Investigation Of Strain Rate And Temperature Coupled Mechanical Behavior And The Constitutive Laws Of Polymeric Soft Materials

Soft polymers have extensive fields of application,e.g.,impact-resistant energy-absorbing elastic inter-layer material of windshield of aircraft and high-speed rail,intelligent flexible elec-tronic materials,soft-robots,medical silicone and hydrogel,as well as biological tissue in na-ture.Soft materials have a complicate microscopic multi-phase geometric structure,whereby their macroscopic physical and mechanical properties are sophisticated,and therefore determin-ing the macro-microscope constitutive law remains challenging.At the moment,developing a three-dimensional constitutive model to characterise the diverse properties of soft materials is a hot topic studied by domestic and overseas scholars.The aim of this work is to analyse and classify the mechanical behaviour of several typical soft materials from macroscope,to develop a universally applicable constitutive model,failure criterion and evolution equation to reproduce different types of mechanical behaviour,and to explain its mechanism.The contributions of this thesis are:(1)A series of mechanical experiments were carried out on transparent polyurethane,VHB acrylic polymer,and Ecoflex silicone rubber series,including high,medium,and low strain rate ranges tensile failure tests,cyclic loading and unloading tests,relaxation tests,etc.Based on these tests,the common and different behaviour of the mechanical response of these three ma-terials are investigated.It is found that VHB polymer has typical viscoelastic properties.It has obvious strain rate and temperature dependence,and the loading-unloading path could be exactly the same as the primary stretch after unloading? meanwhile,Ecoflex doesn't exhibit significant viscoelastic behaviour and the reloading path is not the same as the primary stretch,belonging to softening behaviour? and polyurethane has both viscoelastic and softening behaviour,where its constitutive law can be regarded as the combination of these behaviours.(2)Based on the systematic experiment results,a temperature and strain rate coupled thermo-hyper-viscoelastic constitutive model is developed with regard to VHB,while a hardness-dependent stress softening constitutive model is developed for Ecoflex series.Com-bining these two types of constitutive laws and based on the concept of stress softening accumu-lation and effective maximum strain Invariant,a time-dependent stress softening and thermo-hyper-viscoelastic model is developed for Polyurethane,the corresponding material parameters are identified,and the adaptability of the model is validated.Our proposed model reproduces the mechanical response of Polyurethane with an acceptable accuracy.This model is able to describe most mechanical behaviour of soft materials such as time- and temperature-dependent behaviour as well as stress softening behaviour.It is believed that this model has a broad appli-cability.This research provides comprehensive experimental data as well as a fairly accurate con-stitutive description of soft polymers,and therefore provides theories for simulation based on Finite Element Method,which offers a economical and efficient research tool of the designing,Manufacturing as well as application of soft polymers.