use of a simple, inexpensive pressure sensor to measure hydrostatic resin pressure during processing...

Use of a Simple, Inexpensive Pressure Sensor to Measure Hydrostatic Resin Pressure During Processing

of Composite Laminates

KATHERINE LYNCH, PASCAL HUBERT, and ANOUSH POURSAHIlP

Composites Group, Department of Metals and Materials Engineering The Unwersity of British Columbia

Vancouver, British Columbia, V6T 124, Canada

A simple and cheap method of measuring the resin pressure within a composite laminate during processing is presented. The method consists of using a small diameter, long needle filled with inert fluid and connected to an external pressure sensor, to measure the resin pressure at a point inside a composite laminate. This method can be used to investigate resin flow, laminate compaction, the control of voids, and in several composite material processing methods such as autoclave processing, hot press curing and resin transfer molding. The sensors are suitable for research and development or troubleshootug, but not for production. Sensor assemblies were developed and tested to show that their response is reproducible, linear and stable with temperature and time. Resin pressure profiles for two AS4/3501-6 laminates were generated and compared. The experimental results were also compared to the resin flow simulation of a general processing model for composites, COMPRO. It is shown that the resin profile in the laminate is influ- enced by the presence of the bleeder cloth and the vacuum bag pressure. A si@- cant pressure drop corresponded to the point of minimum viscosity of the resin. Finally, the resin pressure was stabilized when the resin reached gelation.

1. INTRODUCTION take more and more of the applied load. Thus in this

n general, increased understanding of laminate pro- I cessing enables cure cycles to be optimized to produce a good quality laminate in the shortest amount of time. In addition, specific measurements of para- meters indicative of flow, such as laminate resin pressure profiles and mass loss, are useful for the validation of process models. The ability to measure the resin pressure inside laminates would be extremely useful in identifying and solving processing problems, such as uncontrolled resin flow or voids, and would aid in reducing the number of rejected parts.

Mathematical models of resin flow in composite laminates have been developed with various approaches ranging fmm lubrication theory to soil mechanics and with different simplifications and assumptions related to the resin flow and the shape and packing of the fibers [see review by Hubert and Poursartip (l)]. The most realistic mathematical model currently used is the model based on the fiber consolidation experiments of Gutowski (2). The compacting laminate is treated as a porous consolidating bed of fibers. According to this view. as the laminate is compacted, the interaction between fibers increases and the fibers

model, there is a resin pressire gradient throughout the entire laminate, until complete compaction occurs and the resin pressure drops to zero. Differential equations developed for soil mechanics, which govern three-dimensional flow within a consolidating porous bed, are solved. The resin pressure determined from these equations is then used in Darcy’s law for flow through a fully saturated porous medium, to determine the resin velocity at any point in the laminate, at any time during the processing cycle. Smith and Poursartip (3) coded the flow model of Gutowski into a computer model, Lamcure, along with a heat transfer model developed by LQOS and Springer (4). More re- cently, this flow model was implemented in a two-dimensional finite element processing model for complex shaped laminates, COhfPRO (5, 6). The validation of the theories used in these models rely on experimental data, particularly the evolution of resin hydrostatic pressure and more generally, a measure of the flow and compaction behavior of the laminate during cure.

Determining the flow behavior in curing composite laminates is a difficult task. The scale of fibers and resin is small with dimensions of the order of 10 pm, the fibers are not ideally placed in uniform ranks, and

POLYMER COMPOSflS, AUGUST 7 9 9 9 , Vd. 20, No. 4 581

Katherine Lynch, Pascal Hubert, and Anoush Poursartip

the flow is affected by numerous factors including temperature, heating rate, and pressure difference. For autoclave processing, matters are further compli- cated since resin flows both parallel and perpendicu- lar to the toolplate. Several methods have been used to try to understand and measure fluid flow in composite laminates, including mass measurements of the bleeder layers (4, 7, 8). study of fluid flow through ideal cylinder beds and aligned fibers (9), and molecu- lar "tagging" of resins (10).

