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An experimental investigation of consolidation in thermoplastic filament winding
An experimental investigation of consolidation in thermoplastic filament winding Suranjan Roychowdhury
and Suresh G. Advani
Consolidation is a phase in composites manufacturing during which heat and pressure are applied with the intention of obtaining a monolithic structure from discrete plies, simultaneously remove voids and volatiles, achieve desired fibre volume fractions in the part and obtain correct dimensional tolerances. Final performance of a part will depend on its consolidated state. In most manufacturing processes consolidation follows after all the individual plies are laid in a desired fashion. However, in in-situ manufacturing techniques such as filament winding and pultrusion, the consolidation occurs as the plies are laid down. Hence the relative importance of mass, momentum and energy transport are different in such processes. This paper conducts an experimental investigation of consolidation in thermoplastic filament winding. Various approaches to characterize consolidation and the suitability of these approaches in describing consolidation quality in the filament winding process is evaluated. Experiments were conducted to identify dominant mechanisms during consolidation and also the role played by process variables on the quality of consolidation. The results show that the macro phenomena, such as resin flow, are dominant in determining consolidation quality in thermoplastic filament winding, within the processing window used. Further, it was seen that within a given range of process variables, increasing tension and temperature enhanced consolidation, while too high or too low a winding speed proved to be detrimental to obtaining effective consolidation. Keywords:
consolidation;
heat;
pressure;
thermoplastics;
filament
INTRODUCTION Filament winding is a manufacturing method wherein bundles of resin-coated fibres, fed through a crosshead, are wound around a rotating mandrel of the requisite size and geometry and consolidated. Consolidation is a step in the processing of composites during which heat and pressure are applied to discrete plies, to achieve desired fibre volume fractions and dimensional tolerances and to remove voids and volatiles, thus forming a monolithic structure. A few thermoplastic filament winding processes use an external pressure device, such as a roller, to assist in applying consolidation pressure. It is an attractive manufacturing process for continuous fibre thermoset and thermoplastic matrix composites due to its potential for high production rates and low costs. This manufacturing technique is primarily used to make structures of simple geometries that need to withstand high internal pressures such as reactor vessels, pipes and tanks. Structures that are subject to high torsional stresses, such as drive shafts, can also be manufactured by this procedure as one can tailor the placement of the reinforcement to counteract any applied stress field. The manufacturing method has advanced to wind complex, non-symmetric geometries. 0956-7143/91/020097-08
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@ 1991
winding
There are three types of filament winding, namely, wet winding, dry winding and prepreg or tape winding’. The wet winding process consists of passing fibre tows through a bath of the molten matrix and winding the resin-impregnated fibre tows around a mandrel of the desired shape. In the case of thermosets, wet winding must be followed by a cure cycle in an autoclave to facilitate the consolidation of the wound layers and to allow crosslinking of the matrix resin. Wet winding of thermoplastics is not generally practised due to the very high viscosities of thermoplastic melts. However, research is proceeding to enable wet winding of thermoplastics using thermoplastic resins dissolved in suitable solvents, or by combining the winding with a pultrusion operation. The dry winding procedure involves laying down the fibre tows around the mandrel and then impregnating the resulting fibre preform using an impregnation procedure such as resin transfer moulding. The prepreg winding method consists of winding preimpregnated fibres on a mandrel and consolidating in situ, which is feasible in the case of thermoplastics, or carrying out the consolidation in an autoclave, which can be done for both types of polymer composites. Prepreg winding is most commonly used for thermoplastics because the facility of consolidating the structure in situ promises Butterworth-Heinemann
Ltd 97
great savings in production time and cost. In-situ consolidation refers to situations in which consolidation is accomplished concurrently with laying down the ply. Thermoplastic filament winding is generally carried out without the use of a consolidation roller, especially for winding large parts. The use of a roller can enhance part quality but its use is difficult when winding large parts of complex geometries. Processes in which consolidation is accomplished in situ are termed ‘on-line’ processes. In general, on-line processes consist of the following steps : resin impregnation of fibre tows, preheating, prepreg lay-down and consolidation, and part removal. Thermoplastic filament winding and other manufacturing techniques that fall under the broad category of on-line processing are subject to consolidation conditions that are different from the conditions that prevail in autoclave and compression moulding type of processes, where consolidation occurs after the desired lay-up and the pressure and temperature change is gradual. In contrast, most on-line processes see very sharp fluctuations in temperature and applied pressure, with the material being at processing temperatures and pressures for extremely short periods of time. This imposes the constraint of consolidating the structure in very short times, unlike autoclave/bleeder ply moulding and compression moulding, where rates and times of temperature and pressure application can be adjusted to obtain optimum consolidation. In on-line processes, the rate of application of consolidation pressure is difficult to control because the pressure applied is a function of the winding tension and the mandrel radius and is brought to bear on the material as soon as it makes contact with the mandrel surface. The polymer matrix is subjected to high shear stresses during most on-line processes, which is another difference from commonly used processes such as autoclave and compression moulding2-4. Hence, the time scales and the relative importance of mechanisms of transport of mass, momentum and energy in on-line processes will be much different than post lay-up consolidation processes.
CONSOLIDATION
MECHANICS
The mechanics of consolidation are different in on-line processes such as filament winding and pultrusion, where consolidation occurs as the plies are laid down, as compared to post lay-up consolidation in which consolidation follows after all the individual plies are laid in a desired arrangement. In either case, the objective is to attain good consolidation as it will determine the final performance of the manufactured part. The mechanics of the consolidation process can be described as follows5-7. During the consolidation of a polymer composite, the applied load causes the spatial gaps between adjacent, irregular ply surfaces to be removed and, as a result, the plies come into intimate contact2. At this stage, the applied load is borne entirely by the matrix and a pressure gradient results between the interior of the composite and its free surfaces. The consequent outflow of the resin causes compaction of the structure, increasing the fibre volume fraction. The fibre volume fraction increases to a point where the fibres begin to come into contact with each other thus forming a fibre network. Further compaction of the composite
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causes the fibre network to be compressed thus causing a partial load transfer from the resin to the fibre, reducing the resin pressure. With increasing outflow of resin and compression of the fibre network a situation is reached when the applied load is taken entirely by the fibres and the resin pressure becomes negligible. The consolidation process is completed with the attainment of proper bonding between the plies. The consolidation achieved in terms of bonding between plies, fibre volume fraction, thickness of the part or the void content as compared to a theoretical value for a particular application of the part, may be termed as the extent of consolidation or level of consolidation. During consolidation the resin pressure may fall steeply ; in particular under conditions where the resin can flow out freely from the laminate’. Void formation and growth are facilitated by low resin pressures. A primary consideration during consolidation is to ensure that the resin pressure does not fall too low. Also consolidation of geometries with single curvatures involves the introduction of an additional issue of fibre migration 8,9. This effect is caused by unbalanced forces acting on the fibres due to the curvature causing them to move towards the inner radius. Consolidation mechanics and the extent of consolidation are determined by not only material and process parameters but also by the type of manufacturing process (on-line or post lay-up) and the form of the initial material. There are several types of polymer composite material forms available and their use depends on the particular processing method and the geometry of the part. Most thermoset composites are available in the form of prepregs, with the resin in an uncured or partially cured (generally B-staged) state. On the other hand, thermoplastic composite materials are available in a variety of forms as tow, fully or partially impregnated, as commingled fabric and as powder preforms. The schematic representation is shown in Figure 1. Consolidation requirements vary depending on material type (thermoset matrix or thermoplastic matrix) and form (prepreg, commingled, powder preform etc. ). Unlike fully impregnated materials, commingled materials and powder preforms require in-situ impregnation of the fibre reinforcement during the consolidation process, which may change the relative importance of transport mechanisms during the process. On-line processes involving the use of such materials have to ensure that sufficient time is available for both impregnation and consolidation. Thermoplastic prepregs exhibit low tack and drape, even at relatively high temperatures, causing them to be rigid and to have low conformability, thus requiring longer consolidation times. The high viscosities of thermoplastic matrices also play a major role in defining the efficiency of consolidation. Level of consolidation also depends on the process variables used during the manufacturing of the part. The crucial variables in any manufacturing situation are the applied pressure, temperature and time of application. A processing window needs to be established for each material form and class of manufacturing processes to obtain well consolidated parts, of repeatable quality, of acceptable interlaminar shear strength and void content. This requires knowledge of the phenomena dominating consolidation and the effect of process variables in determining consolidation quality.
