Abstract
The present work has investigated the fracture toughness of a model DGEBA epoxy system subject to Hidro-Thermal aging. A Photoelastic Stress Analysis technique has been implemented, showing the evolution of stresses arising throughout the water uptake process due to the non-uniform swelling of the material. Gravimetric and Dynamic Mechanical Thermal Analyses have further complemented the characterization, showing the onset of plasticization effects with aging. The correlation of all previous characterizations has allowed to conclude that an increase of KIC fracture toughness is obtained at the fully saturated condition. In particular Photoelasticity has also revealed the onset of relevant swelling induced stresses during the first stages of water absorption, leading to an increase of fracture toughness due to compressive stresses settling near the crack tip. A stress free condition is instead reestablished at the later stages of absorption, suggesting that the increased toughness of the saturated material is an effect of the modifications induced by aging on the polymer structure.
Highlights
Glossy polymer structures, such as thermoset matrices used in Fiber Reinforced Plastic (FRP) composites, are in general prone to solvents absorption
The present work has investigated the fracture toughness of a model DGEBA epoxy system subject to Hidro-Thermal aging
Preparation of samples and plan of experiments The resin system was prepared by mixing a monomer base: 2,2-bis[4-(glycidyloxy)phenyl]propane (DGEBA) and a 4,4' diamino-diphenyl sulfone (DDS) curing agent
Summary
Glossy polymer structures, such as thermoset matrices used in Fiber Reinforced Plastic (FRP) composites, are in general prone to solvents absorption. The absorbed water is for instance differentiated into free and bonded water [1,4,5,6,7], with the bonded water chemically reacting with sites of the polymer chain, e.g. by means of hydrolysis Such chemical interactions can modify the network structure, increasing the molecular chain mobility (plasticization), and modifying the free volume, which in turns can interfere with the amount of absorbed free water. Another consequence of such diffusion-reaction changing scenario is the swelling deformation of the network [7,8], which can have important mechanical consequences, especially when the polymer is somewhat constrained to other more inert materials [7]
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