OPTICAL METHOD FOR DETERMINATION OF THE CRACK TIP PLASTIC ZONE SIZES AND SHAPE IN ALUMINUM ALLOYS

Authors

  • Дмитрий Костенюк National Aviation University, Kiev, Ukraine

DOI:

https://doi.org/10.20535/2305-9001.2015.74.49122

Keywords:

plastic zone, fatigue crack, cladding layer

Abstract

The paper presents a method for determination of the crack tip plastic zone size and shape by the metallurgical microscope, which based on changing of the metal surface (polished before the crack propagation) image that occurs due to the focus change. Because the images of the plastic deformed and the undeformed regions of the metal change differently when the microscope focus change (in the paper defocus value equals ±250 µm, focal length equals 25mm, and magnification is 80X), the plastic zone boundary can be determined. The difference in the way images change is explained by evolution of deformation relief on the surface of aluminum alloy due to plastic deformation, the image of deformation relief doesn’t enable determination of plastic zone boundary when the image is in focus, that’s why the defocus is required. The size of the monotonic plastic zone at the tip of a fatigue crack experimentally measured for clad and unclad aluminum alloy using the method. The results demonstrate that the presence of the cladding layer doesn’t influence the plastic zone size. The monotonic plastic zone size calculated with the linear elastic fracture mechanics equation as a distance normal to the crack plane from the crack tip to the plastic zone boundary. The experimental plastic zone sizes are compared with the analytical solutions using the Wilcoxon signed-rank test. This comparison helps to infer that the experimental results of plastic zone size are in good agreement with the results estimated by analytical calculation. The extension of the method to other materials needs further investigation.

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Published

2015-10-20

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