PSI - Issue 75

D. Tousse Tchamassi et al. / Procedia Structural Integrity 75 (2025) 450–456 Tousse / Structural Integrity Procedia (2025)

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3.1.3. Fatigue life Whatever the amount of strain applied before the LCF test, the fatigue life behavior obeyed a Manson-Coffin relationship ∆ = with ∆ = (1.23 to 1.54) being the plastic strain amplitude at mid-life, being the number of cycles at fracture and =−0.7 being the Manson-Coffin exponent (Fig. 4a). When plotting the total amount of cumulative plastic strain at fracture, , vs. the fatigue lifetime (Fig. 4b), a linear relationship was fitted, with a slope between 9.0×10 -4 and 3.6×10 -3 ; a common value of 2.5×10 -3 satisfactorily represented most data points, except for the higher fatigue lifetimes, where the uncertainty associated with the Young’s modulus (determined from mid-life cycles) led to a larger uncertainty on the experimental value of . From Fig. 4, the fatigue lifetime behavior of the undeformed material was slightly better than the other three; the amount of applied plastic strain, and the loading path during prior deformation did not significantly affect the lifetime behavior; any applied plastic strain between 2% and 4% reduced the LCF lifetime by about 10% whatever the prescribed total strain amplitude. Regarding LCF lifetime assessment, a 2% prior tensile strain applied in the laboratory to the LCF specimens yielded the same results as prior plastic deformation applied in the factory by expanding the tube to make a connection. This provides a simple and efficient tool for future alloy design.

Cumulative plastic strain

Fig. 4. Effect of testing conditions on the fatigue lifetime. (a) Manson-Coffin plot with corresponding fitted curves; (b) cumulative plastic strain at fracture as a function of fatigue lifetime.

3.2. Damage development and fracture

3.2.1. Fracture surface observations Fig. 5 shows the fracture surface of a specimen taken from the expanded material fatigue tested (± 0.6%) then tensile fractured at -80°C. Several flat fatigue cracks were observed; all of them initiated from the side surface (arrows in Fig. 5a) and propagated into the bulk along the radial direction of the specimen. The central part of the fracture surface, linking fatigue cracks together, was slanted, indicating that fatigue cracks nucleation sites were located at various distances from the specimen ends. Fig. 5b illustrates the typical morphology of the fatigue fracture surface. Some mating occurred from place to place, owing to the low value of the load ratio (-1).

Fig. 5. Fracture surface of the LCF specimen of expanded material tested with a strain amplitude of ±0.6%. (a) Low magnification view; some fatigue cracks are indicated with arrows; (b) enlarged view of the crack at the top right corner of (a).