microvoid coalescence

MVC proceeds in three stages: nucleation, growth, and coalescence of microvoids. The nucleation of microvoids can be caused by particle cracking or interfacial failure between precipitate particles and the matrix. Additionally, microvoids often form at grain boundaries or inclusions within the material.{{Cite book|title=The science and engineering of materials|last=Askeland, Donald R.|others=Wright, Wendelin J.|isbn=978-1-305-07676-1|edition= Seventh|location=Boston, MA|pages=236–237|oclc=903959750|date = January 2015}}{{Cite book|title=Mechanical properties of engineered materials|last=Soboyejo|first=W.O|date=2003|publisher=Marcel Dekker|isbn=0-203-91039-7|pages=393–394|oclc=54091550}} Microvoids grow during plastic flow of the matrix, and microvoids coalesce when adjacent microvoids link together or the material between microvoids experiences necking. Microvoid coalescence leads to fracture.Hertzberg, Richard W. Deformation and Fracture Mechanics of Engineering Materials, Fourth Edition. John Wiley and Sons, Inc, Hoboken, NJ: 1996. Void growth rates can be predicted assuming continuum plasticity using the Rice-Tracey model:{{Cite book|url=https://www.worldcat.org/oclc/190802556|title=Comprehensive structural integrity|date=2003|publisher=Elsevier/Pergamon|others=Milne, I., Ritchie, R. O., Karihaloo, B. L.|isbn=978-0-08-049073-1|edition= 1st|location=Amsterdam|pages=186–192|oclc=190802556}}

$\backslash ln\backslash left(\backslash frac\{\backslash bar\{R\}\}\{R\_0\}\backslash right)\; =\; \backslash int\backslash limits\_\{0\}^\{\backslash epsilon\_q\}\; A\backslash left(\backslash frac\{3\backslash sigma\_m\}\{2\backslash sigma\_\{ys\}\}\backslash right)d\backslash epsilon\_v^p$

where $A$ is a constant typically equal to 0.283 (but dependent upon the stress triaxiality), $\backslash sigma\_\{ys\}$ is the yield stress, $\backslash sigma\_m$ is the mean stress, $\backslash epsilon\_q$ is the equivalent Von Mises plastic strain, $R\_o$ is the particle size, and $\backslash bar\{R\}$ produced by the stress triaxality:

$\backslash bar\{R\}=\backslash frac\{R\_1\; +\; R\_2\; +\; R\_3\}\{3\}$

MVC can result in three distinct fracture morphologies based on the type of loading at failure. Tensile loading results in equiaxed dimples, which are spherical depressions a few micrometres in diameter that coalesce normal to the loading axis. Shear stresses will result elongated dimples, which are parabolic depressions that coalesce in planes of maximum shear stress. The depressions point back to the crack origin, and shear influenced failure will produce depressions that point in opposite directions on opposing fracture surfaces. Combined tension and bending will also produce the elongated dimple morphology, but the directions of the depressions will be in the same direction on both fracture surfaces.

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Category:Fracture mechanics

Category:Materials degradation