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cohesive element definition in ABAQUS

Hi there,

 I am sure a lot of you have been using ABAQUS to define cohesive elements. I have seen a lot of forums here and a lot of them specified the Alfano 2001 paper as a great reference. Here is the title of the paper incase anyone is wondering which paper:

G. Alfano and M.A. Crisfield, Finite element interface models for the delamination analysis of laminated composites: mechanical and computational issues, International Journal for Numerical Methods in Engineering 50 (2001), pp. 1701–1736. 

 ABAQUS itself uses this paper to describe its usage of cohesive elements.

I follow the theory of traction separation very well and understand almost everything that ABAQUS has done to implement the the 2 crack model of Alfano 2001 paper. However the biggest confusion I have is how they define the elasticity of the cohesive element.

 ABAQUS requires an input for Knn, Kss, Ktt or E1, G1, G2. This is the strangest part of the element definition. All other literature defines it in terms of traction and separation (ANSYS also defines it in terms of T_o and d_o). Hence you have to somehow convert you traction-separation data into traction-strain data and provide a stiffness matrix. Hence, out of nowhere, ABAQUS pulls out 850 MPa for all stiffness values i.e. E1 or Knn G1 or Kss and G2 or Ktt as the appropriate values. This confuses me thoroughly as these numbers do not appear in Alfano's paper at all. Hence I am wondering how ABAQUS came up with 850 MPa and what is the logic behind that! Is it the interlaminar shear strength of the HTA913 Carbon/Epoxy composite used?

 By any chance, has anyone figured out exactly how to define the elasticity properties of the cohesive elements in abaqus when you only have traction-separation data? I will greatly appreciate it if anyone can answer this quesion.

 Roodge.

Comments

Ktt is a constant relating the traction to the separation of the top and bottom surface of the interface. The unit of Ktt is N/m^3. It is not as same as stress's unit. Turon et al. gave a detailed discussion on how to determine the stiffness, Ktt etc.

see Turon A, Davila CG, Camanho PP,
Costa J. An engieering solution for mesh size effects in the simulation of
delamination using cohesive zone models. Engineering fracture mechanics
2007;74:1665-1682

Yi

Hi everyone

I think the unit of Knn Kss and Ktt should be the same with elastic constants (like MPa, or N/m^2 etc.)

Please see Abaqus Analysis uses' manual (29.5.6 Defining the constitutive response of cohesive elements using a traction-separation description) for details. Subtitle is Linear elastic traction-separation behavior.

Best wishes

Jiadi

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