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The influence of flow-induced anisotropy on the impact behaviour of injection-moulded short-fibre composites
Composites Science and Technology 29 (1987) 89-102
The Influence of Flow-induced Anisotropy on the Impact Behaviour of Injection-moulded Short-fibre Composites P. J. H o g g Department of Materials, Queen Mary College, Mile End Road, London, E1 4NS (UK) (Received 14 November 1986; accepted 22 December 1986)
S UMMA R Y The objective of this paper was to develop further the concept of subcomponent testing, as suggested by Stephenson, Turner and Whale, to cover an assessment of the impact properties of polymer and composite components. To this end disks of polypropylene and glass-fibre-reinforced polypropylene were cut from plaques moulded with a range of gateing systems and subjected to mechanical testing at a range of speeds. Slow indentation testing could not distinguish between disks with or without a central weld-line, whereas instrumen tedfalling-weight impact tests produced different failure modes and significant changes in the amount of energy absorbed between specimen subsets. The concept of sub-component testing is supported by these results while the need for the sub-components to be tested over a realistic range of conditions is highlighted.
INTRODUCTION It has long been recognised that flow-induced orientation in thermoplastic mouldings can introduce significant anisotropy into a component, particularly if the plastic is reinforced by fibres. 1 - 3 A conceptual awareness by both industry and academy has not however resulted in a design methodology that recognises the p h e n o m e n o n without introducing overcautious safety factors with an attendant cost and performance penalty. A promising solution to the problem of producing relevant design data for reinforced injection-moulded thermoplastics was offered by the 89 Composites Science and Technology 0266-3538/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
90
P.J.
Hogg
philosophy of idealised, sub-component testing proposed by Stephenson, Turner and Whale in a series of papers. 4- 6 These workers reasoned that most products could be considered as assemblies of a restricted number of geometric forms such as plates and struts, and that a knowledge of the behaviour of these sub-component parts and the effects of flow geometry on their behaviour could provide an adequate understanding of the performance of the whole structure. Stephenson e t al. reported on the development of a novel bending test for plate specimens which revealed the differing degrees of anisotropy that resulted from a range of gate-die combinations for both reinforced polypropylene and nylon. The concept of a mean plate-stiffness was introduced as a suitable design parameter that normalised the effects of anisotropy. This enabled sensible, inter-material comparisons of stiffness to be made without attendant reservations stemming from differing processing routes. The work of Stephenson e t al. was concerned primarily with the shortand long-term stiffness of plates. Stiffness rather than strength is probably the most important design criterion for ductile polymers since a component is likely to fail in service by exceeding an allowable strain criterion well before the point of ductile failure is reached. Strength properties do become important, however, under service conditions likely to result in a ductile-tobrittle transition, namely at low temperatures and high rates of strain. In this work, the concept of sub-component testing is explored further by studying plates tested under impact conditions. The successful development of the instrumented falling-dart impact test has meant that plate specimens processed by any desired route and containing any specified moulding feature may be tested, in impact, under conditions in which the forces, deflections and energies involved are continuously monitored, v- lo In this programme plaques of reinforced and unreinforced polypropylene were studied with one subset of specimens moulded to incorporate a 'knit', or weld-line, and the other subset free from any such flow discontinuity. Initial experiments characterised the effect of the weld-line on the inherent stiffness anisotropy within the plates using an approach similar to that of Stephenson et al. Subsequent tests examined the strength of the plates under both slow and impact test regimes to examine the influence of this anisotropy on the fracture process.
M A T E R I A L S A N D SPECIMEN P R E P A R A T I O N The materials used were proprietary injection moulding grades of polypropylene and polypropylene with 30% by weight of short glass fibres.
Impact behaviour of injection-moulded short-fibre composites
91
The reinforced and unreinforced material was injection moulded at Brunel University to form plaque specimens 10cm square and 6 m m thick. Two subsets of specimens were produced that differed solely in the type of gateing system used. The mould cavities were identical in both cases. The first subset utilised a weir-gating system while the second subset was moulded with a parallel double gate, as represented schematically in Fig. 1. Disks of 9 cm diameter were cut from the square plaques for use in subsequent mechanical tests, with care being taken to identify the orientation of the disk with respect to the injection direction. A line was drawn passing through the centre of the disks, parallel to the injection direction, and was labelled the 0 ° orientation line. Specimen subsets were identified by the letters W G for weir-gate, DG for double-gate, P for unreinforced polymer and F for fibre-reinforced polymer.
Testmethods
Two major test procedures were used to compare the mechanical properties of the various subsets of specimens. The first test was a flexural bend test which involved subjecting the disk to three-line bending, Fig. 2. The disks were mounted symmetrically on the bending fixture with the upper loading beam acting through the centre of the specimen. The disks were subjected to small, elastic deflections ( < 2 mm) at a cross-head speed of 3 × 10-5 m/s ( ~ 2 mm/min), subsequently unloaded, and the stiffness recorded. This procedure was carried out with the 0 ° orientation line of the disks at various angles, 0, to the upper loading beam. In this way the variation of stiffness with 0 was recorded, illustrating the anisotropic nature of the specimen. In the second series of tests, disks were mounted unclamped on a circular
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The general loading configuration used for the bend tests.
