I want to write a VUMAT subroutine for progressive impact damage. In a failure criterion, there is a characteristic length. How can I calculate this?
A friend told me to write the element length, but if I want to change element size, what should I do then? Should I use solid element? There is not a description of a characteristic element length for it in abaqus.
Dear Mohammad,
ABAQUS furnish you a "geometric" quantity in each gauss point, that is the characteristic element lenght (CELENT). It is a sort of "average" dimension for the "current" element. It is NOT the average lenght of all elements!
What I mean is that for each element, ABAQUS compute the CELENT as I have told you, and give you this information in the VUMAT, for each element, in each Gauss point. You do not have to calculate it. It is in input.
I do not know what you want to do in your constitutive model. In general, CELENT is used when you have a model which depends on the element lenght, such as, for instance, when your model involves the fracture energy: in this case you have as input the strains, you need the displacements (which are nodal variables, and are not available at the Gauss point), and you can estimate an "average" displacement in the current gauss point using, for example, an "average" strain for your element multiplied for the CELENT. It is very simplified, bot I hope this can help you. Good luck!
Andrea
l is the characteristics length that would keep a constant energy release rate per unit area of crack and make the final results independent of FE mesh size.
for detailed description..u can refer following paper
Modelling damage evolution in composite laminates subjected to low velocity impact
Y.Shi,T.Swait,C.Soutis
In Abaqus software, there is a definition of mesh characterize length, please search in the Abaqus user's manual, in the manual there are some sentence to define the characterize length as
"The definition of the characteristic length depends on the element geometry and formulation: it is a typical length of a line across an element for a first-order element; it is half of the same typical length for a second-order element. For beams and trusses it is a characteristic length along the element axis. For membranes and shells it is a characteristic length in the reference surface. For axisymmetric elements it is a characteristic length in the r–z plane only. For cohesive elements it is equal to the constitutive thickness. This definition of the characteristic length is used because the direction in which fracture occurs is not known in advance. Therefore, elements with large aspect ratios will have rather different behavior depending on the direction in which they crack: some mesh sensitivity remains because of this effect, and elements that have aspect ratios close to unity are recommended."
Please check the manual and I hope this can help you~
For plane element is the square root of the area. For solid element is Volume^(1/3). For linear elements is the element lenght.
@ Andrea
you are right, but if you refine the mesh how should you calculate it?and how you apply it in program?
Dear Mohammad,
ABAQUS furnish you a "geometric" quantity in each gauss point, that is the characteristic element lenght (CELENT). It is a sort of "average" dimension for the "current" element. It is NOT the average lenght of all elements!
What I mean is that for each element, ABAQUS compute the CELENT as I have told you, and give you this information in the VUMAT, for each element, in each Gauss point. You do not have to calculate it. It is in input.
I do not know what you want to do in your constitutive model. In general, CELENT is used when you have a model which depends on the element lenght, such as, for instance, when your model involves the fracture energy: in this case you have as input the strains, you need the displacements (which are nodal variables, and are not available at the Gauss point), and you can estimate an "average" displacement in the current gauss point using, for example, an "average" strain for your element multiplied for the CELENT. It is very simplified, bot I hope this can help you. Good luck!
Andrea
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Thank you Andrea, your answer is enlightening one.
Q.1: May I have your opinion on band width (lb) according to Crack Band Model (CBM) and Extended Crack Band Model (ECBM) ?
Q.2: How does it effect if (lb) is greater than or less than characteristic length of element (l*) ?
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