The material model /MAT/LAW24 (CONC) in Radioss can be used for concrete materials with or without steel reinforcements but also for rocks. The model is robust and is generally a first choice for various cases and applications.
Even if it is a complex model, only a minimal set of input parameters is mandatory to run it, as all the other parameters have default values, automatically computed based on usual/average concrete behaviour.
The mandatory parameters are the following ones:
• Initial Density: rho
• Poisson Coefficient: ν
• Young Modulus: Ec
• Compression Strength: fc
The triaxial failure is based on Ottosen surface model [1], fully determined by providing 4 failure points which are described with 5 parameters. The mandatory one is the compression strength fc. The other four are optional and are expressed as ratios to compression strength, with default values chosen from the usual/average concrete characteristics:
• Direct Tensile Strength (ratio): ft/fc = 0.10 by default.
• Biaxial Compression Strength (ratio): fb/fc = 1.20 by default.
• Confined Compression Strength (ratio) (tri-axial test): f2/fc = 4.00 or 7.00 by default, under
• Confined pressure (ratio): s0/fc =1.25 by default.
The ideal situation to fully determine the 3D failure envelope is to get experimental data for all these values:
Or, if other failure tests are available for different triaxialities, giving different failure points, the previous parameters should be adjusted so that the failure surface passes through these points.
Based on Han & Chen work [2], the initial yield surface is built from the failure surface by applying a pressure dependent scale factor k.
In the tension zone (-σ_{m }≤ ρ_{t}), the yield surface is coincident with the failure surface and the material is elastic up to the failure (fragile mode).
For high pressure confinements (-σ_{m }≥ f_{k}), the yield surface gradually closes on the hydrostatic axis (cap region), leading to an infinite plasticity region.
During material loading, if this initial yield surface is reached, the hardening process starts, and the yield surface expands (the new stress-state is located on this updated yield surface).
In the principal stress space, the failure and yield surfaces have typically the following shapes:
The failure surface is not conical like with Drucker-Prager models but has 3 lobes.
You can use the tool provided here to plot the failure and initial yield surfaces. You first need to set the failure surface parameters:
Strenght Limit Parameters |
||||
fc |
30 |
Mpa |
||
ft/fc |
0.1 |
ft = |
3.00 |
MPa |
fb/fc |
1.2 |
fb = |
36.00 |
MPa |
f2/fc |
7 |
f2 = |
210.00 |
MPa |
so/fc |
1.25 |
so = |
37.50 |
MPa |
Only the uniaxial compression strength fc is mandatory. The other parameters have default values (which are the same than the Radioss solver). The 5 failure parameters are fully controlling the shape of the failure envelope which can be visualized through the meridional and octahedral profiles (section cuts of the failure surface, respectively along and normal to the hydrostatic axis).
The equivalent Ottosen parameters are given in a table, for comparison with other models or solvers. You can also find them in the listing file 0000.out generated by Radioss.
Then, the yield parameters can be set. This is optional as all the parameters have default values:
Current Principal Stress State Panel:
The last panel allows you to define a stress state with 3 principal stress values.
The Lode coordinates and pressure corresponding to this stress state are computed and displayed:
You can then visualize this stress state (green coloured) with respect to the failure and yield envelopes.
Remarks:
The ranges of the plots (in the top plot zones in the excel file) are adjusted to fit the stress state position, thus they may correspond to zoomed views compared with the previous plots (the bottom plot zones in the excel file). The current state is also displayed in the bottom right plot zone, giving an indication of the zoom level.
The meridional profiles are plotted for the current Lode angle, which means that the cut plane along the hydrostatic axis is turned by this angle as below:
The green octahedral profile corresponds to the current pressure.
If you want to modify the file, you can remove the protection with the following password: Altair2024
See also:
Concrete Material (LAW24) (User Guide)
[1] Ottosen N.S. “Non-linear Finite Element Analysis of Concrete Structures” Ris. National Laboratory DK 4000 Roskilde Denmark, May 1980.
[2] Han D.J., Chen W.F. “A non-uniform hardening plasticity model for concrete materials”, Mechanics of Materials, 1985.