Microwave curing of carbon-epoxy composites: process development and material evaluation
File(s)
Author(s)
Kwak, Museok
Type
Thesis
Abstract
Conventional heating methods offer limited opportunities in cycle time reduction due to the low thermal conductivity of polymer matrices. Microwaves can offer advantages in this sense, however past limitations in hardware, software and lack of suitable operating procedures led to inconsistent results and conclusions.
In the present study 2.4 mm, 4.8 mm and 60 mm thick out-of-autoclave unidirectional carbon-epoxy composites were cured using a homogeneous microwave system at 2.45 GHz. A methodology to measure the microwave penetration depth, and to cure carbon fibre reinforced polymer composites without arcing (thus producing reliable and consistent results) were proposed.
The results of the curing methodology adopted was demonstrated by measuring the degree of crosslink, axial and transverse tension, axial compression, in-plane shear, indentation, mode I interlaminar fracture and fibre push-out testing.
With a suitable microwave cure cycle, the axial tensile performance was similar to those cured using a conventional process. The transverse tensile and in-plane shear strength of microwave-cured samples were lower than those produced using conventional means. In axial compression tests, the microwavecured
samples exhibited a significant increase in strength. In interlaminar fracture testing,
conventional oven-cured samples showed lower fracture toughness values with little variation with crack propagation, whereas the microwave-cured samples showed an increasing trend with crack propagation, and a greater average toughness value. The fibre-matrix interfacial shear strength was
also greater in the microwave-cured samples. The increase in fibre-matrix interfacial shear strength was proportional to the increase in the interlaminar fracture toughness.
The changes in properties were due to a change at the microscopic level of the matrix, where the preferential microwave heating of the carbon fibres produced a microstructure where the epoxy closest to the fibres reached a higher degree of cure compared with the rest of the matrix, as well as an increased fibre-matrix interfacial bond. Away from the fibre the matrix was more rubbery, thus producing a multi-phase brittle-ductile region.
The contribution to knowledge in the thesis was the conclusion of pragmatic studies, which now
permits the manufacture of unidirectional composites cured using microwaves without the damage and inconsistencies that occurred using previous methodologies.
In the present study 2.4 mm, 4.8 mm and 60 mm thick out-of-autoclave unidirectional carbon-epoxy composites were cured using a homogeneous microwave system at 2.45 GHz. A methodology to measure the microwave penetration depth, and to cure carbon fibre reinforced polymer composites without arcing (thus producing reliable and consistent results) were proposed.
The results of the curing methodology adopted was demonstrated by measuring the degree of crosslink, axial and transverse tension, axial compression, in-plane shear, indentation, mode I interlaminar fracture and fibre push-out testing.
With a suitable microwave cure cycle, the axial tensile performance was similar to those cured using a conventional process. The transverse tensile and in-plane shear strength of microwave-cured samples were lower than those produced using conventional means. In axial compression tests, the microwavecured
samples exhibited a significant increase in strength. In interlaminar fracture testing,
conventional oven-cured samples showed lower fracture toughness values with little variation with crack propagation, whereas the microwave-cured samples showed an increasing trend with crack propagation, and a greater average toughness value. The fibre-matrix interfacial shear strength was
also greater in the microwave-cured samples. The increase in fibre-matrix interfacial shear strength was proportional to the increase in the interlaminar fracture toughness.
The changes in properties were due to a change at the microscopic level of the matrix, where the preferential microwave heating of the carbon fibres produced a microstructure where the epoxy closest to the fibres reached a higher degree of cure compared with the rest of the matrix, as well as an increased fibre-matrix interfacial bond. Away from the fibre the matrix was more rubbery, thus producing a multi-phase brittle-ductile region.
The contribution to knowledge in the thesis was the conclusion of pragmatic studies, which now
permits the manufacture of unidirectional composites cured using microwaves without the damage and inconsistencies that occurred using previous methodologies.
Version
Open Access
Date Issued
2015-09
Date Awarded
2016-08
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Robinson, Paul
Wise, Roger
Sponsor
TWI Ltd
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)