PSI - Issue 65

Kuskov K.V. et al. / Procedia Structural Integrity 65 (2024) 133–138 Kuskov K.V./ Structural Integrity Procedia 00 (2024) 000–000

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82 "Calculating methods for fatigue strength characteristics" indicate only the amount of roughness without reference to its direction. Thus, the study subject of the roughness direction influence is poorly known.

2. Material and methods

The purpose of this study is to research the effect of the roughness direction relative to the acting force on the fatigue strength of various structural steels. The studied materials as the selected steels are 35, 45, 30KhGSA and 40Kh steels. In all cases the samples are made from a single bar with a length from 5.5 to 6.0 meters. Figure 1 shows the test sample. The theoretical stress concentration factor is calculated in Ansys software. It is 1.019. The samples are manufactured on a CNC machine with a constant coolant supply. The samples are polished with P1000 sandpaper according to ISO 6344-1 to a roughness value of R a = 0.232 μm after manufacturing. The roughness is measured using the Hommel-Etamic W5-Set device.

Fig. 1. Test sample.

All samples are divided into two series of three pieces. They are subjected to final treatment with P1200 sandpaper. The treatment is perpendicular to the forming cylindrical part of the sample for the first series. It is in parallel for the second series. In both cases, the polishing is carried out with an entry for at least 3 mm on the toroidal part of the sample. The final roughness of R a for all series is 0.11 μm. The standard tensile tests and hardness measurements are performed using the Rockwell method in advance of the fatigue tests. The results are shown in Table 1. The data obtained correspond to the characteristics specified in GOST 1050-20, GOST 4543-2016 and GOST 8479-70.

Table 1. Results of tensile tests.

Steel Grade

Yield Strength 0.2 % σ t, , МPа

Tensile Strength σ v , МPа

Reduction of Area ψ, %

Elongation δ, %

Hardness Rockwell B (Hardness Rockwell C), unit

35 (GOST 1050-20) 45(GOST 1050-20)

454.6 417.9 838.4

681.9 764.5

51.8 46.8 45.8

32.1 27.9 35.3

87.5 90.5

30KhGSA (GOST 4543-2016)

1133.3

(32.5)

40Kh (GOST 8479-70)

557.4

912.2

57.6

28.1

97

Fatigue tests are carried out on a servo-hydraulic universal machine TRMC 250S-1. The Locati method for accelerated fatigue tests is used as a basis. The tests are carried out at a temperature of 20-24 ° C in the air. The stress ratio R = -1, the form of loading is sinusoidal, the type of loading is tension - compression. The initial amplitude for all samples is 0.3*σ t . The increment of the loading amplitude is 0.1*σ t . The number of cycles for each stage is 72000. The tests are carried out until the samples are completely destroyed.