Coupled Traction Law

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The Traction Law

Exptraction.jpg

This traction law uses either the triangular cohesive law or the exponential cohesive law but couples the normal and tangential displacements by using an effective displacement. This implementation follows methods derived in a paper by Högberg.[1] The Högberg effective displacement is defined by

      [math]\displaystyle{ \lambda = \sqrt{\left(\frac{u_n}{\delta_c^{(n)}}\right)^2 +\left(\frac{u_t}{\delta_c^{(t)}}\right)^2} }[/math]

where u is displacement, δc is displacement at failure (see figure), and "n" and "t" refer to normal an tangential directions. In terms of λ, the normal and tangential tractions are:

      [math]\displaystyle{ \left(T^{(n)},T^{(t)}\right) = \frac{H(\lambda_\omega)}{\lambda_\omega} \left(\frac{\sigma_{c}^{(n)} u_n}{\delta_c^{(n)}},\frac{\sigma_{c}^{(t)} u_t}{\delta_c^{(t)}}\right) }[/math]

where λω is a damage parameter equal to the maximum value of λ during a simulation. These traction apply after initiation and correctly account for elastic unloading and reloading. Prior to initiation, deformations are elastic. Initiation using an elliptical failure criterion occurs when λ reaches λp(θ) defined by

      [math]\displaystyle{ \frac{1}{ \lambda_p(\theta)} = \sqrt{\left(\frac{\delta_c^{(n)}\sin\theta}{\delta_e^{(n)}}\right)^2 + \left(\frac{\delta_c^{(t)}\cos\theta}{\delta_e^{(t)}}\right)^2} \quad {\rm with} \quad \theta = \frac{\delta_c^{(t)}u_n}{\delta_c^{(n)}u_t} }[/math]

and δe is displacement at end of the elastic regime.[1] Finally, Högberg assumed H(λ) is defined by linear decay from 1 when λ=λp(θ) to zero when λ=1:

      [math]\displaystyle{ H(\lambda) = \frac{1-\lambda}{1-\lambda_p(\theta)} }[/math]

This implementation has extended the Höberg approach to also allow an exponential decay using:

      [math]\displaystyle{ H(\lambda) = \frac{e^{-\alpha\frac{\lambda-\lambda_p(\theta)}{1-\lambda_p(\theta)}}-e^{-\alpha}}{1-e^{-\alpha}} }[/math]

See figure above for effect of α on the decay rate (note that α approaching zero is identical to linear decay).

Failure

Under pure mode I or pure mode II, a coupled traction law and its failure are identical to a triangular cohesive law or an exponential cohesive law. Under mixed-mode conditions, however, these laws differ. Triangular and exponential cohesive law have to impose a mixed-mode failure criterion. This model does not need such a criterion. It fails naturally when λ reaches one. Despite this advantage of failing "naturally", the Högberg model[1] has limitations explained in the next section.

Property Limitations

The Höberg model[1] seemingly depends on independent normal and traction law properties that could be entered as for triangular or the exponential cohesive laws. In fact Ref. [1] claims independent properties are allowed. A new analysis of cohesive zone modeling,[2] however, has revealed that the Högberg model is always invalid for mixed-mode loading unless properties are chosen such that

      [math]\displaystyle{ \frac{\delta_e^{(n)}}{\delta_c^{(n)}} = \frac{\delta_e^{(t)}}{\delta_c^{(t)}} }[/math]

In other words, the ratio of displacement at peak stress to displacment and failure must be the same for both normal and tangential directions. When using the exponential extension of the Högberg model the α coefficient must also be the same for normal and tangential directions.

Note that this implementation lets you choose properties that make the modeling invalid. That option is allowed for comparison of the published model to other models. If you do select invalid properties, the output file will include a warning. The preferred model for simulations of mixed-mode failure, which remains valid for independent normal and tangential laws, is the Mixed Mode Traction Law.

Alternate Effective Displacement Method

Other mixed-mode cohesive laws have used unscaled effective displacement models or simply define it is magnitude of the crack opening displacement:

      [math]\displaystyle{ \lambda = \sqrt{u_n^2+u_t^2} }[/math]

Examples are found in Comanho and Dàvila[3] and Comacho and Ortiz[4] (these models are implemented in Abaqus cohesive elements). These models are not separately implementeded in NairnMPM because they are superfluous. They can be used with this model simply by choosing

      [math]\displaystyle{ \delta_c^{(n)} = \delta_c^{(t)} }[/math]

Combining this restriction with the Högerg limitation, means this alternate effective displacement model is invalid unless:

      [math]\displaystyle{ \frac{\delta_e^{(n)}}{\delta_e^{(t)}} = \frac{\delta_c^{(n)}}{\delta_c^{(t)}} = 1 }[/math]

or both elastic and critical displacements in the normal and tangential cohesive laws are identical (the law magnitudes, or normal and tangential strengths, can differ). Implementations of this approach, such as in Abaqus, unfortunately let users select properties that invalidate the modeling.

Traction Law Properties

The following properties are used to create a coupled traction law:

Property Description Units Default
(Triangular Laws) Enter properties to define normal and tangential cohesive laws assuming triangular traction laws varies none
model Choose whether to model using linear decay (12) or exponential decay (34). The numerical values are material IDs for linear and exponential traction laws none 12
alpha When using exponential decay (model=34), this parameter selects the α coefficient for the decay rate. Note that when using exponential decay, this implementation first finds law properties assuming triangular traction laws and then recalculates Gc based on triangular values or stress and displacements. none none

As described above, this implementation lets you select properties that invalidate the method when used to model mixed-mode failure. If you choose invalid properties, a warning is printed in the output files.

Traction History Variables

This material tracks the following traction history variables

  1. Maximum value of λ (or λω (the value is <0 until initiation of damage)
  2. Cumulative work energy
  3. Normal crack opening displacement (un)
  4. Tangential crack opening displacement (ut)

References

  1. 1.0 1.1 1.2 1.3 1.4 J. L. H ögberg, "Mixed mode cohesive law," International Journal of Fracture, 141, 549–559 (2006).
  2. J. A. Nairn and Y. E. Aimene "A re-evaluation of mixed-mode cohesive zone modeling based on strength concepts instead of traction laws" in preparation (2020).
  3. P. P. Camanho and C. G. Dàvila, "Mixed-mode decohesion finite elements for the simulation of delamination in composite materials," Technical Report, NASA/TM-2002-211737 (2002).
  4. G. T. Camacho and M. Ortiz, "Computational modelling of impact damage in brittle materials," Int. J. Solids Struct., 33, 2899–2938 (1996).