Research On The Contact Mechanical Behavior Of Porous Hydrogel Soft Robots

Hydrogel soft robots often face various contact problems during operation and movement.Hydrogel soft materials have complex mechanical constitutes,and large local deformations are prone to occur under external loads,which makes the mechanical behavior of the hydrogel soft robot in the contact process difficult to predict.At present,there are few theoretical analysis methods for the large deformation contact problem of soft materials.The complex nonlinear characteristics in the contact process make it very difficult to solve the equation.In this paper,numerical and theoretical methods are used to study the contact problem of hydrogels,starting from the material model of hydrogel,large deformation contact theory model,static contact behavior and dynamic contact behavior.The specific research content and conclusions are as follows:(1)Based on the multi-phase and single-phase physical properties of the hydrogel,two material models of the porous medium and super-elastic body of the hydrogel are given.First,the constitutive equation of the model is deduced,the application scope of the model is analyzed,and the specific measures for the application of the model in finite element are introduced.The porous media model is used to calculate the internal liquid flow of the hydrogel under external load,and the influence of liquid flow on the overall load bearing capacity of the hydrogel is analyzed.(2)Based on the classic Hertz contact theory,the incremental contact theoretical model is proposed,which takes into account the influence of hydrogel material nonlinearity and large deformation geometric nonlinearity on the contact process,and uses a discrete method to convert large deformation problems into small deformations.The modified elastic modulus formula is used to calculate the change in the elastic modulus of the hydrogel during the contact process.Then,using the Hertz contact theory calculation formula and the incremental accumulation method to solve the contact force of the large deformation contact problem.The comparison with the finite element calculation results verifies the correctness of the model.(3)The static contact process of hydrogel is studied,and the influence of friction coefficient on the stress distribution and contact state in the contact area is discussed.The results show that the friction coefficient will cause the stress on the contact surface to increase,causing two high stress areas inside the hydrogel,one caused by contact pressure and one caused by friction;in addition,with a small friction coefficient,the hydrogel contact surface.All points on the contact surface are in the limit state of static friction to dynamic friction;while in oblique contact,under any friction coefficient,all points will not be in the limit state of static friction to dynamic friction.(4)Based on the neo-Hookean model,the dynamic contact behavior of the hydrogel was studied,and the classic planar single-rod Painlevé paradox model was used.The study found that there are three contact states of sliding,viscous and separation during the dynamic contact of hydrogels.The change of friction coefficient will cause the change of contact state.When the friction coefficient is greater than the critical friction coefficient,the contact surface will appear viscous,which is manifested by the self-locking phenomenon of the hydrogel rod.Unlike the self-locking situation of the rigid rod,the hydrogel rod will roll when it is selflocking.Friction can also cause structural vibration,and the vibration amplitude is related to the friction coefficient and the negative slope of the tangential velocity.By analyzing the energy conversion process of the hydrogel rod,it is found that the mechanical energy of the system is not conserved.Part of the mechanical energy change is dissipated by friction as power consumption,and part is converted into heat energy due to the energy consumption of the material.