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Hogg
support ring (inner diameter, 4cm) and subjected to central loading by a 2 c m diameter hemispherical indentor. These tests were performed at 3 × 10-5 m/s using a Schenk-Trebel tensile testing machine and at 3 m/s using a CEAST instrumented falling-dart machine. Full details of the instrumented impact facility are presented elsewhere. 11 All testing was performed at 20 + 2°C. Results A. Bending tests
Some of the specimens tested were not perfectly flat. Consequently the initial portion of the load-deflection curve was shallow while the specimen 'bedded in' during loading. For the purposes of measuring the stiffness of the disk these areas were ignored. The stiffness of the disks from each subset was measured at 0, 15, 30, 45, 60, 75 and 90 ° to the 0 ° injection line. Within each subset there was a degree of inter-specimen variation both of the absolute values of stiffness recorded and also in the detail of the stiffness-orientation relationship. The stiffness trends shown in Fig. 3 are for individual specimens and are not average curves for the complete subset. Nevertheless, they provide a good qualitative picture of the anisotropic nature of the specimens. There are some similarities in the orientation dependence of stiffness between the reinforced (F) and unreinforced (P) polypropylene i
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Impact behaviour of injection-moulded short-fibre composites
93
moulded using a weir-gate (WG), and also between the two grades moulded using a double gate (DG). The most significant observation, however, is that the orientation dependence is different between unreinforced grades moulded with weir or double gates (P-WG, P-DG) with similar differences existing between the fibre-reinforced materials (F-WG, F-DG).
B. Central loading tests--slow Both the weir-gated and double-gated subsets of the unreinforced polypropylene specimens behaved in a ductile manner under slow central loading (cross-head speed 3 x 10-s m/s) and produced identical force-displacement curves, Fig. 4. The plastic appeared to yield after about P-WG
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2 mm deflection and this was followed by considerable drawing and work hardening. The specimens were not taken to complete rupture in these tests because of the difficulties of extricating the indentor from the deformed plastic disks. The appearance of the unreinforced disks after about 7 mm deflection is shown in Fig. 5. The weir-gated and double-gated glass-fibre-reinforced disks als0 behaved in a similar fashion during central loading at 3 x 10- 5 m/s. A slight difference was observed in the force~tisplacement curves for each subset, as illustrated by the representative curves shown in Fig. 6. The failure process,
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P. J. Hogg
Fig. 5. Polypropylene disks after central loading to approximately 7 mm deflection showing identical behaviour from the weir-gated disk (left) and the double-gated disk (right).
however, appeared to be identical in all cases. Cracks appeared on the tensile surface of the disks, emanating from the centre of the specimen, after about 1.5 mm deflection. These cracks (typically four or five at regular intervals) propagated slowly to the edge of the supported ring with increased deflection. As the deformation continued many small cracks appeared on the upper surface of the specimen which began to coalesce and allow total rupture after approximately 8 mm deflection, Fig. 7. i
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Impact behaviour of injection-moulded short-fibre composites
95
Fig. 7. Fibre-reinforced specimensafter slow indentation tests. The pair of disks on the left were taken from wier-gated plaques, those on the right from double-gated plaques. The upper pair of specimens show tensile loading surface and the bottom disks the surface loaded in compression.
The energy required for complete penetration o f the fibre-reinforced specimens o f both subsets was in the order of 20 J.
C. Central loading tests--impact U n d e r impact conditions with a striker velocity of 3 m/s the unreinforced specimens exhibited a brittle fracture in marked contrast to the ductile behaviour observed at slow testing speeds. Both subsets (weir-gated and double-gated) produced similar fractures, Fig. 8, that essentially consisted of a n u m b e r o f straight cracks running across the specimen and passing through the centre. Occasionally a circular crack was also observed at the centre of the specimen as shown in Fig. 8a. The double-gated specimens always possessed one crack that had formed along the 0 ° orientation line, corresponding to the position of the central weld-line. Whilst the fractures appeared similar, the force-deflection curves produced during impact differed significantly between each subset of
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Hogg
Fig. 8.
Unreinforced polypropylene disks after impact tests at 3 m/s. The disk on the left was taken from a weir-gated plaque, that on the right from a double-gated plaque.
unreinforced specimens. As would be expected the force-deflection curves revealed an increase in stiffness and a marked reduction in strain-to-failure compared to the slow tests, Fig. 4. Under impact conditions, however, the double-gated subset failed at a lower load and strain than the weir-gated specimens. Superposition of the representative force-deflection curves in Fig. 4 would show that up to the point of failure in the double-gated specimen, the curves are equivalent. The parameters characterising the impact of the unreinforced polypropylene are clearly the maximum force and total energy absorbed during the impact event. These values are recorded in Table 1 and show that weir-gated specimens absorbed twice as much energy as double-gated specimens during failure. The fibre-reinforced specimens under impact conditions exhibited differences compared with specimens tested at slow speeds and also between each subset. The differences between each subset were not however revealed solely by examination of the force-deflection curves, as the nature of the fracture itself differed markedly in each case. The weir-gated specimens fractured with a clean hole on the upper, impact surface of the plate, aproximating in size to the impactor diameter, and a larger, more irregular, circular crack on the lower tensile surface, which was itself traversed by three or four radial cracks, Fig. 9. This usually resulted in a central plug of
Impact behaviour of injection-moulded short-fibre composites
97
TABLE 1
Impact Results
Gate system
Total energy (J)
Maximum force (N)
Initial peak (N)
Unreinforced polypropylene
W.G. D.G.
4.07 (0.93) 1.95 (0.37)
2 514 (327) 1 586 (279)
---
Reinforced polypropylene
W.G. D.G.