A different approach to understanding the resin flow in a composite is to determine the resin pressure within the laminate. The pressure difference across the laminate is the driving force for resin flow and therefore fundamental to its understanding. The absolute resin pressure at any point in the laminate is also vital to the study and control of voids. A simple approach consists of monitoring hydrostatic pressure at the interface between the tool and the part (1 1, 12). This gives information about the pressure at only one point on the tool. An attempt to measure the resin pressure within composite laminates with flat, postage stamp-sized (0.34 mm X 15 mrn X 20 mm) sensors was investigated with limited success (13). Several problems were found with these flat sensors. They displayed time-dependent, hysteritic responses to pressurization and depressurization. Also, the sensor responses were very noisy, and there was a large variation in response from sensor to sensor. In addition, there is uncertainty as to what these sensors measure since they cover a relatively large area in the plane of the composite. If the fibers are pressing against the sensors, then a combination of the fiber bed pressure and the resin hydrostatic pressure would be registered. These problems led Smith (13) to conclude that the flat sensors could be used only in a qualitative fashion. Fiber-optic sensors have also been developed to monitor pressure and temperature during cure (14). The probes have a diameter of 100 pm and are permanently embedded in the part after cure.

. Fiber-optic sensors are typically used to monitor temperature and pressure profiles during pultrusion (15). The major problem with this technology is the very high cost of the sensors and the associated instru- mentation. Hopefully, the costs will decrease significantly in the future.

In this paper, an inexpensive pressure sensing device for measuring resin pressure within a laminate is presented and this device is used to investigate the resin pressure distribution in composite laminates during cure. Furthermore, the experimental results are compared with simulations conducted with the process model COMPRO.

2. DEVELOPMENT OF PRESSURE SENSORS

Measuring the pressure in composite laminates has proven to be a challenge, mainly because of the physi- cal nature of the laminate. A typical lamina is about 0.125 mm thick. Therefore a pressure sensing device

should be close to this size in order to minimize its impact on the conditions that it is trying to report. In addition, a laminate contains both solid elements, the fibers, and a surrounding fluid, the resin. Thus, previ- ously there have been problems determining what an embedded sensor is measuring: the fiber bed pressure, the resin hydrostatic pressure, or a combination of both. The operation of the sensor assemblies described in this paper is sirnilar in principle to a static pressure tap. A small fluid-filled tube extends into the composite laminate. The hydrostatic pressure at the tip of the tube is transmitted by the fluid inside to a sensor mounted outside the laminate. The present sensors will be used in quasi-static applications where fluid dynamics effects are not relevant and can be ignored.

There are two main benefits to this type of sensor. The first is the sensor's ability to measure only the resin hydrostatic pressure. The end of the tube is cut at 90" to the applied force on the laminate. This means that even if the tube deforms, the only pressure that can be transmitted to the sensor is the hydrostatic resin pressure, since it acts in all directions. Before gelation of the resin in the composite, deformation of the tube will also not affect the sensor reading, because the fluid will simply flow out the end of the tube into the laminate. The second benefit is the size and shape of the sensor assembly because the long, thin shape of the tube is very unobtrusive to the flow within the laminate. These tubes easily fit within two layers of a typical composite laminate.

The elements of the tube sensor include a tube, resmoir, plug and a pressure sensing device as shown schematically in Flg. 1. The tubing used in the experiments reported here was 1 mm OD (19 gauge) needle tubing with a 90" tip. The pressure sensing devices used are SenSym SClOOA diaphragm sensors, which function as Wheatstone bridges. These sensors were chosen because they are inexpensive, and reported very good thermal stability (0.1% FSO) and linearity (0.25% FSO). The sensors were connected as resistors and thus produced output in Ohms. For use in a laminate, the sensor assembly is filled with fluid taking care that no air is trapped inside. Uncatalyzed epoxy resin was used for the fluid, since its similar c o m p i - tion should reduce the risk of incompatibilities such as chemical reactions and non-wetting between the sensor fluid and the laminate resin.

The response of the sensors, without the tube assemblies, was tested in several ways, before the sensors were used in composite laminates. The sensors were subjected to varying pressures at both room and high temperatures, and to constant pressure with varying temperatures. A typical sensor response to pressure at room temperature is shown in Rg. 2. The pressurization and depressurization curves overlap closely, with negligible hysteresis. There was not much variation between sensors but each sensor was cali- brated separately to accurately obtain the relationship between resistance and pressure. The sensors were

582 POLYMER COMPOSITES, AUGUST 1999, Vol. 20, No. 4

Hydrostatic Resin Pressure

Q. I. schematic of the tube sensor assembly.

found to be stable with time, and to respond mmmally to changing temperature. This temperature insensitivi- ty is illustrated by Rg. 3, which combines one sensor’s responses to pressure ramps at both 26°C and 80°C.

