PSI - Issue 5

Takahide Sakagami et al. / Procedia Structural Integrity 5 (2017) 1370–1376 Takahide Sakagami/ Structural Integrity Procedia 00 (2017) 000 – 000

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The obtained results of the self-reference lock-in measurement indicating the stress distributions are shown in Fig. 6. To avoid the problem of the infrared-measurement area being hidden by the angle-stiffener plate, an observation window was made in one of the plates, as shown in the photograph. It is found from Fig. 6(a) that a high stress concentration was observed at the crack tip before the repair. It is found from Fig. 6(b) that a high stress concentration was still observed at the crack tip, however the maximum stress value in the vicinity of the crack tip is drastically decreased after the repair work. Stress values were evaluated at 16 points along the straight line on crack propagation direction from the crack tip. The results are shown in Fig. 7. The stress values in the figure were averaged from the experimental data conducted for 5 times under the same loading conditions. It is found from the figure that stress values were decreased by 25-30 % after the repair work demonstrating the effectiveness of stiffening-plate repair. The stress intensity factors were calculated from stress distributions measured by the TSA technique for quantitatively evaluating the suppressing effect of fatigue crack propagation by the repair work. The value of the stress intensity factor range  K before the repair work was 14.2 MPa√m . In contrast  K was decreased to 10.7 MPa√m after the repair work. The relationship between stress intensity factor range  K and crack propagation rate was obtained for JIS SM490 steel by Ohta et al. (1995) as follows. / = (∆ − ∆ ℎ ) , = 4.8 × 10 −13 , = 3.68, ∆ ℎ = 9.3 (3) According to the above relationship, the estimated crack propagation rate was 2.31 × 10 −8 mm/cycle before the repair, and 1.04 × 10 −8 mm/cycle after the repair. It was found that the crack propagation rate was reduced by 55% by the stress mitigation effect of the crack repair that leads to the life prolonging of the cracked steel structures. 6. Conclusions In this study, the TSA technique was applied for on-site measurement of stress distributions around fatigue cracks, and the future crack propagation behavior was estimated by the fracture mechanics approach. Experimental studies were conducted for laboratory specimens which modeled a part of welded structure in steel bridges. The stress intensity factors were calculated from stress distributions measured by the TSA technique. Relationship between stress intensity factor ranges and crack propagation rates was obtained. It is found that the obtained relationship shows a good correspondence with the Paris law. Further the TSA technique was applied to evaluate the effectiveness of repair or reinforcement for defective portions. Severity reduction in stress distribution around the fatigue crack after treatment was confirmed for actual steel bridge members by the TSA technique. Crack propagation rate was estimated from the stress intensity factor calculated from on-site stress measurement data. As the result, 55% reduction in crack propagation rate was ascertained indicating the positive effect of the crack repair.

Acknowledgements

The authors would like to acknowledge that the research works shown in this paper were partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (B: 26289009). The authors also would like to thank T. Fujimoto, T. Higashi, Y. Terauchi and R. Ichihashi for their technical assistance with the experiments and data processing.

References

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