Autoclaving is a process that ensures the highest quality of carbon fiber reinforced polymer (CFRP) composite structures used in aviation. During the autoclave process, consolidation of prepreg laminas through simultaneous elevated pressure and temperature results in a uniform high-end material system. This work focuses on analyzing in a fundamental way the applications of pressure and temperature separately during prepreg consolidation. A controlled pressure vessel (press-clave) has been designed that applies pressure during the curing process while the temperature is being applied locally by heat blankets. This vessel gives the ability to design manufacturing processes with different pressures while applying temperature at desired regions of the composite. The pressure role on the curing extent and its effect on the interlayer region are also tested in order to evaluate the consolidation of prepregs to a completely uniform material. Such studies may also be used to provide insight into the morphology of interlayer reinforcement concepts, which are widely used in the featherweight composites. Specimens manufactured by press-clave, which separates pressure from heat, are analytically tested and compared to autoclaved specimens in order to demonstrate the suitability of the press-clave to manufacture high-quality composites with excessively reduced cost.
Autoclaving is among several manufacturing methods for polymeric composites. It is a process mainly utilized in the field of aviation as it guarantees the highest quality for composite structures. The autoclaving process brings the individual prepreg plies together and consolidates them through pressure into a uniform solid material. Elevated temperatures are necessary to initiate and complete the curing reaction for thermoset-based prepregs. This work focuses on an effort to analyze in a fundamental way the applications of pressure and temperature separately during prepreg consolidation. A controlled pressure vessel has been designed that applies pressure during the curing process while the temperature is being applied locally by heat blankets. This vessel gives the ability to design manufacturing processes with different pressures while applying temperature at desired regions of the composite. The role of pressure on the curing extent and its effect on the interlayer region are also tested in order to evaluate the consolidation of prepregs to a completely uniform material. Such studies may also be used to provide insight into the morphology of the interlayer reinforcement concepts such as
The combination of heat and pressure applied on prepreg plies is necessary in order to consolidate them into a fiber-reinforced polymer composite based on fabricated prepregs. Furthermore, the curing reaction initiation and completion of thermosetting prepreg materials requires elevated temperatures. Several techniques, such as press- or autoclave operations, are available to conduct this manufacturing operation [
Although autoclaving is the manufacturing process used for almost all the polymeric composite parts utilized in aerospace field, in our days, the economic situation requires solutions that will be more affordable than the conventional autoclave processing without sacrificing quality. Efforts are made to manufacture materials of the same quality but with less expensive methods such as vacuum-assisted resin transfer molding (VARTM) [
Such an affordable method is the pressurized vessel or “repair clave,” such as the one manufactured by Heatcon Composite Systems in Seattle, WA, USA. The main characteristic of this clave is the separation of pressure and temperature while curing. Despite its first purpose of being utilized as a repair technique in aviation, this work brings the beginning of a breakthrough to prove that this technique can be used for manufacturing as well, without major differences in the final material product quality. Carbon fiber prepreg panels were manufactured in the clave under different pressures, and they were tested in dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) to investigate the pressure effect on the dynamic mechanical properties as well as on the curing quality of the materials. The description of future work needed to further investigate this effort is also mentioned in this paper [
High-quality polymeric composites manufacturing requires high-pressure procedures of 70 to 90 psi such as autoclaving, which is used in high tech applications and mainly in aviation. The autoclave is a device that can generate a controlled pressure and temperature environment. While several autoclave types are available, all consist primarily of three units: a pressure vessel, a heating/cooling system, and a control unit. When prepreg-based composites are processed using autoclaves, they undergo several steps prior to the actual autoclaving. These steps usually follow a strict procedure because they have been found to affect the final properties of the composite [
Autoclave entire lay-up [
