Analysis and optimization of temperature distribution in carbon fiber reinforced composite materials during microwave curing process

TL;DR: In this paper, the influence of ply orientation and thickness on through-thickness temperature distribution of carbon fiber reinforced composite laminates was investigated in two different types of epoxy systems.

About: This article is published in Journal of Materials Processing Technology.The article was published on 2014-03-01 and is currently open access. It has received 65 citations till now. The article focuses on the topics: Epoxy & Microwave.

Summary

1. Introduction

• Carbon fiber/epoxy composite materials with high mechanical strength, light weight and excellent high-temperature characteristics have gained substantial interest and been widely used in various commercial applications, especially in the aerospace industry.
• Li et al. (2013) and Shyha et al. (2010) reported that the application of high performance composites increased dramatically in aerospace industry and are expanding.
• Thus, microwave curing technologies have been investigated as an efficient energy delivering and uniform heating method to composite materials.
• Until now, the temperature gradient in the thickness direction of carbon fiber reinforced composite materials has still not been measured quantitatively.

2.1 Materials Used

• The laminates used in this experiment are T300/6509 and T300/10128 carbon fiber/epoxy unidirectional prepreg.
• The resin content of both prepreg is 33%.
• The two materials are typically used for the production of high performance composite structures, such as composite fairing, panels and wind ribs of aircrafts.
• They respectively represent the average toughness with low glass transition temperature epoxy resins, and high toughness with high glass transition temperature epoxy resins.
• Other supporting materials are supplied by Airtech Company, such as vacuum bag L500Y, release film Peelplysc01, breather B150 and perforated release film RP3.

2.2 Sample Preparation

• Cutting and lay-up of two types of prepreg.
• It was adhered along the edge of composite materials and compacted using vacuum pressure, as shown in Fig.
• This setup allowed the microwave processing of composites without arcing and had a better performance.
• The temperature at the top surface, center and bottom surface of the composite laminates were measured by embedded FBG sensors in the materials.
• For the ply orientation experiment, the samples had the same thickness (7.5 mm), but with different ply orientations.

2.3 The Curing Procedure and Devices Used

• Accommodate the vacuum pipe and FBG sensors.
• Two electromagnetic microwave generators have a variable power output of 0 to 1.5 KW at a frequency of 2.45 GHz and are installed at the bottom of the multiple mode applicator.
• The temperature was measured by an SM125 optical sensing interrogator, and was transferred to the PC through the Internet.
• The technological process is explained in a schematic diagram, as shown in Fig. Boey (1992) applied vacuum bags for consolidation, and less void contents were observed in microwave cured composites.

2.4 Temperature Measurement

• Electromagnetic wave cannot heat metal materials, but generate electrical current in the metal conduit.
• Degamber and Fernando (2003) declared that FBG sensors essentially consist of serious intra-core gratings.
• Where B is the reflected Bragg wavelength, n is the average refractive index of the fiber core, and D is the grating spacing.
• Therefore, the temperature of a composite part in a microwave oven can be calculated accurately through measuring the movement of center wavelength caused by external thermal strain.
• A method was proposed to solve this problem.

3. Results and Discussion

• To verify whether the measuring approach mentioned above would affect the temperature distribution, a non-contact Infrared thermometer was used to measure the temperature at the surfaces of composite materials.
• Temperature differences of the measured results of the two samples were within 0.6 ± 0.1°C.
• Due to the complexity and immaturity of microwave curing process, the appropriate process parameters need to be investigated to guarantee the reliability of experimental results.
• A representative temperature profile of a composite sample measured by FBG sensors in the thickness direction is shown in Fig.
• The large temperature differences were measured during dwell stages (i.e., certain temperature was set and kept constant for a period).

3.1 Temperature Distribution Influenced by Ply Orientation

• Through employing the verified process parameters mentioned above, the temperature distribution of carbon fiber reinforced composite laminates influenced by ply orientation was investigated.
• The statistical results are designated in the top of every column.
• With regards to the T300/6509 composite materials, the maximum temperature difference between statistical results and target temperature values are almost the same, which are shown in Fig. 6 (a) and (b).
• The center and bottom temperatures are higher than the target temperature.
• The complex [0/+45/-45/90] ply orientation samples have the temperature values close to the [0] lay-up composite laminates.

3.2 Temperature Distribution Influenced by Ply Thickness

• Apart from the influence of ply orientation, the influence of ply thickness is also considered in this research.
• The stacking sequence of three samples is [0/90].
• The measured surface temperatures of three samples are lower than that of the middle and bottom, just as the experiment results shown in Fig.6.
• The obvious temperature difference in composite laminates may impair the advantages of microwave curing technology.
• The optimisation methods will be discussed later in this article.

