PSI - Issue 26

Gonzalo M. Dominguez Almaraz et al. / Procedia Structural Integrity 26 (2020) 20–27 Almaraz et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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solid particles, vibrations, aerodynamic and centrifugal forces and large thermal stresses associated to thermal gradients [1-3]. Very often, this metallic alloy is subjected to severe corrosion attack during use [4-6]; then, particular interest is focused on the analysis of fatigue endurance under corrosion attack through an international norm of induced corrosion. This study is destined to compare the ultrasonic fatigue endurance of Inconel 718 the raw material, against the results obtained on pre-corroded specimens.

Nomenclature C

Plastic constraint factor ESA European Space Agency Hz Hertz I-718 Inconel 718 K I Stress intensity factor K IC Fracture toughness MPa Mega – pascals N Number of cycles R Stress ratio r p Plastic radius  x

Stress components in the X axis Stress components in the Y axis

 y

 yield Yield stress

2. Experimental procedure and set up

Round bars of I-718 were received with dimensions of 0.5 inch in diameter and 2.5 inches in length; the chemical composition (in weight) of this alloy is listed in Table 1.

Table 1. Chemical composition (in weight) of Inconel alloy 718. Element Weight (%) Al 0.62 Ti 0.94 Mo 2.89 Nb 5.13 Cr 18.46 Fe 18.7 Ni Balance

Bi, Pb, Se, S, Ca, B, P, Ta, Mg, C, Cu, Si, Mn and Co

Less than 1 %

Ultrasonic fatigue tests were carried out under the resonance condition; that is, the natural frequency of vibration along the hourglass shape specimen must be close to the frequency of the excitation source (20 KHz). Under resonance, a stationary elastic wave is developed along the specimen inducing high stress at the neck section and maximum displacement at its ends  7, 8  . In Figure 1 is presented the modal analysis and dimensions (mm) of the Inconel 718 specimen under resonance condition. The natural frequency of vibration of Inconel 718 testing specimen was obtained by modal analysis, this value was 20101.7 Hz (Figure 1); such value is close to excitation frequency of the source (20 KHz) . Ultrasonic fatigue tests can be carried out with a deviation of ±250 Hz in regard the nominal frequency of vibration of the source of 20 KHz. All ultrasonic fatigue tests were obtained at room temperature, environmental humidity and load ratio R = -1.