Progressive damage and tensile failure of open-hole carbon fiber polymer composites reinforced by carbon nanotubes

Progressive damage and tensile failure of open-hole carbon fiber/carbon nanotube reinforced composites with different off-axis angles were detected by acoustic emission and Micro-CT. K-mean clustering algorithm can be utilized to post-process signals of acoustic emission. Three critical points including linear growth point, near maximum load point and failure point, were selected to study progressive damage. Results indicated that stiffness variable decreased with the increase of off-axis angle. When the off-axis angle was 0 degrees, higher initial stress and catastrophic brittle fracture characteristics can be obtained, whereas pseudo-ductile behavior can be observed when off-axis angles were 30 degrees and 45 degrees because of the alignment of yarn (fiber trellising) and the matrix plasticity. Due to the symmetrical structure of plain texture, there was a very interesting phenomenon that the tensile strength of composite specimen with 45 degrees off-axis angle was 23.65% higher than that of the specimen with an off-axis angle of 30 degrees. In the second stage of loading, the slope drop sharply when the load reached first key point, and Kaiser Effect can be observed. Acoustic emission technology and Micro-CT method are beneficial to damage assessment of complex composites.