The sensor response is both linear and reproducible, despite the difference in testing temperatures. This re- producibility was verified by retesting these sensors, after they were used in a laminate run, and had therefore been through an entire cure cycle. On comparing the results from this pressure cycle with the resistance curves from before the laminate run (Rg. 4). excellent agreement is obtained with a maximum difference for any sensor (shown in Rg. 4) of 5 Ohms at 448 kPa. This translates to 16 kPa, or a 3.5% variation. This consistent linear behavior made it possible to convert the resistance data to pressure for each sensor by the use of a simple, linear we-fitling routine. Thus before a sensor was used, it was subjected to a pressure cycle and its calibration equation determined from a plot of resistance versus pressure.

The response of the sensor assembly to a fluid hydrostatic pressure change was tested. A sensor tube was placed in a reservoir containing silicone oil having a viscosity of 0.1-1 Pa.s, which is similar to most epoxy resins at typical curing temperature. The reservoir and the sensor assembly were placed in an autoclave where a pressure cycle was applied. Rgure 5 shows the normalized pressure gradient and sensor

0 4

Rg. 2 Qpical sensor response to pressure at room temperature.

0 4 100 150 200 250 300 350 400 450 500 550 -

Rg. 3. 80°C.

Q p i m l sensor response to pressure at 26°C and [..*-.BsforsLaminateI - A f t ~ ~ L u n i ~ ( c I I

Q. 4. Comparison of typical sensor response before and a- Laminate I.

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Applied Pressure ...... Sensor Response

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Fig. 5. Response of typical sensor and tube assembly.

voltage with time. From Fig. 5, a maximum lag of 20 seconds is observed at low pressure, and the lag decreases with increasing pressure. The lag measured is very small compared to the rate of change of pressure in most practical applications, which occur over a pe- riod of minutes. Thus, we can conclude that the response of the sensor with the tube assembly and the transmitting fluid is fast enough to monitor pressure changes in the laminate.

Hg. 6. Location of sensors for (4) ami in ate I and ami in ate n.

3. LAMINATE EXP- Two experiments with 10 cm X 10 cm, AS4/3501-6

carbon fiber reinforced epoxy laminates will be described. They are designated Laminates I and I1 re- spectively. The laminates were cured in a laboratory autoclave, and each laminate was laid up with layers of bleeder cloth placed on top of the laminate to absorb the excess resin duxing compaction. The laminate lay- up was [0/90,/0],, with a nominal thickness of 6.5 mm and each had tube sensors positioned: at the interface between the bleeder and the top ply, 1/4, 1/2 and 3/4 through the laminate thickness, and between the bottom ply and the toolplate (see Rg. 6). This stacking sequence was chosen so that the tubes could be laid up between two plies with the same fiber orien- tation, thus minimizing the disturbance of the fiber compaction and resin flow. Vacuum bag sealant was then used to dam the edges of each laminate, and to prevent resin flow out of the laminate along the fibers, parallel to the plane of the toolplate. Laminate I was laid up with 36 plies of bleeder cloth to absorb the excess resin from the laminate (Fig. 6a). For Laminate XI, the number bleeder layers was increased to 48, and another sensor was placed near the top of the bleeder layers, and called the upper bleeder sensor (Fig. 6b). Rgwe 7 shows a photograph of a laminate with the sensors coming out of the bag, before cure.

@)

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FQ. 7. Photograph showing sensors and bag before cure for Lamina& I.

To prevent any disturbance in the laminate from one sensor tube affecting the response of any other sensor, the tubes were laid up such that only one tube was in a given vertical line. As shown in Fig. 6, the tubes were separated horizontally by at least 10 mm, and all of the tube ends were 6 mm from the centerline of the laminate. After cure, both laminates were sectioned, polished and inspected under an opti- cal microscope. The distortion of the laminate around a sensor tube extended approximately 1 mm in all directions. A micrograph showing the tip of one sensor in Laminate I illustrates the relative size of the tube and the laminate plies (Rg. 8).

