A Study On The Hme-Coupled Constitutives And Fractures With Engineering Applications

In recent years,polymer has been widely used in engineering applications,including plastic packages in producing micro electric packages,and adhesive joints in aerospace engineering etc. As the polymer is hydrophilic,it will absorb the moisture in high humid environment. The process accelerates as the temperature is rising. Recently,the failure caused by moisture and heat was often observed in package solder reflow and adhesive joints which absorbed moisture. As experiments reveals,the moisture diffusion,heat conduction and mechanical response in polymers are mutually affecting behaviors. A fact that stimulates the effort in developing a set of full coupling constitutive relations with which will have the problem modeled. For the polymer showing time-dependent character in many situations,the viscoelasticity should necessarily be taken into account in the general model. Further,as the conventional J integral is no longer valid for hydrothermal materials cases,investigations on new energy-type fracture criteria are specifically needed.In the present thesis,the constitutive equations of the coupling problem involving heat and moisture effects (HME) were developed on continuum thermodynamics basis,in virtue of Helmholtz free energy. In the endeavor just mentioned,a group of internal variables was introduced to characterize the viscous behaviors of polymers. For isotropic cases,the specific coupling constitutive equations were degenerated from those for general cases thus obtained in the context. With the stress (strain) and coupling coefficient being ignored,the coupling heat conduction equation and the coupling moisture diffusion equation would degenerate to Fourier heat conduction model and Pick diffusion model respectively.In this thesis,the modified critical fracture resistance was defined by excluding the viscoelastic dissipation,which is related to the intrinsic fracture energy. According to the energy balance considerations,the fracture driving energy is uniquely provided by free energy. Hence,it was named free energy release rate in this thesis in tribute to the concept of strain energy release rate. The expression of free energy release rate was derived in this thesis as well. As revealed,it is a path -independent integral and was extended to the multi-field of extensive quantity. With the viscous dissipation being ignored,the free energy release rate would be equivalent to strain energy release rate. As such,the expression of elastic strain energy release rate,which involved the hydrothermal effects,was derived. And,it was employed for delamination analysis of electric packages in the subsequent context.The "popcorn" fracture analysis was studied by introducing reasonable simplifications and taking advantage of the strain energy release rate expression involving the hydrothermal effect. According to the result of analysis,it is advisable to invoke the general evaporation model should the avoidance of the overestimation on the strain energy release rate for very thin package cases be desired. Another important conclusion the present study reached is that the controlling of diffusion coefficient DO and evaporation coefficient F0 is efficient for the deduction of the possibility of the "popcorn" delaminations,in specific with controlling evaporation coefficient.In the appendix,the author of the present thesis included the three parameters of Schapery's viscoelastic model based on the work of Roy et al. In this aspect,FEM program was developed for implementation of the modified model. By use of this FEM program,analysis of the moisture diffusion in adhesive joint was also conducted.