Photon Factory Activity Report 2018 #36 (2019)

BL-6A/2017G582

Fatigue Behavior of High-strength Fiber from Liquid Crystalline Polymer
Masayoshi IDO and Masatoshi SHIOYA*Tokyo Institute of Technology, 2-12-1-S8-34 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
1 Introduction
Fatigue behavior of high-strength fibers from liquid
crystalline polymers is considered to be completely
different from that of general-purpose plastics because ofthe microfibrillar fiber structure of the former. This study
investigates the progress of the fatigue damages for thistype of fiber, poly-p-phenylene benzobisoxazole (PBO)
fiber, based on the structure analysis using thesynchrotron radiation small-angle X-ray scattering(SAXS) and the single-fiber axial compression test. The
reason for performing the axial compression test is thatthe axial compressive strength is sensitive to the damagesof the fiber structure. The details of the present study will
be published in another paper [1].
2 Experimental
A PBO fiber with the tensile strength of 5.8 GPa, thetensile modulus of 180 GPa, the strain-to-failure of 3.5 %and the density of 1.54 g cm-3 was used for the study.
For the specimens of SAXS measurements, tensilefatigue deformation was applied to the bundles of ten
filaments of PBO fiber at the frequency 10 Hz with themaximum stress of 4.0 GPa and the minimum stressagainst the maximum stress ratio of 0.25. For thespecimens of the single-fiber axial compression tests,
tensile-fatigue deformation was applied to the single
filaments of PBO fiber at the frequency 10 Hz with themaximum stress of 4.0 and 2.0 GPa and the minimumstress against the maximum stress ratio of 0.25.
For the original fiber without fatigue test and thefatigue tested fibers, the SAXS measurements wereperformed in the static state and during tensile
deformation at the beamline BL6A of High EnergyAccelerator Research Organization, Japan. The SAXS
was detected using a CCD camera (C7300, Hamamatsu
Photonics) attached with an image intensifier. The X-ray
wavelength, the beam cross section sizes and thespecimen to detector distance were 0.15 nm, 0.3 mm by
0.3 mm and 2.1 m, respectively.
The single-fiber axial compression tests wereperformed according to the method shown in a previouspaper [2]. A single filament of the PBO fiber was bonded
on a small steel plate using an adhesive and thecompressive load was applied to the protruded fiber end
with the length of about 16 μm using an indenter having amirror-finished surface.
3 Results and Discussion
Fig. 1 shows the SAXS pattern of the original PBO
fiber without fatigue test where the fiber axis is in the upand-down direction of the figure. The scattering pattern inthe form of the elongated equatorial streak indicates thatthe scattering bodies in the fiber have a large aspect ratio
and their longitudinal directions are highly oriented in thefiber axis direction. In the spinning process of the PBO
fiber, the PBO solution with nematic liquid crystalline
characteristics is extruded into the solidification liquid
during which the microfibrils and microvoids are formed[3, 4]. The scattering bodies in the PBO fiber which
produces the SAXS pattern shown in Fig. 1 are not themicrofibrils but the microvoids scince the volume fractionof the scattering bodies was measured to be as small as
1.4 %.
The azimuthal intensity distribution of the SAXS
pattern is caused by the finiteness of the longitudinalmicrovoid length and the orientation distribution of themicrovoids. The longitudinal microvoid length wasdetermined from the dependence of the broadening of theazimuthal intensity distribution on the scattering angle.
The SAXS measurements were performed during

Fig. 1: SAXS pattern of original PBO fiber.

Fig. 2: Variation of axial microvoid length with
fatigue cycle number for PBO fiber.
Maximum stress applied during fatigue
cycle was 4.0 GPa.

Photon Factory Activity Report 2018 #36 (2019)

tensile deformation and the longitudinal microvoid length
was determined as a function of the stress and the strain
for the original PBO fiber. As a result, we found that theincrease in the longitudinal microvoid length up to thestress of 3.8 GPa was 21 % which was significantly larger
than the macroscopic fiber strain, 2.3 %, at the samestress. This indicates that the increase in the longitudinalmicrovoid length during tensile deformation is not causedby the affine-like deformation of the whole fiber butoccurs due to the splitting of the microfibrils.
Fig. 2 shows the variation of the longitudinalmicrovoid length with the fatigue cycle number for thePBO fiber. We found that the longitudinal microvoid
length is increased by the fatigue deformation.
By the single-fiber axial compression test, the PBO
fiber failed accompanied by the formation of the kinkband as shown in the SEM images of Fig. 3. Fig. 4 shows
the single-fiber axial compressive strength of the PBO
fiber fatigue tested with various fatigue cycle numbers.
The PBO fiber fatigue tested with the maximum stress of
2 GPa (35 % of the tensile strength) during the fatigue
cycle did not show a marked reduction in the compressive
strength, while the fiber fatigue tested with the maximum
stress of 4 GPa (70 % of the tensile strength) decreased
13 % at the fatigue cycle number of 1.6×104. The
decrease in the axial compressive strength is caused by
the increase in the longitudinal microvoid length. We
concluded that for improving the fatigue resistance of thePBO fiber, the bonding strength between microfibrils
needs to be increased.
4 Summary
The structure analysis using the synchrotron radiation
SAXS and the single-fiber axial compression tests wereperformed for investigating the progress of the fatigue
damages in the PBO fiber. The increase of the fatigue
cycle number enhanced the splitting of microfibrils which
increased the longitudinal microvoid length and as aresult, decreased the single-fiber axial compressive
strength. We concluded that for improving the fatigue
resistance of the PBO fiber, the bonding strength between
microfibrils needs to be increased.
Acknowledgement
The authors are indebted to the staffs working at PF fortheir experimental support.
References[1] M. Ido, R. Kimura, H. Kobayashi, M. Shioya, Journalof Fiber Science and Technology, in preparation.
[2] Y. Sugimoto, M. Shioya, K. Yamamoto, S. Sakurai,Carbon, 50, 2860 (2012).
[3] Y. Cohen, E. L. Thomas, Macromolecules, 21, 433(1988).
[4] T, Kitagawa, H. Murase, K. Yabuki, J. Polym. Sci.
Part B, 36, 39 (1998).
* shioya.m.aa@m.titech.ac.jp

Fig. 3: SEM images of original PBO fiber (a) beforeand (b) after single-fiber axial compression
test.

Fig. 4: Single-fiber axial compressive strength of
PBO fiber fatigue tested with various fatigue
cycle numbers. Maximum stress applied
during the fatigue cycle was (□) 2.0 GPaand (○) 4.0 GPa.