PSI - Issue 28

A.A. Lukyanchuk et al. / Procedia Structural Integrity 28 (2020) 2291–2296 A.A.Lukyanchuk / Structural Integrity Procedia 00 (2019) 000–000

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process to ensure the necessary size and shape of loading. The tests of specimens were carried out under a standard atmosphere of 23/50 according to GOST 12423-2013 [3]. To prevent the overall loss of stability of the specimen during compression, a special fixing device was used, which consisted of two anti-bending plates that were fixed on the specimen with two pairs of bolts, washers, nuts and locknuts. The anti-bending device bolts were tightened using a torque wrench with the following moment: the first row of bolts was tightened to 3 N  m, the second row of bolts to 0.5 N  m. Such a tightening torque was chosen based on tests on the shift of the anti-bending device relative to the specimen, taking into account that the friction force between the specimen and the plates should not exceed 2% of the minimum load during fatigue tests. Figure 4 shows a specimen in the anti-bending device, fixed in the grips of the testing machine, and rows of bolts are numbered. The tests were carried out under various loading conditions until the rupture of the specimens. During the tests, the constancy of the given load amplitude was monitored.

Figure 4 – Impact-damaged specimen in the anti-bending device fixed with the grips of testing machine

Experimental fatigue life was compared with analytical evaluations. Experimental fatigue life was taken in the form of incomplete blocks of the program (the whole part and decimal). Analytical evaluation of fatigue life (for the program block) was obtained using a S-N curve of regular loading with the corresponding asymmetry ( R = -6), and using the linear summation hypothesis of damage [1,4]. The amount of fatigue damage in this case was as the ratio of the experimental to the analytical fatigue life:

N

exp

A



(2)

.

N

eval

Table 1 shows the calculated and experimental parameters of the investigated loading programs for two types of specimens. The second column presents the estimated value of the durability in the flight blocks, calculated as the ratio of 1 to the damage of one flight block of the program. For comparison, the fifth column of the table presents the average experimental value of the durability in flight blocks, which was calculated as the ratio of the average value of the actual experimental durability in cycles to the number of cycles in one flight block of the program. It should be noted that there are recommendations in literature sources that if the value of the sum of fatigue damage A ≥ 0.5, then the using of the linear summation hypothesis of damage is possible in most cases the [5,6,7]. The results of the sum of fatigue damage of the first type of specimens have shown up a non-linear effect appearing under irregular loading with low amplitude loads, because of the value of the sum of fatigue damage A =0.36 and it doesn`t satisfy the re recommendations mentioned above. Such results cannot be explained only by the spread of fatigue properties. This may be due to an unknown mechanism of damage accumulation in a specimen with an impact damage, in which the number of loading cycles is more significant for damage than the level of the acting stresses.