PSI- Issue 9

Yu. Matvienko et al. / Procedia Structural Integrity 9 (2018) 16–21 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Other specimens are subjected to cyclic loading with the parameters   = 300 MPa, R = –0.67 before residual stress determination. A multipurpose servo-hydraulic universal testing machine walter + bai ag , Type LFV 500-HH with loading range 0–500 kN is used for cyclic loading. Uniaxial tensile-compressive load P acts along the weld direction. The number of loading cycles N for each investigated specimen is equal to 2000, 4000 and 7000. Residual stress values are determined at the central point of each specimen by combining the hole-drilling method and electronic speckle-pattern interferometry. The residual stress evolution curves for two principal stress components are shown in Figure 4a. The results obtained clearly indicate residual stress relaxation caused by applied cyclic loading. An influence of revealed redistribution of principal residual stress components on fracture mechanics parameters is of considerable interest. This analysis follows below.

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Fig. 4. Residual stress (a) and CMOD (b) values as a function of the loading cycle number

Crack growth for all specimens is simulated by a sequence of eight narrow notches of width  b = 0.17 mm. The total crack length for each specimen consists of 8 prolongation increments. Interferograms of type shown in Figure 2 are obtained for all specimens. The values of crack opening, stress intensity factor and T-stress are derived according to the procedures described by Pisarev et al. (2017). Initial experimental data give us the dependencies of CMOD, SIF and T-stress values from total crack length for different loading cycles. This information provides the dependencies of fracture mechanics parameters for cracks of the fixed length as a function of the loading cycle number, which are shown in Figure 4b, 5a and 5b for CMOD, SIF and T-stress, respectively. Comparative analysis of presented distributions displays the evolution of deformation and force fracture mechanics parameters as the result of cyclic loading for the first for crack length increments (n = 1, 2, 3, 4). 4. Conclusion The CMOD, SIF and T-stress dependencies against of the loading cycle number are of prime interest for fatigue life estimation of specimens with cold-expanded holes and welded coupons. The remarkable feature inherent in the approach developed resides in the capability of obtaining both deformation and force fracture mechanics parameters for cracks of different lengths propagating under combined influence of acting and residual stresses. A wide set of such dependencies obtained for different cycle parameters bring the new power tool for life-time assessment. The developed approach offers several advantages over traditional methods of life-time assessment of cold-expanded holes that employ crack growth rate estimation. Currently available methods involve in situ measurements of current crack length. Accurate measurement of crack length during fatigue test is quite serious technical problem. Thus, the first