PSI - Issue 28

Y. Matvienko et al. / Procedia Structural Integrity 28 (2020) 584–590 Author name / Structural Integrity Procedia 00 (2019) 000–000

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intensity factor (SIF) values versus number of preliminary fatigue cycles. To perform the measurement procedure after preliminary low-cycle fatigue, all specimens are subjected to uniform uniaxial tension by electro-mechanical testing machine, which is located in the interferometer optical system. Three consecutive narrow notches are performed under the constant external load for each specimen. An electro-mechanical testing machine ( walter + bai ag , Type LFM-L 25) with loading range 0–25 kN serves for applying remote tensile stress  during the measurement procedure. All experimental parameters correspond to constant remote tensile stress  = 53.1 MPa. Interferograms obtained by this way for Specimen T4_15 (CC-group, m N = 3300 cycles) and Specimen T4_X1 (XX-group, m N = 5350 cycles) are shown in Fig. 1a and 1b, respectively.

a b Fig. 1. Typical interference fringe patterns obtained in terms of in-plane displacement components at different stage of low-cycle fatigue. (a) CC group, m N = 3300 cycles; (b) XX-group, m N = 5350 cycles

Obtained information leads to the curves of fracture mechanics parameters versus notch length, which are constructed for different number of cycles. Initial points of the first symmetrical notch belong to the hole edge. Then, these data provide the dependencies of SIF values for notches of fixed lengths against number of cycles, which quantitatively describe a process of fatigue damage accumulation. Thus, a deformation response to small notch length increment serves as an indicator of damage accumulation degree at different stages of fatigue loading. Dependencies of relative SIF values against of lifetime percentage for different parameters of cycles are shown in Fig. 2 and 3. Obtained data evidence that evolution of relative SIF values, which are related to the first symmetrical notch emanating from the centred hole in rectangular plate, can be effectively used as the damage parameter. This follows from the fact that squares of areas lying under all plots in Fig. 2 and 3, calculated by linear stepwise approximation, are equal within 6.7 and 3.7% for different stress ratio and stress range, respectively. In other words, it can be assumed that each individual area represents a parameter definitely related to the end point of fatigue damage accumulation process. Thus, the effect of the stress ratio and stress range on the rate of fatigue damage accumulation can be quantitatively evaluated.