Initiation process of a fatigue crack focusing on the interface between carbon fiber and epoxy matrix
Topic(s) : Special Sessions
Co-authors :
Kosuke TAKAHASHI (JAPAN), Takashi TAKASHI 自宅 NAKAMURA (JAPAN)Abstract :
Fatigue of CFRP laminates is generally characterized by accumulation of the transverse cracks, which are generated in the 90° layers, although the interfacial crack between the carbon fiber and polymer matrix has been believed to be the initial process of the failure. This is because it is difficult to detect and track the nanoscale opening gap of the interfacial crack among the innumerable carbon fibers in the CFRP laminate prior to the propagation to a transverse crack.
In recent years, a high-resolution X-ray microscopic CT, called “Nano-CT”, has attracted attention for detecting the nanoscale damage. It was developed to further improve the spatial and density resolutions of conventional X-ray CT methods. The effectiveness has been shown by the detection of internal cracks generated in the metal alloys under cyclic loads. Therefore, Nano-CT has the possibility to capture the interfacial crack between the carbon fiber and polymer matrix and to clarify the crack propagation behavior.
In this study, dumbbell-shaped epoxy samples with a single carbon fiber or a bundle of carbon fibers embedded in the transverse direction were prepared to capture the interfacial crack using Nano-CT. To track the captured interfacial crack under cyclic loads by Nano-CT, a tabletop fatigue testing machine was developed and installed on the rotation stage along the beamline. The total weight is less than 3 kg by utilizing a piezoelectric actuator, which allows a stroke of 300 µm and a static tensile load of 300 N. The experiments were conducted at the large synchrotron radiation facility SPring-8 located in Hyogo, Japan. The several conditions of fatigue test up to a million cycles in the elastic range were carried out using the prepared epoxy samples.
When the epoxy sample with a single carbon fiber was used, the 3-dimensional shape of interfacial crack was successfully captured after the 1st cycle at the pole positions of the carbon fiber. The maximum length was about the same as the fiber diameter of 7 µm, and it kept propagating at the early stage of the loading cycles. However, the propagation rate was decreasing as the number of loading cycle increased, and the sample did not fracture. On the other hand, the epoxy sample with a bundle of carbon fibers showed the accelerated crack propagation and eventually fractured from the interfacial crack connected with a few of neighboring carbon fibers. These results implied that it is necessary for the interfacial crack to connect with neighboring carbon fibers to keep propagating.
In recent years, a high-resolution X-ray microscopic CT, called “Nano-CT”, has attracted attention for detecting the nanoscale damage. It was developed to further improve the spatial and density resolutions of conventional X-ray CT methods. The effectiveness has been shown by the detection of internal cracks generated in the metal alloys under cyclic loads. Therefore, Nano-CT has the possibility to capture the interfacial crack between the carbon fiber and polymer matrix and to clarify the crack propagation behavior.
In this study, dumbbell-shaped epoxy samples with a single carbon fiber or a bundle of carbon fibers embedded in the transverse direction were prepared to capture the interfacial crack using Nano-CT. To track the captured interfacial crack under cyclic loads by Nano-CT, a tabletop fatigue testing machine was developed and installed on the rotation stage along the beamline. The total weight is less than 3 kg by utilizing a piezoelectric actuator, which allows a stroke of 300 µm and a static tensile load of 300 N. The experiments were conducted at the large synchrotron radiation facility SPring-8 located in Hyogo, Japan. The several conditions of fatigue test up to a million cycles in the elastic range were carried out using the prepared epoxy samples.
When the epoxy sample with a single carbon fiber was used, the 3-dimensional shape of interfacial crack was successfully captured after the 1st cycle at the pole positions of the carbon fiber. The maximum length was about the same as the fiber diameter of 7 µm, and it kept propagating at the early stage of the loading cycles. However, the propagation rate was decreasing as the number of loading cycle increased, and the sample did not fracture. On the other hand, the epoxy sample with a bundle of carbon fibers showed the accelerated crack propagation and eventually fractured from the interfacial crack connected with a few of neighboring carbon fibers. These results implied that it is necessary for the interfacial crack to connect with neighboring carbon fibers to keep propagating.