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3 Zeszyty Naukowe. Mechanika / Politechnika Opolska

3 2005

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Load range influence of a short propagating edge crack using the discrete dislocation technique

Hansson P., Melin S.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2005

z. 82

107-114

The influence of different load cycles on the growth of a short edge crack has been studied using a discrete dislocation technique. The external boundary is modeled with dislocation dipole elements and the plasticity is modeled by discrete dislocations. The crack is located within one grain in a bcc material, and is assumed to grow through a single shear mechanism, due to nucleation and annihilation of discrete dislocations along preferred slip planes. It was found that the applied maximum load in the cycle strongly affected the growth behavior of the crack, both in the case of constant minimum load and the case of constant load range.

In-situ scanning electron microscope study of thermo-mechanical fatigue crack behavior

Jacobsson L., Persson C., Melin S.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2005

z. 82

115-122

An experimental technique to analyze images of fatigue cracks taken in-situ in an environmental scanning electron microscope is described. This technique finds the same spot on the surface of the material in different images and generates a displacement fields that can give information on for example crack opening displacement and the shape of the crack close to the crack tip. The test specimen material used is a typical gas turbine material Inconel 718 and low cycle fatigue crack propagation tests at room temperature are performed.

Dislocation distribution in the plastic zone and growth of a short fatigue crack

Bjerkén C., Melin S.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2005

z. 82

29-36

The growth of a short edge crack under fatigue loading is investigated in detail for more than 11 000 cycles. The developing local plasticity consists of discrete dislocations that are emitted from the crack tip. The edge crack is modelled by a distribution of dislocation dipoles. The competition between global loading and local shielding of the crack tip governs the crack growth. The material modelled is BCC iron and the load is varied with a stress ratio R=-1. The growth rate increases in discrete steps with short periods of retardation from approx. The size of Burgers vector, b, per cycle up to 17 b. The plastic zone changes from having an elongating slender form to include a low-angle boundary.