### Abstract

The plastic blunting process during stage II fatigue crack growth was studied in pure polycrystalline Ni to investigate effects of strain localization and inelastic behavior on the kinematics of crack advance. Correlations were obtained between strain fields ahead of a fatigue crack, crack advance per cycle and crack growth kinetics. Strain fields were quantified using a combination of in situ loading experiments, scanning electron microscopy and digital image correlation for 8 < ΔK < 20 MPa m^{1/2} and a fixed load ratio of 0.1. Results indicate that strain localized along a dominant deformation band, which was usually crystallographic and carried mostly pure shear for large loads and was of mixed character for lower loads. Instances of double deformation bands were observed, with bands acting either in a simultaneous or alternating fashion. It was found that the area integral of the opening strain for values larger than a given threshold, an "integrated" strain, had a power-law relationship with ΔK, with the exponent approximately equal to the Paris exponent (m). Therefore, the crack growth rate was proportional to the integrated strain. An analysis based on this correlation and the presence of dominant shear bands indicated that the integrated strain is related to the accumulated displacement in the band. This, in turn, is proportional to the product of the cyclic plastic zone radius and the average shear strain ahead of the tip, which represents a basic length scale for plastic blunting. Assumptions on the load dependence of these quantities, based on their observed spatial variation, allowed estimating m = 2 fenced(1 + frac(1, 1 + n^{′})), where n′ is the cyclic hardening exponent (0 < n′ < 1). This gives 3 < m < 4, which accounts for about 50% of the observed values of m between 1.5 and 6 for a wide variety of metallic materials.

Original language | English (US) |
---|---|

Pages (from-to) | 1763-1795 |

Number of pages | 33 |

Journal | International Journal of Plasticity |

Volume | 23 |

Issue number | 10-11 |

DOIs | |

State | Published - Oct 2007 |

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### Keywords

- Crack tip plasticity
- Electron microscopy
- Fatigue
- Fracture mechanisms
- Metallic material

### ASJC Scopus subject areas

- Mechanical Engineering

### Cite this

*International Journal of Plasticity*,

*23*(10-11), 1763-1795. https://doi.org/10.1016/j.ijplas.2007.03.009

**Experimental quantification of the plastic blunting process for stage II fatigue crack growth in one-phase metallic materials.** / Peralta, Pedro; Choi, S. H.; Gee, J.

Research output: Contribution to journal › Article

*International Journal of Plasticity*, vol. 23, no. 10-11, pp. 1763-1795. https://doi.org/10.1016/j.ijplas.2007.03.009

}

TY - JOUR

T1 - Experimental quantification of the plastic blunting process for stage II fatigue crack growth in one-phase metallic materials

AU - Peralta, Pedro

AU - Choi, S. H.

AU - Gee, J.

PY - 2007/10

Y1 - 2007/10

N2 - The plastic blunting process during stage II fatigue crack growth was studied in pure polycrystalline Ni to investigate effects of strain localization and inelastic behavior on the kinematics of crack advance. Correlations were obtained between strain fields ahead of a fatigue crack, crack advance per cycle and crack growth kinetics. Strain fields were quantified using a combination of in situ loading experiments, scanning electron microscopy and digital image correlation for 8 < ΔK < 20 MPa m1/2 and a fixed load ratio of 0.1. Results indicate that strain localized along a dominant deformation band, which was usually crystallographic and carried mostly pure shear for large loads and was of mixed character for lower loads. Instances of double deformation bands were observed, with bands acting either in a simultaneous or alternating fashion. It was found that the area integral of the opening strain for values larger than a given threshold, an "integrated" strain, had a power-law relationship with ΔK, with the exponent approximately equal to the Paris exponent (m). Therefore, the crack growth rate was proportional to the integrated strain. An analysis based on this correlation and the presence of dominant shear bands indicated that the integrated strain is related to the accumulated displacement in the band. This, in turn, is proportional to the product of the cyclic plastic zone radius and the average shear strain ahead of the tip, which represents a basic length scale for plastic blunting. Assumptions on the load dependence of these quantities, based on their observed spatial variation, allowed estimating m = 2 fenced(1 + frac(1, 1 + n′)), where n′ is the cyclic hardening exponent (0 < n′ < 1). This gives 3 < m < 4, which accounts for about 50% of the observed values of m between 1.5 and 6 for a wide variety of metallic materials.

AB - The plastic blunting process during stage II fatigue crack growth was studied in pure polycrystalline Ni to investigate effects of strain localization and inelastic behavior on the kinematics of crack advance. Correlations were obtained between strain fields ahead of a fatigue crack, crack advance per cycle and crack growth kinetics. Strain fields were quantified using a combination of in situ loading experiments, scanning electron microscopy and digital image correlation for 8 < ΔK < 20 MPa m1/2 and a fixed load ratio of 0.1. Results indicate that strain localized along a dominant deformation band, which was usually crystallographic and carried mostly pure shear for large loads and was of mixed character for lower loads. Instances of double deformation bands were observed, with bands acting either in a simultaneous or alternating fashion. It was found that the area integral of the opening strain for values larger than a given threshold, an "integrated" strain, had a power-law relationship with ΔK, with the exponent approximately equal to the Paris exponent (m). Therefore, the crack growth rate was proportional to the integrated strain. An analysis based on this correlation and the presence of dominant shear bands indicated that the integrated strain is related to the accumulated displacement in the band. This, in turn, is proportional to the product of the cyclic plastic zone radius and the average shear strain ahead of the tip, which represents a basic length scale for plastic blunting. Assumptions on the load dependence of these quantities, based on their observed spatial variation, allowed estimating m = 2 fenced(1 + frac(1, 1 + n′)), where n′ is the cyclic hardening exponent (0 < n′ < 1). This gives 3 < m < 4, which accounts for about 50% of the observed values of m between 1.5 and 6 for a wide variety of metallic materials.

KW - Crack tip plasticity

KW - Electron microscopy

KW - Fatigue

KW - Fracture mechanisms

KW - Metallic material

UR - http://www.scopus.com/inward/record.url?scp=34547691038&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=34547691038&partnerID=8YFLogxK

U2 - 10.1016/j.ijplas.2007.03.009

DO - 10.1016/j.ijplas.2007.03.009

M3 - Article

VL - 23

SP - 1763

EP - 1795

JO - International Journal of Plasticity

JF - International Journal of Plasticity

SN - 0749-6419

IS - 10-11

ER -