13.43 (2.9) 6.61 (0'90)
2 693 (203) 2298 (103)
2 290 (230) 1 610 (144)
Values quoted are mean results (5 specimens) with standard deviations in parentheses. c r a c k e d material falling a w a y f r o m the disk. T h e d o u b l e - g a t e d specimens failed b y b r e a k i n g into f o u r sections as a result o f two m a j o r o r t h o g o n a l splits o n the disk, one o f which always c o r r e s p o n d e d to the weld-line in the material, Fig. 9. T y p i c a l f o r c e ~ l e f l e c t i o n curves u n d e r i m p a c t c o n d i t i o n s are c o m p a r e d to the c o r r e s p o n d i n g curves for slow d e f o r m a t i o n in Fig. 6. As for the u n r e i n f o r c e d specimens, a g r e a t e r stiffness a n d a lower strain-to-failure are o b s e r v e d at the h i g h e r testing speed. T h e differences b e t w e e n the curves
Fig. 9. Fibre-reinforced polypropylene disks after impact at 3 m/s, viewed from the tensile surface. The disk on the left was taken from a weir-gated plaque, that on the right from a double-gated plaque.
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typical of each subset are, however, more fundamental and the curves themselves are more complex than those of the unreinforced specimens. The large, sudden load drops, position X (Fig. 6), indicate that in these circumstances the subsequent maximum force is not associated with the onset of fracture. The initial peak force at position X is likely to be a more significant quantity. The total energy absorbed during impact is always a convenient term to use to describe the performance of a material even though it is recognised that this term includes energy losses due to non-material effects such as 20
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machine vibration. In the case of the weir-gated fibre-reinforced specimens some additional fraction of that energy is absorbed by frictional work as the striker is driven through the punctured hole in the specimen surface, after complete penetration. Despite these qualifying remarks, the values of maximum force and total energy, together with the initial peak force at position X (Fig. 6) are given in Table 1. For both the unreinforced and reinforced materials, the total energy absorbed by the weir-gated specimens is approximately twice that absorbed by the double-gated specimens, Fig. 10.
Impact behaviour of injection-moulded short-fibre composites
99
DISCUSSION The various bending tests on the disk specimens provided two important results c o m m o n to both reinforced and unreinforced material. Under threeline bending the anisotropy differed in both degree and detail between weirgated and double-gated specimens, while simply-supported, centrally-loaded disks exhibited a c o m m o n elastic stiffness irrespective of gating system used. Both of these results support the findings of Stephenson et al.5'6 which were obtained from similar material but with different specimen dimensions (and hence different flow-dependent structures). Anisotropy in the disks derives from the flow-induced orientation of polymer and fibres which results in plaques possessing a skin-core structure. The skins possess an orientation determined by the local flow-axis and 'frozen in' by rapid cooling on contact with the cavity walls, whereas core regions generally possess an orientation orthogonal to the direction of moulding. 2 Some differences in the detailed local orientations of skin and core may be found between the weir-gated and double-gated specimens at all positions in the plaques, but the major difference lies in the existence of the weld-line in the double-gated plaques. Considerable disruption in the through-thickness orientation pattern would have been produced by the complex flow along this line. Stephenson et al. 4 - 6 also provided results obtained from centre-gated disks showing still further differences in the dependence of stiffness on orientation. While it was anticipated that the existence of the weld-line would not affect the overall elastic stiffness of the plate as measured by the central loading test, it was surprising to find no discernible differences in the behaviour of specimens tested at slow speeds beyond the elastic limit. This was not the case however for the high strain-rate tests, where premature fracture of the double-gated specimen occurred. Behaviour of the unreinforced polymer specimens was independent of moulding conditions up to the point of initial fracture which occurs prematurely in the doublegated specimens, indicating that the weld-line is acting as a stress raiser or notch under conditions where plastic deformation is restricted. The behaviour of the fibre-reinforced plastic was somewhat more complex. The behaviour of the weir-gated and double-gated specimens is reasonably equivalent up to the presumed onset of cracking in the double-gated specimens, whereafter the shapes of the force-time curves differ markedly. The differences undoubtedly reflect the different modes of crack propagation through the specimens that give rise to the very different fractures shown in Fig. 9. The weir-gated specimens produce an impact fracture that is very similar to the fractures produced by both subsets on slow testing. The major difference is the formation of a clean, punched hole on the impact face of the
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disk compared to the irregular nature of the corresponding hole from slow tests. The formation of a reasonably symmetrical fracture suggests that the initial cracks formed at the tensile surface, and therefore on the orientated skin, do not propagate easily through the core layer that possesses a significantly different orientation. Furthermore, the similarity between slow tests and impact tests suggests that while the material appears more brittle at the high rates of test, the fracture path is not substantially altered. In contrast, fracture of the double-gated specimens into four pieces under impact in a similar fashion to the impact fracture of the unreinforced material shows that the weld-line becomes active as a defect, of sufficient magnitude to override any effect of through-thickness variations in fibre orientation on the fracture process. The assignment of peaks on the force-deflection curves to specific fracture events is often not as straightforward as it would appear. A recent publication by Johnson and co-workers 12 has clearly demonstrated how machine architecture, signal processing and impact velocity can interfere with, and modify, the relative prominences of significant and insignificant features on the force~:leflection trace. The degree of extraneous noise associated with the typical forcedeflection curves shown in Figs 4 and 6 is limited. This fact assists the interpretation and strongly suggests that the sudden load drops, at position X (Fig. 6) and at maximum load in Fig. 4, are significant features corresponding to specific fracture events. It is reasonable to propose that in -
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Fig. 11. Maximum forces sustained during impact of disks at 3 m/s. The mean value of the maximum force is represented by the shaded zones, with the unshaded portion indicating the standard deviation. The dotted lines on the bars for the fibre-reinforced specimens indicate the mean value of the force at the initial peak (position X as marked in Fig. 6).
