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. ... Read more

Composites are widely used in structural applications due to their high strength-to-weight ratio, stiffness, and design flexibility, but their sustainability remains a limitation. To address this, flax fibre-reinforced polymer (FFRP) composites offer competitive mechanical performance while being biodegradable and less energy-intensive to produce. However, the use of FFRPs in load-bearing structural applications is constrained, in particular, by the susceptibility of flax fibres to environmental conditions such as temperature and humidity, and by a limited understanding of their delamination behaviour in the primary loading modes. Therefore, this study investigates the interlaminar fracture toughness of FFRP composites under various hygrothermal conditions in Mode I loading.

The experimental analysis was conducted under environmental conditions representative of natural weathering, including hot-wet, hot-dry, room, and cold environments. The samples were initially conditioned at the respective hygrothermal conditions and subjected to quasi-static and fatigue loading in an environmental chamber. The results demonstrate a strong dependence of fracture toughness on the applied hygrothermal conditions, indicating that FFRP composites are highly sensitive to both temperature and relative humidity. Under quasi-static loading, the fracture toughness increased with higher humidity and lower temperature, indicating enhanced crack growth resistance due to moisture and improved
fibre bridging, while a reduction in fracture toughness was observed under low-humidity conditions. Fatigue results showed distinct Paris curves, with a rightward shift observed under high-humidity and low-temperature conditions, indicating improved resistance to fatigue crack propagation, whereas Paris curves corresponding to low-humidity environments shifted leftward, reflecting decreased resistance to fatigue crack growth.

Fractographic analysis using optical microscopy and scanning electron microscopy (SEM) revealed common microstructural features such as technical fibre bridging, fibre pull-out, yarn loosening, fibre patches, scarps, and matrix cracking. The nature of fracture transitioned from ductile under high humidity and elevated temperature to brittle at low temperature, highlighting a shift in the dominant failure mechanism from interfacial debonding to matrix-dominated cracking. Surface roughness measurements, however, exhibited considerable statistical scatter across all environmental conditions, likely due to the strong influence of technical fibre bridging on the measured roughness. Consequently, the observed changes in Mode I interlaminar fracture toughness with humidity and temperature were not clearly reflected in the roughness parameters.

Overall, the findings emphasise the strong dependence of the fracture behaviour of FFRP composites on environmental exposure. Understanding these effects is critical for the reliable design and durability prediction of FFRP composites in structural applications. The results contribute to establishing a foundational understanding of the fracture mechanics of FFRPs.
... Read more

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. ... Read more

Climate change is one of the most urgent global issues, with the construction sector contributing nearly 11% of global CO2 emissions. Cross-laminated timber (CLT), is proposed as a sustainable alternative for concrete and steel that, can reduce carbon emissions. The primary objectives are to identify the main drivers and barriers to using CLT in multi-storey buildings in the Netherlands and to propose strategies to overcome these barriers. The results indicate that timber’s natural ability to store captured carbon dioxide was the most important driver. Besides, CLT's lightweight nature allows for precise, modular prefabrication in factories, enabling quick and safe on-site assembly.

The biggest barriers are related to financial and political aspects, but also to technical and sociocultural aspects. Some of the regulatory barriers are CLT’s unfair representation in the MPG system and its incompatibility with building codes. Additionally, CLT is oftentimes more expensive than concrete and steel, making it less attractive. Moreover, there is still till some extent a lack of knowledge and experience across the AEC industry regarding CLT, leading to perceptions of risk and reluctance to adopt it. Some of the technical challenges are related to fire-safety, acoustics, moisture, and connections.

Five sets of strategies have been developed to overcome the aforementioned barriers. The first set of strategies is aimed at increasing industry-wide knowledge and awareness on CLT. The second set of strategies is focused on changing the industry by promoting collaboration and reorganizing the timber supply chain. The third set of strategies addresses the cost barriers by offering new financial models to overcome the cost surplus of CLT construction. The fourth category is focused on overcoming technical barriers by making technical advancements like improvements and innovations in prefabricated systems and increased standardisation. The last set of strategies is aimed at addressing regulatory barriers. Strategies include revising the MPG system to better account for biogenic carbon storage, expanding the national environmental database to include more timber products, establishing building codes that support timber construction, and setting timber building quotas. ... Read more

This thesis investigates the effects of hygrothermal aging on the mode I and mode II fracture toughness of flax fiber-reinforced polymer composites (FFRP) under quasi-static (QS) and fatigue (F) loading conditions. A key motivation for this study is the potential of FFRP to replace synthetic fiber composites, as FFRP offers competitive mechanical properties while being biodegradable and less energy-intensive to produce. However, one of the main limitations of flax fibers is their susceptibility to environmental conditions such as temperature and humidity.

