PSI - Issue 35

Martin Ferreira Fernandes et al. / Procedia Structural Integrity 35 (2022) 141–149 Martin Ferreira Fernandes et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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α phase and 35.3% of β phase. The yield strength ( σ y ) was about 1000 MPa, according to tensile tests. The specimens were machined from Ti-6Al-4V bars with a diameter of 14.3 mm. The number of machined specimens was 12 for dwell-fatigue tests. Fig. 1b shows the geometry of fatigue and dwell-fatigue test specimens. Dwell-fatigue tests with a load-controlled trapezoidal waveform and dwell period of 10 seconds (Fig. 2a) were performed for the base material at room temperature. For triangular and trapezoidal waveforms, a linear increase to the maximum load during 1 second and a linear decrease to the minimum load were applied. Mechanical tests with trapezoidal waveforms are relevant to simulate loadings during the operation of turbine disks (Billot et al., 2010). The tests were performed with maximum stress values in the range of 0.95 σ y to 1.0 σ y , a stress ratio of R = 0.1, and the similar geometry of fatigue tests (Fig. 1b). The specimens were ground using SiC papers from grades 1000 to 1500. The strain was continuously measured at the maximum stress level stages using an extensometer. The cumulated plastic strain as a function of time and number of cycles for dwell-fatigue tests was obtained. The Weibull distribution was applied to analyze the dwell-fatigue data statistically. Equation 1 shows the probability density function, where m is the scale parameter, n is the shape parameter and x is the fatigue life (Fernandes et al., 2020). The statistical analysis of the stress-life (S-N) linear regression models according to the

ASTM E739 standard was also performed. ( ) = ( ) −1 −( ) ≥ 0, ≥ 0

(1)

The analysis of the fracture surfaces of the fatigue and dwell-fatigue tested specimens was performed through scanning electron microscopy (SEM) using a Tescan model Vega 3 XMU equipment to identify the failure modes for the different tests and loading conditions.

(a)

(b)

Fig. 1. (a) Equiaxed microstructure of Ti-6Al-4V alloy and (b) geometry of fatigue and dwell-fatigue specimens.

Fig. 2. (a) Triangular fatigue loading waveform, (b) Trapezoidal dwell-fatigue loading waveform.