Impact behaviour of injection-moulded short-fibre composites
101
both cases the sudden load drops represents the onset of fracture with the initial cracks propagating cleanly through the unreinforced material. In the case of the reinforced specimens the initial fracture is constrained by factors such as fibre pull-out and through-thickness change in fibre orientation leading to a more complex propagation phase. It is instructive to examine the magnitude of the initial peak forces (position X) generated during impact of the reinforced material with the maximum peak forces generated during impact of the unreinforced materials (Table 1 and Fig. 11). For the double-gated subset, the mean values of these forces are very similar indeed, while the correlation is somewhat poorer for the weir-gated subset. The behaviour of the reinforced plaques, with the inherent defect of the weld-line, may be considered to be initially controlled by behaviour of the unreinforced analogue. This might be restated as saying that the strength and, presumably, the structure of a weldline are similar irrespective of whether fibres are present in the bulk material.
CONCLUSIONS The initial programme has confirmed some of the results of Stephenson et al., namely that flow geometry can radically affect the in-plane stiffness of a plate but that central loading of a plate provides a mean stiffness value independent of such variables at slow testing rates. In addition the following conclusions can be drawn: 1. 2.
3.
The concept of a mean stiffness of a plate that is insensitive to the detailed anisotropy within the plate holds for impact velocities. Weld-lines do not act as defects on centrally-loaded plates at slow testing rates when the material behaves in a ductile fashion, for both reinforced and unreinforced materials. Weld-lines act as defects at high strain-rates resulting in significant reductions in the 'toughness' of plate specimens, both reinforced and unreinforced.
The most important conclusion of this work, however, concerns the philosophy of sub-component testing. It is apparent that the different modes of behaviour exhibited by plaques of the same polymers differing only in their detailed flow-induced anisotropic structures could not be easily predicted on the basis of conventional tests on coupon specimens. The value of the sub-component approach to testing is self-evident. What is, however, now shown to be of equal importance, is that sub-components themselves are tested over a range of conditions. It is perhaps fortunate that the polymers tested exhibited a ductile-to-brittle transition at some testing speed
102
P. J. Hogg
below that chosen for the impact tests. If the polypropylene was ductile at 3 m/s the potential of the weld-line to act as a defect would have been missed.
ACKNOWLEDGEM ENTS The author is grateful to his colleagues, Dr P. E. Reed and Dr S. Turner, of Queen M a r y College for their invaluable insights into impact behaviour and sub-component testing. The assistance of Dr M. J. Foulkes of Brunel University in supplying moulded specimens is also warmly appreciated.
REFERENCES 1. D. Hull, An Introduction to Composites, Cambridge University Press, Cambridge, 1981. 2. S. Turner, Modern design data for short-fiber thermoplastic composites, J. Polym. Sci. Polymer Symposium, 72 (1985), p. 319. 3. G. R. Smith, M. W. Darlington and D. Cammond, Flexural anisotropy of glassfibre reinforced thermoplastic injection mouldings, J. Strain Anal., 13 (1978), p. 221. 4. R. C. Stephenson, S. Turner and M. Whale, The load bearing capability of short-fiber thermoplastics composites--a new practical system of evaluation, Polym. Eng. Soc., 19 (1979), p. 173. 5. R.C. Stephenson, S. Turner and M. Whale, The mean stiffness of thermoplastic composites, Composites, 10 (1979), p. 153. 6. R. C. Stephenson, S. Turner and M. Whale, The assessment of flexural anisotropy and stiffness in thermoplastic-based sheet materials, Plastics and Rubber: Materials and Applications, 5 (1980), p. 7. 7. T. Casiraghi, G. Castiglioni and G. Ajroldi, New developments in assessment of impact resistance evaluation by falling-weight method, Plastics and Rubber Processing and Applications, 4 (1984), p. 369. 8. P. Zoller, Instrumentation for impact testing of plastics, Polymer Testing, 3 (1981), p. 197. 9. S. Turner, P. E. Reed and M. Money, Flexed plate impact testing: some effects of specimen geometry, Plastics and Rubber Processing and Applications, 4 (1984), p. 369. 10. P. E. Reed and S. Turner, Flexed plate impact testing lI: the behaviour of toughened polystyrene, Plastics and Rubber Processing and Applications, fi (1985), p. 109. 11. A. Ahmadnia and P. Hogg, The impact performance of stainless steel SMC laminated macrocomposites, to be published. 12. A. E. Johnson, D. R. Moore, R. S. Prediger, P. E. Reed and S. Turner, The falling-weight impact test applied to some glass-fibre-reinforced Nylons, J. Mat. Sci., 21 (1986), p. 3153.
The Influence of Flow-induced Anisotropy on the Impact Behaviour of Injection-moulded Short-fibre Composites P. J. H o g g Department of Materials, Queen Mary College, Mile End Road, London, E1 4NS (UK) (Received 14 November 1986; accepted 22 December 1986)
S UMMA R Y The objective of this paper was to develop further the concept of subcomponent testing, as suggested by Stephenson, Turner and Whale, to cover an assessment of the impact properties of polymer and composite components. To this end disks of polypropylene and glass-fibre-reinforced polypropylene were cut from plaques moulded with a range of gateing systems and subjected to mechanical testing at a range of speeds. Slow indentation testing could not distinguish between disks with or without a central weld-line, whereas instrumen tedfalling-weight impact tests produced different failure modes and significant changes in the amount of energy absorbed between specimen subsets. The concept of sub-component testing is supported by these results while the need for the sub-components to be tested over a realistic range of conditions is highlighted.