After completion of the vacuum bag, the laminate can be autoclaved using a certain temperature and pressure profile. In composite fabrication, the autoclave is usually pressurized with nitrogen so that fire hazards imposed by the exothermic prepreg materials are reduced [
Generally for autoclaves, the usual processing procedure for thermosetting prepreg curing is to firstly increase pressure and right afterwards heating the autoclave at a chosen heating rate to the desired temperature. According to most fluids behaviour, the viscosity of the matrix will decrease with this temperature rise. As a result, at elevated temperature, the resin will freely flow, facilitating the consolidation process until eventually the chemical cross-linking starts occurring and forming gelation. At this point, the resin will soon change from a liquid into a solid, and it will start preventing viscous flow. It is, therefore, important that the chemorheology of the prepreg resins is known in order to complete consolidation and volatile removal prior to resin gelation. A typical autoclave cycle containing two isothermal dwells is shown in Figure
Typical diagram of temperature and pressure profile for curing in a clave [
Nevertheless, high pressures might eventually drive too much resin out of the fiber bed leading again to void formation, resulting from resin starvation this time. Even though much research has focused on the autoclave consolidation and processing optimization, the developed models are of limited use. So, many variables such as prepreg type, part dimensions, vacuum bag materials, lay-up, and processing conditions influence the consolidation that the composites industry has determined almost all operating conditions by trial and error. Consequently, the aerospace industry has tried to employ standardized procedures. For instance, the majority of these parts are cured using 121°C or 178°C standard cure cycles. In addition, one distinguishes between high-, intermediate-, and low-flow prepreg systems. Considering this information, the lay-up can be adjusted such that the optimum part qualities can be accomplished [
The repair clave (Figure
Heatcon Composite System—Repair Clave Model HCS3100 [
Although the main difference between a repair clave and an autoclave comes from the fact that one is mainly dedicated to repairs of composite parts while the other for manufacturing, it is believed that a repair clave could become a useful tool in manufacturing simple parts at a much more affordable cost. Currently, autoclaves could be used for repairs also, if extensive rebuild of a part is needed. The repair clave can be referred to as an affordable autoclave, as it reaches almost the same pressures, but it permits the use of localized heat which results in much more economical cures. However, one needs to investigate if the quality of a repaired part can be maintained at the standards of a part manufactured through an autoclave.
There are of course many differences that put the two claves apart, but by understanding these differences, one can see at which common point the repair clave might be used as a possible future autoclave for manufacturing simple composite parts (Table
Repair clave and autoclave advantages and disadvantages [
Repair clave (for the model in Figure |
Autoclave | |
---|---|---|
Qualification | Qualified for repairing composite parts | Qualified for manufacturing and repair of composite parts |
Design considerations | For a vessel of 4 m (12 ft) length and 0.9 m (3 ft) diameter less than 2 cm thickness needed | Thickness of vessel has to be large, as when pressure and temperature are applied together the deformation of the vessel is larger (e.g., for a 3 m (9 ft) autoclave, 0.6 m (2 ft) diameter, temperature < 343°C (650°F), a minimum 4.5 cm thickness is needed) |
General installation | The only extra equipment it needs is a commercially available air compressor | Installation is an important consideration: foundation, cooling water supply, electrical supply, gas (if used for heating), and pressurization medium supply and exhaust arrangements. |
Operating pressure | Max to 75 psi (517 kPa) |
Max operating pressure to 85 psi (586 kPa) |
Pressurizing system | Through an external air compressor (e.g., for the model in Figure |
Three pressurization gases are typically used for autoclaves: air, nitrogen, and carbon dioxide |
Heating system | Through heat blankets. Heat blanket thermal uniformity is essential for the curing quality. If the heat-up rates for the heat blankets are not respected, cracks can appear in the structures | Gas heating is regularly used in autoclaves with maximum operating temperatures of 450 to 540°C (850 to 1000°F). Steam heating is often used for autoclaves operating in the 150 to 175°C (300 to 350°F) range. Most small autoclaves (under 2 m, or 6 ft in diameter) are electrically heated. Gas circulation provides mass flow for temperature uniformity and heat transfer to the part load. However, gas stream can cause thermal or mechanical shock on the manufactured parts |
