Can someone give me an overview of moisture and salt water induced failure mechanisms of polymers and glass fibre reinforced polymers (such as glass fibre Epoxy composites)?
Background of my question is the recent observation of failure (cracks and delamination) of resin based composite subjected to repetitive strains in presence of water and salt. The failure is not predicted by mechanical FEM models for ageing/fatigue (Ansys). Hence, I am looking for ideas for an integrated mechanical-chemical model of composite ageing and failure analysis.
Thanks for any suggestion, Marc Atkins
Comment by Steve Abbott on 01 Jul 2011 at 15:20:41
I have a question in a similar direction. We are working with Epoxy based glues for structural bonding in the manufacturing of aero-structures, such as airframes and components. We are more or less able to simulate the mechanical strength of the adhesive with Abaqus. However, we lack proper knowledge on the influence of moisture on the long term behaviour (read: we just follow the predescribed test program without exactly knowing why we are doing things). Hence, I am interested in an overview of theories of water driven failure of epoxy adhesives (or other polymer glues), fillers and reinforcements.
Comment by Composite Analytica on 08 Jul 2011 at 12:48:31
Materials can degrade by chemical surface reaction or internal interfacial reaction. In case of polymer based composite materials, the internal interface reaction of moisture with fibres and fillers is usually the primary concern for the long term since at the proper temperature operation, the matrix is usually chemically resistant. Because this phenomenon is regulated by diffusion rates of water into the matrix and by an internal chemical (equilibrium) reaction, analysis is more complex than the Fick's law based assessment.
A proper analysis of the reaction rate and thermodynamical status of the interface as a function of time and place is however vital for the subsequent mechanical analysis.
Since many recent accelerated ageing cases of composite and polymer materials (for example unexpected failure of - and crack formation in - wind mill rotor blades operated at land and sea), we have developed the CheFEM software program. This program simulates the water diffusion rates through the matrix, in presence of internal chemical reactions and interfacial water accumulation due to no or insufficient wetting. Subsequently the interfacial strength (interfacial fracture energy) is determined as function of time and place. Then, this data can be transferred to a mechanically oriented FEM program like Abaqus or Ansys for integration with overall mechanical analysis.
PS: See the CheFEM and case stories section for more information.