INTRODUCTION It has long been recognised that flow-induced orientation in thermoplastic mouldings can introduce significant anisotropy into a component, particularly if the plastic is reinforced by fibres. 1 - 3 A conceptual awareness by both industry and academy has not however resulted in a design methodology that recognises the p h e n o m e n o n without introducing overcautious safety factors with an attendant cost and performance penalty. A promising solution to the problem of producing relevant design data for reinforced injection-moulded thermoplastics was offered by the 89 Composites Science and Technology 0266-3538/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
90
P.J.
Hogg
philosophy of idealised, sub-component testing proposed by Stephenson, Turner and Whale in a series of papers. 4- 6 These workers reasoned that most products could be considered as assemblies of a restricted number of geometric forms such as plates and struts, and that a knowledge of the behaviour of these sub-component parts and the effects of flow geometry on their behaviour could provide an adequate understanding of the performance of the whole structure. Stephenson e t al. reported on the development of a novel bending test for plate specimens which revealed the differing degrees of anisotropy that resulted from a range of gate-die combinations for both reinforced polypropylene and nylon. The concept of a mean plate-stiffness was introduced as a suitable design parameter that normalised the effects of anisotropy. This enabled sensible, inter-material comparisons of stiffness to be made without attendant reservations stemming from differing processing routes. The work of Stephenson e t al. was concerned primarily with the shortand long-term stiffness of plates. Stiffness rather than strength is probably the most important design criterion for ductile polymers since a component is likely to fail in service by exceeding an allowable strain criterion well before the point of ductile failure is reached. Strength properties do become important, however, under service conditions likely to result in a ductile-tobrittle transition, namely at low temperatures and high rates of strain. In this work, the concept of sub-component testing is explored further by studying plates tested under impact conditions. The successful development of the instrumented falling-dart impact test has meant that plate specimens processed by any desired route and containing any specified moulding feature may be tested, in impact, under conditions in which the forces, deflections and energies involved are continuously monitored, v- lo In this programme plaques of reinforced and unreinforced polypropylene were studied with one subset of specimens moulded to incorporate a 'knit', or weld-line, and the other subset free from any such flow discontinuity. Initial experiments characterised the effect of the weld-line on the inherent stiffness anisotropy within the plates using an approach similar to that of Stephenson et al. Subsequent tests examined the strength of the plates under both slow and impact test regimes to examine the influence of this anisotropy on the fracture process.
M A T E R I A L S A N D SPECIMEN P R E P A R A T I O N The materials used were proprietary injection moulding grades of polypropylene and polypropylene with 30% by weight of short glass fibres.
Impact behaviour of injection-moulded short-fibre composites
91
The reinforced and unreinforced material was injection moulded at Brunel University to form plaque specimens 10cm square and 6 m m thick. Two subsets of specimens were produced that differed solely in the type of gateing system used. The mould cavities were identical in both cases. The first subset utilised a weir-gating system while the second subset was moulded with a parallel double gate, as represented schematically in Fig. 1. Disks of 9 cm diameter were cut from the square plaques for use in subsequent mechanical tests, with care being taken to identify the orientation of the disk with respect to the injection direction. A line was drawn passing through the centre of the disks, parallel to the injection direction, and was labelled the 0 ° orientation line. Specimen subsets were identified by the letters W G for weir-gate, DG for double-gate, P for unreinforced polymer and F for fibre-reinforced polymer.
Testmethods
Two major test procedures were used to compare the mechanical properties of the various subsets of specimens. The first test was a flexural bend test which involved subjecting the disk to three-line bending, Fig. 2. The disks were mounted symmetrically on the bending fixture with the upper loading beam acting through the centre of the specimen. The disks were subjected to small, elastic deflections ( < 2 mm) at a cross-head speed of 3 × 10-5 m/s ( ~ 2 mm/min), subsequently unloaded, and the stiffness recorded. This procedure was carried out with the 0 ° orientation line of the disks at various angles, 0, to the upper loading beam. In this way the variation of stiffness with 0 was recorded, illustrating the anisotropic nature of the specimen. In the second series of tests, disks were mounted unclamped on a circular
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Fig. 2.
The general loading configuration used for the bend tests.
92
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Hogg
support ring (inner diameter, 4cm) and subjected to central loading by a 2 c m diameter hemispherical indentor. These tests were performed at 3 × 10-5 m/s using a Schenk-Trebel tensile testing machine and at 3 m/s using a CEAST instrumented falling-dart machine. Full details of the instrumented impact facility are presented elsewhere. 11 All testing was performed at 20 + 2°C. Results A. Bending tests
Some of the specimens tested were not perfectly flat. Consequently the initial portion of the load-deflection curve was shallow while the specimen 'bedded in' during loading. For the purposes of measuring the stiffness of the disk these areas were ignored. The stiffness of the disks from each subset was measured at 0, 15, 30, 45, 60, 75 and 90 ° to the 0 ° injection line. Within each subset there was a degree of inter-specimen variation both of the absolute values of stiffness recorded and also in the detail of the stiffness-orientation relationship. The stiffness trends shown in Fig. 3 are for individual specimens and are not average curves for the complete subset. Nevertheless, they provide a good qualitative picture of the anisotropic nature of the specimens. There are some similarities in the orientation dependence of stiffness between the reinforced (F) and unreinforced (P) polypropylene i
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ORIENTATION, 0
Fig. 3.
