A Novel Model of Ultrasonic Fatigue Test in Pure Bending

Abstract

The very high cycle fatigue (VHCF) failure of in-service components is mainly caused by the vibration of thin-wall elements at a high frequency. In this work, a novel model of ultrasonic fatigue test was developed to test thin-wall material in bending up to VHCF with an accelerated frequency. The theoretical principle and finite element analysis were introduced for designing a sample that resonated at the frequency of 20 kHz in flexural vibration. In the advantage of the second-order flexural vibration, the gauge section of the sample was in the pure bending condition which prevented the intricate stress condition for thin-wall material as in the root of cantilever or the contact point of three points bending. Moreover, combining the constraint and the loading contact in one small section significantly reduced heating that originated from the friction at an ultrasonic frequency. Both strain gauge and deflection angle methods were applied to verify the controlling of stress amplitude. The fractography observation on Ti6Al4V samples indicated that the characterized fracture obtained from the novel model was the same as that from the conventional bending test.

Keywords: experimental method; flexural vibration; thin plate material; very high cycle fatigue.

Figure 1

Ultrasonic bending fatigue vibration: (a) Schematic diagram of ultrasonic fatigue system; (b) Installation of ultrasonic bending equipment; (c) transition of a longitudinal wave to a transverse wave.

Figure 2

Specimen sketch and simulation result at the resonant frequency. (a) Dimensions of bending test sample; (b) Von-Mises stress field; (c) out-plan displacement field.

Figure 3

Stress amplitude along the axial of the sample. (a) Schematic diagram of uniaxial resonance stress path; (b) stress amplitude along the path with 1 µm given displacement.

Figure 4

Schematic diagram of laser curvature measurement.

Figure 5

Laser measurement reliability verification: (a) Laser measurement standard sample size; (b) laser measurement data.

Figure 6

Thermal image of the specimen during resonance: (a) Without air-cooled; (b) air-cooled.

Figure 7

Statistics diagram for the fatigue failure.

Figure 8

S-N curves of ultrasonic bending fatigue in comparison to the axial fatigue in literature.

Figure 9

Pure bending fracture morphology: (a) σ_a = 555 MPa, N_f = 3.82 × 10⁶ (b) σ_a = 435 MPa, N_f = 1.69 × 10⁷; (c) σ_a = 335 MPa, N_f = 1.47 × 10⁸; (d) σ_a = 305 MPa, N_f = 5.41 × 10⁸.

Figure 10

Pure bending fracture morphology: (σ_a = 335 MPa, N_f = 1.47 × 10⁸) crack initiation morphology: (a) initiation I; (b) initiation II.

Figure 11

Fractography of sample failed by bending: (a) Pure bending (σ_a = 335 MPa, N_f = 1.47 × 10⁸) crack morphology; (b) enlarged image of area A crack propagation region); (c) enlarged image of area B (transient fracture region).