Theoretical Study On Mechanical Property Of The Biological Soft Tissue

Soft-tissue-mechanics,as a branch of biomechanics,mainly investigates the stress-strain relationship and stress-growth relationship of bio-soft-tissues and the relationship between the changes of mechanical properties of bio-soft-tissues and illnesses.The research on mechanical property of bio-soft-tissues is of great significance to the understanding of physiological and pathological process of soft tissues.Bio-soft-tissues are made of several basic materials like collagen fibers,elastic fibers and glycosaminoglycans in spite of their various structures,shapes and physical functions.The mechanical property of bio-soft-tissues depends on the properties,structures and interaction of these basic materials.In this dissertation,micro-mechanical models and four-element viscoelastic structural models with two characteristic times for bio-soft-tissues are developed according to their micro-structure,and the stress-strain relationship,stress relaxation and creep behavior of bio-soft-tissues are investigated.The comparison with the experimental results proves the effectiveness of the developed models in this dissertation.Firstly,the homogeneous cocurrent and homogeneous incongruous structural mechanical models of bio-soft-tissues are established according to the micro structure of parallel collagenous tissue.Then the stress-strain relationship of epimysium in rat tibia is investigated by using the established models,and the influence of the biomechanical parameters on the strain stiffenning behavior is discussed.The investigation indicates: in repeat unit,the increasing repetition of sinusoidal collagen fibers leads to increment of elastic modulus and thus higher stiffness of epimysium.In collagenous tissues with parallel fibers,the increment of elastic modulus of collagen fibers and shear modulus of ground substance leads to obviously higher stiffness of the collagenous tissues.The nonlinear behavior of stress-strain relationship of bio-soft-tissue is well described by the established theoretical models.Secondly,a four-element viscoelastic structural model made up of two dashpots and two linear springs was established and two characteristic times are introduced,from which the constitutive relation of bio-soft-tissues was derived and the stress relaxation,strain stiffening and creep behavior of bio-soft-tissues are studied.The influence of the bio-mechanical parameters and model parameters of bio-soft-tissues on these mechanical properties are discussed in detail.The research result indicates that the stress relaxation behavior of bio-soft-tissues mainly depends on the elastic modulus of ground substance and the relaxation time.As the increment of relaxation time,the speed of stress relaxation slows down and the tissues need longer time to reach steady state.No matter how the physical parameters of collagen fibers and ground substance are selected,the tissues reach equilibrium state when the observation time is 7 times longer than the relaxation time.Stress relaxation could not be obviously influenced by elastic modulus of collagen fibers.The low strain rate could effect the initial stress of tissues,but merely effects the process of stress decay,whereas the high strain rate could effect both the two properties.The strain stiffening property of bio-soft-tissues mainly depends on the gradual aggregation behavior and elasticity modulus of collagen fibers,and are partly effected by the interaction of ground substance and collagen fibers and the speed of strain.The creep behavior of bio-soft-tissues is influenced by elasticity of collagen fibers and viscosity of ground substance.As the increment of viscosity factor,creep process slow down,and the creep behavior takes longer time to reach equilibrium state,which leads to the increment of steady state strain.Shape parameter and scale parameter have certain effect on creep.In low strain level,there is positive correlation between shape parameter ? and aggregation of collagen fibers,whereas in high strain level,there is a negative correlation between ? and aggregation of collagen fibers.As time goes on,more collagen fibers are aggregated to the direction of external load and straitened after reorientation,which leads to the tissues reach steady state in shorter time.With the increment of scale parameter,the aggregation speed of collagen fibers under stress slows down,which leads collagen fibers to generate more strain in axial direction and reach higher steady strain value.Lastly,the viscoelastic structural model made up of four discrete elements is generally discussed.Two methods were selected for all possible four-element viscoelastic structural model,to derive their constitutive relation,stress relaxation and creep function.Direct integration method resulted in stress-strain relationships,creep function and relaxation function with similar mathematical form.Numerical calculation indicates that the results from all models have the same function trend with small quantitative difference,which shows that all the models obeyed universal mechanical laws.For differential equation method,all possible models have unified function form for constitutive equation,stress relaxation and creep,with only difference for model parameter,which shows that these models also describes the same mechanical laws.