The world is facing the challenge of climate change, a problem acknowledged by many governments in the Paris Agreement. All sectors, including the construction industry, must take responsibility for reducing their environmental impact. Using renewable bio-based materials, such as flax fibre composites, offers a promising solution towards reducing the construction sector's environmental footprint. Flax fibre-reinforced polymer composites, which consist of flax fibres embedded in a polymer resin, have gained attention for their potential application in structural elements. However, the lack of standardized long-term performance data, for example, its creep behaviour, limits its broader use in structural applications.

Creep is the gradual deformation of materials under a constant load, which can also affect their strength over extended periods of time. Conventional creep tests are often considered impractical as they are time-consuming and costly. Accelerated creep tests, such as the stepped isostress method (SSM), can offer a faster alternative for predicting long-term creep behaviour. The SSM predicts the long-term creep behaviour of a material by incrementally increasing the applied stress while maintaining constant environmental conditions. This method requires only a single test sample and approximately one day of testing to generate a prediction of the long-term creep master curve for a specific reference stress. However, the SSM is an empirical method that has not yet been standardized in any design codes. This research assesses the reliability of the SSM for flax fibre-reinforced polymer composites through an experimental study. Aiming to support the development of sustainable construction practices and encourage using renewable materials in the building industry.

To validate the SSM predictions, long-term creep tests are conducted using a custom-designed tensile creep test setup, supported by quasi-static tensile tests. Additionally, three different composite laminate configurations are compared, demonstrating the fibres' function as the main load-bearing component in the composite. One of the main findings of this research is the material's sensitivity to temperature. Although the SSM predictions produce creep master curves, these predictions are conservative compared to the long-term creep test results. Moreover, the data handling procedure of the SSM, particularly the horizontal shift, is extremely sensitive and partially subjective. Based on the findings, the current form of the SSM is deemed insufficiently reliable to replace conventional creep tests for structural material characterization. However, its potential as a highly time-efficient testing method should not be overlooked. With further refinement in both the experimental procedure and data processing procedure, SSM could become a more reliable approach, making further research on standardizing its methodology essential.

This research urges the need for further research to improve the reliability of the SSM and to standardize its data processing steps. An essential step in this development is understanding the relationship between the activation volume and the applied stress, which may be material-specific. Clarifying this relationship could help reduce the sensitivity and subjectivity of the horizontal shifting process in the SSM data analysis. Additionally, this research aims to inspire continued investigation into bio-based construction materials, contributing to a more sustainable building industry.

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.