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Research Papers: Materials and Fabrication

Relationship Between Crack-Tip Constraint and Dynamic Fracture Initiation Toughness

[+] Author and Article Information
Yuh J. Chao

 Southeast University, Nanjing, 210096, China; Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208

Cheng Wang

 Hohai University, 1 Xikang Road, Nanjing, 210098, P. R. China

Yil Kim

Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208

Chi-Hui Chien

Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, 80424 Taiwan

J. Pressure Vessel Technol 132(2), 021404 (Mar 31, 2010) (9 pages) doi:10.1115/1.4000724 History: Received January 25, 2009; Revised November 17, 2009; Published March 31, 2010; Online March 31, 2010

Two-dimensional finite element analyses are performed to study the crack-tip constraint in an elastic-plastic, three point bend specimen under dynamic load. Both strain rate-independent and strain rate-dependent materials are considered to elucidate the difference in response due to the material rate effect. It is first demonstrated that the crack-tip stress fields can be adequately characterized by the J-A2 three-term solution within the region of interest 1<r/(J/σo)<5. Consequently, A2 is used as a constraint parameter in constraint evaluations. Results show that the crack-tip constraint decreases with increasing loading rate in rate-independent material. On the other hand, in rate-dependent material, the crack-tip constraint first increases at low loading rate but later decreases at high loading rate. It appears that there is a competition between constraint loss due to dynamic load and constraint gain due to material sensitivity to strain rate. The effect of crack-tip constraint on fracture initiation toughness under dynamic load Kdyn is then studied using a critical stress failure criterion. The results are consistent with experimental data in (a) reduced dynamic fracture initiation toughness, as compared with the static fracture toughness, at low loading rate such as those obtained by ASTM E23 Charpy tests and (b) elevated fracture toughness at high loading rate as frequently reported by experimental researchers.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Effect of temperature and loading condition on fracture toughness of A517 steel (1)

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Figure 2

Stress-strain curves with strain rate

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Figure 3

Three point bend specimen and finite element mesh

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Figure 4

Variation in stress (a) σθθ and (b) σrr with normalized distance ahead of the crack-tip (i.e., at θ=0 deg) for the strain rate-insensitive material at KJ=100 MPa m1/2

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Figure 5

Angular distribution of stress (a) σθθ, (b) σrr, and (c) σrθ at rσο/J=2 for the strain rate-insensitive material at KJ=100 MPa m1/2

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Figure 6

Development of crack-tip constraint represented by A2 under static or dynamic loadings in the strain rate-insensitive material

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Figure 7

Development of crack-tip constraint represented by A2 under static or dynamic loadings in strain rate-sensitive material

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Figure 8

Schematic showing the relation of constraint with static KC and dynamic fracture toughness Kdyn

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Figure 9

Variations in the opening stress at r=1 mm ahead of the crack-tip under static and dynamic loading conditions: (a) in strain rate-insensitive material where the dynamic stress at the critical distance is always less than the static one and (b) in strain rate-sensitive material, the dynamic stress at the critical distance is higher (less) than the static one under low (high) loading rate at the same applied KI

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Figure 10

Variation in Kdyn/KC as a function of loading rate using σC=1140 MPa and rC=1 mm: (a) for strain rate-insensitive material and (b) for strain rate-sensitive material

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Figure 11

Typical loading point displacement as a function of time under the loading rate of 10 GN/m s for strain rate-sensitive material

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