The crack development on the micro- and mesoscopic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

Investigations were conducted to get an insight into the crack development during the carbonization of CFRP components and to understand the mechanisms ruling this cracking. It was found that the major crack types are micro-cracks caused by fibre–matrix-debonding, transversal cracks and partial delaminations. Cracking starts with the onset of pyrolysis as the reaction gases are captured in the still compact material leading to internal pressures and consequently to a channel network. At 490 °C (in the case of a heating rate of 10 K/min) the first micro-cracks evolve due to fibre–matrix-debonding resulting in a homogenisation of the stress distribution. Driven by the high shrinkage in this temperature regime the initiation of transversal cracking takes place soon afterwards at 515 °C resulting in a steep increase in the transversal crack density up to approximately 550 °C. Partial delaminations develop from 520 °C onwards as the crack deflection of the transversal cracks. The development of the latter interrupts the stress transfer between warp and weft fibres, so that the further cracking is mainly driven by local stresses inside the C/C segments between the transversal cracks. Thus the further cracking is dominated by fibre–matrix-debonding occurring in all sizes from the submicro- to the microscopic scale. Although the coarse crack pattern mainly develops up to approximately 570 °C, fibre–matrix-debonding at higher temperatures and especially during cool down plays the major role in terms of crack activity as monitored by acoustic emission.