relation between void content and interlaminar shear

RELATION BETWEEN VOID CONTENT AND MECHANICAL AND IMPACT PROPERTIES IN COMMINGLED E-GLASS/POLYPROPYLENE THERMOPLASTIC COMPOSITES C. Santulli, D. Drezet, R.Brooks, C.D.Rudd School of Mechanical, Materials, Manufacturing Engineering & Management University of Nottingham University Park Nottingham NG7 2RD KEYWORDS: E-glass/polypropylene, compression moulding, impact, mechanical properties, thermography ABSTRACT In this paper, flat plaques of E-glass/polypropylene commingled composites with different fibre architecture (2-D, 3-D) were non-isothermally compression moulded under various moulding conditions and then subjected to a number of different tests. These included drop-weight impact tests with a staircase procedure, Charpy impact tests and interlaminar shear strength. In particular, results from void content measurement using optical microscopy on these materials were put in relation with interlaminar shear strength and impact tests results on these materials. The results of these tests are discussed in the light of the moulding conditions and quality, and conclusions have been drawn regarding optimum moulding conditions for impact performance. Finally, indications on the reliability and possible improvement of the moulding procedure to yield an acceptable moulding quality, even with large thickness, are also provided. INTRODUCTION Thermoplastic composites reinforced with commingled fibres are increasingly used in different applications e.g., in automotive industry, for their energy absorbing properties [1-3]. To produce these laminates, non-isothermal compression moulding is frequently used, because it is a simple and not very expensive manufacturing procedure. However, a number of parameters may affect moulding quality and material consolidation. Some of these parameters are preheating temperature, pressure and tool temperature [4-5]. In addition, the conservation of correct fibre alignment during moulding process is essential to maximise mechanical properties. The assessment of optimal moulding conditions on these materials is therefore important, in particular for structural components, where a thickness exceeding some mm. may be required. Measurement of void content can provide an indication of the moulding quality and consolidation of thermoplastic composites. A number of methods exist, that can yield information on the void percentage in composite laminates, based e.g., on ultrasound, density measurement and image analysis from optical micrographs. EXPERIMENTAL Materials Different E-glass polypropylene commingled laminates with the same fibre content (60% weight). were investigated. In particular, three weave structures were considered: a 3-D balanced laminate weave by Parkhill and two 2-D laminates by Vetrotex (Twintex), a balanced weave (1:1 Twintex) and a directional weave (4:1 Twintex). The weave structure of 3-D Twintex is shown in Fig.1.

Fig.1 3-D Twintex weave structure (50x) Moulding procedure All the materials were first preconsolidated at 1.5 bar for approximately 1 hour at 60°C. After that, the preconsolidated sheets were stacked and moulded in non-isothermal conditions obtaining plaques with dimensions 260x120 mm, with thickness around 3.5 mm. Thickness variations throughout a single plaque did not exceed 7 %. All the moulding parameters were investigated and modified in order to obtain a good consolidation of the material with the most economic combination of time, pressure and temperature values. The conditions that yielded a good material consolidation are summarised in Table 1. Material

Oven set temperature (ºC)

Tool temperature (ºC)

245

Preheating time (min.) 4-6

2-D Twintex (1:1 & 4:1) 3-D Twintex

Consolidation time (sec.)

80

Moulding pressure (bar) 30-60

235

10

80

60

70-90

60-80

Table 1 Compression moulding parameters employed for the different materials Mechanical tests All mechanical tests were carried out in displacement control mode using an Instron 1195 machine at room temperature fitted with a 50 kN load cell. Test Tensile tests

ASTM Standard D-3039

Cross-head speed (mm/min) 0.5

Sample dimensions (mm) Grip length 60x15x3 mm

Sample geometry Dog bone

Three-point bending Interlaminar shear strength

D-790 D-2344

10 1.3

80x10x3 Span=46 mm 24x4x4 Span=16 mm

Flat beam Flat beam

Table 2 Mechanical testing set-up Impact tests Charpy impact tests Charpy impact tests were carried out using an Avery-Denison pendulum, fitted with strikers of three different energies (2.7, 5 and 15 Joules). The pendulum gives impact with a velocity of 3.46 m/s and under a span of 61 mm. The specimens were 80 mm. long and 10 mm. wide with a thickness of 2-4 mm., as prescribed in BS EN ISO 179 standard. Drop-weight impact Square plaques (80 mm side) of all the materials considered have been impacted at energies from 15 to 60 Joules using a Rosand IFW5 falling-weight tower, that allows impact from a maximum height of 3.25 metres, fitted with an anti-rebound device. The specimens were impacted using an hemispherical striker (diameter 12.7 mm) carrying a mass of 25.65 kg. The plaques were pneumatically clamped with a pressure of 0.5 bar using a circular anvil (40 mm internal diameter). IR thermography After impact, the damaged area was observed using an Agema Thermovision 900 IR thermography camera. The thermal image was obtained during cooling immediately following a 2-second heating phase to a maximum temperature of 50°C. The possibilities of infrared thermography on these materials are limited by their poor heat conduction. The variations of temperature shown on their surface are to be ascribed mainly to geometry variation produced by impact damage. The experimental set-up employed for heating is shown in Fig.2.

