Energy-based Method On The Fatigue Properties Of Metal Material

The fatigue of metals is corresponding to the changes of mechanical properties under alternating load.As one of the most common failure modes in engineering,fatigue of metals has been widely studied by industry and academia.With the development of technology,more materials or structures are required to work under multi-physical fields and extreme environments.Whether the traditional empirical fatigue prediction method is still applicable to new materials and extreme environments still requires further study.In this work,the fatigue crack initiation,propagation and life prediction of metallic materials are invesitigated theoretically in the framework of thermodynamics and statistical physics.A creep strain prediction model is developed combined with fracture mechanics and micromechanics.In addition,the fatigue life of Sn-Ag-Cu solder joints in extreme environments is studied experimentaly.The developed model is validated by the experimental data of Sn-Ag-Cu solder and other metal materials.The fatigue failure of metallic materials contains dislocations,lattice defects accumulation and interactions.Fatigue is a dynamically changing process of the Gibbs free energy,the change of defects in the materials could be considered to be a generalized form of phase transformation.The fatigue crack nucleation cycles can be obtained when Gibbs free energy attains the maximum value.The prediction of developed model is compared with experimental data for different metal materials.The phase transformation theory is applied to predict the fatigue crack propagation.The new crack growth rate prediction model is proposed based on the dynamic energy balance relationship in the process of macroscopic crack growth.A new energy parameter is proposed and its relationship with the stress intensity factor is studied.The predicted fatigue crack propagation rates are compared with experimental data for different metallic materials,the results show that the proposed model can predict fatigue crack nucleation and propagation with reasonable accuracyThe fatigue properties of SAC305 solder joints in electronic packaging were investigated experimentally at different aging temperatures and aging time.The possible service environment temperature of solder joints is analyzed.The aging temperature experiments were carried out from-196?(close to the lunar night temperature)to 190?(close to the melting point of SAC305 solder).Then both uniaxial-tension and low-cycle fatigue experiments were carried out for the aged solder joints.A modified Weibull distribution model which consider the effect of aging temperature is proposed to analyze the discreteness of expereiment data.According to the experimental results,the maximum tensile strength of SAC305 solder joints is obtained after 24 hours aging at room temperatures.The maximum tensile strength and longest fatigue life of SAC305 solder joints is obtained under 72 hours aging treatment at-196?.The longest fatigue life of SAC305 solder joints for different aging time at-196?is obtained when aging time is 72 hours.With longer aging time,the fatigue life of SAC305 solder joints decreases.Microstructure analysis of the failure samples shows that the tensile plasticity of SAC305 solder is related to the content of Ag₃Sn in the material,excessive content of Ag₃Sn may cause brittle fracture.The volume of Cu₆Sn₅ in SAC305solder joints increases with the increasing of aging temperature.The relation between the thickness of intermetallics and aging temperature is obtained by analyzing the thickness of intermetallics at different aging temperatures.The tensile fracture mode of SAC305 solder joints at-196?aging(completely brittle fracture)is totally different from 190?aging(ductile fracture).Aging experiments on SAC305 solder joints under different conditions have provided some new recognition of mechanical properties of materials at extreme-low temperatures.A normalized equivalent initial flaw size method is proposed based on the updated Kitagawa–Takahashi diagram considering the small crack growth behavior,which is essential to accurately predict the fatigue crack initiation and propagation.Meanwhile,the updated Kitagawa–Takahashi diagram indicates that small cracks can propagate even with an applied stress below the fatigue limit,which is consistent to the experimental observations.A new fatigue life prediction model is proposed by considering the fatigue crack growth from the normalized equivalent initial flaw size to normalized final crack length when the failure occurs.The parameters of the proposed model have clear physical meaning and can be determined from experiments.Experimental data for aluminum alloys and steels under different test conditions are adopted to verify the proposed model,the theoretical predictions show good agreement compared with the experimental results.Fatigue damage is an irreversible progression which can be represented by the entropy increase,and it is well known that the second law of thermodynamics can describe an irreversible process.Based on the concept of entropy,the second law of thermodynamics can provide the changing direction of system.In the current study,a new entropy increment model is developed based on the frame work of continuum damage mechanics.The proposed model is applied to determine the entropy increment during the fatigue damage process.Based on the relationship between entropy and fatigue life,a new fatigue life prediction model is proposed with clear physical meaning.To verify the proposed model,eight groups of experiments were performed with different aging and experimental conditions.The theoretical predictions show good agreement with the experimental data.The developed theory is extended to the creep process,the increasing of entropy with normalized creep time during creep process is studied.An empirical formula is proposed to describe the relation between entropy increment rate and normalized creep time.A new creep strain prediction model is proposed and benchmared with experimental data of different metallic materials.In general,the theoretical predictions show good agreement with the experimental data.