/FAIL/ALTER
Block Format Keyword An advanced nonlinear stressbased failure criteria for glass applications such as a windshield.
The failure stress is described by parameters defining microcracks and crack propagation speed. With the XFEM approach, the stress is set to zero perpendicular to the crack direction.
Format
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/FAIL/ALTER/mat_ID/unit_ID  
Exp_n  V0  Vc  Ncycles  Irate  mode  
Cr_foil  Cr_air  Cr_core  Cr_edge  grsh4N  grsh3N  
K_{IC}  K_{TH}  Rlen 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

fail_ID 
Definitions
Field  Contents  SI Unit Example 

mat_ID  Material identifier (Integer, maximum 10 digits) 

unit_ID  Unit Identifier (Integer, maximum 10 digits) 

Exp_n  Crack growth exponent for subcritical
crack growth. Default = 16.0 (Real) 

V0  Crack growth velocity for subcritical
crack growth at K_{IC}. Default = 0.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
Vc  Maximum crack propagation velocity
glass. Default = 0.0 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
Ncycles  Stress filtering period in cycles. Only
used when Irate=0. 2
(Integer) 

Irate  Stress rate filtering method.
(Integer) 

mode  Flag to switch failure propagation
models between neighbor elements.
(Integer) 

Cr_foil  Crack depth at bottom surface. Default = 0.0 (Real) 
$\left[\text{m}\right]$ 
Cr_air  Crack depth at top surface. Default = 1.0 (Real) 
$\left[\text{m}\right]$ 
Cr_core  Crack depth in between bottom and
surface integration points. Default = 1.0 (Real) 
$\left[\text{m}\right]$ 
Cr_edge  Crack depth at the edge elements of
windshield. Default = 1.0 (Real) 
$\left[\text{m}\right]$ 
grsh4N  (Optional) Group identifier for 4 node
edge shell elements. Default = 0 (Integer) 

grsh3N  (Optional) Group identifier for 3 node
edge shell elements. Default = 0 (Integer) 

K_{IC}  Fracture toughness. Default = 0.0 (Real) 
$\left[Pa\sqrt{m}\right]$ 
K_{TH}  Fatique threshold. Default = 0.0 (Real) 
$\left[Pa\sqrt{m}\right]$ 
Rlen  Reference length. Default = 1.0 (Real) 
$\left[\text{m}\right]$ 
fail_ID  Failure criteria
identifier. 9 (Integer, maximum 10 digits) 
Comments
 This failure criteria is using the maximum stress as failure criterion. It is computed based on the strength of the material determined by initial cracks and the crack propagation velocity. Depending on mode switch flag, different failure propagation models between neighbor elements may be used.
 When
Irate=0, an exponential moving average filter is
used, and the filtered stress is:
(1) $${\sigma}_{f}\left(t\right)=\alpha \sigma \left(t\right)+\left(1\alpha \right)\sigma \left(t\text{\Delta}t\right)$$Where, ${\sigma}_{f}=\text{filteredstress}$
 $\alpha =\frac{2}{{N}_{cycles}+1}$
 This failure model is compatible only with underintegrated shell elements (I_{shell} =24 and I_{sh3n} =2 are recommended) and not compatible fully integrated shells. Also, although there is no restriction of the shell property that can be used, it is only compatible with one layer shell models.
 The elements defined in the groups grsh4N and grsh3N should be along the edge of the windshield and will receive specific failure weakening.
 This failure model is applied to shell elements that sandwich a polyvinyl butyral (PVB) solid element layer using coincident nodes. The entire assembly models a windshield.
 The shell elements using this failure model should be oriented so their normals point away the from the middle PVB.
 The shell elements should have an offset applied to correctly model bending. This can be done using /PROP/TYPE51 I_{pos}=4.
 The fracture limit depends on the location and the fracture state of surrounding elements. ^{1}
 The fail_ID is used with /STATE/BRICK/FAIL and /INIBRI/FAIL and /PERTURB/FAIL/BIQUAD. 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 .sta file with /STATE/BRICK/FAIL for brick and with /STATE/SHELL/FAIL for shell).