Fatigue damage growth of carbon fiber reinforced form observed by X-ray CT and hybrid NDT methods
Topic(s) : Special Sessions
Co-authors :
Ryuto SANO (JAPAN), Yuta KOGA , Atsushi HOSOI (JAPAN), Kota KAWAHARA , Yoshiki TAKEBE , Katsunori KIMOTO , 宏之 川田 (JAPAN)Abstract :
This study aimed to investigate the fracture mechanism focusing on the three-dimensional internal structure of carbon fiber reinforced forms (CFRFs), using X-ray computed tomography (CT) scans to observe the internal structure. In addition, a hybrid non-destructive testing (NDT) method combining the digital image correlation (DIC), acoustic emission (AE), and infrared thermography (IRT) methods was performed during fatigue tests to investigate the fracture process. The CFRF specimens after fatigue testing were X-ray CT scanned at the locations where large and small strains were observed by the DIC method. The evaluation was focused on the deflection of fibers and the volume fraction of voids. The results showed that there was no significant change in the volume fraction of the voids but the percentage of bent fibers with the tortuosity larger than 1.003 was about 10 % larger in the areas with large strain than with small strain.
Damage behavior during fatigue tests was observed using a hybrid NDT method. For CFRFs with an expansion ratio of 3x, the stiffness reduction and temperature rise suddenly slowed down around the fatigue life ratio of N/Nf = 0.2, and the AE waves decreased. On the other hand, for carbon fiber reinforced thermoplastics (CFRTPs) with an expansion ratio of 1x, that is, without expansion, the stiffness reduction and temperature rise slowed down around the fatigue life ratio of N/Nf = 0.4, and AE waves with large amplitude decreased. When the position of the AE waves was determined, for CFRFs, large AE waves with amplitudes exceeding 60 dB were observed immediately after the start of the test. On the other hand, for the CFRTPs, AE waves were gradually observed starting around the fatigue life ration of N/Nf = 0.4.
Based on the above results and previous studies, the damage growth behavior was discussed. In the case of CFRFs with an expansion ratio of 3x, large damage including fiber fracture is concentrated in the initial stage of the fatigue life ratio of N/Nf = 0.2, and the damage develops at the point. In this process, stress is concentrated at the fiber bending points, and the bent fibers are considered to fracture. Then, the fibers fracture and pull out in a chain of fractures and the resin at the nodal point of the fibers breaks down. Finally, it was suggested that the repeated events may have led to fracture. On the other hand, for CFRTPs with an expansion ratio of 1x, the resin cracks from the fiber ends after the start of the test, and the same phenomenon occurs at various locations while the plastic deformation of the resin slows the crack propagation. From the fatigue life ratio of N/Nf = 0.4, it is assumed that the existing cracks propagated and the fibers pulled out one after another as they lost the support of the resin, leading to fracture.
Damage behavior during fatigue tests was observed using a hybrid NDT method. For CFRFs with an expansion ratio of 3x, the stiffness reduction and temperature rise suddenly slowed down around the fatigue life ratio of N/Nf = 0.2, and the AE waves decreased. On the other hand, for carbon fiber reinforced thermoplastics (CFRTPs) with an expansion ratio of 1x, that is, without expansion, the stiffness reduction and temperature rise slowed down around the fatigue life ratio of N/Nf = 0.4, and AE waves with large amplitude decreased. When the position of the AE waves was determined, for CFRFs, large AE waves with amplitudes exceeding 60 dB were observed immediately after the start of the test. On the other hand, for the CFRTPs, AE waves were gradually observed starting around the fatigue life ration of N/Nf = 0.4.
Based on the above results and previous studies, the damage growth behavior was discussed. In the case of CFRFs with an expansion ratio of 3x, large damage including fiber fracture is concentrated in the initial stage of the fatigue life ratio of N/Nf = 0.2, and the damage develops at the point. In this process, stress is concentrated at the fiber bending points, and the bent fibers are considered to fracture. Then, the fibers fracture and pull out in a chain of fractures and the resin at the nodal point of the fibers breaks down. Finally, it was suggested that the repeated events may have led to fracture. On the other hand, for CFRTPs with an expansion ratio of 1x, the resin cracks from the fiber ends after the start of the test, and the same phenomenon occurs at various locations while the plastic deformation of the resin slows the crack propagation. From the fatigue life ratio of N/Nf = 0.4, it is assumed that the existing cracks propagated and the fibers pulled out one after another as they lost the support of the resin, leading to fracture.