PSI - Issue 42

Martin Matušů et al. / Procedia Structural Integrity 42 (2022) 102 – 109 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

103

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Risitano (2000), or Luong (1998). Heat dissipation during cyclic testing is present at any load amplitude. To examine it in this case, the temperature on the surface of a specimen is recorded using an infrared thermal camera.

Nomenclature σ a

amplitude of stress

amplitude of stress corresponding to the fatigue limit

σ FL

number of cycles to failure

N f

loading frequency during a fatigue experiment

f

∆ T s . T O .

stabilized temperature in the second phase of temperature evolution ambient temperature of the specimen environment coefficients of the Kohout - Věchet S-N curve model

A , B, C, β

limiting energy

Φ

temperature increase rates during fatigue testing in separate phases – initial, stabilized, and crack

R 0 , R s , Rc

initiation phases, respectively. r f radius of curvature a, b 2. Fatigue experiments 2.1. Description of specimens

coefficients of the exponential model Eq. (2)

The material used in this study was 42CrMo4+QT steel with ultimate tensile strength of 1097 MPa and yield stress of 1001.5 MPa. Specimens were manufactured from the central part of bars of a ⌀ 35mm which were all produced in a single heat. The surface roughness Ra = 0.8 µm of the specimen was achieved by turning and grinding. Four distinct specimen geometries A01 – A04 for fatigue testing were designed; see Fig. 1. Hourglass geometries A01 and A03 were produced with an active diameter of Ͷ ƒ† ͺ ǡ ”‡•’‡…–‹˜‡Ž› . Dog bone specimens of the A02 and A04 designs also have active diameters of 4 mm and 8 mm, respectively, but also a prismatic part in the specimen centre of the lengths 16 mm and 30 mm, respectively, to increase the critical volume with the active cross section, see Fig. 1.

Fig. 1. Four different geometries of fatigue specimens: hourglass specimens A01, A03, and dog bone specimens A02 and A04.

2.2. Experimental setup The fatigue life performance of series A01-04 was evaluated on an Amsler HFP422 resonator of 100 kN load range with load-controlled push-pull load in the load ratio of R = -1. The testing frequencies of series A01, A02, A03, and A04 were 130Hz, 100Hz, 160Hz, and 130Hz, respectively. The test end-condition was a drop of 5 Hz in loading frequency. This frequency drop is caused by the decrease in the stiffness of the specimen during the propagation of cracks. Fatigue testing was focused on the region between the 5,000 and 2,000,000 cycles. The run-out condition was set to 10 million load cycles. Run-out specimens were then re-run on an at least 1.36 times higher stress amplitude than the original.