Electrical system | “Plug-in wall” (e.g., electrical supply: 90 Volts AC to 264 Volts AC 47 Hz to 63 Hz, 0.15 Amps; power 50 Watts) | Even small autoclaves are not designed to just plug in and run (e.g., electrical supply: 230 Volts AC/50 Hz (110 Volts on request); power: 450 Watts) |
Vacuum system | Similar system | Similar system |
Control system | Similar system | The cure cycle is controlled by feedback from thermocouples, transducers, and advanced dielectric and ultrasonic sensors. Computer-controlled systems are used, but they are far more complex |
Loading system | Similar system | Similar system |
Safety | Has a pressure relief valve in case of overpressure; because the temperature is separated from the pressure, there is a decreased risk possibility | Use of pressurized gas to cure has redundant safety features on any autoclave because of the potential seriousness of any malfunction |
Affordability | The electrical system, reduced need for safety features, reduced thickness of vessel, and therefore decreased manufacturing costs |
High costs can come from various factors; electrical consuming system, gas used for pressurization (nitrogen can be very expensive and air is very dangerous), requires redundant safety features |
Repaired/manufactured parts | Limited geometries that can be created/repaired due to the 2D heat blanket geometry; also, usage of heat blankets limits the number of plies that can be completely cured | Theoretically, any 3D geometry can be manufactured. Higher number of prepreg plies can be processed at the same time. However, when used for repair, areas not being repaired are subject to high temperatures which may cause deterioration to existing bonds and finishes |
In order to investigate the role of pressure during manufacturing, when it is applied separately from the temperature, and how it affects the final material, four different plates were manufactured under four different process pressures. The four different pressures were 0, 30, 50, and 70 psi. Each panel had eight plies of woven carbon fiber prepreg (type: Cytec Engineered Materials BMS 8–297, HMF 934 Carbon Prepreg Fiberite 934, resin density 1.3 g/cc). The configuration of the plies is (0, 90, ±45)s. Dimensions of the panels were
The prepreg plies were stored in a refrigerator at a temperature below 0°F or −17.7°C. They were cut to the desired dimensions and orientation. After that, the lay-up took place. The plies were placed in the desired configuration, and on top of them, all the layers described in Table
Lay-up before curing in repair clave [
Bagging material |
|
The vacuum has to be at least 22 in Hg 0°C. When the desired vacuum is accomplished, the laid-up panel enters the vessel. The vessel is sealed, and the pressure can be adjusted to the desired level. Since the pressure is at the level required by the experiment, the temperature profile by controlling the heat blanket initiates curing. All the measurement parameters can be controlled by a data acquisition system attached to the vessel. The processing/claving lasts about 4 hours. The first step elevates the temperature up to 177°C with a rate 2.5°C/min, and then it goes through an isothermal period of two hours at 177°C. After two hours in the isothermal phase, the temperature starts reducing with a rate of 2.5°C per min. The pressure is released, and the panel is ready after removing all the lay-up extra layers.
The differential scanning calorimeter that was used for these experiments was a 29 series DSC of TA Instruments, model 291°. Three different temperature elevating rates were used, 0.5, 1, and 5°C/min. For all of those three different rates, the samples were elevated up to 400°C. Taking the panels manufactured in the pressurized vessel “repair clave,” small samples of 5 to 20 mg weight were cut for all the four different panels. Hence, eventually, experiments of the 4 different panels manufactured at 0, 30, 50, and 70 psi were tested in 3 different rates, that is, 0.5, 1, and 5°C/min.
The manufactured samples are also characterized by scanning electron microscopy, through visual examination of surfaces resulting after manufacturing. Samples were cut in dimensions of
The dynamic mechanical analyzer used for this study was a Triton DMA. Two different temperature elevating rates were used, 1 and 5°C/min. For both of these two different rates, the samples were elevated up to 300°C. The samples were tested in 3-point bending experiments. Taking the panels manufactured in the pressurized vessel “repair clave,” samples of approximately
The specimens were tested in a screw Instron machine (model 4505). More than five specimens were tested for each of the four different CFRP panels of 0, 30, 50, and 70 psi manufactured. Specimens of
Furthermore, specimens from the panels were also tested under compression testing in the same Instron machine that flexural testing took place. Specimens of
At first, uncured samples of the prepreg with which the panels were manufactured were tested in DSC. Figure
Uncured samples heat of cure comparison.
