Photon Factory Activity Report 2019 #37 (2020)

AR-NW2A/2019S2-002

Establishment of in situ Observation of crack initiation and propagation in Carbon
Fiber Reinforced Plastic using Synchrotron Radiation X-ray Computed Tomography
1Toshiki Watanabe1, a, *, YasuoTakeichi1, 2, Yasuhiro Niwa1, and Masao Kimura1, 2
Photon Factory, Institute of Materials Structure Science, High Energy Research Organization,
1-1 Oho, Tsukuba, 305-0801, Japan2
Department of Materials Structure Science, School of High Energy Accelerator Science,SOKENDAI (Graduate University for Advanced Studies),
1-1 Oho, Tsukuba, 305-0801, Japana
Present: Kyoto University, Graduate School of Human and Environmental Studies,
Yoshida Nihonmatsu-cho, Sakyo-ku, Kyoto, 606-8316, Japan

1 Introduction
Structural materials such as composites are commonly
subjected to multiple directional loads that can cause
critical damage. Carbon fiber reinforced plastic (CFRP)
composites have multiple components which are strong but
brittle anisotropic carbon fibers and a ductile isotropic
plastic. Therefore, CFRP has the properties of lightweightand high stiffness and is extensively used to reduce theweight of structural materials in the aerospace industry.
The mechanisms of crack initiation and propagation in
CFRP composites have been investigated using various
modelling and experimental techniques. However, owing
to recent advances in the production of CFRP composites
through microscopic design, observation with even asmaller spatial resolution of less than a micrometer isrequired to understand the mechanism of crack initiationand propagation.
In this study, we non-destructively observed crack
initiation and propagation in CFRP using nanoscopic
synchrotron radiation X-CT (SR X-CT).
2 Experiment
The CFRP specimens consisted of T800H carbon
fiber/epoxy (fiber areal weight of 190 g/m2, resin contentof 35 wt%). The specimens the size of 1 mm long with adiameter of 60 μm were prepared by mechanical cutting.
X-CT measurements were carried out by nanoscopic
SR X-CT installed at the NW2A of the Photon Factory
Advanced Ring (PF-AR) in IMSS (KEK, Tsukuba, Japan).
The apparatus can measure by Zernike phase-contrast
imaging method using a phase ring to observe theboundary of carbon fiber and resin. A focusedmonochromatic X-ray beam of 8 keV irradiated to thespecimen and the transmitted X-ray beam was detectedusing a CCD camera. The detected image of a 20 × 20 µm2
field of view and a voxel size of 39 nm was measured.
In situ nanoscopic measurements were performed using
a nanomechanical test stage [1]. A pyramidal diamondindenter was used to apply the stress and initiate cracks inthe specimen (Fig. 1). In situ CT measurements wereperformed by rotating the stage. 151 radiographs werecollected using exposure time of 10 s during rotating of151°. The collected radiographs were reconstructed to a 3D

image (20 μm diameter, 20μm height) by a filtered backprojection method.

Fig. 1 Schematic image of the part of the CFRP specimen
for the nanoscopic SR X-CT measurements [2].
3 Results and Discussion
Fig. 2 shows a series of X-Y cross sections of 3D
volume data from lower to upper of specimen along Zdirection. In other words, the from lower to upper series ofZ-direction is corresponding to increase strains. The crack
initiation occurred by competing two processes offiber/resin interface debonding and in-resin crack initiation.
In the fiber/resin interface debonding process, the cracks
initiated by opening the fiber/resin interfaces andpropagated along the fiber/resin boundary (red triangles inFig. 2b–e). These cracks observed at positions with shortdistances between neighboring fibers. In in-resin crack
initiation process, the small void and cracks were observed(blue triangles in Fig. 2c, d, and f). These cracks werepropagated with plastic deformation.
We observed crack initiation and propagation process inan early stage using nanoscopic SR X-CT with a highresolution of < 50 nm and phase contrast imaging whenmechanical load was applied to CFRP specimen by
inserting pyramidal indenter. This observation suggested
that the crack initiation in CFRP did not simply result fromlocal stresses but instead occur mainly through twocompeting nanoscale mechanisms (fibre/plastic interface
debonding and in-plastic crack initiation). The
mechanisms of crack initiation and propagation arestrongly location dependent on parameters such as thedistances between neighboring fibers and the bonding
strength at the fiber/resin interface under similar applied
local stresses. These inhomogeneities have been suggested
to be corresponded to a lot of factors such as fiberPhoton Factory Activity Report 2019 #37 (2020)alignment irregularities, surface morphologies, and thesize of carbon fibers. Such information is expected tocontribute to the design of CFRPs with reasonable safetymargins for use in airplanes.
The developed techniques have been applied to in situ
observation of CFRP under a stress applied with a wedge[3].

Fig. 2: X-Y cross sections of the 3D volume data fromlower to upper of specimen [2].
Acknowledgement
This work was supported by the Structural Materials forInnovation of the Cross ministerial Strategic Innovation
Promotion Program (SIP) of Japan Science andTechnology (JST). Experiments using synchrotron
radiation were performed with the approval of the Photon
Factory Program Advisory Committee (Proposal Nos.
2016S2-001and 2019S2-002). This work was also partially
supported by JSPS KAKENHI [grant number JP
19H00834].
References[1] B. M. Patterson, et al., Exp. Mech. 56, 1585 (2016).
[2] M. Kimura, et al., Sci. Rep. 9, 19300 (2019).
[3] T. Watanabe, et al., Compos. Sci. Technol. 197, 108244(2020).
* watanabe.toshiki.7z@kyoto-u.ac.jp