Adhesive CZM ANSYS Parameters

Motivation

Perhaps one of the biggest curse to any engineering analyst is the the lack of key parameters to make a good estimate. For adhesive modeling, what parameters should one use? One can try to look up typical values online or from research papers but they are often not exactly what's being used immediately. 

To overcome this difficulty, you could create an experimental test for your adhesive to extract the information you need as described in the paper by Dastjerdi et al [1].

The rest of the blog post details how to interpret the test data. Next, by using extracted parameters, replicate the test in Ansys to get similar results (as expected).  

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.

Simulation Model Goal
Unlike ASTM D1876, the paper by Dastjerdi et al [1] provides a clear way of translating load frame test data into parameters used in ANSYS. The flimsy panels used in the ASTM standard are highly non-linear. The critical contribution of the paper is to replace highly deformable test pieces with a rigid body substrate and providing the math in computing the parameters.

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 [1] 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.

Extracting Parameters from Test Data
Figure 7 & 8 from the paper shows the measured force vs opening and normal traction vs separation respectively. Please see the paper on how to compute the nominal traction vs separation plot.  

Next, I appended in Figure 2 below, the light turquoise line was appended over the green Polyurethane line to approximate the ANSYS load path. The vertical and horizontal appended orange lines helps determine the amplitudes for this path (5.2MPa & 70micro-meter).

Figure 2: Appended Plots from Paper

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

Figure 3: CZM Properties 

Note the peak normal traction stress is used in Fig 3. A key snippet is needed for the initial gradient which specifies the stiffness of the contact elements:

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

Note both the peak stress and separation is included in the above snippet. The negative value tells Ansys to interpret the real constants 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. 

Key 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] or [link]
4. Ansys Knowledge Resource #2052789 [link] model with bonded contacts

Archive File
ANSYS WB v18.2 [Link]

[1] Khayer Dastjerdi, A., Tan, E. & Barthelat, F. Direct Measurement of the Cohesive Law of Adhesives Using a Rigid Double Cantilever Beam Technique. Exp Mech 53, 1763–1772 (2013). https://doi.org/10.1007/s11340-013-9755-0