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.