Fig.2 Heating system for IR thermography

Microscopy Sample preparation Squares of 20 mm. side were cut at 45° from the moulded plaques to view fibre sections in both longitudinal and transverse orientation. After that, they were cast in polyester resin. The potted samples were polished mechanically using glass paper disks, decreasing in grain size from 30 to 10 µm and after that with a fluid containing very fine alumina particles (diameter 1 µm). The final polishing was carried out in an ultrasonic bath. This level of polishing allowed the use of a 50 times magnification to highlight voids under the optical microscope. For every moulding at least three specimens were cut from different regions of the plaque at polished on both sides, so that at least six surfaces per moulding were obtained, yielding a total of not less than forty 50x images for void content measurement. Void measurement A Zeiss Axiolab microscope was used. The image was processed using the Aphelion software to create a binary image to highlight voids. To calculate the void content, the number of pixels in the original grey scale image (size 512x512 pixels captured from CCD camera, corresponding to a 2.5x2.5 mm square) was compared with the number of pixels in the binary image, representing the void content. To obtain a void percentage, an image is grabbed, and then a threshold is applied to it by using the image pixel histogram, in order to highlight the voids. The void proportion is calculated from the ratio between the area of the original image and the area of the threshold image. RESULTS Moulding study To study the influence of moulding parameters on impact and mechanical performance of the laminates, so to finally achieve an optimisation of moulding procedure, a number of temperature measurements have been carried out. Figure 3 represents the temperature of the different layers in a three-layers 2-D Twintex 1:1 during preheating, when the oven temperature was set at 245 °C. The real oven temperature was also measured in two locations, namely the oven internal wall and the internal grid, on which the laminates were physically disposed.

Fig.3 Temperature distribution during preheating in Twintex 1:1 laminate After that, the influence of different moulding parameters on mechanical properties was investigated. However, a clear trend was not always obtained, as is shown in Fig.4, where time at pressure vs. Charpy absorbed energy is plotted. For time at pressure exceeding 50 seconds, no substantial improvements in impact resistance have been observed. It is also important to notice that any consideration made on Charpy tests may be affected by the large scattering shown by Charpy normalised energy values, even exceeding 20% in some cases. The quality of consolidation can be reliably indicated by the value of interlaminar shear strength (ILSS). In Fig.5, referred to 2-D Twintex 1:1 (5 layers), an increase of ILSS by more than 25% is observed, only by passing from a six-minutes preheating cycle, not sufficient for proper consolidation, to an eight-minutes cycle, that yielded an acceptable consolidation quality. For as regards the most significant parameters, Figure 6 shows the influence of the oven-press transfer time on the void content on 2-D balanced Twintex. Void content is clearly affected, when the transfer time exceeds 15 seconds, especially on the high side, reaching a scattering of about 6% for a transfer time of 25 seconds. The higher void content yielded by a prolonged transfer is due to air trapping in the laminate and to non-uniform cooling during transfer.

Fig.4 Influence of the time at pressure on Charpy impact results for Twintex 1:1

Fig.5 Influence of preheating time on interlaminar shear strength Tool temperature is another very sensitive parameter. The application of tool temperatures higher than 60°C yields laminates with a smoother surface and a slight increase in Charpy impact values is also observed on 2-D Twintex (Fig.7) and with much greater evidence on 3-D Twintex (Fig.8). However, any non-uniformity in properties (thickness variations, void content scattering) in the laminates is exalted by the application of higher tool temperatures (exceeding e.g., 100 °C), so that in practice tool temperatures in an interval between 60 and 100 °C were applied. This will be discussed further, when dealing with drop-weight impact damage observation.

Fig.6 Void content vs. oven-press transfer time (2-D Twintex 1:1)

Fig.7 Charpy impact absorbed energy vs. tool temperature (2-D Twintex 1:1)

Fig.8 Charpy impact absorbed energy vs. tool temperature (3-D Twintex) Optimally moulded material properties Once optimised the moulding procedure with the parameters values reported in Table 1, the void content was contained, not exceeding 1.5 (±0.5)%. Most defects are observed as matrix voids or interlaminar debonding, usually not exceeding the order of magnitude of fibres (about 20-30 µm). In Fig.9 matrix defects in Twintex 1:1 are shown, from the picture are also clearly discernible glass fibres (smaller: 17 µm) and polypropylene fibres (larger: 25 µm).