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a
b
d l
Reinforcing
fibre
Matrix Figure 1 Various preforms: (a) commingled thermoplastic resin coated reinforcing ; (c)partially
bundles of reinforcing fibres and the matrix resin ; (b) impregnated tapes ; (d ) fully impregnated prepreg tapes
This paper presents an experimental study of the effects of crucial variables on consolidation in thermoplastic filament wound parts. The first section describes various ways to characterize extent or level of consolidation and experimental techniques for measuring it. In the subsequent section the relative importance of bonding between the plies and matrix flow is characterized to identify the dominant phenomena during the consolidation phase during on-line processing. Further, the effect of process variables on consolidation is investigated using filament wound rings with fully impregnated material form. CHARACTERIZATION CONSOLIDATION
OF STATE
In order to identify the phenomena occurring during the consolidation phase and to understand how these phenomena dictate consolidation quality it is necessary to be able to characterize the extent of consolidation occurring during the process. Consolidation can be characterized on the basis of parameters that are related to the state of the material such as void content or failure strength. Fibre volume fraction or part dimensions as secondary characterization parameters as they do not describe the state of the material directly. Hence optimization of the process should be based on the primary variables. Predictive capabilities to obtain a value for the consolidation state as a function of process parameters will eliminate time consuming and uneconomical optimization of process variables to consolidate parts manufactured under various process regimes. As consolidation constitutes of several phenomena occurring simultaneously or concurrently it is not possible to characterize the consolidation state by a single quantity which will compose the most general and complete description. There are several ways to characterize consolidation. One way to describe the extent of consolidation is to express the change in the level of consolidation as a function of the changes in microstructure of the material. The microstructure
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represents three phases: the fibres, resin and the voids. The consolidation quality improves if one can squeeze out voids and/or maximize fibre content. Thus one may select the changes in the fibre volume fraction or voids to monitor consolidation. However, although this is a necessary condition to enhance consolidation, it may not be sufficient as another phenomenon of good interply bonding between individual layers is also necessary to consolidate a part. The strength of interply bonds can be quantified in terms of adhesive forces which depend on contact area of the ply surfaces and the diffusion of polymer chains across the interface, so the interlaminar shear strength can also serve as a characterization parameter. These approaches require measurement techniques which cannot be monitored on-line or during the process. Another approach is to characterize consolidation on a macroscopic level. The changes in the microstructure of the part are visible on the macroscopic scale in terms of the changes in the dimensions of the part being consolidated, which allows one to express the level of consolidation as a function of the percentage change in the consolidated dimension. Consolidation involves more than one phenomenon and the manufacturing process determines which of the phenomena has the most influence on the consolidation mechanics. Therefore it is the process that will determine which description or descriptions are best suitable to quantify the consolidation state. For example, in a manufacturing process where consolidation is being characterized by monitoring part dimension the achievement of the required dimension(s) of the part does not necessarily translate into the part having acceptable strength or void content. This is especially true of high speed, short process time manufacturing as is exemplified by most on-line processes. MEASUREMENT
TECHNIQUES
The experimental measurement procedure will depend on the entity selected to characterize consolidation. If one chooses consolidation in terms of adhesive strength,
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one needs to measure the bond strength of the consolidated plies. The usual procedure is to carry out a fracture test on a sample specimen and measure the strength value. The extent of consolidation is then given by the ratio of the measured strength to that of the theoretical ultimate strength2~‘0~’ ‘. The theoretical strengths can be calculated from the reptation theory of DeGennes12 and its extension, the healing theory of Wool and co-workers’3*‘4. Strength measurements are carried out by various fracture testing methods, the most common ones being the DCB (double cantilever beam) test, the short beam shear test and the peel test. This method of characterization of consolidation will constitute a general description if there are no voids present in the consolidated specimen. However, as most consolidated parts contain voids, one could measure the void content of a part to monitor the extent of consolidation. Measurement of void content may be carried out by means of density measurements, ultrasonic testing or image analysis. Of these methods, density measurement is the least reliable. This technique involves measuring the actual density of the part using a density column or the buoyancy method and using the discrepancy between the theoretical value of the part density and the measured value to calculate the void content. The inherent error in this procedure is high due to lack of accurate density values which makes it the least reliable of all the methods of determining void content. Ultrasonic testing, which involves detecting the attenuation in a sound wave passed through the part to evaluate the void content, or fibre volume fraction, is limited by the available resolution and prevents detection of small voids. Image analysis involves obtaining an image of the cross-section of the part and then analysing the image to find the void content. The most successful techniques for obtaining images are optical. Voids, or fibres, in a matrix can be seen in an optical micrograph. One can cut and polish cross-sections using standard metallographic techniques. To analyse the images involves creating a photograph or video image using the requisite type of camera attached to a microscope. This image is then digitized using a frame grabber board attached to a personal computer. Using image analysis software based on counting the number of pixels above or below a selected threshold value allows one to determine the void content or fibre volume fraction. The drawback of image analysis is that in order to arrive at a reliable estimate of the void content, a large number of cross-sections must be analysed to obtain a reliable statistical average. This makes it an extremely time and labour intensive process. Further, for accurate results, a sharp contrast is a prerequisite for image analysis. The above preliminary descriptions for the state of the part require elaborate and tedious measurement techniques. Hence a simple approach to monitor consolidation is to measure dimensional changes. This is a useful indicator only if one has previously established the correspondence between part dimensions with levels of acceptable strength and void content for a given set of manufacturing conditions 15p1‘. A similar approach is used to measure consolidation in thermoplastic matrix composites with a compression thermal analysis of the process”. This technique considers the consolidation process to be a viscoelastic phenomenon and uses the consolidation strain Ed, with the original prepreg thickness as the basis, to measure the extent of
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consolidation. This procedure squeezes the prepreg system in a press and monitors the change in the lay-up thickness with an extensometer. This technique is simple and practical and can be used to obtain the consolidation characteristics of a particular thermoplastic material system before using it in an actual manufacturing process. This procedure also supplements information available from DSC and DMA techniques for a single ply specimen. The drawback of this procedure is that it does not account for process-related variations in consolidation behaviour. In this paper we will restrict our description of the extent of consolidation in terms of the void content and interlaminar shear strength. EXPERIMENTAL
