/FAIL/TAB1

Format

Definition

Comments

1. Using Ixfem=0, failure leads to element or layer deletion. In this case, if I_{fail_sh}=1, then P_thick_fail has to be set to zero for proper working failure criteria.
2. Using Ixfem=1 (XFEM formulation), failure leads to element crack:

   XFEM formulation is only compatible with Belytchko (I_shell=1 or 2), I_shell=3 or 4 and QEPH (I_shell=24) shell elements.

   Two XFEM options are available: mono-layer and multi-layer. The XFEM option depends on the property type associated to the failure criterion applied to the material identifier:

   1. If /PROP/SHELL (TYPE1) is used, then mono-layer XFEM will be applied.

      In this case, the whole element thickness is considered as a single layer. The failure criterion is calculated in each integration point but only one single crack can appear in this element. This approach is compatible with all shell flag options (I_{fail_sh}=1, 2 or 3), as well as P_Thick_fail values. The crack direction is determined by the principal constraints in the last failed integration point.

   2. If /PROP/SH_SANDW (TYPE11) is used, then multi-layer XFEM will be applied.

      In this case, each integration point over thickness is considered as a distinct layer. The failure criterion is calculated separately, and the crack direction may be different for each layer. Crack direction in each layer will independently propagate from one element to another. Multi-layer XFEM is not compatible with I_{fail_sh}=1 and P_thick_fail>0. Their values will be automatically set to I_{fail_sh}=2 and P_thick_fail=0.

   Warning: Mono-layer and multi-layer XFEM formulations cannot be mixed in the same model, yet. The choice between them must be made for the whole model.
3. The plastic failure strain is defined as:
   $${\epsilon }_f=Yscale1\cdot Table1({\sigma }^{*},\frac{\dot{\epsilon }}{Xscale1},\xi )\cdot facto{r}_{el}\cdot facto{r}_T$$
   Where,

   $f({\sigma }^{*},\dot{\epsilon },\xi )$

   Described in table1_ID and is calculated by interpolating between the failure strain versus stress triaxiality functions for strain rate $\dot{\epsilon }$ and Lode angle $\xi$ .

   ${\sigma }^{*}\text{=}\frac{{\sigma }_m}{{\sigma }_{VM}}$

   Stress triaxiality

   ${\sigma }_m$

   Hydrostatic stress

   ${\sigma }_{VM}$

   von Mises stress

   The first function from table1_ID is used for strain rate values from 0 to its corresponding strain rate. For strain rates above the last defined function, the failure strain value is extrapolated using the last two curves and their corresponding strain rates.

   It is possible to consider element size in material failure by function fct_ID_el to scale the failure strain depending on the normalized element size with Ch_i_f=1 or 3.

   $$facto{r}_{el}=Fscal{e}_{el}\cdot {\mathrm{f}}_{el}\left(\frac{Siz{e}_{el}}{El\_ref}\right)$$

   Where, ${\mathrm{f}}_{el}\left(\frac{Siz{e}_{el}}{El\_ref}\right)$ is the function of fct_ID_el.

   Element size scale factor is only applied between triaxiality limits defined by Shrf and Biaxf.

   $$Shrf<{\sigma }^{*}<Biaxf$$
   Outside this triaxiality range, the element size scaling is not applied to failure or instability curves.

   Note: If non-local regularization is used (with /NONLOCAL/MAT), the element size scaling factor is not used. If a scaling function is still defined (fct_ID_el > 0), the parameters are scaled using LE_MAX parameter of the non-local card (either specified directly by you or computed from the Rlen parameter value).

   Temperature dependency can be considered in material failure by defining a function to scale the failure strain depending on the normalized temperature:

   $$facto{r}_T=Fscal{e}_T\cdot {\mathrm{f}}_T\left(T^{*}\right)$$

   Here, ${\mathrm{f}}_T\left(T^{*}\right)$ is defined using fct_ID_T and Temperature $T^{*}$ is computed as:

   $$T^{\ast }=\frac{T-T_{ini}}{T_{melt}-T_{ini}}$$

   It is recommended to use /HEAT/MAT to define the thermal parameter for material laws (which support thermo-plasticity).

4. Two different failures (rupture or crack) are introduced in this failure model. The failure criteria is calculated as:
   Element rupture (Ixfem=0):

   • Element rupture (deleted), if
     $$\sum \text{Δ}D>D_{\mathit{crit}}$$

     Where, $D_{crit}$ is the only rupture criterion used when Ixfem=0.