Secondly, dynamic scans from 0 to 400°C, at heating rates of 5°C /min, 1°C/min, and 5°C/min, were carried through for all different samples manufactured under the pressures of 0 psi to 70 psi. The sample weight varied from 3 mg to 20 mg, and generally, two samples were collected for each of the pressure cases in each of the heating rates, in order to verify the results.
Graphs are provided to give an indication of the evaluation of the samples in DSC based on the exothermic peak.
In Figure
DSC scans at 1 and 5°C/min for different pressure cases ((a) 0 psi; (b) 70 psi).
Figure
Eight plies samples comparison for the rate 5°C /min at 0, 30, 50, and 70 psi.
Scanning electron microscopy (Figures
(a) All different samples manufactured in different pressures. (b) Samples manufactured at 0 psi, showing increased void formation.
(a) The entire thickness cross-section of a sample manufactured at 30 psi. (b) Void also formed at 30 psi sample.
(a) The entire thickness cross-section of a sample manufactured at 50 psi. (b) Smaller voids also formed at 50 psi sample.
(a) The entire thickness cross-section of a sample manufactured at 70 psi. (b) Even smaller and less voids formed at 70 psi sample.
Graphs are provided to present the storage modulus (Figure
Storage modulus of carbon fiber prepregs—8 plies manufactured at 0, 30, 50, and 70 psi in 1 and 5°C /min.
It can be seen that there is a slight increase of the modulus while the manufacturing pressure increases, which is expected, as the higher the pressure, the less void formation within the composite. There are other factors such as density change, degree of curing, or glass transition temperature change, which one could think that may be responsible for this modulus increase; nevertheless, the first argument regarding the fewer voids with higher pressure is the most prevailing one as first, the density does not significantly vary with the pressure variation according to thermogravimetric analysis that has been performed [
Normalized moduli comparison of 0 psi, 70 psi, and 70 psi annealed.
Storage modulus of carbon fiber prepregs—8 plies manufactured at 0, 30, 50, and 70 psi in 1 and 5°C/min.
It is observed that the glass transition temperature decreases while the pressure increases. The glass transition reduction at 1°C /min heating rate is in the order of around 30°C (
This can be explained due to internal stresses [
Finally, the
The loss modulus (not shown here) influenced by the elastic energy that is lost to the environment basically decreases as the pressure increases presenting a proportional behavior to
Two different tests were performed for the CFRP specimens manufactured under 0, 30, 50, and 70 psi. As stated in the experimental description, the first testing was flexural mechanical testing (three-point bending). Figure
Flexural testing stress-strain curves for (a) 0 psi, (b) 30 psi, (c) 50 psi, and (d) 70 psi.
Following, in Figure
Stress-strain comparison of CFRP specimens manufactured under 0, 30, 50, and 70 psi derived from flexural testing.
It can be seen that the specimens manufactured under 70 psi have a larger inclination than the specimens manufactured under 0 psi in relation to the strain axis, which is translated to a higher modulus.
The flexural modulus is calculated by (
By calculating the flexural modulus with type 2 applied for the elastic region of the stress-strain curve, the values of the flexural modulus are similar to the ones calculated by (
Equation (
Regarding the compression testing that was performed, Figure
Compression testing stress-strain curves for (a) 0 psi, (b) 30 psi, (c) 50 psi, and (d) 70 psi.
Following, in Figure
Stress-strain comparison of CFRP specimens manufactured under 0, 30, 50, and 70 psi derived from compression testing.