4. Temperature Distribution Analysis and Optimization

• Under the action of an electromagnetic field, the temperature at the surface of carbon fiber reinforced composites is lower than the target temperature, no matter for what kinds of composite laminates.
• As the composite surface has a large contact area with the air and the carbon fiber bundles are good conductors of heat, the heat conduction primarily happens at the top surface of the composite laminate.
• To reduce the temperature difference between the center and bottom of the laminates, the microwave energy need to penetrate into the materials from both the top and the bottom sides.
• The modified microwave curing technology is shown in Fig.8.
• The statistical results of temperature differences of three curing technologies are shown in Table 3.

5. Conclusions

• In order to reduce the no-uniformity of temperature distribution in the thickness direction of carbon fiber reinforced composites, vacuum assisted microwave processing technologies and modified optical sensing systems were applied to the study of this problem.
• The statistical results of temperature values of T300/6509 and T300/10128 composite laminates demonstrated that the ply orientation did not affect the temperature distribution of composites.
• The thickness was an important influencing factor.
• According to the experimental results, the physical mechanisms during microwave curing were analyzed and the vacuum assisted microwave curing technologies were modified to optimize the temperature distribution.
• As a result, he even temperature distribution in the thickness-direction of carbon fiber reinforced composite was achieved using modified microwave processing technologies.

Figures

Table 1 Thermo-mechanical properties of polymer systems.

Fig. 8. The modified microwave curing technologies.

Fig. 1. Sample preparation for the composite curing process

Fig. 3. Vacuum assisted microwave curing process in a multiple model microwave oven.

Table 2 Experimental parameters of ply orientation and thickness

Fig. 7. Statistical temperature difference influenced by thickness with standard deviation: (a)

Fig. 2. The microwave curing and temperature measurement method

Table 3 The temperature differences of three curing technologies.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Analysis and optimization of temperature distribution in carbon fiber reinforced composite materials during microwave curing process" ?

Two different types of epoxy systems have been studied. The maximum temperature difference of the samples formed using themodified microwave curing technologies in this research could be reduced by 79 % to 2. 1 °C. 

Q2. What are the future works in "Analysis and optimization of temperature distribution in carbon fiber reinforced composite materials during microwave curing process" ?

Future studies will be carried out to identify the relationship between several physical mechanisms in microwave curing to get a better temperature distribution and establish the standard process specifications. 

Q3. What is the physical mechanism of microwave curing?

The physical mechanism of microwave curing includes the natural heat convection of air inthe microwave oven, heat conduction inside the composite material and the directly heating action of microwave. 

Q4. What was the result of the composite curing cycle?

Using the vacuum pressure technology and ceramic tooling, the composite samples achieved lower cost in manufacturing and better performance. 

Q5. What is the reason why Bogetti and Gillespie reported that the complex gradients of temperature?

Bogetti and Gillespie (1992) reported that the complex gradients of temperature in thickness direction of composite laminates may cause serious process-induced residual stress and deformation. 

Q6. What is the effect of the microwave on the surface of composites?

Under the action of an electromagnetic field, the temperature at the surface of carbon fiberreinforced composites is lower than the target temperature, no matter for what kinds of composite laminates. 

Q7. What is the effect of microwave curing on the reaction rates of carbon fiber reinforced composites?

Boey and Yap (2001) reported that microwave curing was more effective than conventional heating in enhancing the reaction rates during cross-linking by testing different curing agents in curing. 

Q8. What is the condition to be satisfied to get such a Bragg reflected wavelength?

The condition to be satisfied to get such a Bragg reflected wavelength is known as Bragg condition and is given by Grattan and Meggitt (1997) as: 2B nD （1） 

Q9. What are the thickness values of composite laminates?

The thickness values of composite laminates are 4.5, 7.5 and 22.5mm, corresponding to sample #1, sample #2 and sample #3 respectively. 

Q10. What are the important dwell stages?

The most important dwell stages were studied to analysis the temperature distribution influenced by ply orientation and thickness. 

Q11. What is the definition of a periodic perturbation of the refractive index?

A periodic perturbation of the refractive index can be formed along the fiber length through the change of reflection wavelength influenced by the physical or mechanical characteristics of the grating area. 

Q12. What are the two main solutions for the temperature distribution of composite laminates?

Based on the analysis, in order to uniform the temperature distribution of compositelaminates, different solutions need to be applied. 

Q13. What is the reason why composites are becoming so popular?

For the above reason, alternative processing technologies, such as microwave curingtechnologies have been developed to form composite structures and provide a uniform temperature distribution. 

Q14. How was the vacuum pressure used in the microwave cured composites?

Considering the previous work, the 0.09Mpa vacuum pressure was provided when heating from 100 °C or 90 °C to the end of heat preservation. 

Q15. What was the curing cycle of the composite laminates?

The composite laminates were laid up on a ceramic tooling coated with release agent and the edge areas were covered by sealant tapes.