DETAILS
A filament winding set-up consisting of an infrared strip preheater and an infrared nip point heater (see Figure 2) was used to wind rings under various process conditions. Filament winding was the on-line manufacturing process chosen to be studied as it demonstrates the key consolidation mechanisms seen in in-situ processes. The materials wound were fully impregnated APC~tows, with AS4 carbon fibre reinforcement, supplied by Imperial Chemical Industries, Ltd. (ICI). The tow had a fibre volume fraction of 60 f 2% by volume and an average width of 0.004 m. The rings wound were of 5 inch (0.125 m) inside diameter and consisted of only 90” layers. Process conditions were varied by changing the applied tension, winding speed and temperature. Process temperatures were controlled by changing the power settings of the heaters. The temperature of the two was measured as it emerged from the preheater with an infrared thermometer. Temperature profiles of the two were obtained by inserting a thermocouple at the nip point and allowing it to be wound along with the tow. No consolidation roller was used, hence the tow tension was the only mode of applying consolidation pressure. Extent of consolidation was calculated using both the primary variable descriptions ; void content and interlaminar shear strength. MEASUREMENT VARIABLES
OF PRIMARY
Four rings of 40 layers each were wound for tension set at 0,4, 6,8 and 10 ib (0, 17.79,26.69,35.58 and 44.48 N) respectively. A constant winding speed of 0.459 m s-l Infrared
thermometer
Figure 2 Schematic winding experimental
illustration set-up
of major
components
of the filament
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was used and the nip point and preheater power settings were also kept constant. The rings were removed from the mandrel using an outer sleeve and mechanical force with a hand held hammer, or an Instron. The interlaminar shear strengths of the rings was obtained from the ASTM Short Beam Shear Test (ASTM D 2344) 19. The specimens for these tests were prepared by first smoothing the edges of the rings using coarse grade sandpaper then cutting sections from the rings using a diamond saw. The sections were cut so as to conform to the specifications of the curved ASTM test specimen. The interlaminar shear strength was then measured using a three-point bend fixture on an Instron. The test was conducted at a crosshead speed of 0.05 in min-’ (2.12 x 10-5ms-‘). For each specimen, the test was repeated at four different locations. The interlaminar shear stress was obtained using the various test parameters in the standard expression for the ASTM Short Beam Shear Test. Void content of the rings was measured using optical microscopy and image analysis procedures. Sections were cut from the rings and mounted in epoxy such that the cross-section was exposed. The mounted sections were then polished, following standard polishing procedures, to obtain an optically smooth surface. On observing these polished ring cross-sections through an optical microscope, the voids in the rings could be seen. Quantification of the void content in a given ring was done using a Cambridge Instruments Quantimet 970 Image Analyzer as well as image analysis software available for Macintosh personal computers. The results of the void content in this paper are obtained using the image analysis software. This involved scanning in the image of the ring cross-section and analysing the image for the number of pixels above a threshold value that represented the voids visible in the image. To ensure accuracy of the measured void content, verification was carried out using the Quantimet 970 Image Analyzer. At least seven locations on each ring cross section were analysed to ensure statistical accuracy of the measured void content.
IDENTIFICATION PHENOMENA
OF IMPORTANT
There are several factors which play a role in the consolidation process during thermoplastic composite processing. These include resin flow, polymer chain diffusion, stiffness of the fibre network, diffusivity of air and volatiles in the molten matrix and elasticity of the matrix resin in the melt state. There are complex interactions between all of these factors, which can be classified into macroscopic (macro) or microscopic (micro ) phenomena. The chief macro phenomenon is the flow of the molten matrix resin which is controlled by momentum transfer. The primary micro phenomenon in consolidation is the diffusion of polymer chains across ply interfaces which is a thermodynamic phenomenon and involves the relaxation of surface chains into their Gaussian conformations as a result of applied temperature and time of application’3,‘4. In order to arrive at a fundamental understanding of the consolidation process in thermoplastic composites it is necessary to determine which of the phenomena, resin flow (macro) or polymer chain diffusion (micro), is dominant in determining consolidation quality in thermoplastic filament winding.
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This knowledge would also enable process conditions to be manipulated so as to maximize the effect of the dominant mechanism, thus resulting in optimum consolidation. A study was carried out to establish the relative importance between the two types of phenomena occurring during consolidation to decide which one plays a significant role in obtaining effective consolidation and also to establish their relative contributions under various processing conditions. This study was based on the hypothesis that under conditions of low applied tension at a given temperature diffusion would dominate the consolidation process as resin flow would be minimal. Under large values of applied tension, at the same temperature, substantial resin flow would occur and hence resin flow would be dominant in determining consolidation. This study was carried out using rings wound on the filament winding setup shown in Figure 2. These rings were wound over a range of temperatures, between 290 and 395°C with two rings being wound at each temperature, under conditions of low and high tension; that is 3.0lb (13.34N) and ll.Olb (48.93 N) respectively. As will be discussed later, the choice of primary variables to describe extent of consolidation in this process may be either void content or interlaminar shear strength. Both should give qualitatively consistent results. Hence interlaminar shear strength of these rings was measured using the ASTM Short Beam Sheat test described previously to establish the extent of consolidation.
PARAMETRIC
STUDY
Evaluation of the effect of process variables used in thermoplastic filament winding on the consolidation quality was sought via a series of experiments. The three main process variables in thermoplastic filament winding are the winding tension, the winding temperature and the winding speed. Rings were wound under a variety of process conditions from low to high tension (2.0, 5.5, 8.0 and 11.O lb (8.89,24.46,35.58 and 48.93 N) respectively), temperature (290°C to 390°C) and winding speed (0.2 m s-l to 0.9 m s-l, approximately). Three sets of experiments were carried out to investigate the effect of each process variable with one parameter being varied in each set while keeping the others constant. The extent of consolidation in the rings was evaluated using void content as the scale.
RESULTS The study determined that the primary consolidation characterization parameters were individually sufficient to characterize consolidation completely in the thermoplastic filament winding process. The shear strength and the void contents of the parts were measured and it is seen in Figure 3 that the nature of consolidation quality obtained was consistent using either parameter. The shear strength increased with increasing consolidation quality, while the void content decreased correspondingly, indicating that, independently, either parameter was sufficient to characterize completely consolidation. Although qualitative behaviour of consolidation state can be characterized and measured in terms of void content and/or shear strength, there is a need for a quantitative relationship between the level of consolidation and the
101
6000 ^ B 5 F 5 VI
-
Low
tension
5000
4000
2 z
3000
& I >"
2000
i
a 1000
Tension
(lb)
30
300
320 Winding
3
0
20
Figure 4 Variation high applied tension
0
340
360
temperature
380
400
(“Cl
in shear strength of rings wound under as a function of process temperature
low and
E J 5 03
.
