Study On Mechanical Behaviors Of Hydrogel And Fiber Network Under Multi-Axial Loading

Hydrogels are gaining its popularity in diverse fields of research for their outstanding physical-chemical properties and the sensitive responses to the subtle change of the external stimuli.Although the mechanical properties and behaviors of hydrogels have been studied by many researchers,most of the work mainly focuses on the uniaxial loading condition.Deformation caused by uniaxial loading is only a specific case among all the possible deformation of hydrogels,which is far from enough to characterize the non-linearity of the material completely.Moreover,hydrogels always work under complex stress in reality,meaning that it is of great value and significance to investigate the mechanical behaviors of hydrogels under multi-axial loading.Polymeric fiber network is regarded as the core structure of the hydrogels,and its shape and the properties of cross-linking may have a decisive influence on the macroscopic mechanical behaviors and properties of hydrogels.Therefore,studying the deformation of the polymeric fiber network enables us to better understand the related microscopic mechanisms of the macroscopic mechanical behaviors of hydrogels.Aiming at the two goals mentioned above,biaxial tension experiments with different loading ratios on PVA hydrogels with diverse fiber contents are conducted,and the deformation of fiber network model as well as the corresponding mechanisms under uniaxial and biaxial loading conditions are studied and explained in this thesis.In the end,a new method is put forward,which manages to separate and quantitatively analyze the contributions by polymeric fibers and water to the total stress of hydrogels.The main contents of this thesis are listed below:(1)Biaxial tension experiments with different loading ratios on PVA hydrogels with various fiber contents are conducted.By the observation and analysis of the experimental phenomenon as well as the results of the simulation of the fiber network model,the influences of fiber contents and loading ratios on the mechanical behaviors of PVA hydrogels along with the corresponding mechanism are discussed.(2)The discrete characteristics of macroscopic mechanical behavior of network models with low fiber contents are discussed,and the corresponding mechanism is analyzed.By exploring the influence of the spatial structure attributes and cross-linking characteristics on axial stress of fibers,the mechanical characteristics of fiber in the loading process are studied.Since the inertia force is small enough to be neglected compared to the axial force of the fibers,the inertia effect of fiber network model is eventually eliminated.(3)Fiber network models with diverse fiber contents are used for uniaxial and equi-biaxial tension.The volume change behaviors of the fiber network models under the two loading cases are compared.Through the correlation analysis of the volume change and fiber alignment characteristics,it is proved that the fiber alignment can reflect the relation between the deformation behaviors of the fiber network on both the macroscopic and the microscopic scales.(4)A new method based on the single-phase fiber network model is proposed to simulate the mechanical behavior of the two-phase hydrogel.The effect of polymer fiber and water on the macroscopic mechanical properties of the hydrogel under uniaxial tension and uniaxial compression is studied respectively.