Modelling the needling effect on the stress concentrations of laminated C/C composites

• Needling effect on the stress concentration of carbon/carbon laminates was modelled.
• A new numerical method combining shear-lag model and finite difference method was developed.
• The relationship between needling density, stress concentration and tensile strength was established.
• The detailed mechanism for tensile strength reduction under the condition of high needling density was investigated.

The initial stress concentration resulting from the needle-punching effect is a fundamental issue in the longitudinal strength model for laminated carbon/carbon composites. In the present work, this stress distribution is obtained by combining the finite difference method with the shear-lag model. The effect of the needled hole size on the stress concentration is investigated. The broken width is more significant than the separated length for the improvement of the stress concentration degree. The increase of the fiber volume fraction is shown to decrease the overload area. Random needling distributions are compared to the ordered one. Few interactions of each needled area are found in the ordered distribution and the stress concentration is decreased in these random needling distributions. Finally it is concluded that the enlarging of each needled hole, especially the accumulation of broken fibers, during the cyclic process of overlaying laminates and punching is the prime reason for tensile strength reduction under the condition of high needling density.

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