In this study, we experimentally evaluated the correlation between the microscopic flow and permeability of a glass cloth, which was modified using a silane coupling agent. We focused on the capillary number, which is a parameter determining the microscopic resin impregnation behavior within and between fiber bundles. The capillary numbers were classified into different parameters based on their temperatures and pressures. First, we obtained the temperature condition, and resin viscosity/(surface tensionz⋅contact angle) ratio based on temperature, which was constant for each resin. Under these temperature conditions, the pressure condition of the resin impregnation rate was made constant. The permeability was evaluated for different resins under the conditions of temperatures and pressures making vary the capillary numbers in three conditions. As a result, the permeabilities of the different resin systems were approximately equal. Therefore, it was found that the macroscopic permeability was not significantly influenced by the capillary number representing microscopic resin flow.

The direct observation of the interfacial debonding between carbon fibers and an epoxy matrix was achieved through in-situ tensile and fatigue testing on the beamline of a synchrotron radiation facility, SPring-8. Preliminary tests were conducted using miniature epoxy specimens with multiple embedded carbon fibers under a high magnification digital microscope. The static loading-unloading tests revealed clear interfacial debonding between the carbon fibers and epoxy matrix under a nominal stress of 50 MPa and indicated debonding initiation at 30 MPa. Synchrotron radiation X-ray computed tomography (CT) was employed to observe the internal shape of the debonded interface. Although it was hardly visible under the conventional X-ray absorption contrast CT (micro-CT), the 3-dimensional shape of the debonded interface was clearly observable under the high-resolution phase-contrast X-ray CT (nano-CT), even under a nominal stress of 30 MPa. The debonding between the carbon fibers and epoxy matrix initiated at the interface where the neighboring fibers were closely spaced under static loading. Debonding did not propagate under the cyclic loading; however, further propagation was confirmed along the exterior interface of the fiber bundles.

The microscopic interfacial debonding between fibers and matrices induces microscopic or macroscopic failures, such as transverse cracks or delamination, that affect the structural and mechanical performance of composite materials. Therefore, there has been extensive research using the cohesive element method on the microscale or mesoscale fracture mechanisms at the interface between fibers and matrices. However, the parameters of the cohesive elements were often adjusted to match the experimental results; therefore, the physical implications of these parameters remain ambiguous. The experimentally measured interfacial tensile strength between pitch-based carbon fibers and the matrix, obtained via the cruciform test in the authors’ previous study, was considered as the maximum effective tensile force for the cohesive elements in the present study. Furthermore, the interfacial cohesive energy was calculated using the potential energy obtained from the finite element models of the cruciform specimen before and after interfacial debonding. The maximum effective tensile force and the interfacial cohesive energy were determined to be 41.3 [MPa] and 2.29 [J/m²] in the present study. These parameters were applied to a two-dimensional cross-ply laminate model, and the crack propagation behavior was determined to be nearly identical to that deduced from the in-situ experimental observations. These results demonstrated the establishment of a mesoscale crack-propagation model without the analysis of the parameters of the cohesive elements, based on the parameters experimentally and analytically determined via cruciform tests.

The effects of strain rate on the bending properties of carbon-fiber-reinforced thermoplastics (CFRTP) with polyamide6 (PA6) as a matrix were investigated using three-point bending tests. The results of the tests, in conjunction with acoustic emission (AE) measurements, confirmed the increase in the bending strength, Young’s modulus, and failure strain with an increase in the strain rate. The scale of damage, determined from the AE measurements, also increased with an increase in the strain rate. Dynamic mechanical analysis was performed to elucidate the reason for the strain-rate dependence of the results of the bending tests. The results indicated an increase and decrease in the storage modulus and loss tangent, respectively, with an increase in the strain rate. The frequency dependence of the storage modulus was consistent with the strain-rate dependence of the Young’s modulus. The loss tangent corresponded to the scale of damage, owing to the change in the energy-absorption properties. The crystal structure of the CFRTP before and after the bending tests was evaluated using X-ray diffraction. The results revealed the destruction of α-form crystals and the formation of γ-form crystals during the bending tests. The presence of γ-form crystals results in ductility. Therefore, the formation of γ-form crystals during bending deformation rendered the CFRTP ductile. This resulted in an increase in the failure strain.