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The effect of processing on the microstructure of CFRP
The effect of processing on the microstructure of CFRP S. C. THOMPSON*, H. C. K I M t and F. L. MA TTHEWS* It has proved possible to produce void-free laboratory test specimens of CFRP provided care is taken during the processing. Of particular importance is the dwelling time, ie the time that is allowed to elapse between placing the mould in a heated press and applying the pressure. A dwelling time longer than the ideal is more damaging to the specimen than a dwelling time which is too short.
The basic methods of preparing laboratory specimens of carbon-fibre/resin composites are now well established. However the effects of variations in the basic manufacturing process on the microstructure and mechanical properties of the specimen are, in general, poorly documented, although some basic work has been undertaken in glassfibre/resin systems. 1 The most common form of microstructural defect is the void, a space within the composite caused by air or vapour entrapped during the cure process. Even a small quantity of voids can cause a marked reduction in the mechanical properties, in particular the strength, of a composite. In the project described in this report, carbon-fibrereinforced resin specimens were produced by hot pressing preimpregnated fibre: vacuum impregnation and curing methods were not employed. It was felt that the most critical part of the process was the dwelling time, ie the time that is allowed to elapse between placing the loaded mould into a heated press and applying the pressure to close the mould. Accordingly, specimens were produced from a range of dwelling times and the resulting microstructure examined and strength in bending determined.
SPECIMEN PREPA RATION The mild steel mould used in this series of tests produced finished specimens which were 100 mm long x 19 mm wide x about 3 mm thick. All specimens contained unidirectional fibres parallel to the length. Manufacture of some preliminary specimens showed that void-free specimens could be obtained using a dwelling time of about 10½ min.
*Aeronautics Department, Imperial College of Science and Technology, Prince Consort Road, London SW7, UK tPhysics Department, Chelsea College, Manresa Road, London SW3, UK
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Such specimens are referred to as 'correctly dwelled'. Similarly, specimens made with dwelling times shorter or longer than the ideal are referred to as 'under dwelled' and 'over dwelled' respectively. The material used was Courtaulds' untreated Graffl A carbon fibre, Ciba epoxy novalac LY558/HT973 prepreg of 0.125 mm nominal thickness. Approximately 40 thicknesses of prepreg were needed to till the mould cavity with the plunger fully closed. To ensure adequate pressure during curing the mould was overfilled by 30%, giving a total of 53 thicknesses of prepreg. The loaded mould was placed in a preheated press, the plattens of which were thermostatically controlled to 170°C. After waiting for the chosen dwelling time to elapse the mould was closed with a pressure of 2-8 MN/m 2 and left to cure for one'hour. After this the mould was removed and allowed to cool before extracting the specimen. No post-curing treatment was carried out. The specimens were tested to failure in three-point bending.
DISCUSSION OF RESULTS A large number of photomicrographs were obtained including complzte traverses across the thickness and width of the specimen. Figs 1 - 3 show typical microstructure obtained from a width traverse in the region of an edge. The fibre and void distribution shown is completely representative of the microstructure over the whole cross-section. It is seen from Fig 1 that it is possible to produce a voidfree specimen with correct choice of dwell time, in this case 11½ rain. There are however some resin rich areas, at the interfaces of prepreg layers. Fig 2 shows an under dwelled specimen. The formation is similar to the void-free specimen apart from the few voids which are seen, in general, to be concentrated in the resin rich areas mentioned above.
COMPOSITES. MARCH 1973
cause gross cracking. This latter situation was observed in some over dwelled specimens. It was noted from many tests that the correct dwelling time depends not only on the mould size and material, ie its heat capacity, but also on the quantity of prepreg placed in the mould and the condition, ie the age, of the partly cured resin.
Fig 1
Mierostructure o t correctly dwelled specimen (x30)
Results obtained from the bending tests showed the correctly dwelled specimens to be stronger than the under dwelled and considerably stronger than the over dwelled. The scatter in the data was greatest for the over dwelled and least for the correctly dwelled specimens. These results are consistent with the void fractions obtained from the photomicrographs, ie 1% for under dwelled and 6% for over dwelled specimens. The precise strength figures are in themselves not too meaningful since size, shape and distribution of voids and, of course, fibre alignment over the cross-section can critically affect the failure stress. This conclusion was also reached by Fried 2 and Hand. 3 However, the bending test is simple to perform and is useful in this instance in determining the basic effects of processing. For realistic strength data it would be necessary to test uniformly stressed specimens; in tension for example.
