Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 201 The second group of specimens was manufactured from a rod with mechanical properties shown in Table 2. The same type of specimens was manufactured from the rod for fatigue tests presented in Fig.1. Specimen № and standard Ultimate tensile, σ u , MPa Elongation, δ % Area reduction, Ψ % Impact toughness, kgl/mm 2 1 1048 14.0 14.5 4.1 2 1070 15.2 41.0 4.6 Mean value 1059 14.6 41.2 4.4 Standard ≥ 960 ≥9.0 ≥20.0 ≥ 3.0 Table 2 . Mechanical properties of the rod of titanium alloy VT3-1 at 20°С Fatigue and tension tests The first and the second group of specimens were tested with frequency 35 Hz and 200 Hz, respectively, at 20 o C. Two types of cyclic load tests were performed: rotating-bending and tension with different values of R-ratio and various maximum stress levels. After tests, fractographic analyses were performed on specimens subjected to different stress levels with durability in the range of 3х10 4 - 3х10 7 load cycles: - notched bar NT- and NTP-specimens were fatigued under tension with R-ratio in the range of 0.3 to 0.76 , with maximum stress level in the range of 920 to 680 MPa; - notched T- and TP-specimens experienced rotating-bending with maximum stress level in the range of 750 to 300 МPа; - smooth T-specimens were tested in the range of R-ratio 0.41 to 0.64 , with maximum stress level in the range of 850 to 730 MPa; - smooth TP-specimens experienced tension with R-ratio in the range of 0.3 to 0.67 , with maximum stress level in the range of 920 to 720 MPa. T-specimens have not failed up to 5x10 7 cycles under various positive R-ratios. Then, they were cyclically fatigued in the second stage of tests to their failure with higher stress amplitude, approximately in 2.3 time in accordance with previous stress level amplitude for R=-1.0. All run-out specimens were subjected to monotonic tension on the “Instron” test-machine with low speed of grip. Fractographic investigations All fatigued and monotonically tensed specimens were subjected to fractographic analyses through the scanning electron microscope “EVO40” of the Karl Zeiss instruments. Fatigue striation spacings were measured to reproduce crack growth period and to evaluate the relationship between durability N f and crack growth duration N p . R ESULTS OF INVESTIGATION Mono S-N curves irst of all, S-N curves were constructed for smooth T- and TP-specimens subjected to different tests condition (Fig.2). It is clear that, after tempering, durability of material decreases. This result reflects difference in surface state before and after tempering on the air. Surface layer has more intensive damage after tempering than before. The same result was found for steels [7]. Surface hardening of smooth specimens significantly influences durability, but this effect decreases with durability increasing. The discussed result was discovered for mean stress 600 MPa. Hardening effect can be negligible for long-life specimens because they reach durability area for transition zone from HCF to VHCF regime. In this case, a significant difference does not exist in durability for hardened and not hardened specimens of steels [8]. Notched specimens have not significant difference in regularities of their behavior under cyclic loads that were discussed related to smooth specimens (Fig.3). The T-specimens have a decrease in stress level (on 20-25%) for durability from 10 4 to 2x10 7 load cycles. F