**parks#

Description#

The **parks option indicates that the crack tip energy release rate \(G\) is to be calculated by the perturbation method (c.f. Parks [U8]). The method is only available in 2D calculations.

Parks’ method advances the crack length a small amount in order to evaluate the energy release per crack length unit. The obtained stress intensity factor taken from \(G\) is thus a “global” measure of the mixed mode I and II cracking.

../../_images/parks.fig.svg

It is necessary to define a translated internal zone (in the interior of the contour \(C_1\)). The perturbed zone by the translation is that constituted by the first arrangement of elements about the translated contour (contour between \(C_1\) and \(C_2\)). Beyond the contour \(C_2\) nothing will be modified.

The internal translated contour is fabricated from either an element set or a single node defining the crack tip. These geometrical groups must be defined in the problem.geof file with the meshing commands described in the leading chapters of this manual. For the case of axisymmetric calculations, it is necessary to define the radius of the crack tip point.

Note

The results are stored in the sequential formatted file problem.parks with one line per increment calculated. Be careful that for for structures exploiting symmetry (one half the crack is modeled) the value of \(G\) must be multiplied by a factor of \(2\), while \(K\) must be multiplied by the square root of two.

Syntax#

**parks \(~\,~\,\) perturb (elset | tip) (name1  node_num) \(~\,~\,\) … % continuing with as many lines as \(G\) calculations \(~\,~\,\) [ tip_radius val ] \(~\,~\,\) da (dax day)

name1

the character name of an elset composed of the translation elements.

node_num

integer corresponding to the crack tip.

val

real value designating the radius of the crack tip. This option only applies to axisymmetric geometries.

da

vector describing the virtual advancement of the crack tip.

dax

real value for the advancement of the crack along the \(x\) axis only. This value may be negative.

day

real value for the advancement of the crack along the \(y\) axis only. This value may be negative.

Example#

***post_increment
    **parks
      perturb elset inter
      da (1.e-2 0.)
      da (1.e-4 0.)
      da (1.e-6 0.)
      da (1.e-8 0.)
      da (1.e-10 0.)
      da (1.e-12 0.)
      da (1.e-16 0.)
      da (1.e-20 0.)
      da (1.e-30 0.)
      da (1.e-100 0.)