Fig 2
Microstructure of under dwelled specimen (x30)
Paul and Thompson 4 in their work on glass-fibre composites point out that the most severe void, as far as mechanical properties are concerned, is what they call a planar void. Although no longitudinal sections were examined the transverse sections indicate the presence of planar type voids in the over dwelled specimens. The magnitude of the strength reduction caused by such voids must depend on their orientation, at present unknown, relative to the applied stress, but the results of the bending tests tend to confirm Paul and Thompson's conclusions.
CONCL USI ONS
Fig 3
Microstructure of over dwelled specimen (x30)
The over dwelled specimen shown in Fig 3 demonstrates larger resin rich areas and a greater number of larger voids than the under dwelled specimen. The formation of a void-free microstructure depends critically on the resin viscosity at the time of closing the mould. If pressure is applied either too early or too late, as in under and over dwelled specimens, en;trapped air and gases are less likely to escape from the mould. Obviously early pressing is less damaging since the resin has still to pass through its least viscous state and hence a large amount of air and gas will be removed. The application of pressure when the resin is beginning to set means that very little air or gas will escape from the mould and furthermore the plunger force is likely to
C O M P O S I T E S . M A R C H 1973
O n a laboratory scale'it has proved possible to produce, without the complication of vacuum techniques, specimens which appear to be void-free in the optical microscope and which show superior strength in bending to specimens containing voids. The microstructure depends largely on the dwelling time which in turn depends on the heat capacity of the mould, the quantity of fibre used, and the age of the resin in the prepreg. Specimens that are over dwelled show a more drastic loss of strength t h a n do under dwelled specimens..
REFERENCES 1 Brclant, S., 'The relationship of voids, process and material parameters and performance of filament wound pressure vessels'. Reinforced Plastics 1965, Regional Technical Conference Society of Plastics Engineers lnc (July 1965) 2 Fried, N., 'The response of orthogonal filament wound materials to compressive stress', Proceedings 20th Anniversary Technical Conference, Chicago, Ill, SPI Reinforced Plastics Division (l:ebruary 1965) 3 Hand, W., 'Quality control of filament wound materials for deep submergence vessels', ibid 4 Paul Jr., J. and Thompson, J., 'The importance of voids in the filament wound structure', ibid
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The basic methods of preparing laboratory specimens of carbon-fibre/resin composites are now well established. However the effects of variations in the basic manufacturing process on the microstructure and mechanical properties of the specimen are, in general, poorly documented, although some basic work has been undertaken in glassfibre/resin systems. 1 The most common form of microstructural defect is the void, a space within the composite caused by air or vapour entrapped during the cure process. Even a small quantity of voids can cause a marked reduction in the mechanical properties, in particular the strength, of a composite. In the project described in this report, carbon-fibrereinforced resin specimens were produced by hot pressing preimpregnated fibre: vacuum impregnation and curing methods were not employed. It was felt that the most critical part of the process was the dwelling time, ie the time that is allowed to elapse between placing the loaded mould into a heated press and applying the pressure to close the mould. Accordingly, specimens were produced from a range of dwelling times and the resulting microstructure examined and strength in bending determined.
SPECIMEN PREPA RATION The mild steel mould used in this series of tests produced finished specimens which were 100 mm long x 19 mm wide x about 3 mm thick. All specimens contained unidirectional fibres parallel to the length. Manufacture of some preliminary specimens showed that void-free specimens could be obtained using a dwelling time of about 10½ min.
*Aeronautics Department, Imperial College of Science and Technology, Prince Consort Road, London SW7, UK tPhysics Department, Chelsea College, Manresa Road, London SW3, UK
86
Such specimens are referred to as 'correctly dwelled'. Similarly, specimens made with dwelling times shorter or longer than the ideal are referred to as 'under dwelled' and 'over dwelled' respectively. The material used was Courtaulds' untreated Graffl A carbon fibre, Ciba epoxy novalac LY558/HT973 prepreg of 0.125 mm nominal thickness. Approximately 40 thicknesses of prepreg were needed to till the mould cavity with the plunger fully closed. To ensure adequate pressure during curing the mould was overfilled by 30%, giving a total of 53 thicknesses of prepreg. The loaded mould was placed in a preheated press, the plattens of which were thermostatically controlled to 170°C. After waiting for the chosen dwelling time to elapse the mould was closed with a pressure of 2-8 MN/m 2 and left to cure for one'hour. After this the mould was removed and allowed to cool before extracting the specimen. No post-curing treatment was carried out. The specimens were tested to failure in three-point bending.