10
0 0
0 -1
1
3
5 Tension
7
9
11
(lb)
Figure 3 Characterization of consolidation quality of filament wound rings as a function of applied tension in terms of (a) average shear strength and (b) void content
characterization parameters. It may also be noted that the extent of consolidation is a function of the transport mechanisms operative in the process. Hence, the relative importance of polymer chain diffusion as compared to momentum transfer will play a key role in the choice of the characterization parameter to indicate consolidation quality. Having established that either parameter was sufficient to completely characterize consolidation, the interlaminar shear strength was used to determine the dominant phenomena in determining consolidation, while the void content was used to study the effect of process variables on consolidation in thermoplastic filament winding. The results of the study to determine the dominant phenomena in consolidation are shown in Figure 4. It can be seen-that the interlaminar shear strength increases with increasing tension, or applied pressure, as the temperature is raised. This suggests that the extent of consolidation increases with increasing temperature and pressure. It has been observed that, even at the highest winding speed, the process time is of the order of the bonding (or reptation) time for PEEKS'. Therefore, the increase in interlaminar strength with applied pressure indicates the importance of the flow of the resin in determining consolidation. As the winding tension, or the applied pressure, increases the matrix flow in the interlaminar region increases permitting greater intimate contact between the mating surfaces and fewer interlaminar voids. This is confirmed by the corresponding increase in shear strength. This study shows that
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although increase in temperature results in enhanced diffusion it is the higher resin flow (which is a consequence of higher tension and processing temperature) that results in better levels of consolidation thus suggesting that the matrix flow is a crucial parameter to obtain well consolidated parts. The applied tension (or pressure) required to achieve intimate contact does decrease with increase in the winding temperature but the winding temperature itself is limited by the degradation temperature of the material being wound. Hence upper and lower limits exist for the applied tension defined by the tensile strength of the tow on one side and the applied tension required for consolidation at the degradation temperature of the tow material on the other. The results of the parametric study of process variables on consolidation in filament wound parts are summarized in Figures 5 and 6. Figure 5 clearly shows the qualitative trend of consolidation quality. It increases with increasing tension and temperature in the range of
30 q Tension
= 2.0
lb
Tension
= 5.5
lb
l
;
= 8.0lb
0 Tension
=ll
.Olt
q
20
.
zaJ +z 8 ; ’
q Tension
q l q
10
Q
q
q . q 0
0
0
0
-280
300
320 Winding
Figure values
340 temperature
360
380
400
(‘Cl
5 Void content variation in filament wound rings under of applied tension as a function of process temperature
four
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0.2
0.6
0.4
Winding
speed
Figure 6 Variation of consolidation in terms of void content as a function
0.8 (m s
-1
1 .o
1
quality of filament of winding speed
wound
rings
process variables studied. Here the consolidation quality is measured in terms of voids as opposed to Figure 4 where it was calculated in terms of shear strength. Higher temperatures enhance polymer chain diffusion, and the combined effect of higher tensions and temperatures result in increased resin flow and consequent intimate contact. Figure 6 shows the effect of winding speed on the extent of consolidation. High winding speeds result in shorter residence times for the material at the nip-point heater. This prevents the material from reaching the optimal temperature required for consolidation, resulting in poor consolidation. Low winding speeds translate into very high material temperatures due to the nip-point heating, which can cause partial degradation of the material leaving large voids between patches of dry fibres. From the discussion of the results of this investigation it is clear that the temperature plays a key role in controlling the consolidation quality since it is this variable that determines both the matrix flow and ply bonding through chain diffusion. The applied pressure also plays an important role since it governs the extent of flow at a given temperature. The interlaminar strength obtained is a synergistic effect of the bond strength controlled by temperature and the extent of intimate contact controlled by resin flow and, hence, by applied pressure and temperature. The process time, or the time for which heat and pressure are applied, has a bearing upon the extent of flow and bonding. It serves as a limiting factor below which consolidation is poor thus restricting the winding speed.
Experiments were conducted to identify dominant mechanisms during consolidation and also the role played by process variables on the quality of consolidation. The results show that the macro phenomena such as resin flow are dominant in determining consolidation quality in thermoplastic filament winding processes in which the applied tension on the tow is used as the only source of consolidation pressure. Hence, for a given processing temperature sufficient tow tension or pressure must be applied to ensure complete intimate contact in the process time available Furthermore, the winding temperature must be maintained such that sufficient time is available for bonding to occur without the occurrence of material degradation. These conclusions are borne out by the results shown above, from which it can be seen that, within a given range of process variables increasing tension and temperature enhance consolidation while too high or too low a winding speed tends to be detrimental to obtaining effective consolidation.
ACKNOWLEDGEMENT The use of the filament winding facilities at the Center for Composite Materials, University of Delaware is gratefully acknowledged.
REFERENCES I
2
3 4
5 6
7
8 9
SUMMARY
10 11
In this paper we have investigated experimentally consolidation in filament wound thermoplastic matrix composites. Characterization procedures to quantify consolidation have been described in detail and the techniques of void content and shear strength characterization have been applied to consolidation in a thermoplastic filament winding situation. It has been shown that for the filament winding process either method of quantifying consolidation is sufficient.
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12 13 14
15
Zeler, D.J. ‘Filament winding potential with advanced multi-axis computer controlled filament winding machines’ Am Sot Camp Proc (1988) pp 33-35 Lee, W.I. and Springer, G. ‘A model of the manufacturing process of thermoplastic matrix composites’ J Comp Muter 21 (Nov 1987) pp 1017-1055 Loos, A.C. and Springer, G.S. ‘Curing of epoxy resin composites’ .I Camp Mater 17 (1983) pp 135-169 Halpin, J.C., Kardos, J.L. and Dudukovic M.P. ‘Processing science : an approach for prepreg composite systems’ Pure and Appl Chem 55 No 5 (1983) pp 893-906 Gutowski, T.G., Morigaki, T. and Cai, Z. ‘The consolidation of laminate composites’ J Camp Muter 21 (Feb 1987) pp 172-188 Gutowski, T.G., Cai, Z., Bauer, S., Boucher, D., Kingery, J. and Wineman, S. ‘Consolidation experiments for laminate composites’ J Comp Muter 21 (Feb 1987) pp 650-669 Dave, R., Kardos, J.L. and Dudukovic, M.P. ‘A model for resin flow during composite processing : Part 1-general mathematical development’ Polym Comp 8 No 1 (Feb 1987) Calius, E.P. and Springer, G. ‘Modeling the filament welding process’ SAMPE Proc 31st Int SAMPE Symp (1986) Calius, E.P. and Springer, G. ‘Optimization of the cure window for a large filament wound case’ Report to Chemical Research Projects Office (NASA-Ames Research Center, Mountain View, CA, USA, 1984) Bastien, L. ‘Resistance welding of PEEK composites’ MS Thesis (University of Delaware, July 1990) Bastien, L., Don, R.C. and Gillespie, J.W. ‘Processing and performance of resistance welded thermoplastic composites’ Proc 45th SPI Annual Conference (Feb 1990) DeGennes, P.-G. J Chem Phys 55 (1971) p 572 Wool, R.P. ‘Crack healing in polymers’ ACS Organic Coatings and Plastics Chemistry 40 (1979) p 271 Kim, Y.H. and Wool, R.P. ‘A theory of healing at a polymer-polymer interface’ Macromolecules 16 (1983 ) pp 1115-1120 Muzzy, J.D. ‘Processing of advanced thermoplastic composites’ in Vol 4: The Manufacturing Science of Composites (Gutowski, T.G., Ed.), Proc Manufacturing International ‘88 (1988) ~~27-39
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16 Hou, T.C. ‘A resin flow model for composite prepreg lamination process’ ANTEC ‘86 (Society of Plastics Engineers, 1986) 17 Van West, B.P. ‘The draping and consolidation of commingled fabrics’ PhD Dissertufion (University of Delaware, May 1990) 1S Nelson, K.M., Manson, J-A.E. and Seferis, J.C. ‘Compression thermal analysis of the consolidation process for thermoplastic matrix composites’ J Thermoplastic Camp 3 (July 1990) pp 216-231 19 Annnal Book of ASTM Standards (1989) Part 36 20 Agarwal, V. PhD Dissertation (Materials Science Program, University of Delaware, Newark, DE 19716, June 1991)
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AUTHORS Suranjan Roychowdhury is a graduate student with the Materials Science Program at the University of Delaware and Suresh G. Advani, to whom correspondence should be addressed, is Assistant Professor within the Mechanical Engineering Department, also at the University of Delaware, Newark, DE 19716, USA. (Date received 25 February 1991; accepted in revised form 7 October 1991).