The cure cycle applied to Laminate I is shown in Fig. 9. This cycle was used in order to facilitate comparison of the results to that of previous resin pressure experiments using flat sensors (12). The pressure was released in steps at the end of this cycle in order to ob- serve the response of the sensors to pressure in the

F@. 8. Ph0-h showing a CTOSS- section of an embedded tube in Jhmhfi?Iajk??rcure.

cured laminate. For the same reason a rapid 345 Wa pressure spike was applied at the end of the cycle. For Laminate 11, the cure cycle was essentdly the same as for Laminate I, but without the final pressure spike.

3.1 Lpminrtt I Opening up the vacuum bag after cure showed that

all 36 plies of the bleeder contained resin. Clearly resin flow occurred during the cure cycle. The experimental results from Laminate I are shown in Fig. 10. When the pressure in the autoclave is increased, all of the sensors, with the exception of the interface sensor, react by reading an increasing pressure with time. The time for a sensor to reach its highest pressure depends on where it is in the laminate, with the toolplate sensor first, progressing in sequence to the 3/4 point sensor, the 1/2 point sensor, and finally the 1/4 point sensor. At the start of the test the laminate behaves as a solid because the resin is too viscous to

POLYMER COMPOSITES, AUGUST 1999, Vd . 20, No. 4

Katherine Lynch Pascal Hubert, and Amush Poursartip

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8 500 v

E 400 a E 300 a 200

100

0

I- Autoclave - - - - - - Vacuum - - - Temperature I m. 9. Cure cycle for Laminate I.

flow and therefore bears elastically the pressure applied to it. At this stage, the fibers are generally sur- rounded by resin, there is little contact between fibers, and so they bear little of the load. The interface sen-

sor, because of its placement on the surface of the laminate next to the bleeder, does not have resin at its tip and reads the pressure in the bleeder, i.e. the vacuum pressure.

800 I 700

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0 , 0 30 60 90 120 150 180 210 240 270

Time (min)

li4 Point I Autoclave - Vacuum - - - - - - Interface ---- I- 1/2 Point - - - 3/4 Point - Tooblate ~ ~~

Hg. 10. Resin pressure throughout the cure cyc&for Laminate I.

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At approximately 35 minutes after pressurization (t = 50 minutes), all the sensors in the laminate show a drop in resin pressure. At the same time, the interface sensor shows an abrupt increase in resin pressure that corresponds to the decrease in the other sensors. The viscosity of the resin is now low enough that it begins to flow. As the resin flows out from between the fibers, the fibers start to interact and take the applied pressure and thus the resin pressure drops. The order of the sensors, with decreasing pressure, at any point in time is toolplate, 3/4 point, 1/2 point, 1/4 point, interface. Thus a pressure gradient is present in the laminate. The resin moves up through the laminate, and into the bleeder, past the interface sensor. As the end of the interface sensor tube is covered now by this resin, it begins to read a resin pressure as well. All of the sensors (including that at the interface)

level off at about one hundred minutes and maintain their relative values for the duration of the test. As the laminate compacts, the fibers are forced together, making it more difficult for the resin to pass through. At the same time, the resin is continuously curing, and becoming increasingly viscous. At some point, the resin can no longer be forced through the fiber bed by the pressure gradient across the laminate, and flow stops.

3.2 Laminate II wure I 1 shows the experimental results from Lam-

inate 11. The results from Laminate I1 are similar to

those of Laminate I (see Flg. lo), but not as consistent. As before, all of the sensors show increasing pressure, after the application of autoclave pressure. In this case however, the interface sensor also reacts almost immediately. although it is very noisy, as does the upper bleeder sensor. It is thought that the interface and upper bleeder sensors read pressures higher than the vacuum registered by the external vacuum gauge owing to a small leak in the vacuum bag. If this leak were at a position removed from the vacuum line, a pressure difference could exist horizontally across the laminate, resulting in the conflicting readings.

