Flaw Sensitivity And Tensile Fatigue Of Poly(Vinyl Alcohol) Hydrogels

Tensile fatigue behavior is commonly overlooked as researchers pursue the toughest hydrogels. This work describes a poly(vinyl alcohol) (PVA) hydrogel prepared through freezing-thawing (FT) processing to achieve varied monotonic strength and toughness. The monotonic tensile responses of relatively strong and weak versions of the hydrogel are studied with cylindrical hole and crack-like flaws of different sizes to develop an understanding of monotonic strength in the presence of two different, extreme defect types. The monotonic strength of the samples with cylindrical defects is reasonably predicted using nominal stress which accounts for a loss of load-bearing area, while linear-elastic fracture mechanics gives a first-order approximation of the impact of crack-like flaw size on monotonic strength. A subset of key defected samples are further subjected to cyclic loading and fatigue failure at varying stress amplitude. The cylindrical defect samples outperformed cracked samples in fatigue, and the utilization of four FT cycles instead of two improved both monotonic toughness and fatigue properties. This work represents the first tensile fatigue analysis on defected hydrogel materials, sheds light on the behavior of hydrogels in cyclic loading environments, and evaluates both the monotonic toughness and fatigue behavior of soft materials with and without defects.