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and Suresh G. Advani
Consolidation is a phase in composites manufacturing during which heat and pressure are applied with the intention of obtaining a monolithic structure from discrete plies, simultaneously remove voids and volatiles, achieve desired fibre volume fractions in the part and obtain correct dimensional tolerances. Final performance of a part will depend on its consolidated state. In most manufacturing processes consolidation follows after all the individual plies are laid in a desired fashion. However, in in-situ manufacturing techniques such as filament winding and pultrusion, the consolidation occurs as the plies are laid down. Hence the relative importance of mass, momentum and energy transport are different in such processes. This paper conducts an experimental investigation of consolidation in thermoplastic filament winding. Various approaches to characterize consolidation and the suitability of these approaches in describing consolidation quality in the filament winding process is evaluated. Experiments were conducted to identify dominant mechanisms during consolidation and also the role played by process variables on the quality of consolidation. The results show that the macro phenomena, such as resin flow, are dominant in determining consolidation quality in thermoplastic filament winding, within the processing window used. Further, it was seen that within a given range of process variables, increasing tension and temperature enhanced consolidation, while too high or too low a winding speed proved to be detrimental to obtaining effective consolidation. Keywords:
consolidation;
heat;
pressure;
thermoplastics;
filament
INTRODUCTION Filament winding is a manufacturing method wherein bundles of resin-coated fibres, fed through a crosshead, are wound around a rotating mandrel of the requisite size and geometry and consolidated. Consolidation is a step in the processing of composites during which heat and pressure are applied to discrete plies, to achieve desired fibre volume fractions and dimensional tolerances and to remove voids and volatiles, thus forming a monolithic structure. A few thermoplastic filament winding processes use an external pressure device, such as a roller, to assist in applying consolidation pressure. It is an attractive manufacturing process for continuous fibre thermoset and thermoplastic matrix composites due to its potential for high production rates and low costs. This manufacturing technique is primarily used to make structures of simple geometries that need to withstand high internal pressures such as reactor vessels, pipes and tanks. Structures that are subject to high torsional stresses, such as drive shafts, can also be manufactured by this procedure as one can tailor the placement of the reinforcement to counteract any applied stress field. The manufacturing method has advanced to wind complex, non-symmetric geometries. 0956-7143/91/020097-08
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@ 1991
winding
There are three types of filament winding, namely, wet winding, dry winding and prepreg or tape winding’. The wet winding process consists of passing fibre tows through a bath of the molten matrix and winding the resin-impregnated fibre tows around a mandrel of the desired shape. In the case of thermosets, wet winding must be followed by a cure cycle in an autoclave to facilitate the consolidation of the wound layers and to allow crosslinking of the matrix resin. Wet winding of thermoplastics is not generally practised due to the very high viscosities of thermoplastic melts. However, research is proceeding to enable wet winding of thermoplastics using thermoplastic resins dissolved in suitable solvents, or by combining the winding with a pultrusion operation. The dry winding procedure involves laying down the fibre tows around the mandrel and then impregnating the resulting fibre preform using an impregnation procedure such as resin transfer moulding. The prepreg winding method consists of winding preimpregnated fibres on a mandrel and consolidating in situ, which is feasible in the case of thermoplastics, or carrying out the consolidation in an autoclave, which can be done for both types of polymer composites. Prepreg winding is most commonly used for thermoplastics because the facility of consolidating the structure in situ promises Butterworth-Heinemann
Ltd 97
great savings in production time and cost. In-situ consolidation refers to situations in which consolidation is accomplished concurrently with laying down the ply. Thermoplastic filament winding is generally carried out without the use of a consolidation roller, especially for winding large parts. The use of a roller can enhance part quality but its use is difficult when winding large parts of complex geometries. Processes in which consolidation is accomplished in situ are termed ‘on-line’ processes. In general, on-line processes consist of the following steps : resin impregnation of fibre tows, preheating, prepreg lay-down and consolidation, and part removal. Thermoplastic filament winding and other manufacturing techniques that fall under the broad category of on-line processing are subject to consolidation conditions that are different from the conditions that prevail in autoclave and compression moulding type of processes, where consolidation occurs after the desired lay-up and the pressure and temperature change is gradual. In contrast, most on-line processes see very sharp fluctuations in temperature and applied pressure, with the material being at processing temperatures and pressures for extremely short periods of time. This imposes the constraint of consolidating the structure in very short times, unlike autoclave/bleeder ply moulding and compression moulding, where rates and times of temperature and pressure application can be adjusted to obtain optimum consolidation. In on-line processes, the rate of application of consolidation pressure is difficult to control because the pressure applied is a function of the winding tension and the mandrel radius and is brought to bear on the material as soon as it makes contact with the mandrel surface. The polymer matrix is subjected to high shear stresses during most on-line processes, which is another difference from commonly used processes such as autoclave and compression moulding2-4. Hence, the time scales and the relative importance of mechanisms of transport of mass, momentum and energy in on-line processes will be much different than post lay-up consolidation processes.
CONSOLIDATION
MECHANICS
The mechanics of consolidation are different in on-line processes such as filament winding and pultrusion, where consolidation occurs as the plies are laid down, as compared to post lay-up consolidation in which consolidation follows after all the individual plies are laid in a desired arrangement. In either case, the objective is to attain good consolidation as it will determine the final performance of the manufactured part. The mechanics of the consolidation process can be described as follows5-7. During the consolidation of a polymer composite, the applied load causes the spatial gaps between adjacent, irregular ply surfaces to be removed and, as a result, the plies come into intimate contact2. At this stage, the applied load is borne entirely by the matrix and a pressure gradient results between the interior of the composite and its free surfaces. The consequent outflow of the resin causes compaction of the structure, increasing the fibre volume fraction. The fibre volume fraction increases to a point where the fibres begin to come into contact with each other thus forming a fibre network. Further compaction of the composite
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causes the fibre network to be compressed thus causing a partial load transfer from the resin to the fibre, reducing the resin pressure. With increasing outflow of resin and compression of the fibre network a situation is reached when the applied load is taken entirely by the fibres and the resin pressure becomes negligible. The consolidation process is completed with the attainment of proper bonding between the plies. The consolidation achieved in terms of bonding between plies, fibre volume fraction, thickness of the part or the void content as compared to a theoretical value for a particular application of the part, may be termed as the extent of consolidation or level of consolidation. During consolidation the resin pressure may fall steeply ; in particular under conditions where the resin can flow out freely from the laminate’. Void formation and growth are facilitated by low resin pressures. A primary consideration during consolidation is to ensure that the resin pressure does not fall too low. Also consolidation of geometries with single curvatures involves the introduction of an additional issue of fibre migration 8,9. This effect is caused by unbalanced forces acting on the fibres due to the curvature causing them to move towards the inner radius. Consolidation mechanics and the extent of consolidation are determined by not only material and process parameters but also by the type of manufacturing process (on-line or post lay-up) and the form of the initial material. There are several types of polymer composite material forms available and their use depends on the particular processing method and the geometry of the part. Most thermoset composites are available in the form of prepregs, with the resin in an uncured or partially cured (generally B-staged) state. On the other hand, thermoplastic composite materials are available in a variety of forms as tow, fully or partially impregnated, as commingled fabric and as powder preforms. The schematic representation is shown in Figure 1. Consolidation requirements vary depending on material type (thermoset matrix or thermoplastic matrix) and form (prepreg, commingled, powder preform etc. ). Unlike fully impregnated materials, commingled materials and powder preforms require in-situ impregnation of the fibre reinforcement during the consolidation process, which may change the relative importance of transport mechanisms during the process. On-line processes involving the use of such materials have to ensure that sufficient time is available for both impregnation and consolidation. Thermoplastic prepregs exhibit low tack and drape, even at relatively high temperatures, causing them to be rigid and to have low conformability, thus requiring longer consolidation times. The high viscosities of thermoplastic matrices also play a major role in defining the efficiency of consolidation. Level of consolidation also depends on the process variables used during the manufacturing of the part. The crucial variables in any manufacturing situation are the applied pressure, temperature and time of application. A processing window needs to be established for each material form and class of manufacturing processes to obtain well consolidated parts, of repeatable quality, of acceptable interlaminar shear strength and void content. This requires knowledge of the phenomena dominating consolidation and the effect of process variables in determining consolidation quality.
