MEC207 Composites Virtual Practical Dr Joel Foreman
The mechanical performance of composite materials is often the most important criterion determining whether they will be used in a particular application. In tension, carbon fibres have a higher specific tensile modulus and strength than almost any other material. To test the mechanical performance, flat plates of a typical carbon fibre composite have been laid up from pre-preg, enclosed in a vacuum bag and cured in a convection oven. The pre-preg was unidirectional so the plates have all the fibres oriented in the same direction. Small rectilinear samples of area approximately 100 x 10 mm are cut from the plates at three different angles. Each is then pulled to break in a universal testing frame where a known load is applied to the samples and the resultant extension is recorded. recorded. The types of sample are cut from the plate at 0, 45 and 90 o to the direction of the fibres. The 0o sample has all the fibres oriented along the testing direction of the sample; the 90o sample has the fibres oriented perpendicular to the testing direction of the sample. The 0o sample is manufactured with a lower thickness than the 45 and 90o samples because they were too strong for the machine! The load extension data is provided for the three samples. The cross sectional area of the 0o sample is 6.55 mm2; 45o is 12.63 mm2; 90o is 11.62 mm 2. The original gauge length of the sample is 50 mm. Use the data provided to calculate the Young’s modulus for the three samples and samples and enter your values in the boxes provided.
Unidirectional 0o
132GPa
[5 marks]
Unidirectional 45o
10.7GPa
[5 marks]
Unidirectional 90o
8.26GPa
[5 marks]
The 45o data is more difficult to interpret as the strain measurement software wasn’t working properly resulting in ‘wavy’ lines. You will have to make a judgement call on how to interpret this data.
Briefly explain the differences in the Young’s modulus values you have obtained obtained for the three samples. Your answer must be less than 500 words. [5 marks]
Carbon fibre is an anisotropic material, meaning that its strength changes depending on the direction its loaded. This is due to it being comprised of many individual fibres all running together in one direction to make up a solid material. each of these fibres is 16 - 22 micrometers long (1). These fibres, when all running and under stress in the same direction have a very high tensile strength as very strong C-C covalent bonds need to be broken in each fibre for a failure to happen. However, if the stress is applied perpendicular to the direction of the fibres, it is only the much weaker intermolecular (Van der Waals) forces between the carbon chains that have to be broken. This means that much less stress perpendicular to the fibres is required for a failure to occur. This is shown in the calculated values of youngs modulus. A lower youngs modulus means less force is required to give a certain extension. The youngs modulus is significantly higher when the stress is applied in the direction of the fibres, as the covalent bond energies are much higher and so require much more force to break, and thus stretch the carbon fibre. At 45 and 90 degrees the the fibres it is only the much weaker Van der Waals forces that require breaking, and so much less force is required to break these forces, and thus in these directions the material extends a lot more with a given force. The value for youngs modulus at 45 and 90 degrees are very similar. This is because the strong covalent C-C bonds are in one specific direction only, and so are only strong in this direction. The weaker Van der Waals forces operate in all directions, and so even at 45 degrees, the covalent bonds in the fibres are not taking on any stress because there are none running in this direction, all the stress is taken by the intermolecular forces between the fibres. To avoid this problem of anisotropy, in a lot of carbon fibre materials the fibres are woven at 90 degrees to each other, similar to in clothing, and these gives it high strength in all directions.
(1) W.J. Cantwell, J Morton (1991). "The impact resistance of composite materials a review". Composites 22 (5): 34762. doi:10.1016/0010-4361(91)90549-V.