Fig.9 Presence of matrix defects in Twintex 1:1 (200x magnification)

For optimally moulded materials, some mechanical properties are presented in Table 3. Materials 2-D Twintex 1:1 2-D Twintex 4:1 3-D Twintex

Tensile strength (MPa) 240 380 (long.)-120 (trans.) 320

Tensile modulus (MPa) 14 22 (long.)- 8 (trans.) 13

ILSS (MPa) 35 40 (long) – 23 (trans.) 38

Moreover, Charpy impact and 3-point bending results are plotted in Fig.10 and 11 respectively. The average Charpy impact was again comparable for 2-D Twintex 4:1 (longitudinal) and 3-D Twintex, whilst 2-D Twintex 1:1 was considerably lower, as expected. In contrast, 3-D Twintex shows lower bending properties than expected, only slightly superior to 2-D Twintex 1:1.

Fig.10 Charpy impact tests results

Fig.11 Three-point bending results

Impact properties Considering laminates with the same thickness (around 3.5 mm.), the impact performance of the three weave structures was characterised by comparing force-deflection curves obtained for impact at the same energy. In Fig.12 the curves obtained for impact at 20 Joules are shown: 3-D Twintex shows a higher peak force than the 2-D materials, indicating its higher stiffness, due to the effect of fibres disposed in through thickness direction.

Fig.12 Drop-weight impact force- deflection curves for the three materials (energy=20 Joules) Impact damage pattern in unidirectional 2-D Twintex 4:1 follows fibre orientation, as shown in Fig.13. In addition, the different fibre architecture of the three materials affects the way damage propagates into the materials, resulting in a limited depth of impact damage penetration, but in a larger area of impact damage absorption in 3-D Twintex in comparison with 2-D Twintex 1:1 (see Table 4).

Fig.13 Damaged areas for 20 Joules energy impact on the three materials. Dimensions of the observed area are 50x50 mm Material Damage Penetration Damage Dissipation

3-D Low High

2-D Tw. 4:1 High Low

2-D Tw. 1:1 Low Medium

Table 4 Impact effect on the different materials The effect of different tool temperatures on impact damage was also investigated. It is difficult to obtain a clear trend, especially for 2-D Twintex 4:1 (Fig.14), due to the directionality of its damage pattern. However, as already discussed above, the application of tool temperatures of 80 and 100°C result in more limited impact damage, although the scattering of properties between samples can be considerable, due to the specific moulding conditions (e.g., transfer time), that have a much greater influence on higher tool temperatures.

Fig.14 Impact damaged areas on the three materials for different tool temperatures (60-100 °C) and for different impact energies (35-45 Joules). Dimensions of the observed area are 45x25 mm. CONCLUSIONS Different moulding parameters are critical for the attainment of sufficient impact properties on Eglass polypropylene composites. The application of higher transfer time or not appropriate tool temperature may also affect material microstructure, resulting in a higher void content. If the material is optimally moulded, a 3-D fibre architecture yields higher impact properties, although the same is not clearly observable for static properties (e.g., tensile, bending, IL shear strength). ACKNOWLEDGEMENTS The support of Ford Motor Company Limited, Jaguar Cars Limited, MIRA, BMW Group, Borealis, ESI (Engineering Systems International), DOW Automotive, Magna Interior Systems, Mapleline, Park Hill Textiles, Polynorm Plastics, Security Composites, Symalit, Vetrotex International S.A, Warwick University, DTI (Department of Trade and Industry) and the EPRSC (Engineering and Physical Sciences Research Council, UK) is gratefully acknowledged.

References [1] C. SANTULLI, R. BROOKS, A.C. LONG, C.D. RUDD, M.J. WILSON, N.A. WARRIOR, ACP2000 Conference, Dunton, December 2000. [2] B. SADASIVAM, J.G. CHERNG, P.K. MALLICK, J. Reinf. Plast. Comp.19, 2000, p.124. [3] S.J. RIOS, R. ARROWOOD, 44th Int. SAMPE Symp., May 1999. [4] C.E. WILKS, Processing Technologies for Woven Glass/Polypropylene Composites, PhD Thesis, University of Nottingham, November 1999. [5] M.D. WAKEMAN, Non-isothermal compression moulding of glass fibre reinforced polypropylene composites, PhD Thesis, University of Nottingham, March 1997.