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a
b
d l
Reinforcing
fibre
Matrix Figure 1 Various preforms: (a) commingled thermoplastic resin coated reinforcing ; (c)partially
bundles of reinforcing fibres and the matrix resin ; (b) impregnated tapes ; (d ) fully impregnated prepreg tapes
This paper presents an experimental study of the effects of crucial variables on consolidation in thermoplastic filament wound parts. The first section describes various ways to characterize extent or level of consolidation and experimental techniques for measuring it. In the subsequent section the relative importance of bonding between the plies and matrix flow is characterized to identify the dominant phenomena during the consolidation phase during on-line processing. Further, the effect of process variables on consolidation is investigated using filament wound rings with fully impregnated material form. CHARACTERIZATION CONSOLIDATION
OF STATE
In order to identify the phenomena occurring during the consolidation phase and to understand how these phenomena dictate consolidation quality it is necessary to be able to characterize the extent of consolidation occurring during the process. Consolidation can be characterized on the basis of parameters that are related to the state of the material such as void content or failure strength. Fibre volume fraction or part dimensions as secondary characterization parameters as they do not describe the state of the material directly. Hence optimization of the process should be based on the primary variables. Predictive capabilities to obtain a value for the consolidation state as a function of process parameters will eliminate time consuming and uneconomical optimization of process variables to consolidate parts manufactured under various process regimes. As consolidation constitutes of several phenomena occurring simultaneously or concurrently it is not possible to characterize the consolidation state by a single quantity which will compose the most general and complete description. There are several ways to characterize consolidation. One way to describe the extent of consolidation is to express the change in the level of consolidation as a function of the changes in microstructure of the material. The microstructure
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represents three phases: the fibres, resin and the voids. The consolidation quality improves if one can squeeze out voids and/or maximize fibre content. Thus one may select the changes in the fibre volume fraction or voids to monitor consolidation. However, although this is a necessary condition to enhance consolidation, it may not be sufficient as another phenomenon of good interply bonding between individual layers is also necessary to consolidate a part. The strength of interply bonds can be quantified in terms of adhesive forces which depend on contact area of the ply surfaces and the diffusion of polymer chains across the interface, so the interlaminar shear strength can also serve as a characterization parameter. These approaches require measurement techniques which cannot be monitored on-line or during the process. Another approach is to characterize consolidation on a macroscopic level. The changes in the microstructure of the part are visible on the macroscopic scale in terms of the changes in the dimensions of the part being consolidated, which allows one to express the level of consolidation as a function of the percentage change in the consolidated dimension. Consolidation involves more than one phenomenon and the manufacturing process determines which of the phenomena has the most influence on the consolidation mechanics. Therefore it is the process that will determine which description or descriptions are best suitable to quantify the consolidation state. For example, in a manufacturing process where consolidation is being characterized by monitoring part dimension the achievement of the required dimension(s) of the part does not necessarily translate into the part having acceptable strength or void content. This is especially true of high speed, short process time manufacturing as is exemplified by most on-line processes. MEASUREMENT
TECHNIQUES
The experimental measurement procedure will depend on the entity selected to characterize consolidation. If one chooses consolidation in terms of adhesive strength,
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one needs to measure the bond strength of the consolidated plies. The usual procedure is to carry out a fracture test on a sample specimen and measure the strength value. The extent of consolidation is then given by the ratio of the measured strength to that of the theoretical ultimate strength2~‘0~’ ‘. The theoretical strengths can be calculated from the reptation theory of DeGennes12 and its extension, the healing theory of Wool and co-workers’3*‘4. Strength measurements are carried out by various fracture testing methods, the most common ones being the DCB (double cantilever beam) test, the short beam shear test and the peel test. This method of characterization of consolidation will constitute a general description if there are no voids present in the consolidated specimen. However, as most consolidated parts contain voids, one could measure the void content of a part to monitor the extent of consolidation. Measurement of void content may be carried out by means of density measurements, ultrasonic testing or image analysis. Of these methods, density measurement is the least reliable. This technique involves measuring the actual density of the part using a density column or the buoyancy method and using the discrepancy between the theoretical value of the part density and the measured value to calculate the void content. The inherent error in this procedure is high due to lack of accurate density values which makes it the least reliable of all the methods of determining void content. Ultrasonic testing, which involves detecting the attenuation in a sound wave passed through the part to evaluate the void content, or fibre volume fraction, is limited by the available resolution and prevents detection of small voids. Image analysis involves obtaining an image of the cross-section of the part and then analysing the image to find the void content. The most successful techniques for obtaining images are optical. Voids, or fibres, in a matrix can be seen in an optical micrograph. One can cut and polish cross-sections using standard metallographic techniques. To analyse the images involves creating a photograph or video image using the requisite type of camera attached to a microscope. This image is then digitized using a frame grabber board attached to a personal computer. Using image analysis software based on counting the number of pixels above or below a selected threshold value allows one to determine the void content or fibre volume fraction. The drawback of image analysis is that in order to arrive at a reliable estimate of the void content, a large number of cross-sections must be analysed to obtain a reliable statistical average. This makes it an extremely time and labour intensive process. Further, for accurate results, a sharp contrast is a prerequisite for image analysis. The above preliminary descriptions for the state of the part require elaborate and tedious measurement techniques. Hence a simple approach to monitor consolidation is to measure dimensional changes. This is a useful indicator only if one has previously established the correspondence between part dimensions with levels of acceptable strength and void content for a given set of manufacturing conditions 15p1‘. A similar approach is used to measure consolidation in thermoplastic matrix composites with a compression thermal analysis of the process”. This technique considers the consolidation process to be a viscoelastic phenomenon and uses the consolidation strain Ed, with the original prepreg thickness as the basis, to measure the extent of
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consolidation. This procedure squeezes the prepreg system in a press and monitors the change in the lay-up thickness with an extensometer. This technique is simple and practical and can be used to obtain the consolidation characteristics of a particular thermoplastic material system before using it in an actual manufacturing process. This procedure also supplements information available from DSC and DMA techniques for a single ply specimen. The drawback of this procedure is that it does not account for process-related variations in consolidation behaviour. In this paper we will restrict our description of the extent of consolidation in terms of the void content and interlaminar shear strength. EXPERIMENTAL
DETAILS
A filament winding set-up consisting of an infrared strip preheater and an infrared nip point heater (see Figure 2) was used to wind rings under various process conditions. Filament winding was the on-line manufacturing process chosen to be studied as it demonstrates the key consolidation mechanisms seen in in-situ processes. The materials wound were fully impregnated APC~tows, with AS4 carbon fibre reinforcement, supplied by Imperial Chemical Industries, Ltd. (ICI). The tow had a fibre volume fraction of 60 f 2% by volume and an average width of 0.004 m. The rings wound were of 5 inch (0.125 m) inside diameter and consisted of only 90” layers. Process conditions were varied by changing the applied tension, winding speed and temperature. Process temperatures were controlled by changing the power settings of the heaters. The temperature of the two was measured as it emerged from the preheater with an infrared thermometer. Temperature profiles of the two were obtained by inserting a thermocouple at the nip point and allowing it to be wound along with the tow. No consolidation roller was used, hence the tow tension was the only mode of applying consolidation pressure. Extent of consolidation was calculated using both the primary variable descriptions ; void content and interlaminar shear strength. MEASUREMENT VARIABLES
