Hygroscopic stress and strain

[simulation_engineer_2022]

Hi everyone!

I want to know how to model hygroscopic stress and strain in Abaqus in case of fiber reinforced composites. I've found moisture swelling model in Abaqus documentation but I'm not sure if it is what I need.

The case is the following:
Structure is made primarily of fiber reinforced composite on Earth (where it absorbs moisture) and then is sent to the orbit, where it desorbs moisture. The problem is to find hygroscopic strain and stress during absorption and desorption processes.

What could be material properties, boundary conditions and loads, procedures I could use as built-in and what I should add as subroutine? This problem is new for me but very important to solve.

Thanks in advance!

 

[FEA way]

I would start by checking research papers and theses on this topic. They can give you some ideas. The moisture swelling model in Abaqus is meant for porous media but could be helpful in your case depending on the physics of your particular problem and your needs. You could also try implementing this with subroutines - their variety should be sufficient to model this phenomenon. For example, the UEXPAN subroutine may help you combine thermal and hygroscopic strains. Also, keep in mind that there's a possibility to perform a coupled moisture-stress analysis using a predefined field of normalized concentrations.

 

[SWComposites]

a) there are no built in material properties for composites
b) finding hygroscopic (moisture swelling) properties for composites will not be easy; try searching research papers (scholar.google.com) and NASA documents (Langley technical reports server)
c) you can analyze hygroscopic effects the same way as thermal strain and induced forces are analyzed, just substitute hygroscopic properties for thermal expansion properties, moisture levels for temperature, etc - just get the units correct!)
d) if you have never done a thermal loads analysis then you are in for a very steep learning curve, good luck

 

[simulation_engineer_2022]

Thanks everybody for replying!

c) you can analyze hygroscopic effects the same way as thermal strain and induced forces are analyzed, just substitute hygroscopic properties for thermal expansion properties, moisture levels for temperature, etc - just get the units correct!)

[ul]
[li]I've found this workaround but what if I need to analyze both thermal and hygroscopic effects simultaneously? [/li]

[li]And how should I interpret thermal loads and BCs in hygroscopic analysis? I'm familiar with thermal analysis, but not with hygroscopic one.[/li]
[/ul]

 

[SWComposites]

what I suggest you do (and it what we used to do long ago when messing with FE models) is to build and run very simple "test case" models (even as simple as one element) that you can analyze by hand to compare to the FEM results. Start simple and add complexity and be sure you fully understand the results at each step. e.g., run thermal load by itself, then hydro load by itself, then combine them, etc.

 

[simulation_engineer_2022]

Yes, this approach is absolutely clear and straightforward and I usually build models starting from very simple cases.

But I'm telling about combining two analyses, how can I conduct thermal and hygroscopic analyses simultaneously? You can not add more than one thermal expansion coefficient at a time, but if one needs to consider both thermal and hygroscopic effects - thermal expansion coefficient is to be in material model as well as hygroscopic expansion coefficient.

 

[SWComposites]

You could perhaps run the thermal and hygro analyses as separate analysis subcases, then superimpose the results (this assumes you are running a linear analysis).

Or if you have a constant temperature delta and moisture delta, then maybe combine them and create a combined “CTEeffective”, as
CTEeffective = CTE*deltaT + CME*%moisture, with input temperature change = 1

 

[FEA way]

Check the article "Nonlinear Stress Modeling Scheme to Analyze Semiconductor Packages Subjected to Combined Thermal and Hygroscopic Loading" by S. Yoon et al. and "FE Model of Moisture Absorption by Adhesive Joints between Composites" thesis by V.B. Mora.