/FAIL/BIQUAD
Block Format Keyword
/FAIL/BIQUAD - Strain based Failure Model
Description
A simplified non-linear strain based failure criteria with linear damage accumulation. The failure strain is described by two parabolic functions calculated using curve fitting from up to 5 user input failure strains. A perturbation of the failure limit for each element can be activated using
/PERTURB/FAIL/BIQUAD. This failure criteria is developed in a partnership with Christian Cremer from FORD.
Flag Definition
(Integer, maximum 10 digits) Comments
1. This failure criteria is defined using failure plastic strain versus stress triaxiality (state of stress).
This allows for the different plastic failure strains that materials exhibit depending on loading condition. The curve is described by 2 parabolic functions that intersect at the triaxiality value of
1
3
which is uniaxial tension.
The parameters for the 2 parabolic failure strain curves versus the state of stress (stress triaxiality) are calculated iteratively by RADIOSS during the initialization phase using the input c1-c5 values. The calculated parabolic failure curve parameters a, b, c, d, e and f, can be reviewed in the starter output file.
If the calculated parabolic failure strain curves have negative failure strain values, these negative values will be replaced by a failure strain of 1E-6 which give very high damage accumulation and brittle behavior.
This failure criteria is usable for all elasto-plastic material models with shells and solids. 2. The plastic strain at failure from physical tests can be input as c1 – c5.
3. If failure strain data is not available then the material flag, M-Flag, can be used to select
predefined failure values for some materials.
If M-Flag > 0, the entered c1, c2, c4 and c5 values will not be used and instead will be calculated as follows using the predefined ratio values from the table:
113c r c =⋅ 223c r c =⋅ 33c c = 443c r c =⋅ 553c r c =⋅
M-Flag
Roughly
Corresponds to Material c3 (Default) r1  r2  r4  r5  1
Mild steel
0.60
3.5
1.6
0.6
1.5
P l a n e  s t r a i n -T e n s i o n
2 HSS steel 0.50    4.
3    1.
4 0.6    1.6
3 UHSS steel 0.12    5.2    3.1 0.8    3.5
4 Aluminum AA5182 0.30    5.0    1.0 0.4 0.8
5 Aluminum AA6082-
T6
0.17 7.8    3.5 0.6    2.8
6 Plastic PA6GF30 0.10    3.6 0.6 0.5 0.6
7 Plastic PP T40 0.11 10.0    2.7 0.6 0.7
99 Self-defined values
(optional line) 0.30 Optional
input
Optional
input
Optional
input
Optional
input
*Neither Altair nor the authors assume any responsibility for the validity, accuracy or any results obtained from these values. The user must verify his own values by reasonable test results.
Usage is only recommended for early design exploration.
If the M-Flag=99, failure strain ratios r1, r2, r4 and r5 must be input in a following additional line 4.Damage is accumulated linearly and can be post-processed in the animation files using the
output request /ANIM/SHELL/DAMA/ALL or /ANIM/BRICK/DAMA/111. Failure occurs when
damage reaches a value of 1.0
5.If /PERTURB/FAIL/BIQUAD is used, M-Flag>0 must be used and the scatter will be applied to the
c3-value only. The resulting c1, c2, c4, and c5 values will be calculated using the failure strain ratios thus applying the perturbation noise to the entire failure strain curve.
6.Special features are activated by this flag:
S-Flag = 1: failure curves created as shown in Comment 1
S-Flag = 2: Ensures value c4 as global minimum. To achieve this, the second equation is split into 2 separate quadratic sub-functions where the minimum value of the curves is at c4 where *
σ=.
block truncatedThe Inst_start value can be estimated as the (true plastic) strain at maximum stress, from the uniaxial tension test.
7.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 option).
See Also:
/FAIL/JOHNSON
/FAIL/TAB1
/PERTURB/FAIL/BIQUAD

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