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Adhesive CZM ANSYS Parameters

How to Extract CZM Parameters From Test Data?
A few years ago, I was pulled in to do some adhesive modeling. The project leaders later decided against using adhesives but it peaked my interest on how to simulate adhesives using Cohesive Zone Modeling (CZM). Mode I Peel direction failure was a concern and glue manufacturers sometimes lists the peel strength per ASTM D 1876. Unfortunately, from those values alone, it is difficult to determine the needed parameters for use in ANSYS.

Resources
1. CAEAI presentation [link] provides good background information
2. PADT The Focus #56 by Rod Scholl [link] explained the parameters clearly
3. Paper by A. Khayer Dastjerdi & E. Tan & F. Barthelat [link] referenced in this blog
4. Ansys Knowledge Resource #2052789 [link] model with bonded contacts

Simulation Model Goal
Unlike ASTM D1876, the paper by Dastjerdi et al provides a clear way of translating load frame test data into parameters used in ANSYS. ASTM's flimsy panels that are highly non-linear are eschewed in favor for the simplicity of rigid body substrate.

Figure 1: Fixtures used in CZM Adhesive Characterization

The second piece of the puzzle is SR #2052789 which provides many tips on the model setup requirements (e.g. Never update contact stiffness, use fracture object etc). Their attached project file was used as a template for this blog post's project.

The paper by Dastjerdi et al shows how one could calculate the normal traction vs separation from their experimental setup. The experimental setup and corresponding test results were also described well enough to be replicated in ANSYS. As both test data and calculated cohesive laws curve are available in the paper, we could extract CZM CBDD parameters then try to replicate the test in ANSYS to see how well the calculated force measures up to test data. This would be done for the Polyurethane adhesive.

Parameter Setup
Figure 7 & 8 from the paper shows the measured force vs opening and normal traction vs separation respectively. In the Figure 2 below, the light turquoise line was appended to approximate the ANSYS load path. Orange lines helps determine some amplitudes for this path.

Figure 2: Appended Plots from Paper

The CZM Separation-Distance Debonding parameters approximated from the turquoise line are:
Figure 3: CZM Properties 

A key snippet is needed for the initial gradient:
Figure 4: Stiffness Specification Location

The snippet is:
rmod,cid,3, -0.07428e12 ! FKN = 5.2e6/70e-6
rmod,cid,11,-0.07428e12 ! FKOP = 5.2e6/70e-6

The negative value tells Ansys to interpret FKN & FKOP as absolute value and not based on the underlying element stiffness/dimensions. Other details can be found in the archived model.

Results & Discussion
Figure 5 has the calculated force peaks at 97.5N at 0.12mm opening.
Figure 5: Calculated Force

It's a bit smaller than the peak ~112N at ~0.14mm shown in the paper but it's not far in the left field. A bit more parameter tweaking should better match the ANSYS results to the test forces.

Note that a shear test (Mode II) is advised to determine the tangential properties which were (wrongly) assumed to be the same in the above example.

Archive File
ANSYS WB v18.2 [Link]

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