Research Papers

Characterization of Hydrogen-Induced Contact Fracture in High-Strength Steel

[+] Author and Article Information
Akio Yonezu

Department of Precision Mechanics,
Chuo University,
1-13-27 Kasuga,
Bunkyo, Tokyo 112-8551,Japan
e-mail: yonezu@mech.chuo-u.ac.jp

Michihiro Niwa

Department of Precision Mechanics,
Chuo University,
1-13-27 Kasuga,
Bunkyo, Tokyo 112-8551,Japan

Xi Chen

Department of Earth
and Environmental Engineering,
Columbia University,
500 W 120th Street,
New York, NY 10027
International Center for Applied Mechanics,
SV Laboratory,
School of Aerospace,
Xi'an Jiaotong University,
Xi'an 710049, China
e-mail: xichen@columbia.edu

This study also confirmed the size of plastic zone at the maximum indentation force. The depth is about 200 μm which is much smaller than the specimen thickness (1 mm), suggesting that the boundary of bottom surface does not affect the stress field surrounding the impression for crack nucleation. Therefore, boundary effect can be ignored.

As preliminary computation, this study investigated how CZM parameter (maximum stress and crack growth resistance) affect the radial crack length. The maximum stress was varied in 0.6, 0.75, and 0.9 GPa (which covers the critical stress in Fig. 8), while the crack growth resistance (stress intensity factor) was changed in 3.5, 4.0 and 7.0 MPa m1/2. The result suggested that the crack length was strongly depending on the input value of crack growth resistance, while it showed less dependence of the critical stress of crack nucleation. Therefore, the maximum stress for CZM set to be constant with 0.65 GPa in this study.

The trend of Figs. 8 and 11 show different. As described in Sec. 4.2, although the critical stress for crack nucleation significantly decreases (compared with tensile strength of 2.45 GPa) due to hydrogen embrittlement, such a decreasing rate becomes minor when the hydrogen content is large. This trend is observed in Fig. 8. On the contrary, Kth is usually sensitive to hydrogen content, even if large hydrogen content (up to 49 ppm) [8].

1Corresponding authors.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 21, 2014; final manuscript received December 16, 2014; published online January 30, 2015. Assoc. Editor: Toshio Nakamura.

J. Eng. Mater. Technol 137(2), 021007 (Apr 01, 2015) (7 pages) Paper No: MATS-14-1017; doi: 10.1115/1.4029530 History: Received January 21, 2014; Revised December 16, 2014; Online January 30, 2015

This study investigated the hydrogen embrittlement (HE) cracking behavior produced by local contact loading of high-strength steel. When a spherical impression was applied to a hydrogen-absorbed high-strength steel, HE induces contact fracture, where radial cracks are initiated and propagated from the indentation impression. The length of the radial crack was found to be dependent on the hydrogen content in the steel as well as the applied contact force. A combined experimental/computational investigation was conducted in order to clarify the mechanism of hydrogen-induced contact fracture. In the computation, crack propagation was simulated using a cohesive zone model (CZM) in finite element method (FEM), in order to elucidate stress criterion of the present HE crack. It was found that the normal tensile stress was developed around impression, and it initiated and propagated the HE crack. It was also revealed that the hydrogen content enhanced contact fracture damage, especially the resistance of crack propagation (i.e., threshold stress intensity factor, Kth). The findings may be useful for countermeasure of contact fracture coupled with hydrogen in high-strength steel. Such phenomenon is potentially experienced in various contact components in hydrogen environment.

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Fig. 1

Micrographs of the spherical impressions produced in hydrogen-charged specimens with different hydrogen content CH: (a) 11.8 ppm (charging time: 12 hr), (b) 24.8 ppm (24 hr), (c) 33.7 ppm (48 hr), and (d) 49.1 ppm (72 hr). They also include the magnified view of crack initiation site.

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Fig. 2

Distance from the impression center to radial crack initiation site with respected to hydrogen content CH (this figure also includes impression radius) (a). Panel (b) shows the distance from the impression center to end point of radial crack with respect to hydrogen content CH.

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Fig. 3

Two-dimensional FEM model for spherical indentation

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Fig. 4

Three-dimensional FEM model with cohesive zone element (CZM) for radial crack propagation

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Fig. 5

Contour map of circumferential stress σθθ at the indentation force of 150 N

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Fig. 6

Surface distribution of σθθ as a function of radial distance r

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Fig. 7

Changes in maximum σθθ as a function of radial distance. The indentation force is also shown in this figure.

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Fig. 8

Critical stress for crack nucleation estimated by crack initiation site (in Fig. 2(a)) and maximum σθθ stress (in Fig. 7)

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Fig. 9

Contour map of crack nucleation area in the model of K = 4.5 MPa m1/2 (a) and 5.5 MPa m1/2 (b), in Fmax = 300 N case. Panel (c) shows the result of K = 4.5 MPa m1/2 in Fmax = 200 N case.

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Fig. 10

Changes in simulated crack length (distance to the crack end point) as function of input value of crack growth resistance (Kth) for the maximum force of 200 N and 300 N

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Fig. 11

Estimated Kth with respect to hydrogen content CH

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Fig. 12

Estimated Kth with the comparison of between spherical indentation and Vickers indentation. Previous study is Yonezu et al. [8].




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