The use of Glass Fiber Reinforced Composites is increasing across the world in its use in booming industries such as wind energy, construction and electronics due to the advantages it offers. A replacement for glass fiber composites is being researched widely in Europe because of sustainability reasons. Bio-based composites such as Flax Fiber Reinforced Composites offer higher degradability and better mechanical properties for the same specific weight ratio as glass fiber composites. Although flax composites are an attractive alternative to traditional, petroleum-based composites, they have higher susceptibility to moisture degradation due to the presence of hygroscopic natural fibers. The moisture degradation of flax composites negatively affects its durability and strength performance.

In this thesis work, existing work employing the use of state of the art numerical techniques used to model hygrothermal aging of glass fiber composites is used to model moisture aging mechanisms seen in flax composites. A literature review is performed to identify the crucial degradation mechanisms affecting flax composites due to moisture aging. Interface debonding between phases is found to be a possible crucial damage mechanism due to moisture uptake. Presently, there is a lack of experimental studies on the effect of moisture degradation effect on the interface the significance of the same in the loading configuration of transverse flax-epoxy composites in literature. There is also presently a lack of numerical modelling efforts that describe the complex multi-physics and multi-scale nature of natural fibers and bio-based composites consistently and wholistically in literature. The goal of this thesis is to carry out exploratory work in the numerical modelling of swelling and tensile behaviour of transverse flax composites, based on existing modelling techniques for GFRP and supported by parameters from literature and experiments.

The swelling and transverse tensile behaviour of the flax fiber reinforced composites is modelled at the micro-scale. The state-of-the-art numerical framework used to model moisture degradation and quasi-static and fatigue behaviour in glass fiber composites is adapted for simplified micro-scale modelling using a Representational Volume Element (RVE). The sensitivity of the modelling parameters obtained from literature is studied to understand the effects of geometry variation and material degradation on the swelling strain in a linear-elastic micro-scale RVE analysis.

An experimental campaign is carried out to obtain relevant parameters for the numerical model that is missing from literature. The aim of the experimental campaign is also to characterize the moisture aging behaviour of transverse flax composites and its constituent phases. The material stiffness degradation of transverse flax composites and epoxy is studied with tensile tests before and after moisture uptake in the climate chamber. The swelling co-efficients of both transverse flax composites and epoxy are calculated. Microscopy imaging is used to qualitatively assess damage mechanisms due to moisture uptake and the fracture surface from tensile tests.

An elasto-plastic material for epoxy is calibrated with the tensile test experiments performed on the epoxy. A final iteration of the swelling numerical model based on the experimental benchmark is presented. The transverse tensile tests for composites from experiments are numerically simulated. The capabilities and the shortcoming of the final model are discussed for improvements in future work.