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CheFEM is used for analysis of chemical resistance, diffusion and forthcoming mechanical retention of polymer based materials. As such, the software enables lifetime prediction of composites, coatings, laminates and seals during exposure to any chemical or chemical mixture in any state: liquid, gas or supercritical. Furthermore, the tool is used for failure and permeation (e.g. in packaging) assessments. CheFEM can be used in addition to other existing mechanically oriented FEM packages.
On a chemical-physical level, CheFEM rigorously quantifies surface related phenomena (solubility thermodynamics, surface corrosion rates, blister formation) and internal retention mechanisms (mass/temperature/UV diffusion, chemical potential driven corrosion, plasticizing, interfacial stress swelling shears, crack formations). CheFEM consists of the following four modules:
100 FEM/FD Sequencer
This module is used for the mathematical modeling of the application/equipment dimensions in terms of Finite Elements (FEM) and Finite Differences (FD). The spatial domain ranges from nano (e.g. carbon nano particles), to micro (e.g. a glass fibre), to macroscopic (e.g. a coating on top of metal) distance. The chemical-physical parameters defined in the following three modules are determined in function of this spatial domain and the time domain of interest (stationary and instationary solutions). |
101 Lattice Based Thermodynamics & Diffusion ( )
This module deals with (multicomponent) chemical potential and solubility in different material layers as function of temperature, system pressure and stress on the element under consideration. Moreover, diffusion rates are calculated. Subsequently, permeation rates, time lag, degree of plasticizing and swelling stresses are determined. Effects of fatigue and ageing are included.
102 Surface & Chemical Activity Driven Corrosion ( )
Although polymers can exhibit high surface corrosion resistance, chemical reactivity of internal interfaces or the reinforcement materials themselves, can drive loss of interfacial strength. Since the strength of composites is largely dependent on the interfacial robustness, this module is of vital importance for service life analysis. The routine uses chemical potential data from the previous module in combination with chemical reaction kinetics in the specific element under consideration.
103 Gibbs Free Restraint ( )
This module is based on Gibbs Free - Fracture analysis of cast materials, surfaces of - and interfaces between - materials. The module predicts whether surfaces restrain the swelling stress and chemical degradation. If not, it predicts the extent of fracture.
Effects of fatigue and ageing of material components are incorporated. |
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Below the three CheFEM modules are explained more thoroughly:
101 Lattice Based Thermodynamics & Diffusion Module
102 Surface & Chemical Activity Driven Corrosion Module
103 Gibbs Free Restraint Module
Please note that the underlying database contains raw data from our own experiments (excluding disclosed laboratory analysis for our clients, third parties, etc.) and from reputable scientific articles. The CheFEM library contains specialistic information on rates of diffusion, corrosion resistance and forthcoming mechanical retention with regard to plastics, laminate materials and composites. Obviously, MATWEB and IDES like data such as "1% swelling after 24 hours of ASTM D471 water exposure according to producer" is insufficient for our assessments. See some examples of the underlying library at tables.
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| 101 |
Lattice Based Thermodynamics & Diffusion Module |
| The following diffusion and thermodynamics modes influence sorption behaviour, diffusion rates, swelling stress and forthcoming penetration time, time lag (especially time lag for multilayer and laminate materials) and permeation rates. |
| 101.1 |
Both S (solubility) and D (diffusion coefficient) are independent of chemical activity. In standard conditions this applies for solvents that have very low chemical compatibility with the polymer. Then, solubility is governed by Henry's Law. For mass transfer (e.g. Water Vapour Transmission in polymers like Polyethylene or Helium in PTFE / PVDF) and diffusion coefficient calculations, commonly used Fick's first and second laws can be used. |
| 101.2 |
With increasing activity, S increases while D decreases. This behaviour occurs in polymers like PDMS (Polydimethylsiloxane) elastomer and Polyurethane rubber. It results from a tendency of tighter clustering of the polymer by hydrogen bond formation. This behaviour has been observed with Water and Carbon Dioxide in ambient conditions (PDMS rubber) as well as in supercritical conditions (PVDF plastic). |
| 101.3 |
Both S and D increase as a function of chemical activity. This occurs in case of strong solvent - polymer interaction, hence enthalpy and entropy of the solvent have a significant influence on solubility. Then, solubility thermodynamics are for example governed by: Flory-Huggins for liquid solvents, Sanchez-Lacombe for supercritical solvents. Usually effects of swelling of the polymer matrix must be included in diffusivity, time lag and mass transfer calculations. Since chemical potentials must be used, Maxwell-Stefan theory is most appropriate for diffusion coefficient and/or mass transfer calculations. Examples are Gasoline and Toluene diffusion in polymers like Epoxy in ambient conditions and Hydrogen Sulfide diffusion in EPDM or Nitrile Rubber in supercritical conditions. |
| 101.4 |
Both S and D increase as a function of chemical activity. However, the sorption isotherms are sigmoidal, governed by Brunauer-Emett-Teller (BET) adsorption isotherm. Examples are: Water absorption in Nylon 4,6 and Nylon 6,6 plastics(mainly due to free Amide groups in the Polyamide). Due to use of chemical potentials, Maxwell-Stefan diffusion equation is most appropriate. |
| 101.5 |
With increasing activity, S decreases while D increases. This is the Dual Mode Sorption and Transport Model. Sorption and diffusion behaviour of carbon dioxide and Water in glassy polymers and matrix composites have been successfully interpreted in terms of this model (to a certain pressure range). Usually Fick model in combination with local accumulation term can be used. Examples are Water absorption in Vinyl Ester resin or hydrophobic Polyimide polymers. |
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| 102 |
Surface & Chemical Activity Driven Corrosion Module |
| 102.1 |
Surface corrosion driven by external surface concentration of chemicals and radiation. Most commonly used mechanically oriented simulation programmes, think of Abaqus or Ansys have included this surface corrosion rate for metals, but not for polymer surfaces and not for interfaces between different components. |
| 102.2 |
Chemical activity (chemical potential, fugacity) driven corrosion. Internal chemical degradation (initially) driven by diffusion and chemical activity. Might be followed by concentration controlled chemical degradation. Think of Sodium Hydroxide attack of glass fibres, although properly shielded by resin or a thermoplastic materials (e.g. PEEK or PPS polymers).
