Insertion of aligned CNTs into the intralaminar regions of conventional prepreg-based CFRP laminates using magnetically responsive CNTs

Although carbon fiber reinforced plastics (CFRPs) are widely used in aerospace, automotive, and energy industries due to their excellent in-plane mechanical properties, they exhibit relatively weak out-of-plane properties. Current methods of improving the out-of-plane properties with CNTs are limited to adding random or aligned CNTs between conventional CFRP plies or integrating randomly oriented CNTs throughout the entire CFRP volume using prepregs made with CNT/resin mixtures. Even in such manufacturing approaches, methods of quantitatively characterizing CNT morphology (concentration, alignment, distribution, etc.) in CFRPs are lacking. This work addresses both limitations by using magnetically responsive Ni-coated CNTs. The Ni-coated CNTs were inserted, with through-thickness alignment, into conventional CFRP prepregs by applying external magnetic fields during manufacturing. Aligned CNTs were observed in regions of the prepreg 10s of microns away from the prepreg interface, where the Ni-CNTs were originally inserted. The Ni-coated CNTs within laminates were confirmed to be aligned based on anisotropic saturation magnetization 34 % larger in the out-of-plane direction versus the in-plane direction, as measured with a vibrating sample magnetometer. In short beam shear testing, the CNT-CFRPs exhibited a more ductile post-peak behavior in comparison to the same CFRP without CNTs. The ratio of the post-peak work to total work in the aligned CNT-CFRPs was ∼30 % larger than in the same CFRP with unaligned CNTs and ∼250 % larger than in the same CFRP without CNTs, confirming the beneficial effect of aligned CNTs in resisting interlaminar crack growth.