Combined mechanical and environmental ageing of FRPs - Hygrothermal fatigue case study

icaf2023 Tracking Number 148

icaf2023 poster

FRP composites are used in aerospace, marine, and civil engineering structural applications where the materials constituting these structures are often exposed to outdoor environment or particular climate. Aircraft fuselage, wind-turbine blades, ship hulls, and bridge decks are examples of large-scale structures that must be designed considering both environmental and mechanical ageing to ensure structural safety. The mentioned structures are commonly made of FRP with carbon or glass fibre reinforcement. Decades of research and operation of those materials have shown that FRPs are suitable in terms of durability with, in most situations, limited assessment of environmental loading effects on the structural integrity during laboratory testing. However, with rising concerns on climate change and the significant impact of engineering structures on the global CO2 emissions, using traditional FRP composites does not meet the requirements of low embodied-energy and easily disposable or recyclable materials. Bio-based fibres or resins satisfy those requirements with one example of such being flax fibre reinforced polymer composites. Flax fibres are a sustainable alternative that could replace synthetic fibres in FRP composites thanks to their renewability, low embodied energy, ease of disposal, competitive mechanical and high damping properties. Example of this is the use of flax fibre composites in satellite for clean burn upon re-entry to the atmosphere (Natural fibre reinforced satellite panel by ESA and Bcomp). While carbon and glass fibres are relatively inert to water, bio-based material alternatives are usually hygroscopic and susceptible to environmental humidity. Since existing lifetime prediction models for FRPs often neglect hygrothermal effects, they cannot be directly and confidently applied to bio-based FRPs or to FRPs subjected to harsh climate conditions. In the last decade, research on flax FRPs mainly focused on the characterisation of mechanical properties, fatigue life and effect of environmental ageing on the residual properties. The literature shows that environmental ageing has an effect on the damage in flax FRP with strength and stiffness moderately affected. Literature also shows that damage due to cyclic mechanical loading (fatigue) is similar to damage due to environmental ageing. Although environmental and fatigue effects have been studied separately, to this day, no comprehensive investigation on the interaction of these two has been carried out. To ensure the competitive durability and service life of biobased FRPs compared to synthetic composites, the effect of environmental loadings such as moisture and temperature cycles concomitant to mechanical loading must be considered in the structural design. The aim of the current study is to identify the phenomenological commonalities between the damage due to environmental and cyclic mechanical loadings (fatigue) when they occur sequentially or simultaneously. Understanding the interaction between damage mechanisms originated from environmental and mechanical loadings and how this affects the residual material properties is pivotal for the adoption of biobased FRPs in structural applications in terms of durability of such structures. This study critically discusses current understanding of this interaction in existing literature and proposes an experimental methodology that aims to set the foundation for further development of reliable life prediction models for FRPs regarding environmental effects. This ultimately leads to safer and more durable design of large-scale engineering structures with bio-based materials as constituents having low environmental impact.