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| 103 |
Gibbs Free Restraint Module |
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The Gibbs Free Restraint module is based on a Gibbs Free - Fracture analysis. This approach allows integrated analysis of interfacial behaviour of composite materials exposed to static / dynamic loads and swelling driven by mass uptake and temperature gradients. The interfaces under consideration can be macroscopic interfaces, such as Epoxy and Steel, microscopic interfaces, such as Polyurethane and Glass Fibre, or nanoscopic interfaces, think of a Carbon Nanotube within a polymer matrix.
The module is capable of incorporating effects of fatigue and ageing.
It includes the phenomena listed below.
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| 103.1 |
Debonding and possible subsequent delamination, due to:
- insufficient initial adhesion (due to voids, improper wetting);
- inhomogeneous or reactive surface (osmotic blister driving delamination);
- intrinsic stress (temperature, pressure, swelling);
- improper shielding of interface due to composite formulation or long term
degradation (i.e. UV light) of protective layer (capillary pathways);
- improper dispersion of filler material (conglomerates);
- non optimized shape or dimensions of filler material;
- external loads and impacts.
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| 103.2 |
Environmental Stress Cracking (ESC) including Hydrogen Initiated Stress Cracking (HISC). |
| 103.3 |
Polymer and composite fracture as a result of Rapid Gas Decompression (RGD). RGD might give rise to nanoscopic material failure by swelling discontinuities during depressurization and temperature drop (Joule - Thomson effect). Secondly, voids in the material can drive failure. Voids in the used definition can be accumulated chemicals inside large cavities, an amorphous filler and/or chemicals in the space around a filler, fibre, etc. These spaces can result from insufficient wetting and/or swelling strains beyond the interfacial strength. Especially the pressure potenital of gases being operated around the critical point can give rise to failure.
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For a predictive service life and failure tool like CheFEM, real-life and laboratory validation of predicted behaviour is of vital importance. In this regard, some key facts of the computer simulation are listed below. Contact us for validation data concerning your specific case.
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CheFEM prediction results show high degree of correlation to - properly carried out accelerated and long term - laboratory durability test results.
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All CheFEM modulus, such as thermodynamic routines, diffusion and mechanical - temperature and swelling stress routines are based on well established and referenced scientific works. Please contact us for more information on the physical - chemical basis of the routines.
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In many research projects, specific issues and outcomes require further validation. In this case, we can carry out the required laboratory experiments. Please visit the laboratory analysis section to learn more.
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Forthcoming assessments contain the appropriate scientific and laboratory references, as well as applied formulae, equations and industry standards (no black box simulations).
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For free online CheFEM case studies, please visit the cases section.
CheFEM has - among others - been used for permeation, service life and failure modes assessments in the following industrial applications:
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Polymer - metal based packaging for electronics and medical packaging. |
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Multilayer (polymer laminate with inorganic plasma barrier layer) Multicomponent Carbon Dioxide (in presence of moisture and other flue gas components) membrane design. |
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Water diffusion based lifetime analysis of flexible polymer based solar cell. |
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Water vapour transmission and chemical degradation assessment of flax - resin based bio composite containment. |
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Service life prediction of glass reinforced composite rod for concrete reinforcement
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bridge, sluice, dam, and road application). |
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Service life of internal liners / coatings of concrete demineralized Water tanks
(nuclear power plant). |
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Pipeline retrofitting and residual service life assessment. |
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Simultaneous migration and diffusion of Oxygen, Water and Acid through multilayer ( Aluminum barrier layer - EVOH and PP polymers) laminate for food packaging. |
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We invite you to contact us for more information on CheFEM.
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