It can be seen that there are no large differences in the inclination of the stress-strain curves at the elastic region. This means that the modulus of CFRPs manufactured under different pressures does not present significant changes in compression loads from low to higher pressures while manufacturing. The compression modulus is derived from (
The moduli and strength behaviour can also be seen in Table
Comparison of specimens manufactured in press-clave and autoclave.
0 psi | 30 psi | 50 psi | 70 psi | 85 psi autoclave processing | |
---|---|---|---|---|---|
Flexural modulus (GPa) | 41.8 |
47 |
51.67 |
58.67 |
60 |
Flexural strength (MPa) | 448.8 |
522.46 |
547.9 |
613.4 |
618 |
Compression modulus (GPa) | 8.88 |
9.96 |
9.29 |
9.02 |
10.4 |
Compression strength (MPa) | 42.1 |
40.47 |
39.97 |
43.13 |
44.5 |
The results presented in Table
The analysis made throughout this section of innovative manufacturing with DSC, DMA, flexural and compression, and SEM characterizations gives the picture of the quality of composites manufactured through the press clave. DSC testing assures that composites are properly cured; DMA examines the viscoelastic behaviour of composites; flexural testing exhibits the stiffness improvement analogously with the higher manufacturing pressure; SEM gives information on the surface quality of the samples manufactured under different pressures. Furthermore, compression tests show that both modulus and strength under compression do not vary, and as a result, they are not affected by the different manufacturing pressure. Consequently, the well-cured specimens under the press-clave, the acceptable surface that they present in SEM characterization, and most importantly the slight differences in flexural strength and modulus under high-pressure manufacturing demonstrate that the press clave can be utilized as a decent alternative manufacturing process of the extremely expensive autoclave. Of course, this high-quality manufacturing through the press clave is achieved by the use of heat blankets, which are necessary for achieving the appropriate curing temperature profile while manufacturing and the thermal uniformity within the part while curing. The latter is very accurate in conventional autoclaves as it is very important for high-quality CFRP parts. Thus, the heat blanket is a mandatory tool in press clave manufacturing for accurate thermal uniformity within the part that is manufactured. Nevertheless, besides the obvious financial and other benefits from the utilization of heat blankets, their limitation compared to the heating system of an autoclave is the three-dimensional and the complex geometry manufacturing of composite parts.
The DSC results were not conclusive with respect to expected percentages of curing. It could not be concluded that the percentage of cure would be higher with a higher pressure, and also it was noticed that the differences from one pressure to another were quite small. These small differences could appear also because of the way the lay-up is done or the way the vacuum was maintained through the curing process. It is also possible that the differences are so small because of the small number of plies subjected to the experiments. An increased number of plies and an increased number of experiments could offer more definitive answers with respect to the influence of pressure on the manufacturing process of composite parts. In the future, it is planned to further investigate the press clave manufacturing technique and specifically test several specimens in DSC that have been manufactured with increased number of plies and more experiments to investigate the percentage of curing in several levels of the composite thickness, which will be demanded since the number of plies is increased.
Furthermore, the dynamic mechanical analysis DMA gives the expected results regarding the storage modulus reaction with the manufacturing pressure increase. The higher the manufacturing pressure the higher the modulus. However, there is a significant decrease in glass transition temperature as the manufacturing pressure, increases. This happens due to internal stresses formed in the materials when pressure is higher. Then, these internal stresses are relieved during heating and soften the material, resulting in a reduced
Additionally, scanning electron microscopy observation was performed for samples from all pressures. The results verified the storage modulus increase, as the higher the pressure, the less void formation. Finally, as far as the mechanical testing is concerned, the specimens under flexural testing exhibited a significant increase of both the flexural modulus and strength as the manufacturing pressure increases. On the other hand, the compression testing showed that the compression modulus is not dependent on the manufacturing pressure, and thus, besides a slight increase from 0 to 70 psi which is considered negligible, it presents relatively similar values for all the specimens. Similar behavior of manufacturing pressure independence is presented by the compression strength as well. The mechanical comparison with autoclaved samples exhibits the press clave potential as a curing alternative of the highly costly autoclave.