the Pure layer composites on account of the heat and time exposure. Below the critical processing temperature of 185 C, only a slight loss in the characteristic values is evident. Zeitschrift Kunststofftechnik 9 (2013) 6 287

Zug-E-Modul E t [MPa] 6000 5000 4000 3000 2000 1000 0 Figure 8: Depiction of the Young's modulus of the material systems for the three process settings Mono-extrudates SNW 17 and SG 30/30 I (175 C/60s) II (180 C/120s) III (185 C/180s) In contrast to the Pure layer composites, the mono-extruded layer composites display a more considerable heat and time-sensitive material behaviour. With an increase of the pressing temperature and time, respectively the compaction degree, a clear increase of the Young's moduli can be observed, which are boosted to up to 50% with an increasing degree of compaction and extend from 2000 to 3000 MPa for SNW 17 and from 1800 to 2500 MPa for SG 30/30, see Figure 8. In order to generate a sufficient matrix phase, a brief period of heat introduction above melting temperature is necessary for the mono-extrudates in order to trigger a fibre impregnation and indispensable flow mechanisms, such as the matrix percolation and squeeze flow. With an increasing processing temperature, the interlaminar bond between the textile layers is improved, as shown by the Young's moduli in Figure 8. The results can be summarised as follows: Although the composites in coextruded woven tapes have considerably higher Young's moduli, the monoextruded material systems cannot be devaluated immediately. A key advantage of mono-extrudates is their temperature and time-sensitive behaviour which makes them more suitable for the thermal grading process. In addition, these materials, which are primarily used in earthworks and road building for reinforcements and drainages, are more readily available. In the light of the considerably lower material prices of SNW 17 (0.004 /GPa) and SG 30/30 (0.007 /GPa) compared to the considerably higher-priced Pure at 0.011 /GPa, it is possible to offer a more advantageous value for money ratio with respect to the Young's modulus. Zeitschrift Kunststofftechnik 9 (2013) 6 288

4 DISCUSSION Summarising the ultrasonic investigation and the evaluation of the tensile stiffnesses, it is necessary to make a distinction between mono-extrudates and co-extrudates in the case of textile semi-finished products used to produce SRPP. An increasing heat input, leads to an increase of the bond strength in the mono-extrudate system-based composites due to a better adhesion between the matrix and the embedded fibres or tapes. However, with the co-extrudates composite (Pure) the tensile stiffness decreases with an increasing processing temperature, since the tape core of the fabric is thermally weakened beyond the co-extruded outer layer. Figure 9 compares the measured Young s Modulus and results of the non-destructive ultrasonic testing. Figure 9: Comparison between the Young s Modulus and results of the ultrasonic testing As far as the ultrasonic investigations are concerned, a rise in the amplitudes can be observed with an increasing degree of textile compaction in the hotcompaction process, regarding setting I (175/60s), setting II (180 C/120s) and setting III (185 C/180s). Ultimately, fewer acoustic waves are scattered through the more compacted material and hence allows a better detection of the back wall. With a higher ultrasonic amplitude the Young s Modulus of SNW17 and SG 30/30 based composites increases, while the modulus of the pure based composite decreases, which is in agreement with the material behaviour during the hot-compaction process. In this regard, the presented results indicate that it is possible to determine the compaction degree of SRPP by means of non-destructive ultrasonic testing method instead of applying destructive testing methods. Zeitschrift Kunststofftechnik 9 (2013) 6 289

5 CONCLUSION This paper proves that ultrasonic testing is an appropriate method to visualise the compaction ratio of SRPP composites. Several composites with different compaction degrees were successfully produced by varying the processing parameters of the sensitive hot-compaction process. The parameters for the ultrasonic testing were selected to further enable a comparison of the different materials. The results of the ultrasonic testing are in a good agreement with the measured Young s modulus. This study validates that ultrasonic testing can identify different compaction zones within a component in a non-destructive manner, without the need for more elaborate or even destructive testing methods. In addition, with a suitable compaction, the C-Scans allow the analysis of retained fibre structures. Hence, conclusions on the prevailing macrostructure can be drawn. Acknowledgements The authors would like to thank the German Research Foundation for their financial support of the scientific work involved in testing self-reinforced thermoplastic layer composites in the context of the subprojects A4 and A5 of the Transregio 30 collaborative research centre entitled "Process-integrated production of functionally graded structures on the basis of thermo-mechanically coupled phenomena". Zeitschrift Kunststofftechnik 9 (2013) 6 290

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Stichworte: Eigenverstärkte Polypropylen Verbundwerkstoffe, Heißkompaktierung, Ultraschallprüfung Keywords: Self-reinforced polypropylene composites, Hot-compaction, Ultrasonic testing Autor/author: Prof. Dr.-Ing. Hans-Peter Heim 1 Prof. Dr.-Ing. Dipl.-Wirt. Ing. Wolfgang Tillmann 2 Dipl.-Ing. Angela Ries 1 Dipl.-Ing. Norman Sievers 2 Dipl.-Ing. Björn Rohde 1 Dr.-Ing. Dipl.-Phys. Reiner Zielke 2 1 Lehrstuhl Kunststofftechnik Institut für Werkstofftechnik Universität Kassel Mönchebergstraße 3 34109 Kassel 2 Lehrstuhl für Werkstofftechnologie Technische Universität Dortmund Leonhard-Euler-Straße 2 44227 Dortmund Herausgeber/Editor: Europa/Europe Prof. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein, verantwortlich Lehrstuhl für Kunststofftechnik Universität Erlangen-Nürnberg Am Weichselgarten 9 91058 Erlangen Deutschland Phone: +49/(0)9131/85-29703 Fax.: +49/(0)9131/85-29709 E-Mail-Adresse: ehrenstein@lkt.uni-erlangen.de Verlag/Publisher: Carl-Hanser-Verlag Jürgen Harth Ltg. Online-Services & E-Commerce, Fachbuchanzeigen und Elektronische Lizenzen Kolbergerstrasse 22 81679 Muenchen Tel.: 089/99 830-300 Fax: 089/99 830-156 E-mail-Adresse: harth@hanser.de E-Mail: heim@uni-kassel.de Webseite: www.ifw-kassel.de Tel.: +49(0)561/804-3670 Fax: +49(0) 561/804-3672 Amerika/The Americas Prof. Prof. h.c Dr. Tim A. Osswald, responsible Polymer Engineering Center, Director University of Wisconsin-Madison 1513 University Avenue Madison, WI 53706 USA Phone: +1/608 263 9538 Fax.: +1/608 265 2316 E-Mail-Adresse: osswald@engr.wisc.edu Beirat/Editorial Board: Professoren des Wissenschaftlichen Arbeitskreises Kunststofftechnik/ Professors of the Scientific Alliance of Polymer Technology Zeitschrift Kunststofftechnik 9 (2013) 6 294