Fracture behavior of unidirectional commingled-yarn-based carbon fiber/polyamide 6 composite under three-point bending

Flexure-response and fracture behavior of unidirectional commingled-yarn-based carbon fiber (CF)/polyamide (PA) 6 composite were investigated using three-point edgewise and flatwise bend tests. Difference of the experimental flexure data from that expected was attributed to poorly bonded fiber/matrix interfaces and deviated fiber bundle orientations. For the fracture test, four kinds of notch direction were adopted: edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel (ZT) and perpendicular (ZT) to this direction. Strength and absorbed fracture energy for L and ZL specimens exhibited high sensitivity to notching. ZL specimens exhibited the lowest values of the critical stress intensity factor K-c which were slightly superior to those of unfilled PA6 matrix. T specimens showed the largest values of the strain energy release rate G(c) which were mostly occupied by the plastic fracture energy: the large plastic energy coincided with generation of the crushing-mode failure on the compression side. Enlargement of the compression area was analyzed by means of the rigidity reduction resulting from the fracture progression.