PSI - Issue 19

Hiroyuki Oguma et al. / Procedia Structural Integrity 19 (2019) 224–230 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6 shows the fatigue strength at 10 7 cycles under each stress ratio condition. The fatigue strength at 10 7 cycles was defined as the average of the stress of sampled data at 10 7 cycles and the minimum stress among the fracture data below 10 7 cycles. Regardless of the surface treatment conditions, the fatigue strength goes down slightly with increasing the mean stress between R = − 1 and 0.1. However, the fatigue strength dramatically decrease when the stress ratio is larger than 0.5.

Fig. 6. Fatigue strength at 10 7 cycles.

4. Observation results and discussion

To investigate the strength properties in the tensile and fatigue tests, fracture parts of the specimens were observed with a digital microscope. Fracture parts obtained in the tensile tests are shown in Fig. 7. In the as-received specimens, cohesive failures were mainly observed. The cohesive failures were also observed in the sandblasted specimens, but fracture of the surface layer occurred in the adherend plate. Magnified photograph of the fracture part as framed box in Fig. 7 (b) is shown in Fig. 8. Corrugated surface along the glass fibers was observed, and this result indicated that sandblast treatment damaged the surface of the adherend; therefore, the static strength of the sandblasted specimens became lower than that of the as-received specimens. However, the sandblast treatment makes homogeneous surface, and the variation of the strength value became smaller as mentioned in section 3.1 .

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Fig. 7. Fracture surfaces obtained in the tensile tests (a) as-received; (b) sandblasted.