Flexural and interlaminar shear strength properties of carbon fibre/epoxy composites cured thermally and with microwave radiation

Abstract

The ease of heating an epoxy resin with microwaves depends, among other factors, on the dielectric properties of its components at the frequency of the radiation used. The majority of the papers published on the microwave curing of reinforced epoxy resin composites have used widely available DGEBA type resins and amine hardeners such as 4,4'-diaminodiphenylsulphone (DDS). This paper investigates the use of two epoxy systems where the choice of resin and hardener was based on their measured dielectric loss factors. System 1 contained a resin and hardener with higher loss factors than those used in System 2. The two systems were formulated with polyetherimide (PEI) as a toughening agent. Unidirectional carbon fibre prepregs were prepared from both systems. Composites were laid up from these prepregs which were then cured in three different ways: autoclave curing, partial autoclave curing followed by microwave post-curing, and microwave curing. System 1 composites had greater flexural properties and interlaminar shear strengths than System 2 composites when autoclave cured. Flexural properties and interlaminar shear strengths were greater for System 2 in the microwave post-cured composites. When fully microwave cured the properties were similar. In the microwave cured composites the flexural and interlaminar shear properties were influenced by the structure of the phase separated PEI and the void content. AbstractThe ease of heating an epoxy resin with microwaves depends, among other factors, on the dielectric properties of its components at the frequency of the radiation used. The majority of the papers published on the microwave curing of reinforced epoxy resin composites have used widely available DGEBA type resins and amine hardeners such as 4,4'-diaminodiphenylsulphone (DDS). This paper investigates the use of two epoxy systems where the choice of resin and hardener was based on their measured dielectric loss factors. System 1 contained a resin and hardener with higher loss factors than those used in System 2.The two systems were formulated with polyetherimide (PEI) as a toughening agent. Unidirectional carbon fibre prepregs were prepared from both systems. Composites were laid up from these prepregs which were then cured in three different ways: autoclave curing, partial autoclave curing followed by microwave post-curing, and microwave curing. System 1 composites had greater flexural properties and interlaminar shear strengths than System 2 composites when autoclave cured. Flexural properties and interlaminar shear strengths were greater for System 2 in the microwave post-cured composites. When fully microwave cured the properties were similar. In the microwave cured composites the flexural and interlaminar shear properties were influenced by the structure of the phase separated PEI and the void content.

Optimization of curing cycle in carbon fiber-reinforced laminates: Void distribution and mechanical properties

“…6c. The porosity increased from the surfaces towards the interior of the panel in this case and the void volume fraction in the center of the 1 It should be noted that the average laminate thickness was kj4.6 mm but kj0.5 mm in each surface were not included in the tomographic analysis. panel for unidirectional laminates was much higher than that found in multiaxial panels subjected to the same curing cycle.…”

“…A prepreg raw sample of 25 mm in diameter and 4 plies in thickness was placed between the plates of the rheometer and subjected to an oscillatory shear strain of 0.05% and 1 Hz frequency while imposing a specified temperature cycle. The dynamic viscoelastic response of the composite prepreg in terms of the storage and loss moduli, C and G", respectively, is modified by the cross-linking reactions and the evolution of the complex viscosity if (modulus of real and imaginary parts) can be determined as if | C' + iC" (1) where m is the frequency of the oscillatory strain. The presence of the fibers in the prepreg increased storage component of the shear modulus and it was not possible to determine the gel point using the standard criterion for neat resins, i.e.…”

“…Consolidation is normally carried out in an autoclave because the high hydrostatic pressure (>0.7 MPa) ensures full compaction of the laminate, leading to a maximum fiber volume fraction and negligible porosity and, thus, to optimum mechanical properties. Autoclave processing has, however, major drawbacks from the industrial viewpoint (long processing time, limited production rate and component size, high capital investment) and there is an enormous interest in optimizing out-of-autoclave processing routes so high performance composites can be processed at higher rates and lower cost [1][2][3][4][5].…”

Effect of laminate edge conditions on the formation of microvoids in composite laminates

“…<2%) are most often manufactured by the autoclave process [6][7][8]. Yet the composites industry is increasingly considering lower cost alternative processes such as hot-press molding because autoclave curing is expensive partially due to the high cost of the required equipment [8][9][10][11]. However, before hot-press molding or other out-of-autoclave processes can be implemented as an alternative to autoclave curing, techniques must be developed to allow for components to be fabricated with a comparable high quality.…”

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

“…But for carbon fiber reinforced composites, Nightingale and Day (2002) reported that the presence of carbon fibers in the composite may affect the penetrating depth of the microwave energy, because of the electrical conductivity and electromagnetic reflection of carbon fiber bundles. The experiments accomplished by Hunyar et al (2006) also showed that the anisotropic properties of composite materials may affect the temperature distribution.…”