Issue 75

M. Nikhamkin et alii, Fracture and Structural Integrity, 75 (2026) 390-398; DOI: 10.3221/IGF-ESIS.75.28

Figure 7: Example of a sample thermogram. The black rectangle is region A, in which the temperature field is recorded.

Figure 8: Dependence of the maximum temperature increase in region A on the number of loading cycles at different stress values σ max . To determine the fatigue limit, the rates of temperature rise at the beginning of loading blocks were calculated in four variants: maximum θ̇ max and average θ̇ av values in region A, each determined by differentiation over 6000 and 12 000 cycles. Fig. 9 shows the dependencies of these parameters on σ max . Fatigue limits were determined as described above from the intersection point of bilinear approximations of the two branches of the heating indicator versus stress. To formalize the selection of the branch separation point for the dependencies θ̇ max ( σ max ) and θ̇ av ( σ max ), the criterion of maximum combined coefficient of determination R 2 Σ (1) was used [16, 22]. Fig. 10 presents R 2 Σ values for different separation points k (k is the serial number of the loading block). The highest value corresponds to k=4. Tab. 1 summarises the fatigue limits obtained by infrared thermography for the four heating indicators. All four variants give nearly identical fatigue limits; the mean value is 383 MPa, with a coefficient of variation of 0.46 %.

Heating indicator

Fatigue limit, MPa

384.7 384.5 381.3

θ̇ av ( Δ N = 6000 cycles) θ̇ av ( Δ N = 12 000 cycles) θ̇ max ( Δ N = 6000 cycles) θ̇ max ( Δ N = 12 000 cycles)

381.9 Table 1: Fatigue - limit values obtained by infrared thermography.

Validation of the developed accelerated thermographic technique for determining the fatigue limit was performed using traditional fatigue tests with an S–N curve. Tests were conducted on a batch of Inconel 625 specimens produced by WAAM.

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