Gigacycle fatigue in high strength steels

Abstract

This paper reviews the research results to date on gigacycle fatigue caused by internal fractures in high strength steels. Firstly, accelerated fatigue testing was realized using ultrasonic fatigue testing at 20 kHz, which completes 10⁹ cycles in one day, unlike the 3-4 months needed for conventional fatigue testing. Although the frequency effect was anticipated to be problematic, it proved negligible under conditions in which internal fractures occurred. Later, many unique characteristics of internal fractures were elucidated. For example, hydrogen has dramatically greater effects on internal fractures than on conventional surface fractures. Mean stress effects are more serious in titanium alloys than in high strength steels. Size effects were notable in high strength steels. These distinctive characteristics required a unique model to be able to predict gigacycle fatigue strength, which first required elucidation of its mechanisms. To this aim, the author attempted to measure the crack growth rates of small internal cracks using the beach mark method. The results revealed that the crack growth of small internal cracks controls internal fractures. In calculating the crack growth life, however, it was found that the conventional crack growth law overestimates the effects of inclusion size. To rectify this problem, a new model using a new crack growth law was proposed, which predicts more realistic fatigue life curves. As a result, predictions were derived for several grades of high strength steels.

Keywords: 10 Engineering and Structural materials; 106 Metallic materials; Fatigue; Mechanical property; Structural materials; crack growth; inclusion; internal fracture; steel.

Graphical abstract

Figure 1.

Typical fatigue test results for conventional steel [2].

Figure 2.

Typical fatigue test results for high strength steel [1].

Figure 3.

Two-fold S-N curve concept.

Figure 4.

Typical ultrasonic fatigue test results for high strength steels [22,25].

Figure 5.

Typical ultrasonic fatigue test results for titanium alloys [27,46].

Figure 6.

Effect of tensile strength on fatigue strength [1].

Figure 7.

10⁹ cycles fatigue strength as a function of inclusion size [40]. Reprinted by permission from Springer Nature, Metallurgical and Materials Transactions A, Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes, Y. Furuya, H. Hirukawa, T. Kimura et al, 2007.

Figure 8.

Typical fatigue test results for hydrogen-charged specimens of low-alloy steels [42]. Reproduced by permission of the Iron and Steel Institute of Japan, Hisashi Hirukawa, Yoshiyuki Furuya, Gigacycle fatigue properties of hydrogen charged high strength steels. Tetsu to Hagane, 99, 494–501, Copyright 2013.

Figure 9.

Comparisons of fatigue strength between Ti-6Al-4V alloys and steels [27, 46].

Figure 10.

Endurance limit diagrams [27,46,53]. Figure 10(b) reprinted from Materials & Design, 32(3), Yoshiyuki Furuya, Takayuki Abe, Effect of mean stress on fatigue properties of 1800 MPa-class spring steels, 1101–1107, Copyright (2011), with permission from Elsevier.

Figure 11.

Size effects in gigacycle fatigue of high strength steels [50].  Reprinted from Materials Science and Engineering: A, 528, Yoshiyuki Furuya, Notable size effects on very high cycle fatigue properties of high strength steel, 5234–5240, Copyright (2011), with permission from Elsevier.

Figure 12.

Typical waveform used in the repeated two-step fatigue tests.  Reprinted from Materials Science and Engineering: A, 528, Yoshiyuki Furuya, Notable size effects on very high cycle fatigue properties of high strength steel, 5234–5240, Copyright (2011), with permission from Elsevier.

Figure 13.

Typical beach marks observed in ref [76]. Reprinted from Materials Science and Engineering: A, 678, Yoshiyuki Furuya, Small internal fatigue crack growth rate measured by beach marks, 260–266, Copyright (2016), with permission from Elsevier.

Figure 14.

Internal crack growth rates measured by beach marks [76]. Reprinted from Materials Science and Engineering: A, 678, Yoshiyuki Furuya, Small internal fatigue crack growth rate measured by beach marks, 260–266, Copyright (2016), with permission from Elsevier.

Figure 15.

Example of predictions based on the Tanaka-Akiniwa model [77]. Reprinted from Materials Science and Engineering: A, 743, Yoshiyuki Furuya, A new model for predicting the gigacycle fatigue strength of high strength steels, 445–452, Copyright (2019), with permission from Elsevier.

Figure 16.

R ²-values for various values of α in the new model [77]. Reprinted from Materials Science and Engineering: A, 743, Yoshiyuki Furuya, A new model for predicting the gigacycle fatigue strength of high strength steels, 445–452, Copyright (2019), with permission from Elsevier.

Figure 17.

Predictions based on the new model using the same data as in Figure 15 [77]. Reprinted from Materials Science and Engineering: A, 743, Yoshiyuki Furuya, A new model for predicting the gigacycle fatigue strength of high strength steels, 445–452, Copyright (2019), with permission from Elsevier.

Figure 18.

Predicted fatigue strength [77]. Reprinted from Materials Science and Engineering: A, 743, Yoshiyuki Furuya, A new model for predicting the gigacycle fatigue strength of high strength steels, 445–452, Copyright (2019), with permission from Elsevier.

Figure 19.

Endurance limit diagram with the predictions in ref [77]. Reprinted from Materials Science and Engineering: A, 743, Yoshiyuki Furuya, A new model for predicting the gigacycle fatigue strength of high strength steels, 445–452, Copyright (2019), with permission from Elsevier.