The variation of stiffness with orientation for disks tested in three-line bending.
Impact behaviour of injection-moulded short-fibre composites
93
moulded using a weir-gate (WG), and also between the two grades moulded using a double gate (DG). The most significant observation, however, is that the orientation dependence is different between unreinforced grades moulded with weir or double gates (P-WG, P-DG) with similar differences existing between the fibre-reinforced materials (F-WG, F-DG).
B. Central loading tests--slow Both the weir-gated and double-gated subsets of the unreinforced polypropylene specimens behaved in a ductile manner under slow central loading (cross-head speed 3 x 10-s m/s) and produced identical force-displacement curves, Fig. 4. The plastic appeared to yield after about P-WG
P-DG 3x10-r' m / s ~
3 z
//
3 m/=
O u. 2
1
2
/ 3
i
i
i
i
4
5
6
7
DEFLECTION,
a
mm
1
2
3
4
DEFLECTION,
5
6
J 7
mm
b
Fig. 4. Force~leflection curves for unreinforced specimens subjected to central loading at slow rates (3 x 10-5 m/s) and impact rates (3 m/s) (a) weir-gated plaques, (b) double-gated plaques.
2 mm deflection and this was followed by considerable drawing and work hardening. The specimens were not taken to complete rupture in these tests because of the difficulties of extricating the indentor from the deformed plastic disks. The appearance of the unreinforced disks after about 7 mm deflection is shown in Fig. 5. The weir-gated and double-gated glass-fibre-reinforced disks als0 behaved in a similar fashion during central loading at 3 x 10- 5 m/s. A slight difference was observed in the force~tisplacement curves for each subset, as illustrated by the representative curves shown in Fig. 6. The failure process,
94
P. J. Hogg
Fig. 5. Polypropylene disks after central loading to approximately 7 mm deflection showing identical behaviour from the weir-gated disk (left) and the double-gated disk (right).
however, appeared to be identical in all cases. Cracks appeared on the tensile surface of the disks, emanating from the centre of the specimen, after about 1.5 mm deflection. These cracks (typically four or five at regular intervals) propagated slowly to the edge of the supported ring with increased deflection. As the deformation continued many small cracks appeared on the upper surface of the specimen which began to coalesce and allow total rupture after approximately 8 mm deflection, Fig. 7. i
i
i
i
f
i
i
F WG
i
i
i
F-DG i
3x10_Sm/s
Z 2 ~J (I 0LI.
3m/s
1
I
2
3
4
5
DEFLECTION ry7m
6
7
8
i
i
1
2
i
i ~
~'8
3 4 5 DEFLECTION, mm
i
i
i
6
7
8
b Fig. 6. Force~deflection curves for fibre-reinforced specimens subjected to central loading at slow rates (3 × 10- 5 m/s) and impact rates (3 m/s) (a) weir-gated plaques, (b) double-gated plaques.
Impact behaviour of injection-moulded short-fibre composites
95
Fig. 7. Fibre-reinforced specimensafter slow indentation tests. The pair of disks on the left were taken from wier-gated plaques, those on the right from double-gated plaques. The upper pair of specimens show tensile loading surface and the bottom disks the surface loaded in compression.
The energy required for complete penetration o f the fibre-reinforced specimens o f both subsets was in the order of 20 J.
C. Central loading tests--impact U n d e r impact conditions with a striker velocity of 3 m/s the unreinforced specimens exhibited a brittle fracture in marked contrast to the ductile behaviour observed at slow testing speeds. Both subsets (weir-gated and double-gated) produced similar fractures, Fig. 8, that essentially consisted of a n u m b e r o f straight cracks running across the specimen and passing through the centre. Occasionally a circular crack was also observed at the centre of the specimen as shown in Fig. 8a. The double-gated specimens always possessed one crack that had formed along the 0 ° orientation line, corresponding to the position of the central weld-line. Whilst the fractures appeared similar, the force-deflection curves produced during impact differed significantly between each subset of
96
P.J.
Hogg
Fig. 8.
Unreinforced polypropylene disks after impact tests at 3 m/s. The disk on the left was taken from a weir-gated plaque, that on the right from a double-gated plaque.
unreinforced specimens. As would be expected the force-deflection curves revealed an increase in stiffness and a marked reduction in strain-to-failure compared to the slow tests, Fig. 4. Under impact conditions, however, the double-gated subset failed at a lower load and strain than the weir-gated specimens. Superposition of the representative force-deflection curves in Fig. 4 would show that up to the point of failure in the double-gated specimen, the curves are equivalent. The parameters characterising the impact of the unreinforced polypropylene are clearly the maximum force and total energy absorbed during the impact event. These values are recorded in Table 1 and show that weir-gated specimens absorbed twice as much energy as double-gated specimens during failure. The fibre-reinforced specimens under impact conditions exhibited differences compared with specimens tested at slow speeds and also between each subset. The differences between each subset were not however revealed solely by examination of the force-deflection curves, as the nature of the fracture itself differed markedly in each case. The weir-gated specimens fractured with a clean hole on the upper, impact surface of the plate, aproximating in size to the impactor diameter, and a larger, more irregular, circular crack on the lower tensile surface, which was itself traversed by three or four radial cracks, Fig. 9. This usually resulted in a central plug of
Impact behaviour of injection-moulded short-fibre composites
97
TABLE 1
Impact Results
Gate system
Total energy (J)
Maximum force (N)
Initial peak (N)
Unreinforced polypropylene
W.G. D.G.
