This programme has contributed to understanding of various composites compressive failure modes, and to addressing quantitative requirements on identified key constituents. Experimental and theoretical developments have demonstrated that compression is clearly matrix dominated for composites made out of low inertia fibres such as High Strength and Intermediate Modulus carbon or such as glass. The 30% target has been shown to be directly dependent of an improved neat resin shear behaviour, by increasing preferentially the yielding strength, but also the modulus. The fibre/matrix interface shear load transfer ability appears to be of second order for compression. The initial fibres misalignment was also identified as an important parameter, which controls the filaments micro-buckling phenomenon. However, ICOMP experimental trials to install pre-tension in plies are not seen as industrially applicable. For designs driven by compression, the recomendations are to use product forms which minimize fibres waviness.
Therefore, propositions of innovative materials have been concentrated on new matrices development. As polymer experts have stated that the organic chemistry has probably reached an absolute limit with respect to shear properties, the introduction of fine particules in tough resins was investigated to match the shear behaviour requirements. An encouraging demonstration has been performed using a low class ductile polymer system. The addition of whiskers into the matrix increased both the neat resin strength and modulus, which enhanced the unidirectional composite compression in the forecast proportion. The lack of particules and resin optimisation, within ICOMP time frame, did not lead to an improved high class aeronautical material. However, this innovative route appears to be the only solution to combine prepregs processability, material cost and compression improvement. Furthermore, experimental indications suggested that this gain will directly be transposed into design criteria, such as compression after impact or compression of laminates containing a bolt, without degrading tensile properties and delamination resistance.
Dedicated inspection methods for in-situ damage monitoring or initial geometrical imperfections assessment were developed. A reliable compression test was a key preliminary request, which led to propose an innovative procedure to measure intrinsic unidirectional laminate compressive properties. The tremendous improvement brought by the new specimen has already been taken into account for future standards.
The main mechanical improvement of carbon fibre/organic matrix composites has been based for the last then years on high tensile strength fibres development. This research has led to a tremendous increase in composite tensile properties, but no significant progress has been noticed in compression, which often becomes the design limitation in complex structures. The preliminary work of this study is to develop a simulation tool to assess the raw materials influences on the compression failure modes of continuous fibres composites for high loaded structures. After a detailed analysis and an optimisation of the materials parameters, new constituents requirements will be addressed to a material manufacturer to improve the composites compression strength. Prototype composites will be elaborated out of innovative constituents (fibres and matrices) and the general properties will be checked to define their industrial potential. Independent works have suggested the key importance of the matrix in the composites compression behaviour. Because of chemistry difficulties to modify the resin mechanical characteristics without lowering the other properties such as hot/wet resistance, a goal of 30% increase in composites compression strength can be reasonably targeted.
Funding SchemeCSC - Cost-sharing contracts
GU14 6TD Farnborough
8316 PR Marknesse
4099 Porto Codex