PSI - Issue 16

Oleksiy Nemchuk et al. / Procedia Structural Integrity 16 (2019) 245–251 Oleksiy Nemchuk et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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5

Figure 2 illustrates the position of all tested steel fragments on the crane. The fragments with the highest Δ  e value are indicated in red while the loves ones in green. It should be noted that we consider just the range of cyclic stresses Δ  e which is important factor of exhaustion of steel plasticity and decrease of its brittle fracture resistance. The change of Δ  e from 45 до 145 MPa for the same loading scheme indicates essential difference in operational stresses for different crane units and, respectively, a different level of operational degradation could be achieved, namely, unequal mechanical properties of steel from different parts of the crane. Results of SSRT testing are shown in Table 2. There is no indication that parameters of strength and plasticity are sensitive to the level of Δ  e .

Table 2. Strength and plasticity parameters of some crane units Steel fragment (from Table 1) Crane unit Δσ e σ UTS ,

σ Y

δ

RA

МPа

%

2 9

Jib

110

437 434

271 312

33 29

68 70

Back column wall

70

Comprehensive research was carried out for impact toughness evaluation (Table 3). Higher KCV values in the range 150 – 363 J/cm 2 were obtained for longitudinal specimens while the values for transversal ones were essentially lower, 45 – 137 J/cm 2 . From the one hand, these results illustrate general regularities of higher resistance to brittle fracture of longitudinal specimens since in this case the fracture plane intersects material texture formed by rolling. On the other hand, it was shown by Krechkovs’ka and Student (2017) for heat resistant steels of power engineering, and by Zvirko et al. (2018) for pipe steels of main pipelines that their long term operation leads to intensive degradation of their brittle fracture resistance if fracture plane is parallel to rolling direction. This could be explained by enhanced susceptibility to delamination between fibers of rolling texture. This regularity becomes more pronounced if metal is subjected to hydrogenation in a coarse of operation, since hydrogen recombination in steel defects at the boundaries of inclusions elongated due to rolling procedure is supposed to be a reason of cohesion disturbance between matrix and inclusions.

Table 3. The values KCV of the steel from different crane units, J/cm 2 Steel fragment (from Table 1) Δσ e , МPа

Specimen orientation relative to rolling Longitudinal Transverse

1 2 3 4 5 6 7 8 9

130 110

159 220 299 310 303 363 150 189 227 312

60 95

48 55 45 50

114 123 127 177

120 145

45 53

70

125 137

10

100

The dependencies KCV – Δσ e are built for the specimens of different orientation regarding the rolling direction. (Fig. 3). The decrease in brittle fracture resistance with the rise of service loading is revealed for the both types of specimens (longitudinal and transversal). This indicates an essential effect of operational cyclic loading on in-service changes in a material state.