Crack path in torsion loading in very high cycle fatigue regime

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Abstract

Torsion fatigue tests have been conducted at 20 kHz ultrasonic fatigue testing systems, and compared to the torsion fatigue data generated on 35 Hz conventional fatigue test machine to determine if there are any frequency effects, for steels including D38MSV5S steel and 100C6 steel. Results indicated that the SN curves exhibit decrease in fatigue strength beyond 107 cycles. The initiation in the Gigacycle regime is related to defects sometimes located beneath the surface which shows a competition between the maximum shear at the surface and the stress concentration under the surface, even in torsion.

Introduction

Recent work using ultrasonic test system has shown that many materials, including some steel, aluminum alloys and titanium alloys, exhibit a sharp decrease in fatigue strength between fatigue lives of 106 and 109 cycles [1], [2], [3], [4], [5]. However, it is much more difficult to carry out fatigue tests in torsion in the Gigacycle regime where the fatigue life of many car components is ranging The first step to study Gigacycle fatigue is to design a good device, which is an important part of this study.

Structural components used in rotating parts in engines, are usually required to be designed using a lifetime failure-free criterion for a very large number of cycles, or an endurance limit. Fatigue data in tension loading are insufficient for assessing high cycle fatigue limit in torsion loading [1], [2]. Thus, it is necessary to investigate the torsion fatigue performance in very high cycle regime. Since 1993, Stanzl-Tschegg et al. [1] designed and constructed an ultrasonic torsion fatigue test system at a frequency of 21 kHz. However, much of the available fatigue properties today are in axial loading. A novel piezoelectric torsion fatigue testing machine was developed, in our laboratory, in which a torsion fatigue specimen is driven into a 20 kHz high frequency resonance mode. The SN curve, the origin of the fatigue crack, and the fatigue initiation modeling are discussed up to 1010 cycles.

Section snippets

Materials

The materials used in this study were, a D38MSV5S steel and a bearing steel 100C6, having the nominal compositions as shown in Table 1, and mechanical properties given in Table 2.

The typical microstructure in 100C6 steel is a fine martensite with small inclusion (MnS). The microstructure of D38MSV5S shows ferrite (50%) and perlite (50%) (Fig. 1).

Introduction to the Gigacycle fatigue regime

When the fatigue life is below 105 cycles, general plastic deformation of the specimen bulk governs the initiation. When the fatigue life is between 106

Experimental results

The stress-life (SN) curves for D38MSV5S steel and 100C6 steel are shown respectively in Fig. 8a and b. In the figures, solid marks indicate the specimens tested at 35 Hz, the open ones indicated specimens tested at 20 kHz ultrasonic torsion fatigue test machine. The SN curves show that fatigue failure of the two alloys may occur between 106 and 1010 cycles.

For D38MSV5S steel, fatigue lifetime increases as the stress amplitude decreases in the life range. Stress amplitude decreases continually

Fracture analysis and discussion

In torsion fatigue test [6], [7] shear crack nucleation and growth are followed by crack growth on planes of maximum principal stress amplitude. But the crack initiation is not the same for the two alloys.

Fatigue crack initiation in the Gigacycle regime for D38MSV5S steel is always from the surface of specimens, in torsion, even in the Gigacycle regime. In axial loading, the initiation is in the subsurface beyond 107 cycles, starting from a super grain (Fig. 9, Fig. 10).

In 100C6 bearing steel,

Conclusion

For D38MSV5S steel, under a cyclic torsion loading, fatigue crack initiated from surface of the centre of specimen where the shear stress is maximum. Fracture occurs normal to the 45° tensile plane, producing a conical fracture surface, what ever is the fatigue life.

For 100C6 steel, the fatigue crack initiation location depends strongly of the number of cycles to failures. Roughly speaking, it is observed that the initiation is located at the surface of the specimen when the fatigue life does

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