Delamination is a common failure mode in composites, and conducting fracture testing under mode I and mode II conditions is crucial for designing durable components. Double Cantilever Beam (DCB) and End-Loaded Split (ELS) specimens were manufactured for mode I and mode II tests, respectively. Subsequently, hygrothermal aging was simulated by subjecting the specimens to one or two cycles of humidification and drying at elevated temperatures within a climate chamber. Quasi-static testing was performed on unaged, 1-cycle aged, and 2-cycle aged specimens, while fatigue testing was conducted exclusively on unaged and 1-cycle aged specimens.

Testing resulted in significant plastic deformation of the specimens, this was attributed to their insufficient stiffness. This invalidated the assumption of Linear Elastic Fracture Mechanics (LEFM). To better capture these effects, the analysis was conducted using the J-integral, based on non-linear fracture mechanics. While the J-integral cannot account for all observed effects, it provides for a more realistic approximation for comparative evaluation of fracture toughness between aging states.

The results reveal that in mode I QS testing, the initiation fracture toughness on average improved by 19% after one aging cycle, with no further increase observed after a second cycle, while mode II QS fracture toughness was insensitive to aging. In mode I fatigue testing, a reduction in delamination growth resistance was observed after one aging cycle. Mode II fatigue testing exhibited substantial variability within aging states, making it challenging to determine the influence of aging, although a reduction in variability was noted after aging. The increase in QS initiation fracture toughness is likely due to the plasticization of fibers and matrix.

These results indicate that aging does not have a straightforward effect on fracture toughness, as its impact varies between modes and regions of crack growth. These findings provide valuable insights for the design of FFRP and other biofiber composites, contributing to the development of more sustainable materials.
... Read more

Fibre metal laminates (FML) were initially conceived as a hybrid material, aiming to create synergy between the impact resistance of metals and excellent fatigue resistance of fibre reinforced polymers. The purpose of this approach was to overcome the limitations of single-material structures. However, despite its considerable promise, the use of the FML concept has primarily been confined to aerospace applications and heavily relies on synthetic fibres that carry significant environmental implications. Hence, given the growing concerns about climate change and the challenges posed by recycling glass fibre composites, a new generation of FMLs with a reduced carbon footprint should be envisaged.

Research on flax fibre composites reveals convincing mechanical properties and remarkable damping capacities. However, the broader adoption of these composites remains restricted primarily due to issues related to low impact resistance, moisture absorption and flammability concerns. The FML concept presents a viable solution to surmount these constraints, consequently facilitating the integration of these materials into primary structures. Hence, the research endeavour aimed to attain comprehensive insights into FLAx REinforced aluminium (FLARE), particularly focusing on its impact resistance and vibration damping capabilities, which are believed to be the principal benefits of this hybrid material.

The research goal was divided into three distinct research tasks: conducting experimental analyses to characterise the damping behaviour of FLARE, evaluating the impact resistance through experimental means, and validating predictive tools to offer initial insights into the design principles governing such a FML. FLARE, along with flax fibre reinforced epoxy (FFRE) and GLARE specimens, were manufactured using wet layup combined with vacuum bagging techniques.

First, tensile tests were conducted to validate the applicability of the metal volume fraction (MVF) approach in predicting the mechanical properties of FLARE. Intriguingly, the well-known non-linear behaviour exhibited by flax was not observed in the case of FLARE. The results revealed that while the MVF method provided a satisfactory approximation, it was the "inelastic" modulus of FFRE that predominantly contributed to the stiffness of FLARE.

Dynamic mechanical analysis and vibration beam tests were carried out to assess the influence of incorporating metallic layers on the vibration damping characteristics of flax fibre composites. The investigation revealed that the metallic layer predominantly governs the damping behaviour of the FML. Notably, an inverse rule of mixture emerged as the most effective means of approximating its loss factor.

Low-velocity impact tests were conducted to gain insights into the impact response of FLARE in comparison to conventional FMLs. The analysis indicated that the aluminium layers play a significant role in energy absorption, whereas the composite strength emerges as the critical factor influencing impact resistance. A quasi-static analytical model was also assessed, offering an initial estimation of the impact response, yet it warrants further refinement.

In conclusion, the FML concept holds promise for FLARE, but its application requires a novel approach compared to previous methods, to render FLARE viable for practical real-world applications. ... Read more