Study On The Mechanical Properties Of Urea-formaldehyde Resin Microcapsules

Generally,urea-formaldehyde(UF)microcapsules filled containing healing agent,such as epoxy resin,are applied different self-healing systems.In the present study,these microcapsules are used for achieving self-healing in cementitious composites.On the one hand,given that the preparation,storage and application of microcapsules they have to be kept mechanically and chemically stable,and the shell structure must maintain intact during the process of mixing,vibration and curing.On the other hand,as the starting and propagation of microcracks,until they penetrate the microcapsules which releases healing agent into the crack plane through capillary action,finally the self-healing process depends on polymerization reaction between the microcapsule contained healing agent and catalyst.Consequently,the mechanical properties of microcapsules play a key role in achieving self-healing of cementitious composites.In this study,UF microcapsules containing the epoxy resin E-51 and butyl glycidyl ether(BGE)were synthesized using an in situ polymerization method.The elastic properties of a single UF microcapsule,including Young's modulus(E)and hardness(H),were determined through a nanoindentation technology based on the elastic contact theory using a Berkovich indenter.Moreover,to investigate the structural effects of microcapsules,a diamond plate indenter was used,and the Young's modulus of the microcapsule wall was calculated through the ordinary least square optimization method according to three analytical solutions on the basis of thin shell theory,namely Reissner,Pogorelov,and Lukasiewicz methods.The results of the contact and thin shell theory were quite similar.In addition,the finite element method was applied to simulate the mechanical response of the microcapsules using the results obtained.Additionally,sinking-in and piling-up are discussed for nanoindentation using Berkovich indenter,and the mechanical responding of hollow and solid spherical shell are analysed by finite element method(FEM).When the deformation was large(indentation depth >900nm),the structural effect became significant.To determine elastic-plastic parameters for a microcapsule,such as Young's modulus,yield stress,and hardening coefficients,based on different mechanical constitutive models,parameters identification needs to be implemented using inverse analysis.In the present study a new approach by coupling finite element method and optimization procedure was proposed to determine the constitutive parameters of urea-formaldehyde microcapsules.In this study three types of elastic-plastic constitutive models were considered,namely,power law hardening model,elastic-perfectly plastic model and elastic-perfectly plastic with linear hardening model.The nonlinear optimization algorithm is applied to find the minimum or maximum of the multivariable objective function,which is defined as the norm of the difference between the numerical and experimental results.The efficiency and robustness of the proposed method are verified with different initial values,numbers of elements,different Possion's ratio and tangential contact friction coefficient.The force-displacement curves from numerical simulation show a good agreement with the experimental data,which indicates the proposed approach and the mechanical parameters determined are reliable.Specifically,the optimization results(Young's modulus and yield stress)and dimensionless analytical solutions for elastic-perfectly plastic model are compared.Studying the long-term mechanical properties of UF microcapsules is necessary since organic polymers have viscoelastic behavior.UF microcapsules show a force relaxation after the process of compression and holding tests.3-parameters and 5-parameters Maxwell models and Burger model are applied for understanding the viscoelastic behavior of UF microcapsules,and characterizing the viscoelastic responding quantitatively.