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

2293

3

min      . max 6

R

(1)

For the first type of specimens, the loading programs had the following parameters:  The first program had two steps of loading with the minimum cycle stress  min 1 = 0.63  c and  min 2 = 0.56  c for the first and second steps, respectively.  The second program had three steps of loading with the minimum cycle stress  min 1 = 0.63  c ,  min 2 = 0.56  c and  min 3 = 0.49  c , for the first, second and third steps, respectively. For the second type of specimens, the loading programs had the following parameters:  The first program had two steps of loading with the minimum cycle stress  min 1 = 0.68  c and  min 2 = 0.62  c for the first and second steps, respectively.  The second program had three steps of loading with the minimum cycle stress  min 1 = 0.68  c ,  min 2 = 0.62  c and  min 3 = 0.57  c , for the first, second and third steps, respectively. The average value of static strength  c was obtained during preliminary tests of samples of material with impact damage for static strength. Figure 3 shows diagrams of program blocks. The first loading program contained cycles with two different levels, in the form of two steps. The percentage contribution to damage relative to the program block, according to it S-N curve, is the same. The second loading program, in addition to the steps of the first program, contained lower level cycles. The loading level of such cycles corresponded to the fatigue life of more than 5×10 6 cycles. The number of cycles of the third step of loading exceeded the total number of cycles of the first and second steps more than 2 times. Figure 2 shows the diagrams of one block of the first and second programs, where n 1 ,n 2 ,n 3 stand for number of cycles of the first, the second and the third step, respectively.

Figure 3 – Block diagrams of the first and second programs

3. Results. Fatigue tests were carried out on the electro-hydraulic testing machines, which allow the specimen to be stretched and compressed at a constant speed and provide load and displacement measurements with an accuracy of ± 1% of the measured value. The grips provide reliable fastening of specimens in a position when their axis of symmetry coincides with the axis of loading. The operation of the testing machines was monitored throughout the entire testing