One explanation for the slow response of the toolplate and 1/2 point sensors in this laminate is that if the fibers around the tip of a sensor tube do not come together tightly there will be a gap that is ini tdy not filled with resin. This could occur because of too large of a Illlsalignment of the tube with the fiber axes. In such a case, the sensor will register the pressure in the gap, and not in the resin. In Laminate II, all the sensors, ex- cept the interface and the upper bleeder sensor, eventually reach the applied pressure. Then at approximately 35 minutes after pressurization (t = 50 minutes) the laminate sensor readings start to drop and, as in Laminate I, the interface sensor shows an increase in pressure corresponding to the decreases registered by the other sensors. Once again, a resin pressure gradient is evident aRer the sensor responses level off, with the sensors again in the order (with decreasing pressure): toolplate, 3/4 point, 1/2 point, 1/4 point, interface. Thus, the differences between the two tests can be

attributed to a slight loss of vacuum across the vacu-

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Vacuum - -Upper Bleeder - - - - - - Interface

---. 1/4 Point -.-- 1/2 Point --- 3/4 Point Toolplate

- Autoclave -

Q. 1 1. Resin pressure throughout the cure cycle for Laminate U.

POLYMER COMPOSITES, AUGUST 1999, Vol. 20, No. 4 587


um bag, as described earlier. Both the interface and upper bleeder sensors read increasing pressures with the pressurization of the autoclave, and the upper bleeder sensor shows an elevated pressure for the duration of the test. Overall, there was less flow and less of a drop in the resin pressure throughout Laminate II. Less of a vacuum results in less of a pressure gradient across the laminate, which translates to less driving force for resin flow.

3.3 P~-GelatJOIl Behavior

At the end of each laminate run the resin is solid. Thus the resin pressure is no longer transmitted from fluid resin to sensor fluid, as occurred before the resin gelled. The applied pressure was reduced in steps, and in Laminate I raised briefly to 446 kPa and released, in order to investigate the response of the sensors. This response was then examined to determine what the sensors measure after gelation of the resin occurs.

The 1/4 point sensor in Laminate I (Rg. 10) and the toolplate, 1/4 point, and 1/2 point sensors in Lami- nate II (Rg. 11) do not step down consistently with the other sensors, and appear to react to the autoclave temperature, decreasing with decreasing temperature. Examination of the tubes for these sensors revealed that all but the Laminate II 1/2 point sensor tube had become sealed, no doubt by the migration of catalyzed resin (or catalyst) up the tubes. The 1/2 point sensor tube was likely partially blocked also, however, this could not be verified. None of the tubes that consistently reacted to the pressure drops was found to be sealed. Once a tube becomes sealed with cured resin, no pressure change can be felt by the diaphragm because the sensor assembly has become a fixed volume system. As the temperature drops, the liquid resin in the tube shrinks, thereby releasing some of the pressure against the sensor diaphragm. A drop in sensor resistance after a tube assembly has become blocked indicates a drop in temperature, not a drop in pressure. The majority of the sensors in all the laminates respond to the stepping down of the applied pressure at the end of the cure cycle. The laminates are now solid and cannot transmit pressure directly to the sensor assemblies. However, the tubes are still filled with fluid and so, for each sensor tube, there must be a boundary between these two media, formed when the resin solidifies.

The spacing of the crosslinked network of the solid epoxy matrix is determined by the resin pressure at the time of gelation, at that point in the laminate. Resin that gelled at a lower pressure will have a larger network spacing than resin that gelled at a higher pressure. As the autoclave pressure is reduced, the laminate expands elastically, increasing the network spacing and pulling the boundary slightly away from the tube, thereby releasing some of the pressure on the sensor diaphragm. For a given autoclave pressure drop, the pressure change throughout the laminate will vary, because of the resin pressure gradient across the laminate thickness. Resin that gelled at a

588

lower pressure will expand less, because it experi- ences a smaller pressure change. At the end of the Laminate I run, the unsealed sensors all correctly register the applied pressure throughout the short pressure spike. Nevertheless, the sensor behavior past the resin gelation is not an issue, since the purpose of the sensor is to measure resin hydrostatic pressure during flow prior gelation when the resin is still fluid.

4. MODEL SIMULATIONS

The experiments were simulated using a finite element processing model, COMPRO, developed for autoclave composite structures manufacturing (5). Figure 12 shows the finite element mesh and the boundary conditions used. One-dimensional heat transfer, flow and compaction was assumed, therefore only a slice of the laminate-tool-bleeder assembly was modeled. Convective heat transfer conditions were applied at the tool and bleeder surfaces exposed to the autoclave air (Td. Resin flow was restricted to the laminate re- gion and free flow into the bleeder was triggered by setting a fixed resin pressure (PvAJ at the bleeder- laminate interface. The cure cycle shown in Fig. 9 was

Fig. 12. Mesh &$nition for simulations. HT = 6 mm H , = 7- and^, = 5 and 6 m j i ~ ~ a m i n a t e I andn mspciiueb-

POLYMER COMPOSITES, AUGUST 1999, Vol. 20, No. 4


Table 1. Predicted Time of Pressure Drop (Minutes) at Different Locations in the Laminate, Compared to the Experiemental Results.