OF PRIMARY
Four rings of 40 layers each were wound for tension set at 0,4, 6,8 and 10 ib (0, 17.79,26.69,35.58 and 44.48 N) respectively. A constant winding speed of 0.459 m s-l Infrared
thermometer
Figure 2 Schematic winding experimental
illustration set-up
of major
components
of the filament
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was used and the nip point and preheater power settings were also kept constant. The rings were removed from the mandrel using an outer sleeve and mechanical force with a hand held hammer, or an Instron. The interlaminar shear strengths of the rings was obtained from the ASTM Short Beam Shear Test (ASTM D 2344) 19. The specimens for these tests were prepared by first smoothing the edges of the rings using coarse grade sandpaper then cutting sections from the rings using a diamond saw. The sections were cut so as to conform to the specifications of the curved ASTM test specimen. The interlaminar shear strength was then measured using a three-point bend fixture on an Instron. The test was conducted at a crosshead speed of 0.05 in min-’ (2.12 x 10-5ms-‘). For each specimen, the test was repeated at four different locations. The interlaminar shear stress was obtained using the various test parameters in the standard expression for the ASTM Short Beam Shear Test. Void content of the rings was measured using optical microscopy and image analysis procedures. Sections were cut from the rings and mounted in epoxy such that the cross-section was exposed. The mounted sections were then polished, following standard polishing procedures, to obtain an optically smooth surface. On observing these polished ring cross-sections through an optical microscope, the voids in the rings could be seen. Quantification of the void content in a given ring was done using a Cambridge Instruments Quantimet 970 Image Analyzer as well as image analysis software available for Macintosh personal computers. The results of the void content in this paper are obtained using the image analysis software. This involved scanning in the image of the ring cross-section and analysing the image for the number of pixels above a threshold value that represented the voids visible in the image. To ensure accuracy of the measured void content, verification was carried out using the Quantimet 970 Image Analyzer. At least seven locations on each ring cross section were analysed to ensure statistical accuracy of the measured void content.
IDENTIFICATION PHENOMENA
OF IMPORTANT
There are several factors which play a role in the consolidation process during thermoplastic composite processing. These include resin flow, polymer chain diffusion, stiffness of the fibre network, diffusivity of air and volatiles in the molten matrix and elasticity of the matrix resin in the melt state. There are complex interactions between all of these factors, which can be classified into macroscopic (macro) or microscopic (micro ) phenomena. The chief macro phenomenon is the flow of the molten matrix resin which is controlled by momentum transfer. The primary micro phenomenon in consolidation is the diffusion of polymer chains across ply interfaces which is a thermodynamic phenomenon and involves the relaxation of surface chains into their Gaussian conformations as a result of applied temperature and time of application’3,‘4. In order to arrive at a fundamental understanding of the consolidation process in thermoplastic composites it is necessary to determine which of the phenomena, resin flow (macro) or polymer chain diffusion (micro), is dominant in determining consolidation quality in thermoplastic filament winding.
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This knowledge would also enable process conditions to be manipulated so as to maximize the effect of the dominant mechanism, thus resulting in optimum consolidation. A study was carried out to establish the relative importance between the two types of phenomena occurring during consolidation to decide which one plays a significant role in obtaining effective consolidation and also to establish their relative contributions under various processing conditions. This study was based on the hypothesis that under conditions of low applied tension at a given temperature diffusion would dominate the consolidation process as resin flow would be minimal. Under large values of applied tension, at the same temperature, substantial resin flow would occur and hence resin flow would be dominant in determining consolidation. This study was carried out using rings wound on the filament winding setup shown in Figure 2. These rings were wound over a range of temperatures, between 290 and 395°C with two rings being wound at each temperature, under conditions of low and high tension; that is 3.0lb (13.34N) and ll.Olb (48.93 N) respectively. As will be discussed later, the choice of primary variables to describe extent of consolidation in this process may be either void content or interlaminar shear strength. Both should give qualitatively consistent results. Hence interlaminar shear strength of these rings was measured using the ASTM Short Beam Sheat test described previously to establish the extent of consolidation.
PARAMETRIC
STUDY
Evaluation of the effect of process variables used in thermoplastic filament winding on the consolidation quality was sought via a series of experiments. The three main process variables in thermoplastic filament winding are the winding tension, the winding temperature and the winding speed. Rings were wound under a variety of process conditions from low to high tension (2.0, 5.5, 8.0 and 11.O lb (8.89,24.46,35.58 and 48.93 N) respectively), temperature (290°C to 390°C) and winding speed (0.2 m s-l to 0.9 m s-l, approximately). Three sets of experiments were carried out to investigate the effect of each process variable with one parameter being varied in each set while keeping the others constant. The extent of consolidation in the rings was evaluated using void content as the scale.
RESULTS The study determined that the primary consolidation characterization parameters were individually sufficient to characterize consolidation completely in the thermoplastic filament winding process. The shear strength and the void contents of the parts were measured and it is seen in Figure 3 that the nature of consolidation quality obtained was consistent using either parameter. The shear strength increased with increasing consolidation quality, while the void content decreased correspondingly, indicating that, independently, either parameter was sufficient to characterize completely consolidation. Although qualitative behaviour of consolidation state can be characterized and measured in terms of void content and/or shear strength, there is a need for a quantitative relationship between the level of consolidation and the
101
6000 ^ B 5 F 5 VI
-
Low
tension
5000
4000
2 z
3000
& I >"
2000
i
a 1000
Tension
(lb)
30
300
320 Winding
3
0
20
Figure 4 Variation high applied tension
0
340
360
temperature
380
400
(“Cl
in shear strength of rings wound under as a function of process temperature
low and
E J 5 03
.
10
0 0
0 -1
1
3
5 Tension
7
9
11
(lb)
Figure 3 Characterization of consolidation quality of filament wound rings as a function of applied tension in terms of (a) average shear strength and (b) void content
characterization parameters. It may also be noted that the extent of consolidation is a function of the transport mechanisms operative in the process. Hence, the relative importance of polymer chain diffusion as compared to momentum transfer will play a key role in the choice of the characterization parameter to indicate consolidation quality. Having established that either parameter was sufficient to completely characterize consolidation, the interlaminar shear strength was used to determine the dominant phenomena in determining consolidation, while the void content was used to study the effect of process variables on consolidation in thermoplastic filament winding. The results of the study to determine the dominant phenomena in consolidation are shown in Figure 4. It can be seen-that the interlaminar shear strength increases with increasing tension, or applied pressure, as the temperature is raised. This suggests that the extent of consolidation increases with increasing temperature and pressure. It has been observed that, even at the highest winding speed, the process time is of the order of the bonding (or reptation) time for PEEKS'. Therefore, the increase in interlaminar strength with applied pressure indicates the importance of the flow of the resin in determining consolidation. As the winding tension, or the applied pressure, increases the matrix flow in the interlaminar region increases permitting greater intimate contact between the mating surfaces and fewer interlaminar voids. This is confirmed by the corresponding increase in shear strength. This study shows that
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although increase in temperature results in enhanced diffusion it is the higher resin flow (which is a consequence of higher tension and processing temperature) that results in better levels of consolidation thus suggesting that the matrix flow is a crucial parameter to obtain well consolidated parts. The applied tension (or pressure) required to achieve intimate contact does decrease with increase in the winding temperature but the winding temperature itself is limited by the degradation temperature of the material being wound. Hence upper and lower limits exist for the applied tension defined by the tensile strength of the tow on one side and the applied tension required for consolidation at the degradation temperature of the tow material on the other. The results of the parametric study of process variables on consolidation in filament wound parts are summarized in Figures 5 and 6. Figure 5 clearly shows the qualitative trend of consolidation quality. It increases with increasing tension and temperature in the range of
30 q Tension
= 2.0
lb
Tension
= 5.5
lb
l
;
= 8.0lb
0 Tension
=ll
.Olt
q
20
.
