PSI - Issue 2_B

Noriyo Horikawa et al. / Procedia Structural Integrity 2 (2016) 293–300 Horikawa, N. et al./ Structural Integrity Procedia 00 (2016) 000–000

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bands, shown as black strips in this figure, confirm their formation during wrapping. In the previous study (Horikawa et al. (2013)), the authors defined the kink band density as the number of kink bands found per unit length of individual fiber. This study presents a quantitative evaluation of the relation between the value of the density and the tensile strength. The number of kink bands per fiber length of 250 µ m was counted under the optical microscope and converted to a number per 100 µ m to define the kink band density. As shown by the numerals in the figure, the number of kink bands was counted by assuming a black strip as a single defect. The bands in each test specimen were counted outside the gauge region (A in Fig. 5.) before the tensile tests.

2.3. Specimen shape

Because PBO fiber is extremely fine, paper tabs as defined in JIS R 7606 (gauge length 12.5 mm) were used for the tensile test of single fibers. Figure 5 is a diagram of a single fiber specimen. A single fiber was removed from the bundle, wrapped around the bar, and fixed at each end to the paper tabs with epoxy adhesive to create a specimen. Since PBO fiber loses strength under exposure to visible light, the specimens were stored in the dark until the time of the tensile test.

2.4. Single fiber tensile test (Monofilament Tensile Test)

A small table-top tester (SHIMADZU CORPORATION, EZ-Test) was used for the tensile tests of single fibers. For the test, the paper tabs attached to a fiber were fixed in the chucks of the tester and cut with scissors at each end, and the load was applied. The load was measured with a 5 N load cell, and the cross-head motion was measured simultaneously. The tension rate was set at 0.5 mm/min and the number of specimens tested under each condition was set at 40, in anticipation of scatter in the strengths. There is also scatter in the diameters of PBO fibers, so the diameter of each specimen was measured on a SEM image and used to calculate the tensile strength of that specimen. The diameters were measured with the SEM after completion of the tensile test in order to avoid deformation and defects due to heating of the fiber with the electron beam. These measurements were carried out in portions of the fibers outside the gauge region, and thus not subject to the actual tensile forces (A in Fig. 5). Fibers in which the failure occurred at the bonded locations rather than within the gauge region were eliminated from the dataset. 3. Experimental Results and Discussion

3.1. Tensile strength of PBO fiber containing kink bands

Figure 6 is a plot of the Weibull probability of the tensile strengths of PBO fibers incorporating kink bands at all rod diameters. The strengths of the relatively weak PBO fibers incorporating kink bands form a curve on the graph, but the overall pattern is linear. This result suggests that the bands comply with a two-parameter Weibull distribution. Table 3 lists the shape parameters and the scale parameters. The shape parameters for the diameter of steel bar 0.65

Cut off

Gripping area

Single PBO fiber

Glue

Gauge length 12.5 mm

A; Measurement area of kink band and fiber diameter

Fig. 5. Tensile specimen of single PBO fiber.