Location Laminate I Laminate I1 -

Experiment Free Flow Bleeder Flow Experiment Free Flow Bleeder Flow Simulation Simulation Simulation Simulation

114 Point 36 112 Point 41 314 Point 48 Toolplate 54

35 35 41 41 46 46 48 48

40 40 47 45 53 52 67 57

40 45 52 57

applied to the model and two bleeder-laminate interface resin pressure conditions were simulated. The first case, referred as free flow, was to set PvAc to the measured vacuum bag pressure. This is the approach commonly used by the majority of flow models. The second case, referred as bleeder flow, consisted of setting PvAc to the pressure measured by the interface sensor for Laminate I and 11. This condition is equiva- lent to including the bleeder resistance to resin flow.

Material properties used in the model were obtained from various sources in the literature or measured on the prepreg used in the experiments. The resin initial degree of cure was measured with DSC (dynamic scanning calorimeter) scans and a rather advanced value of 0.26 was found. The initial fiber volume fraction was measured by resin digestion at 43%. The pa- rameters for the resin cure kinetics and viscosity models were taken from Loos and Springer (4). The fiber bed compaction curve was taken from Dave et al. (16) and the fiber bed permeability model used can be

found in Gutowski et al. (17). The resin pressure was extracted from nodal locations corresponding to the sensor positions. Finally, predicted resin mass losses and final fiber volume fractions were also compared with the experiments.

4.1 b i n preutve

The predicted resin pressure profiles are presented in Figs. 13-16 and are compared to the experimental results. In general, good agreement is obtained when the bleeder flow boundary condition was applied to the model (Figs. 14 and la. However, for the free flow condition, the resin pressures predicted are significantly lower then the measured pressures (Figs. 13 and 15). The onset of the pressure drop is not influ- enced by the pressure boundary condition, as shown in Tabk 1 and good agreement with the experiments was found. As shown in Rg. 17, the variation of the measured

resin profile can be closely related to the resin viscosity

800 1 I

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I ir

0 30 60 90 120 150

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0 Interface 0 1/4 Point A 112 Point X 314 Point Toolplate -COMPRO I Rg. 13. Measured and predicted resin p r e s m variation for Laminate I wmpared with the simuhho . n us@freeflow wnditions at the bleeder.


Katherine Lynch Pascal Hubert, and Anoush Poursartip

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Time (min) I 1 I 0 Interface 0 1/4Point A 1/2 Point X 3/4Point X Toolplate -COMPRO I

Rg. 14. Measured and predicted resin pressure Uariation for Laminate I compared with the shudatw . n using bleederflow wnditbns atthebleeder.

variation. Flow begins as the resin viscosity decreases to approximately 10 Pa.s. When the resin reaches minimum viscosity, the flow rate is maximum and the fastest pressure drop is recorded by the pressure sensor. Finally, as the resin gels, the viscosity increases, which eventually stops the flow and no further resin

pressure variation is measured. The final resin pressure profiles predicted and measured through the thickness are shown in Fug. 18. Again, good agreement was found for the bleeder flow boundary condition. This result illustrates the importance of applying the proper flow boundary condition when simulating lami-

c e a E a

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the bIeeder.



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Q. 16. M e a s u r e d and predicted resin pressure variation for Laminate II wmpared with the simulation using bleederflow conditions atthebleecier.

nate compaction in bleed conditions. The compacted bleeder layers offer a significant resistance to flow of

the incoming resin from the laminate. Although the bleeder cloth has a low permeability compared to the

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Interface - 1/4 Point - 1/2 Point *3/4 Point -Toolplate -Viscosity I Q. 17. M e a s u r e d resin pressure variation fm Laminate I with corresponding resin viscosity predicted by COMPRO. l%e gel point of the resin is also shown at lo0 Pas.

