PSI - Issue 42

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Pedro R. da Costa et al. / Procedia Structural Integrity 42 (2022) 1560–1566 edr . a st / Structural Integrity Procedia 00 (2019) 00 – 000

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Fig. 3. Displacement to stress relation obtained by analytical, experimental, and numerical methods: (A) tension-compression; (B) pure torsion; (C) multiaxial Tension/Torsion.

2.2. Fatigue fracture surfaces In conventional fatigue methods the experimental tests come to a stop when a full fracture of the specimen is reached. The fatigue fracture surface is directly available for its analysis upon taking out the specimen from the machine. Because all three ultrasonic methods work on resonance and vibration concepts, their experimental fatigue test stops before the final fracture. The specimens require to be fully fractured in a tensile machine. All obtained fatigue fracture surfaces were analyzed by optical microscope and scanning electron microscope (SEM). From the optical microscope the crack angle of initiation was determined. For uniaxial specimens a 0º crack initiation was obtained. In the torsion specimen a small 0º crack initiation was observed, followed by a 45º propagation. Regarding the tension-torsion specimens, the crack initiation angle varied with the shear/axial stress ratio. The crack initiation angle showed to increase for higher shear/axial ratios. Figure 4 compares three different fatigue fracture surfaces between the three ultrasonic fatigue conducted methods. All specimens presented surface crack initiation with the exception of one tension-compression that presented crack initiation from a spherical inclusion. Energy Dispersive Spectroscopy (EDS) quantified the chemical composition of the inclusion to be composed of aluminium oxide with considerable percentages of aluminium, oxygen, calcium and manganese. Similar shaped and chemical composed spherical inclusions were found in Jiang et al. (2016) ultrasonic fatigue tested structural steel and Tridello et al. (2017) AISI H13 steel. In pure torsion specimens, crack initiation occurred at a point with a low angle associated with the highest shear stress plane mode II. This crack initiation region presents a smooth surface. Afterwards, the crack propagates at an increasing angle with radial marks (figure 4.C shear bands). Some specimens presented a crack bifurcation as the one shown in figure 4 after some initial propagation. Tension-torsion specimens showed a similar crack initiation region morphology to the obtained in tension compression fatigue specimens. A small dark region surrounded the pinpointed crack initiation location. The final fracture surface radial marks have similar morphologies to the p ure torsional specimen’s propagation .