X-ray nanoimaging of a transversely embedded carbon fiber in epoxy matrix under static and cyclic loads

Abstract

The initial stage of fatigue failure has not been thoroughly clarified for carbon fiber reinforced plastics (CFRPs). Although the initiation of fatigue cracks has been regarded to be interfacial debonding between the carbon fiber and polymer matrix, their detection among numerous carbon fibers, whose diameter is only 7 µm, is extremely difficult. In this study, a single carbon fiber was transversely embedded in a dumbbell-shaped epoxy sample to focus on the interfacial debonding and was observed using synchrotron radiation (SR) X-ray computed tomography (CT). A tabletop fatigue testing machine driven by a piezoelectric actuator was developed to apply static and cyclic loads along the beamline. SR X-ray multiscale CT imaging was conducted by switching between an absorption-contrast projection method (micro-CT) and a phase-contrast imaging-type X-ray microscopic CT (nano-CT). The carbon fiber was entirely captured by micro-CT and then magnified at both ends on the free surfaces. Nano-CT clearly visualized the interfacial debonding under 30 MPa static tensile load and the implication of the coalescence of nano-voids along the interface under 50 MPa. Under cyclic loads, the interfacial debonding gradually progressed under a 8–40 MPa sinusoidal stress after 10,000 cycles, whereas it did not propagate under a stress below 30 MPa.

Lock-in thermographic measurement and model-based evaluation of effective thermal diffusivity to quantify early stage fatigue damage of CFRP

“…If the degree of fatigue progression can be evaluated in situ by applying nondestructive testing, it is possible to estimate the remaining service life. The fatigue progression rate is considered to be correlated with microdamage such as micro-voids, micro-cracks, and fiber/resin interface delamination that occur in CFRP [2,3,4]. The authors proposed a fatigue damage evaluation method using active thermography, which has been applied as a non-destructive inspection method for aircraft composite materials.…”

Interfacial Structure Analyses of Single Carbon Fiber-Embedded Polymers by In Situ X-ray Scattering and Birefringence Measurements

“…32 Takahashi et al observed localized debonding and voiding in a single-CF epoxy resin using synchrotron radiation X-ray computed tomography upon applying stress or cyclic stress to the sample. 36 As a different method to investigate the interface between (CF/matrix polymer), Raman spectroscopy and dynamic birefringence measurements have been performed to investigate stress transfer near the CF during the elongation process. 37−42 Finite elemental analyses of the stress field have also been performed, and various models have been reported to account for fiber rupture, plastic deformation, crack propagation, and shear lag.…”

“…Synchrotron radiation X-ray can be used to evaluate not only CFs but also the orientation of the polymer chains and craze formation during uniaxial elongation. − Furthermore, when the microbeam X-ray is used for this measurement, information on the interface between (CF/matrix polymer) may be obtained by mapping. − Schneider et al reported spatial distribution of failed interface and voids near the broken fiber edges in the epoxy matrix near the glass fibers in a stretched sample using microbeam small-angle X-ray scattering (SAXS) . Takahashi et al observed localized debonding and voiding in a single-CF epoxy resin using synchrotron radiation X-ray computed tomography upon applying stress or cyclic stress to the sample . As a different method to investigate the interface between (CF/matrix polymer), Raman spectroscopy and dynamic birefringence measurements have been performed to investigate stress transfer near the CF during the elongation process. − Finite elemental analyses of the stress field have also been performed, and various models have been reported to account for fiber rupture, plastic deformation, crack propagation, and shear lag. − …”