J integral

The J-integral represents a way to calculate the strain energy release rate, or work (energy) per unit fracture surface area, in a material.Van Vliet, Krystyn J. (2006); "3.032 Mechanical Behavior of Materials", [http://www.stellar.mit.edu/S/course/3/fa06/3.032/index.html] ] The theoretical concept of J-integral was developed in 1967 by Cherepanov [G. P. Cherepanov, The propagation of cracks in a continuous medium, Journal of Applied Mathematics and Mechanics, 31(3), 1967, pp. 503-512.] and in 1968 by Jim RiceJ. R. Rice, A Path Independent Integral and the Approximate Analysis of Strain Concentration by Notches and Cracks, Journal of Applied Mechanics, 35, 1968, pp. 379-386.] independently, who showed that an energetic contour path integral (called J) was independent of the path around a crack.

Later, experimental methods were developed, which allowed measurement of critical fracture properties using laboratory-scale specimens for materials in which sample sizes are too small and for which the assumptions of Linear Elastic Fracture Mechanics (LEFM) do not hold, and to infer a critical value of fracture energy $J\_\{Ic\}$. The quantity $J\_\{1c\}$ defines the point at which large-scale plastic yielding during propagation takes place under mode one loading. ] Physically the J-integral is related to the area under a curve of load versus load point displacement.Meyers and Chawla (1999): "Mechanical Behavior of Materials," 445-448.] .

Two-dimensional J-Integral

The two-dimensional J-integral was originally defined as ] (see Figure 1 for an illustration):$J\; :=\; int\_\{Gamma\}\; left(W~dx\_2\; -\; mathbf\{t\}cdotcfrac\{partialmathbf\{u\{partial\; x\_1\}~ds\; ight)$where $W(x\_1,x\_2)$ is the strain energy density, $x\_1,\; x\_2$ are the coordinate directions, is the traction vector, $mathbf\{n\}$ is the normal to the curve $Gamma$, $sigma$ is the Cauchy stress tensor, and $mathbf\{u\}$ is the displacement vector. The strain energy density is given by:The J-Integral around a crack tip is frequently expressed in a more general form (and in index notation) as:$J\_i\; :=\; lim\_\{epsilon\; ightarrow\; 0\}\; int\_\{Gamma\_epsilon\}\; left(W\; n\_i\; -\; n\_jsigma\_\{jk\}~cfrac\{partial\; u\_k\}\{partial\; x\_i\}\; ight)\; dGamma$ where $J\_i$ is the component of the J-integral for crack opening in the $x\_i$ direction and $epsilon$ is a small reqion around the crack tip.Using Green's theorem we can show that this integral is zero when the boundary $Gamma$ is closed and encloses a region that contains no singularities and is simply connected. If the faces of the crack do not have any tractions on them then the J-integral is also path independent.

Rice also showed that the value of the J-integral represents the energy release rate for planar crack growth.The J-integral was developed because of the difficulties involved in computing the stress close to a crack in a nonlinear elastic or elastic-plastic material. Rice showed that if monotonic loading was assumed (without any plastic unloading) then the J-integral could be used to compute the energy release rate of plastic materials too.

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J-Integral and Fracture Toughness

The J-integral can be described as follows ]

:$J=oint\_\{C\}\; frac\; \{F\}\{A\}frac\{du\}\{dl\_0\}=int\_\{\}^\{\}sigma\; dvarepsilon,$

where
* "F" is the force applied at the crack tip
* "A" is the area of the crack tip
* "$frac\{du\}\{dl\_0\}$" is the change in energy per unit length
* "$sigma$" is the stress
* "$dvarepsilon$" is the change in the strain caused by the stress

Fracture toughness is then calculated from the following equation ]

:$J\_\{1c\}\; =\; K\_\{1c\}^2(frac\{1-v^2\}\{E\})$

where
*"$K\_\{1c\}$" is the fracture toughness in mode one loading
*"v" is the Poisson's ratio
*"E" is the Young's Modulus of the material

ee also

* Fracture toughness
* Toughness
* Fracture Mechanics

References

External links

* J. R. Rice, " [http://esag.harvard.edu/rice/015_Rice_PathIndepInt_JAM68.pdf A Path Independent Integral and the Approximate Analysis of Strain Concentration by Notches and Cracks] ", Journal of Applied Mechanics, 35, 1968, pp. 379-386.
* Van Vliet, Krystyn J. (2006); "3.032 Mechanical Behavior of Materials", [http://www.stellar.mit.edu/S/course/3/fa06/3.032/index.html]
* [http://hdl.handle.net/1813/3075 Fracture Mechanics Notes] by Prof. Alan Zehnder (from Cornell University)
* [http://imechanica.org/node/755 Nonlinear Fracture Mechanics Notes] by Prof. John Hutchinson (from Harvard University)
* [http://imechanica.org/node/903 Notes on Fracture of Thin Films and Multilayers] by Prof. John Hutchinson (from Harvard University)
* [http://www.seas.harvard.edu/suo/papers/17.pdf Mixed mode cracking in layered materials] by Profs. John Hutchinson and Zhigang Suo (from Harvard University)
* [http://www.mate.tue.nl/~piet/edu/frm/sht/bmsht.html Fracture Mechanics] by Prof. Piet Schreurs (from TU Eindhoven, Netherlands)
* [http://www.dsto.defence.gov.au/publications/1880/DSTO-GD-0103.pdf Introduction to Fracture Mechanics] by Dr. C. H. Wang (DSTO - Australia)
* [http://imechanica.org/node/2621 Fracture mechanics course notes] by Prof. Rui Huang (from Univ. of Texas at Austin)