Damage Analysis Of Automotive Coatings Based On Continuum Damage Mechanics

Research on automotive coatings is of great importance not only because they candecorate vehicles with all kinds of colors but also can protect vehicle from corrosion, beingscratched and stone chipping ect.The main contents of this paper are as follows:The first chapter firstly introduces the significance of investigating automotive coatings,which is research background. Based on the background, a brief history of automotivecoatings, including the classification of coatings, typical materials used in automotivecoatings, the functionalities of different coatings and their relative advantages anddisadvantages, the advances of coating processes and the globalization of automotivecoatings industry, were presented. An extensive literature research was carried out and thestate-of-the-art of automotive coatings was summarized, based on which research problemsand questions were figured out and research contents were arranged.The second chapter the engineering background of the formation of continuum damagemechanics. Comparisons were made between continuum damage mechanics and traditionalcontinuum mechanics (e.g. elasticity, plasticity) and micromechanics (molecular mechanics,quantum mechanics), the advantages and disadvantages of these mechanics were briefed andthe interrelationship was made as well. The most important terminology of continuumdamage mechanics, damage variable (that was defined by various literature differently), wasthen presented. The method for employing the three equivalent assumptions to introducedamage variable into constitutive equations, to predict and calculate the stress and strain ofengineering problems, was introduced at the end of this chapter.The third chapter firstly states that the premise of analyzing the damage and failure ofautomotive coatings is for formulating a constitutive model that is suitable for automotivecoatings. According to the formulating methodology presented in the earlier chapter, damagevariable and other parameters that reflect creep and hardening were proposed to describecumulative effects of microstructural changes in continuum material model. Since there areseveral undetermined material constants in the above mentioned material model,evolutionary programming is introduced in the following section, and by which materialconstants are fitted and determined for five typical automotive body materials. Due to the fact that there are only a few typical material models in most finite element software such asABAQUS, user-defined material model must be implemented into finite element solverthrough the interface (e.g. VUMAT in ABAQUS) of user subroutine that provided by finiteelement software. Consequently, the structure and grammar of VUMAT was introduced inthe following section. Discretion was made by investigating the structure of the proposedconstitutive model, and FORTRAN code for the numerical implementation of the abovematerial model into ABAQUS/Explicit was also developed.The fourth chapter firstly discusses the necessity of finite element modeling of automotivecoatings system. After that the effect of geometry size on coatings performance (load bearingcapacity, mar resistance) was investigated by considering the exclusive structure ofautomotive coatings, and it was found that typical automotive coating system is quitedifferent from TiN/Cu system that has obvious size effects, the determination of geometrysize of automotive coating system hardly affects the prediction of mechanical properties.Based on this conclusion and the geometry feature of automotive coating system (thethickness of coating and substrates is normally suited between20μm~40μm and0.8mm~2mm respectively), a series of finite element analyses were carried out to investigatemechanical properties of automotive coating system. The results show that damagemechanics based material model is not only suitable to compute stress distributions ofcoatings and substrates, but also can be used to predict damage evolution. Subsequently, theinterrelationships between load bearing capacity/mar resistance and coating material/coatingthickness were investigated. It was found that load bearing capacity/mar resistance increaseswith decreasing/increasing coating thickness respectively, and both of the two propertieskeep unchanged even if the coating material is substituted.The fifth chapter summarizes the whole thesis and posts further insights on it. Theresearch contents, methodology, results and research highlights are summarized. Thedrawbacks and omissions of the thesis and further improvements for this research werepresented finally.