PSI - Issue 33

Julio A. Ruiz Vilchez et al. / Procedia Structural Integrity 33 (2021) 658–664 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Ultrasonic fatigue tests were carried out in a machine designed and developed in our laboratory, which presents the following features: high frequency generator (20 KHz), with 1100 watts of power, connected to piezoelectric transducer to convert ultrasonic electronic signal to ultrasonic mechanical vibration, coupled to an amplifier horn. Displacement controlled tests were implemented under ultrasonic fatigue; the calibration was carried out with an inductive proximity sensor to measure the displacements at the free end of specimen, with  2 m  of precision. Induced stress at the neck section of specimens was evaluated by numerical simulation, applying the corresponding measured displacement at the free end of specimen. Testing temperature at the narrow section of specimen may be controlled using cooling air, oil or water; nevertheless, no cooling system was used for the ultrasonic fatigue tests in the maraging 300 steel. The failure criteria for tested specimens was fixed with the stopping of the ultrasonic fatigue machine: the specimen’s stiffness is enough modified to leave the resonance condition, when failure occurs. Figure 4 presents the ultrasonic fatigue machine used for this study.

Fig. 4. Ultrasonic fatigue machine for testing the maraging 300 steel.

3. Experimental results Ultrasonic fatigue tests have been obtained at room temperature, with zero mean stress (R = -1), without control of environmental humidity and without cooling system. The sequence of testing was as follows: 10 seconds at the lowest stress of the ultrasonic equipment (195 MPa), corresponding to 10 volts for the ultrasonic generator; then, increasing the voltage during 10 seconds to attain the nominal applied load. Three nominal applied loads were used to characterize the fatigue endurance of this steel, under ultrasonic regime: 389, 486 and 585 MPa. In Figure 5 are plotted the ultrasonic fatigue results obtained for the three stress levels. Fatigue endurance for the highest stress level is close to 6 – 11 million of cycles, 25 – 70 million of cycles for the middle-applied load and 300 – 900 million of cycles for the low applied load. Concerning the crack initiation and propagation in this material under the described applied load, the origin of failure on this steel is associated with inclusions, such as : Ti(N,C), or Al2O3, Wang et al. (2010) , in which the size of the inclusion is related to the detrimental effect on fatigue life. Some authors suggest, Abe et al. (2004), that the controlled reinforcement with inclusions on the high strength steel base material, leads to the improvement of its fatigue endurance. The concepts of optical dark area (ODA), or fine granular area (FGA), related to fatigue endurance of high strength steels, have been studied by some authors, Sun et al. (2013), Sander et al. (2013), Li et al. (2014), in which the geometrical properties and size of crack initiation inclusion, Dominguez (2008), together with the applied load regime, determine the fatigue behavior of high strength steels. Nevertheless, other investigations suggest that additional factors, such as the physic-chemical interaction between the crack initiation inclusion and matrix, can induce modification in fatigue endurance of high strength steels, Spriestersbach et al. (2014).