zaJ +z 8 ; ’
q Tension
q l q
10
Q
q
q . q 0
0
0
0
-280
300
320 Winding
Figure values
340 temperature
360
380
400
(‘Cl
5 Void content variation in filament wound rings under of applied tension as a function of process temperature
four
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0.2
0.6
0.4
Winding
speed
Figure 6 Variation of consolidation in terms of void content as a function
0.8 (m s
-1
1 .o
1
quality of filament of winding speed
wound
rings
process variables studied. Here the consolidation quality is measured in terms of voids as opposed to Figure 4 where it was calculated in terms of shear strength. Higher temperatures enhance polymer chain diffusion, and the combined effect of higher tensions and temperatures result in increased resin flow and consequent intimate contact. Figure 6 shows the effect of winding speed on the extent of consolidation. High winding speeds result in shorter residence times for the material at the nip-point heater. This prevents the material from reaching the optimal temperature required for consolidation, resulting in poor consolidation. Low winding speeds translate into very high material temperatures due to the nip-point heating, which can cause partial degradation of the material leaving large voids between patches of dry fibres. From the discussion of the results of this investigation it is clear that the temperature plays a key role in controlling the consolidation quality since it is this variable that determines both the matrix flow and ply bonding through chain diffusion. The applied pressure also plays an important role since it governs the extent of flow at a given temperature. The interlaminar strength obtained is a synergistic effect of the bond strength controlled by temperature and the extent of intimate contact controlled by resin flow and, hence, by applied pressure and temperature. The process time, or the time for which heat and pressure are applied, has a bearing upon the extent of flow and bonding. It serves as a limiting factor below which consolidation is poor thus restricting the winding speed.
Experiments were conducted to identify dominant mechanisms during consolidation and also the role played by process variables on the quality of consolidation. The results show that the macro phenomena such as resin flow are dominant in determining consolidation quality in thermoplastic filament winding processes in which the applied tension on the tow is used as the only source of consolidation pressure. Hence, for a given processing temperature sufficient tow tension or pressure must be applied to ensure complete intimate contact in the process time available Furthermore, the winding temperature must be maintained such that sufficient time is available for bonding to occur without the occurrence of material degradation. These conclusions are borne out by the results shown above, from which it can be seen that, within a given range of process variables increasing tension and temperature enhance consolidation while too high or too low a winding speed tends to be detrimental to obtaining effective consolidation.
ACKNOWLEDGEMENT The use of the filament winding facilities at the Center for Composite Materials, University of Delaware is gratefully acknowledged.
REFERENCES I
2
3 4
5 6
7
8 9
SUMMARY
10 11
In this paper we have investigated experimentally consolidation in filament wound thermoplastic matrix composites. Characterization procedures to quantify consolidation have been described in detail and the techniques of void content and shear strength characterization have been applied to consolidation in a thermoplastic filament winding situation. It has been shown that for the filament winding process either method of quantifying consolidation is sufficient.
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12 13 14
15
Zeler, D.J. ‘Filament winding potential with advanced multi-axis computer controlled filament winding machines’ Am Sot Camp Proc (1988) pp 33-35 Lee, W.I. and Springer, G. ‘A model of the manufacturing process of thermoplastic matrix composites’ J Comp Muter 21 (Nov 1987) pp 1017-1055 Loos, A.C. and Springer, G.S. ‘Curing of epoxy resin composites’ .I Camp Mater 17 (1983) pp 135-169 Halpin, J.C., Kardos, J.L. and Dudukovic M.P. ‘Processing science : an approach for prepreg composite systems’ Pure and Appl Chem 55 No 5 (1983) pp 893-906 Gutowski, T.G., Morigaki, T. and Cai, Z. ‘The consolidation of laminate composites’ J Camp Muter 21 (Feb 1987) pp 172-188 Gutowski, T.G., Cai, Z., Bauer, S., Boucher, D., Kingery, J. and Wineman, S. ‘Consolidation experiments for laminate composites’ J Comp Muter 21 (Feb 1987) pp 650-669 Dave, R., Kardos, J.L. and Dudukovic, M.P. ‘A model for resin flow during composite processing : Part 1-general mathematical development’ Polym Comp 8 No 1 (Feb 1987) Calius, E.P. and Springer, G. ‘Modeling the filament welding process’ SAMPE Proc 31st Int SAMPE Symp (1986) Calius, E.P. and Springer, G. ‘Optimization of the cure window for a large filament wound case’ Report to Chemical Research Projects Office (NASA-Ames Research Center, Mountain View, CA, USA, 1984) Bastien, L. ‘Resistance welding of PEEK composites’ MS Thesis (University of Delaware, July 1990) Bastien, L., Don, R.C. and Gillespie, J.W. ‘Processing and performance of resistance welded thermoplastic composites’ Proc 45th SPI Annual Conference (Feb 1990) DeGennes, P.-G. J Chem Phys 55 (1971) p 572 Wool, R.P. ‘Crack healing in polymers’ ACS Organic Coatings and Plastics Chemistry 40 (1979) p 271 Kim, Y.H. and Wool, R.P. ‘A theory of healing at a polymer-polymer interface’ Macromolecules 16 (1983 ) pp 1115-1120 Muzzy, J.D. ‘Processing of advanced thermoplastic composites’ in Vol 4: The Manufacturing Science of Composites (Gutowski, T.G., Ed.), Proc Manufacturing International ‘88 (1988) ~~27-39
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16 Hou, T.C. ‘A resin flow model for composite prepreg lamination process’ ANTEC ‘86 (Society of Plastics Engineers, 1986) 17 Van West, B.P. ‘The draping and consolidation of commingled fabrics’ PhD Dissertufion (University of Delaware, May 1990) 1S Nelson, K.M., Manson, J-A.E. and Seferis, J.C. ‘Compression thermal analysis of the consolidation process for thermoplastic matrix composites’ J Thermoplastic Camp 3 (July 1990) pp 216-231 19 Annnal Book of ASTM Standards (1989) Part 36 20 Agarwal, V. PhD Dissertation (Materials Science Program, University of Delaware, Newark, DE 19716, June 1991)
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AUTHORS Suranjan Roychowdhury is a graduate student with the Materials Science Program at the University of Delaware and Suresh G. Advani, to whom correspondence should be addressed, is Assistant Professor within the Mechanical Engineering Department, also at the University of Delaware, Newark, DE 19716, USA. (Date received 25 February 1991; accepted in revised form 7 October 1991).
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