4.07 (0.93) 1.95 (0.37)
2 514 (327) 1 586 (279)
---
Reinforced polypropylene
W.G. D.G.
13.43 (2.9) 6.61 (0'90)
2 693 (203) 2298 (103)
2 290 (230) 1 610 (144)
Values quoted are mean results (5 specimens) with standard deviations in parentheses. c r a c k e d material falling a w a y f r o m the disk. T h e d o u b l e - g a t e d specimens failed b y b r e a k i n g into f o u r sections as a result o f two m a j o r o r t h o g o n a l splits o n the disk, one o f which always c o r r e s p o n d e d to the weld-line in the material, Fig. 9. T y p i c a l f o r c e ~ l e f l e c t i o n curves u n d e r i m p a c t c o n d i t i o n s are c o m p a r e d to the c o r r e s p o n d i n g curves for slow d e f o r m a t i o n in Fig. 6. As for the u n r e i n f o r c e d specimens, a g r e a t e r stiffness a n d a lower strain-to-failure are o b s e r v e d at the h i g h e r testing speed. T h e differences b e t w e e n the curves
Fig. 9. Fibre-reinforced polypropylene disks after impact at 3 m/s, viewed from the tensile surface. The disk on the left was taken from a weir-gated plaque, that on the right from a double-gated plaque.
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P. J. Hogg
typical of each subset are, however, more fundamental and the curves themselves are more complex than those of the unreinforced specimens. The large, sudden load drops, position X (Fig. 6), indicate that in these circumstances the subsequent maximum force is not associated with the onset of fracture. The initial peak force at position X is likely to be a more significant quantity. The total energy absorbed during impact is always a convenient term to use to describe the performance of a material even though it is recognised that this term includes energy losses due to non-material effects such as 20
15
© rr'lO w
z LLI
F
J
,< I'--
0
P-5
P P
WG
DG
Fig. 10. Bar graph illustrating the total energy absorbed by disks during impact at 3 m/s. The mean values are represented by the shaded sections, with the unshaded portion of the bars indicating the standard deviation.
machine vibration. In the case of the weir-gated fibre-reinforced specimens some additional fraction of that energy is absorbed by frictional work as the striker is driven through the punctured hole in the specimen surface, after complete penetration. Despite these qualifying remarks, the values of maximum force and total energy, together with the initial peak force at position X (Fig. 6) are given in Table 1. For both the unreinforced and reinforced materials, the total energy absorbed by the weir-gated specimens is approximately twice that absorbed by the double-gated specimens, Fig. 10.
Impact behaviour of injection-moulded short-fibre composites
99
DISCUSSION The various bending tests on the disk specimens provided two important results c o m m o n to both reinforced and unreinforced material. Under threeline bending the anisotropy differed in both degree and detail between weirgated and double-gated specimens, while simply-supported, centrally-loaded disks exhibited a c o m m o n elastic stiffness irrespective of gating system used. Both of these results support the findings of Stephenson et al.5'6 which were obtained from similar material but with different specimen dimensions (and hence different flow-dependent structures). Anisotropy in the disks derives from the flow-induced orientation of polymer and fibres which results in plaques possessing a skin-core structure. The skins possess an orientation determined by the local flow-axis and 'frozen in' by rapid cooling on contact with the cavity walls, whereas core regions generally possess an orientation orthogonal to the direction of moulding. 2 Some differences in the detailed local orientations of skin and core may be found between the weir-gated and double-gated specimens at all positions in the plaques, but the major difference lies in the existence of the weld-line in the double-gated plaques. Considerable disruption in the through-thickness orientation pattern would have been produced by the complex flow along this line. Stephenson et al. 4 - 6 also provided results obtained from centre-gated disks showing still further differences in the dependence of stiffness on orientation. While it was anticipated that the existence of the weld-line would not affect the overall elastic stiffness of the plate as measured by the central loading test, it was surprising to find no discernible differences in the behaviour of specimens tested at slow speeds beyond the elastic limit. This was not the case however for the high strain-rate tests, where premature fracture of the double-gated specimen occurred. Behaviour of the unreinforced polymer specimens was independent of moulding conditions up to the point of initial fracture which occurs prematurely in the doublegated specimens, indicating that the weld-line is acting as a stress raiser or notch under conditions where plastic deformation is restricted. The behaviour of the fibre-reinforced plastic was somewhat more complex. The behaviour of the weir-gated and double-gated specimens is reasonably equivalent up to the presumed onset of cracking in the double-gated specimens, whereafter the shapes of the force-time curves differ markedly. The differences undoubtedly reflect the different modes of crack propagation through the specimens that give rise to the very different fractures shown in Fig. 9. The weir-gated specimens produce an impact fracture that is very similar to the fractures produced by both subsets on slow testing. The major difference is the formation of a clean, punched hole on the impact face of the
100
P. J. Hogg
disk compared to the irregular nature of the corresponding hole from slow tests. The formation of a reasonably symmetrical fracture suggests that the initial cracks formed at the tensile surface, and therefore on the orientated skin, do not propagate easily through the core layer that possesses a significantly different orientation. Furthermore, the similarity between slow tests and impact tests suggests that while the material appears more brittle at the high rates of test, the fracture path is not substantially altered. In contrast, fracture of the double-gated specimens into four pieces under impact in a similar fashion to the impact fracture of the unreinforced material shows that the weld-line becomes active as a defect, of sufficient magnitude to override any effect of through-thickness variations in fibre orientation on the fracture process. The assignment of peaks on the force-deflection curves to specific fracture events is often not as straightforward as it would appear. A recent publication by Johnson and co-workers 12 has clearly demonstrated how machine architecture, signal processing and impact velocity can interfere with, and modify, the relative prominences of significant and insignificant features on the force~:leflection trace. The degree of extraneous noise associated with the typical forcedeflection curves shown in Figs 4 and 6 is limited. This fact assists the interpretation and strongly suggests that the sudden load drops, at position X (Fig. 6) and at maximum load in Fig. 4, are significant features corresponding to specific fracture events. It is reasonable to propose that in -
Z
D
F
2
Ill
of r © LI1
<
WG
DG
Fig. 11. Maximum forces sustained during impact of disks at 3 m/s. The mean value of the maximum force is represented by the shaded zones, with the unshaded portion indicating the standard deviation. The dotted lines on the bars for the fibre-reinforced specimens indicate the mean value of the force at the initial peak (position X as marked in Fig. 6).