   Element crack (Ixfem=1):

   • Element cracked, if:
     $$\sum \text{Δ}D>D_{\mathit{crit}}$$

     in case no failed neighbors for this element. $D_{crit}$ is used for new crack initialization.

     $$\sum \text{Δ}D>D_{\mathit{adv}}$$

     in case there is failed neighbors for this element, $D_{adv}$ is used for crack advancement.

     Element is deleted, if a second crack arrives to the same element.

   Note: $D_{adv}$ should always be less than $D_{crit}$ ( $D_{adv}$ < $D_{crit}$ ). If not, then $D_{adv}$ is set to $D_{crit}$ _crit ( $D_{adv}$ = $D_{crit}$ ).
5. Damage accumulation is computed in Radioss one of two different ways:
   • With parameter input, if fct_ID_d = 0:
     $$\text{Δ}D=\frac{\text{Δ}{\epsilon }_p}{{\epsilon }_f}\cdot n\cdot D_p{}^{\left(1-\frac{1}{n}\right)}$$
     Where,

     $\text{Δ}{\epsilon }_p$

     Change in plastic strain of the integration point.

     ${\epsilon }_f$

     Plastic failure strain.

     D_p and n

     Damage parameters.

   • With curve input, if fct_ID_d ≠ 0:
     $$\text{Δ}D=\frac{\text{Δ}{\epsilon }_p}{{\epsilon }_f}\cdot {\mathrm{f}}_d$$

     Where, ${\mathrm{f}}_d$ is the damage scale factor as a function of current damage defined in fct_ID_d.

6. P_thick_fail is only compatible with shell elements (except, shells with /PROP/TYPE11 (SH_SANDW)) and is only used when I_{fail_sh}=2 or I_{fail_sh}=3. If Ixfem=1, P_thick_fail is only compatible with mono-layer XFEM formulation. 1
7. When P_thick_fail is used, the shell complete rupture occurs when the thickness of broken layers is greater than the ratio of shell total thickness. Any P_thickfail value defined in the shell properties is ignored and the value entered in this failure model is used instead.

   Only adjacent layers that fail consecutively are accounted for the thickness sum (usually from one of external skin to the mid-surface).

8. The first variable of table1_ID is the plastic failure strain versus stress triaxiality function, the second variable is strain rate and the third is the Lode angle parameter $\xi$ (for solids).

   For shell, only 2D tables are available (no dependency of Lode angle).

9. Instability (diffuse necking):
   • Only available for shells
   • The fading exponent describes the softening behavior and starts of instability (diffuse necking). The recommended value of Fad_exp is 5 to 10.

     If Fad_exp < 0 and Ch_i_f=2 or 3, then the absolute value of the fading exponent is a function identifier fct_ID_el which defines the fading exponent as a function of element length.

   • The start of instability can be described as a function or constant value:
     • table2_ID is a function of instability strain versus triaxiality where the instability strain defines when diffuse necking starts. Strain rate dependency for diffuse necking could be considered as well using dimension=2 in /TABLE.
       $${\epsilon }_f=Yscale2\cdot Table2({\sigma }^{*},\frac{\dot{\epsilon }}{Xscale2})\cdot facto{r}_{el}\cdot facto{r}_T$$
     • If table2_ID is not defined, inst_start is used as constant flat line for instability starting value over the triaxiality, where the default value is D_p.

       
       
       

   • The diffuse necking softening is based according to this equation:
     $${\sigma }_{reduced}=\sigma \cdot \left(1-{\left(\frac{D_{instability}-inst\_start}{1-inst\_start}\right)}^{Fad\_\exp }\right)$$

     Where, $D_{instability}={\displaystyle \sum \frac{\text{Δ}{\epsilon }_p}{{\epsilon }_f}}$ with ${\epsilon }_f$ being the diffuse necking strain.

     Currently, diffuse necking (material instability) in /FAIL/TAB1 could be used with material laws greater than 28.

10. The fail_ID is used with /STATE/BRICK/FAIL and /INIBRI/FAIL. There is no default value. If the line is blank, no value will be output for failure model variables in the /INIBRI/FAIL (written in the .sta file with /STATE/BRICK/FAIL option).