DISCUSSION OF RESULTS A large number of photomicrographs were obtained including complzte traverses across the thickness and width of the specimen. Figs 1 - 3 show typical microstructure obtained from a width traverse in the region of an edge. The fibre and void distribution shown is completely representative of the microstructure over the whole cross-section. It is seen from Fig 1 that it is possible to produce a voidfree specimen with correct choice of dwell time, in this case 11½ rain. There are however some resin rich areas, at the interfaces of prepreg layers. Fig 2 shows an under dwelled specimen. The formation is similar to the void-free specimen apart from the few voids which are seen, in general, to be concentrated in the resin rich areas mentioned above.
COMPOSITES. MARCH 1973
cause gross cracking. This latter situation was observed in some over dwelled specimens. It was noted from many tests that the correct dwelling time depends not only on the mould size and material, ie its heat capacity, but also on the quantity of prepreg placed in the mould and the condition, ie the age, of the partly cured resin.
Fig 1
Mierostructure o t correctly dwelled specimen (x30)
Results obtained from the bending tests showed the correctly dwelled specimens to be stronger than the under dwelled and considerably stronger than the over dwelled. The scatter in the data was greatest for the over dwelled and least for the correctly dwelled specimens. These results are consistent with the void fractions obtained from the photomicrographs, ie 1% for under dwelled and 6% for over dwelled specimens. The precise strength figures are in themselves not too meaningful since size, shape and distribution of voids and, of course, fibre alignment over the cross-section can critically affect the failure stress. This conclusion was also reached by Fried 2 and Hand. 3 However, the bending test is simple to perform and is useful in this instance in determining the basic effects of processing. For realistic strength data it would be necessary to test uniformly stressed specimens; in tension for example.
Fig 2
Microstructure of under dwelled specimen (x30)
Paul and Thompson 4 in their work on glass-fibre composites point out that the most severe void, as far as mechanical properties are concerned, is what they call a planar void. Although no longitudinal sections were examined the transverse sections indicate the presence of planar type voids in the over dwelled specimens. The magnitude of the strength reduction caused by such voids must depend on their orientation, at present unknown, relative to the applied stress, but the results of the bending tests tend to confirm Paul and Thompson's conclusions.
CONCL USI ONS
Fig 3
Microstructure of over dwelled specimen (x30)
The over dwelled specimen shown in Fig 3 demonstrates larger resin rich areas and a greater number of larger voids than the under dwelled specimen. The formation of a void-free microstructure depends critically on the resin viscosity at the time of closing the mould. If pressure is applied either too early or too late, as in under and over dwelled specimens, en;trapped air and gases are less likely to escape from the mould. Obviously early pressing is less damaging since the resin has still to pass through its least viscous state and hence a large amount of air and gas will be removed. The application of pressure when the resin is beginning to set means that very little air or gas will escape from the mould and furthermore the plunger force is likely to
C O M P O S I T E S . M A R C H 1973
O n a laboratory scale'it has proved possible to produce, without the complication of vacuum techniques, specimens which appear to be void-free in the optical microscope and which show superior strength in bending to specimens containing voids. The microstructure depends largely on the dwelling time which in turn depends on the heat capacity of the mould, the quantity of fibre used, and the age of the resin in the prepreg. Specimens that are over dwelled show a more drastic loss of strength t h a n do under dwelled specimens..
REFERENCES 1 Brclant, S., 'The relationship of voids, process and material parameters and performance of filament wound pressure vessels'. Reinforced Plastics 1965, Regional Technical Conference Society of Plastics Engineers lnc (July 1965) 2 Fried, N., 'The response of orthogonal filament wound materials to compressive stress', Proceedings 20th Anniversary Technical Conference, Chicago, Ill, SPI Reinforced Plastics Division (l:ebruary 1965) 3 Hand, W., 'Quality control of filament wound materials for deep submergence vessels', ibid 4 Paul Jr., J. and Thompson, J., 'The importance of voids in the filament wound structure', ibid
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