Impact behaviour of injection-moulded short-fibre composites
101
both cases the sudden load drops represents the onset of fracture with the initial cracks propagating cleanly through the unreinforced material. In the case of the reinforced specimens the initial fracture is constrained by factors such as fibre pull-out and through-thickness change in fibre orientation leading to a more complex propagation phase. It is instructive to examine the magnitude of the initial peak forces (position X) generated during impact of the reinforced material with the maximum peak forces generated during impact of the unreinforced materials (Table 1 and Fig. 11). For the double-gated subset, the mean values of these forces are very similar indeed, while the correlation is somewhat poorer for the weir-gated subset. The behaviour of the reinforced plaques, with the inherent defect of the weld-line, may be considered to be initially controlled by behaviour of the unreinforced analogue. This might be restated as saying that the strength and, presumably, the structure of a weldline are similar irrespective of whether fibres are present in the bulk material.
CONCLUSIONS The initial programme has confirmed some of the results of Stephenson et al., namely that flow geometry can radically affect the in-plane stiffness of a plate but that central loading of a plate provides a mean stiffness value independent of such variables at slow testing rates. In addition the following conclusions can be drawn: 1. 2.
3.
The concept of a mean stiffness of a plate that is insensitive to the detailed anisotropy within the plate holds for impact velocities. Weld-lines do not act as defects on centrally-loaded plates at slow testing rates when the material behaves in a ductile fashion, for both reinforced and unreinforced materials. Weld-lines act as defects at high strain-rates resulting in significant reductions in the 'toughness' of plate specimens, both reinforced and unreinforced.
The most important conclusion of this work, however, concerns the philosophy of sub-component testing. It is apparent that the different modes of behaviour exhibited by plaques of the same polymers differing only in their detailed flow-induced anisotropic structures could not be easily predicted on the basis of conventional tests on coupon specimens. The value of the sub-component approach to testing is self-evident. What is, however, now shown to be of equal importance, is that sub-components themselves are tested over a range of conditions. It is perhaps fortunate that the polymers tested exhibited a ductile-to-brittle transition at some testing speed
102
P. J. Hogg
below that chosen for the impact tests. If the polypropylene was ductile at 3 m/s the potential of the weld-line to act as a defect would have been missed.
ACKNOWLEDGEM ENTS The author is grateful to his colleagues, Dr P. E. Reed and Dr S. Turner, of Queen M a r y College for their invaluable insights into impact behaviour and sub-component testing. The assistance of Dr M. J. Foulkes of Brunel University in supplying moulded specimens is also warmly appreciated.
REFERENCES 1. D. Hull, An Introduction to Composites, Cambridge University Press, Cambridge, 1981. 2. S. Turner, Modern design data for short-fiber thermoplastic composites, J. Polym. Sci. Polymer Symposium, 72 (1985), p. 319. 3. G. R. Smith, M. W. Darlington and D. Cammond, Flexural anisotropy of glassfibre reinforced thermoplastic injection mouldings, J. Strain Anal., 13 (1978), p. 221. 4. R. C. Stephenson, S. Turner and M. Whale, The load bearing capability of short-fiber thermoplastics composites--a new practical system of evaluation, Polym. Eng. Soc., 19 (1979), p. 173. 5. R.C. Stephenson, S. Turner and M. Whale, The mean stiffness of thermoplastic composites, Composites, 10 (1979), p. 153. 6. R. C. Stephenson, S. Turner and M. Whale, The assessment of flexural anisotropy and stiffness in thermoplastic-based sheet materials, Plastics and Rubber: Materials and Applications, 5 (1980), p. 7. 7. T. Casiraghi, G. Castiglioni and G. Ajroldi, New developments in assessment of impact resistance evaluation by falling-weight method, Plastics and Rubber Processing and Applications, 4 (1984), p. 369. 8. P. Zoller, Instrumentation for impact testing of plastics, Polymer Testing, 3 (1981), p. 197. 9. S. Turner, P. E. Reed and M. Money, Flexed plate impact testing: some effects of specimen geometry, Plastics and Rubber Processing and Applications, 4 (1984), p. 369. 10. P. E. Reed and S. Turner, Flexed plate impact testing lI: the behaviour of toughened polystyrene, Plastics and Rubber Processing and Applications, fi (1985), p. 109. 11. A. Ahmadnia and P. Hogg, The impact performance of stainless steel SMC laminated macrocomposites, to be published. 12. A. E. Johnson, D. R. Moore, R. S. Prediger, P. E. Reed and S. Turner, The falling-weight impact test applied to some glass-fibre-reinforced Nylons, J. Mat. Sci., 21 (1986), p. 3153.