13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 4.3. Friction force measurement Figure 10(a) shows an example of the relationship between the load and the strain at N = 4.0 × 104 cycles. In order to measure the hysteresis loop of the strain with high accuracy, a subtraction circuit was developed. The input data were the strain at a jig and the strain at a specimen, as measured by strain gauges. These strains were converted to electric potentials and subtracted in the circuit. Figure 10(b) shows that the relationship between the load and the subtracted strain. From Fig. 10(b), the hysteresis loop was observed during a load cycle. As seen from a comparison of Fig. 10(b) with Fig. 6, the hysteresis loop was the same at a low load, whereas a difference existed at a high load. At present, the exact reason for this result is not known. However, through an analysis of the obtained load-strain hysteresis loop, the friction force and values of ΔKIIeff and ΔKIIth could be determined in a future study. 102 103 104 105 106 107 0 0.2 0.4 0.6 Number of cycles, N Displacement range, Δδ [mm] Figure 7. Relationship between displacement range of pull-rod and number of cycles 102 103 104 105 106 107 0 0.1 0.2 0.3 Number of cycles, N Potential difference, ΔE/E0 Figure 8. Relationship between potential difference and number of cycles Figure 9. Fracture surface of Mode II fatigue crack 500 μm
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