POLYMER COMPOSIES, AUGUST 1999, Vd. 20, No. 4 591


Interface 1/4 Point 1/2 Point 3/4 Point Toolplate

Position

0 Experiment --- COMPRO Free Flow - COMPRO Bleeder Flow

Fig. 18. stable resin pressure pmms a m 120 minutes compared with simUIati0 Rs.

fiber bed at atmospheric pressure, when placed under the autoclave pressure, the bleeder is compacted and its permeability decreases significantly (18).

4.2 Reain Masa Lou and Fiber Volume Fraction 5. DISCUSSION

Nevertheless, the trends observed experimentally are well predicted by the model and are consistent with the resin pressure profiles.

The resin mass loss ratio and the fiber volume fraction predicted are compared to the experimental results in Table 2. Again, better agreement is found for the bleeder flow condition. However, the difference with the free flow condition is not as large as the difference in the resin pressure results. In general, lower resin losses and consequently lower fiber volume fraction are predicted by the model. It is important to mention that the accuracy of the prediction of the laminate mass losses is strongly related to the fiber bed compaction curve used in the simulation (6). Since the compaction curve used in the simulations was taken from a published reference, it is quite possible that the prepreg used in the experiments has a slightly different compaction curve.

The sensor developed in this work has since been modified to operate in more robust industrial conditions. Smaller needles were used and silicone oil was used as the pressure transmission fluid. These new sensors were tested in large laminates having non- uniform geometry and containing honeycomb cores. The performance of these modified sensors was not satisfactory and this was attributed to several factors. First, the prepreg used had a high initial fiber volume fraction. Second, the resin system used had a higher viscosity and third, the laminates were cured under no-bleed conditions. All these conditions resulted in less resin flow and made the reading of resin pressure more difficult. Furthermore, the silicone oil tends to flow into the laminate causing undesirable contami-

Table 2. Predicted Resin Mass Loss Ratio (%) and Final Fiber Volume Fraction (%), Compared to the Experimental Results.

Location Laminate I Laminate II

Experiment Free Flow Bleeder Flow Experiment Free Flow Bleeder Flow Simulation Simulation Simulation Simulation

22 19 61

26 21 26 21 64 59

16 14 55

26 15 26 15 64 53

~

'Recun mass loss ratio based on laminate mass loss. "Recun mass loss rat10 based on bleeder mass gain.

592 POLYMER COMPOSITES, AUGUST 1999, Vol. 20, No. 4


nation between the laminate plies. To correct that problem, a membrane could be added at the tip of the tube. But this solution would increase significantly the cost and the complexity of the sensor, since one would have to compensate for temperature effects, tube compression and fluid compression. However, the new sensors were used successfully to measure the internal core pressure of the honeycomb panel. It is important to ensure that the additional volume of the needle and pressure sensor cavity is small compared to the cell size, or the effect must be allowed for.

0. CONCLUSIONS

Cheap, reusable pressure sensors were developed to monitor the hydrostatic resin pressure inside a curing composite laminate. The tube sensor assemblies are non-hystentic, absolute devices that produce reproducible, linear results that are stable with time and temperature. If a sensor tube becomes sealed, it will no longer measure pressure, but will be primarily sensitive to temperature changes. The sensors are suitable for research and development or troubleshooting, but not production. The sensors were tested in two flat laminates under bleed conditions. The resin pressure within a laminate increases with the applied pressure until flow begins, at which time the resin pressure drops off progressively through the laminate thickness, from the top surface to the toolplate. A resin pressure gradient exists across the entire laminate thickness until full compaction occurs or the applied pressure is released. The flow module implemented in a finite element processing model, with appropriate material para- meters and boundary conditions, can produce a reasonable estimate of the resin pressure throughout a laminate during cure, and the final laminate fiber volume fraction.

7. ACKNOWLEDGMENTS This work was supported by funding h m the Natural

Sciences and Engineering Fbearch Council of Canada. The authors would like to thank Roger Bennett, Serge Milaire, and Greg Smith at The University if British

Columbia for their assistance with the experiments discussed in this paper, and Dr. Karl Nelson and his colleagues at the Boeing Company for the follow-on industrial evaluation of the sensors.

8. REFERENCES 1. P. Hubert and A. Poursartip, J. Reinf. Plast. Comp., 17, 286 (1998).

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'Computer-Aided Cure Optimization," Proc. 1- .rial


use of a simple, inexpensive pressure sensor to measure hydrostatic resin pressure during processing...

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