Following is information about the settings for fracture analysis, including settings for both fracture-parameter calculation and crack initiation and growth:
A standard CINT command block defines fracture calculation for the existing edge crack in the CT specimen:
CINT,NEW,1 ! DEFINE CRACK ID FOR PRE-MESHED CRACK CINT,TYPE,SIFS ! OUTPUT QUANTITY FOR PRE-MESHED CRACK CINT,CTNC,TIP,_IASSISTNODE ! DEFINE CRACK TIP NODE COMPONENT CINT,NCON,4 ! DEFINE NUMBER OF CONTOURS CINT,NORMAL,0,2 ! DEFINE CRACK PLANE NORMAL CINT,SURF,SURUPPER,SURLOWER ! DEFINE CRACK TOP AND BOTTOM SURFACES
The stress-intensity factor is calculated on four contours. The upper and lower crack surfaces are described by node components SURUPPER and SURLOWER, respectively.
The CINT block is associated with a CGROW command block which manages edge-crack growth:
GROW,NEW,1 ! CRACK GROWTH SET CGROW,CID,1 ! CINT ID FOR CRACK GROWTH CGROW,METHOD,SMART ! CRACK-GROWTH METHOD CGROW,FCOPTION,MTAB,1 ! MATERIAL DATA TABLE CGROW,FCG,METH,LC ! CRACK-GROWTH METHODOLOGY CGROW,FCG,SRAT,0.000000 ! STRESS/LOAD RATIO CGROW,RMCONT,ESIZE,0.0002 ! SET ELEMENT SIZE AT CRACK FRONT TO 0.0002 CGROW,RMCONT,COARSE,MODE ! SET COARSENING LEVEL FOR REMESHING TO MODERATE CGROW,SOPT,DAMX,6E-4 ! SET MAX LENGTH OF GROWTH TO 0.0006
In addition to the basic CGROW commands necessary for specifying crack growth, these control commands specify the details for the crack-growth definition:
CGROW,METHOD,SMART
Specifies the SMART method crack-growth simulation. (VCCT and XFEM are other options.)
CGROW,FCOPTION,MTAB,
VALUELinks crack-growth criteria to the specified crack. For fatigue analysis,
VALUEis a TB material ID. (For static crack-growth analysis,VALUEis the critical J-integral or stress-intensity-factors (SIFS) value.)CGROW,FCG,METH,LC
Specifies the life-cycle method. (Another option for fatigue analysis is the cycle-by-cycle [CBC] method.)
CGROW,RMCONT,ESIZE,
VALUEControls element size at crack fronts, ensuring that meshes around crack fronts are fine enough for accurate fracture-parameter calculation.
CGROW,RMCONT,COARSE,
VALUESets the mesh-coarsening level in the zones where crack fronts pass. Generally, finer meshes are required only to mitigate the stress concentration in crack-front zones. After crack fronts move forward to new positions, the mesh in the old crack-front zones should be coarsened to minimize the total number of elements in the model (thereby improving simulation performance).
In a deformed structure, a new crack could appear potentially at vulnerable positions (such as locations with high stress concentrations).
If a potential crack-initialization zone is not precisely known, the whole model can be checked, but at the cost of a longer solution time (and possibly less accurate results due to meshing, constraints, and other factors).
For this model, based on experience, a high-stress zone will be present at the hole wall after the edge crack in the CT specimen merges with the hole. It is therefore a known potential crack-initialization zone.
ADPCI, CINT, and CGROW command blocks work together to define and control the adaptive crack-initiation and crack -propagation process:
ADCPI command block
ADPCI,DEFINE,1,HOLENCM,1,ELLIPSE ADPCI,GEOM,1,ALEN,0.0025,0.0025
ADPCI,DEFINE defines and specifies the data for crack initiation.
The first 1 value is a user-specified ID for this ADPCI command block.
HOLENCM is a user-specified node component identifying the crack-initiation zone. ELLIPSE specifies an elliptical shape crack.
The second 1 value is a TB material ID specifying the crack-initiation criterion, defined as follows:
MATID=1 TB,CRKI,MATID,1,,PSMAX TBDATA,1,5E5
The maximum principal stress works as the initiation indicator. A new crack is inserted into the initiation zone HOLENCM when the maximum principal stress in the model is
5E5.The ADPCI,GEOM command provides detailed data for the initial crack (such as location, criteria, and crack shape). In this case, the lengths of the major and minor elliptical axes are 0.0025.
Following the ADPCI command block, CINT and CGROW command blocks are required for the adaptive initialized crack.
CINT command block
CINT,NEW,_SIFS CINT,TYPE,SIFS CINT,NCON,4 CINT,UMM,ON CINT,SURF,SURF1,SURF2 CINT,INIT,1
The CINT commands are similar to those in any standard fracture analysis but include a CINT,INIT,1 command specifying an ongoing initiation crack. The command does not provide data for the crack fronts and crack surfaces, as the program calculates them automatically (based on data specified via the preceding ADPCI command block 1).
CGROW command block
This command block controls crack growth following crack initiation, similar to a predefined crack:
CGROW,NEW,2 ! CRACK-GROWTH SET CGROW,CID,2 ! CINT ID FOR CRACK GROWTH CGROW,METHOD,SMART ! CRACK-GROWTH METHOD CGROW,FCOP,MTAB,1 ! MATERIAL DATA TABLE CGROW,FCG,METH,LC ! CRACK-GROWTH METHODOLOGY CGROW,FCG,DAMN,0.000000 ! PROGRAM-SELECTED MINIMUM CRACK-GROWTH INCREMENT CGROW,FCG,DAMX,0.000000 ! PROGRAM-SELECTED MAXIMUM CRACK-GROWTH INCREMENT CGROW,FCG,SRAT,0.000000 ! STRESS/LOAD RATIO CGROW,RMCONT,COARSE,CONS ! CONSERVATIVE MESH-COARSENING OPTION
Conservative mesh coarsening (CGROW,RMCONT,COARSE with
Option= CONS) is used in contrast to the moderate coarsening (Option= MODE) used for the edge crack in the prior CGROW command block.For more information about how the coarsening options affect the analysis, see Results and Discussion.