The Vibrational Characteristics And Instability Patterns Of Soft Materials Under Hypergravity

When an object sits in an environment where the gravity is greater than that on the surface of the Earth,it will be affected by the hypergravity or move with the acceleration larger than gravity.Centrifugal hypergravity is one of the important technologies to create artificial hypergravity through high-speed rotation of centrifuge,which is widely used in industrial engineering,biophysics,geophysics,etc.The essence of the impact of the gravitational field on the object is to make the object receive the corresponding physical force.Studies have shown that altering the gravitational field can not only affect the deformation,movement,and evolution of substances but also change the microstructure and mechanical behaviour of materials.As novel smart materials,soft materials play increasingly important roles in daily life,which are often very sensitive to external stimuli.Under the action of hypergravity acceleration,these soft materials can produce large deformation and bucklings,which significantly affect the function and safety of the device.In addition,studying the deformation and instability of soft matter in a hypergravity environment has important theoretical significance for taikonauts ’ life and health assurance.This doctoral dissertation mainly focuses on the theoretical investigation of the vibration characteristics and instability behaviour of soft matter under the hypergravity field,and combines experiments and finite element simulation to verify the validity of the theory.This dissertation mainly includes the following four aspects:(1)Based on nonlinear elasticity and the associated incremental theory,the vibrational characteristics of rotating homogeneous incompressible soft cylinders are investigated.An accurate and effective numerical solution of vibrational characteristics of soft tubes associated with inhomogeneous rotational fields is obtained by combining the state-space method and approximate laminate technique.The results show that the soft tube under hypergravity exhibits nonlinearly expansion and rotation,which further generates anisotropy and gradient distributed equivalent modulus.Moreover,the existence of Coriolis force makes the natural frequencies bifurcate to forward and backward waves with respect to the rotating direction and deviate significantly from each other for non-zero modes.(2)The buckling behaviour of a soft tube attached to a rotating rigid shaft is studied.The nonlinear elasticity theory is adopted to describe the finite deformation of the tube caused by the centrifugal force,and the linear incremental theory is used to describe the instability of the soft cylinder.The state-space method and approximate laminate technique are used again to obtain the critical rotating speed and buckling modes.The study found that for the infinite soft tube,only circumferential instability occurs;for finite length tube,there are two buckling modes: axial instability,mixed axial and circumferential instability.(3)Based on nonlinear electroelasticity theory,the finite deformation,snap-through instability,and electric breakdown phenomenon of the rotating soft dielectric tube subjected to the combined action of an axial pre-stretch and a radial electric field are studied.The nonlinear governing equations for axisymmetric deformations are derived in general.Numerical examples are used to study the influence of rotating speed,electric field,pre-stretch,and geometrical size on the deformation of the tube.In particular,the snap-through instability is well explained by calculating total energy of the system.Moreover,we reveal the transition and competitive mechanism between snap-through instability and electric breakdown and propose a guidance to control defamation of rotating tubes.(4)The Rayleigh-Taylor instability in soft solids under hypergravity are systematically studied by means of experiment,theoretical analysis and finite element simulation.A finite deformation model was developed to predict the onset of instability and the post-buckling patterns of confined hydrogel cylinders.The results are in good agreement with experiments and finite element simulations.In addition,the instability modes of a free-hanging soft cylinder are obtained by performing finite element simulation.The influence of confinement on soft cylinders under hypergravity is investigated.The result shows that the free-hanging soft cylinder is more prone to buckle and exhibits combined fringe instability and Rayleigh-Taylor instability than the confined cylinder.In summary,these investigations quantitatively illuminate the influence of hypergravity on the static and dynamic response of soft materials and explore the electromechanical coupling regulation method of hypergravity sensitive structures.These results not only provide comprehensive theoretical guidance and scientific basis for ensuring the capability of soft material devices under hypergravity but also benefit interpreting natural instabilities associated with hypergravity,designing and controlling surface patterning in industrial manufacturing.