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Interfacial moisture diffusion using cohesive zone elements

shirangi's picture

Hello every body,

As most of you know, fracture mechanics of polymer materials needs a special consideration of the viscoelastic material properties. Especially under thermomechanical loading the role of glass transition temperature T_g is very important. That is why people try to characterize the material with different methods including stress relaxation based on Time-Temperature superposition or DMA test.

a similar problem regarding the fracture mechanics of polymers appear when the material is under moisture loading. For example in Electronics Packaging the mold compound of the IC packages absorbs moisture and moisture decreases the interfacial strength between the Mold compound and leadframe.

Using cohesive zone elements is usually a good method for investigating the Interfacial Fracture Mechanics of Polymer-Metal interface. But as both temperature and Moisture concentration of the materials change, the critical values of fracture toughness change and make it too difficult to bring these changes in FEM Code. On the other hand moisture diffusion through bimaterial interface is different ( I guess much faster) than diffusion of a bulk material.

Has anybody any experience regarding the following subjects?

1. In a transient thermomechanical analysis due to change in the temperature and moisture content of polymers the fracture toghness of the interface changes. Since the fracture toghness is an Input of the cohesive zone elements, the cohesive zones also must change. Is there any way to update the fracture toughness of the material after each iteration when using cohesive zone elements?

2. What is the difference between the moisture diffusion of a bulk material and moisture diffusion of the interface of polymer-metal interface? How fast does the moisture diffuse through the interface?

3. Is the moisture concentration at the polymer-CU interface different than inside the polymer?  Does the moisture remail at interface?

thanks for your comments,

Hossein

Email: hossein.shirangi@izm.fraunhofer.de

 

Comments

Ying Li's picture

I am not very familiarly with your problem. However, I think your problems could be solved in this way:The fracture toughness of the cohesive element could be changed at the different sub-step in the finite element method (FEM), especially in the ANSYS if you want to calculate the thermal-structure coupled problem. You just need to set different material parameters in the soft wear.

shirangi's picture

Hi Yinlee,

 thanks for your comment. What a bout a coupled moisture-thermomechanical peoblem. For example a moisture-saturated material which is under moisture desorption because of high temperature! moisture desorption increases the strength of interface and temperature rise reduced the interfacial fracture toughness. How can you update cohesive strength during moisture-thermomechanical analysis?

stay Happy

Hossein

Shailendra's picture

Depending on the constituents the metallic phase may corrode/ oxidize under the coupled effects of moisture and temperature. This may be necessary to incorporate as a part of degrading interface because

a. Corroded regions would essentially lose contact with the polymer. 

b. Corrosion leads to volumetric increase that may induce residual stresses.

Overall a very interesting problem to address. 

 

Thanks,  

~Shailendra 

shirangi's picture

Hi Shailendra,

actually you are right, but I am not considering the corrosion of copper lead frame at this moment. It makes the subject more complicated.

stay Happy

Hossein

Joost Vlassak's picture

Dear Hossein,

We have done some work on the diffusion of moisture into film stacks and how it affects adhesion. We have experimental results and a rather simple model that couples subcritical crack growth and diffusion. The model agrees quite well with the experimental results. We did experiments on dielectric (ceramic) film stacks, but I assume you would find similar effects for stacks that incorporate polymers.

You can find one iMechanica entry at http://www.imechanica.org/node/222. Another paper just got published (Youbo Lin, Ting Y. Tsui, and Joost J. Vlassak, "Water diffusion and fracture in organosilicate glass film stacks", Acta Materialia, 55, 2455-2464 (2007)).

Joost J. Vlassak

shirangi's picture

Dear Joost J. Vlassak,

thanks alot for your kind reply. I find your papers very helpful. I hope we can remain in contact and exchange our experiences.

Stay Happy

Hossein Shirangi

aamirmub's picture

Dear Hossein

 I have following comments to offer regarding your questions;

1. You are right that fracture toughness of the interface will change with changing moisture concentration. The fracture toughness at different concentrations can be determined experimentally and the moisture based fracture toughness data can be given in FE software (like msc.marc or abaqus).

2. The mechanism of capillary diffusion is considered responsible for the high diffusion rate at the interface, however, high diffusion rate at interface is still not accepted generally. There is evidence in its favour and against it. I think that it mainly depends on the preparation of underlying surface. The diffusion rates of as high as 10 times of bulk diffusion rate have been used in literature. It is very difficult to measure the interfacial diffusion rate.

3. The moisture concentration at the interface depends on many factors including the polymer and metal or non-metal substrates. Water can be present in oxide layer on metals in form of hydroxl groups or due to hydration.

For detailed study of the subject, I would recommend the research papers of Dr Ian Ashcroft (Loughborough University, UK) and Prof A.D.Crocombe (Surrey University, UK).

Regards

Aamir

shirangi's picture

Hi Aamir,

 

thanks for your hints and very helpful arguments.

stay Happy

Hossein

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