Probabilistic modeling of fatigue related microstructural parameters in aluminum alloys

This paper presents the results of probabilistic modeling of the fatigue related microstructural parameters in unclad 2024-T351 aluminum sheets. The statistical distributions of the constituent particle size, which were obtained from metallographic measurements from polished surfaces, were determined by graphical goodness-of-fit tests. The distributions of the crack-nucleating particle sizes were determined using the data measured from various fatigue fracture surfaces. Initially, an extreme value theory based model was investigated to correlate the overall particle distribution with its fatigue subsets. Furthermore, a new Monte Carlo simulation was developed to determine the fatigue subsets using the microstructural parameters such as particle size, grain size, and grain orientation distributions, in association with qualitative criteria on fatigue crack nucleation and growth mechanisms.     

Determination of the length of deformation waves in the high-rate deformation of metals

A simple method has been devised for determining the length of a deformation wave in a solid. The method is based on the phenomenon of metalcutting and makes use of the interference of coherent waves. A hypothesis regarding the wave propagation of mechanical and thermal energy is proven experimentally, and it is established that the frequency of these oscillations increases with an increase in deformation rate.Translated from Problemy Prochnosti, No. 3, pp. 88–95, March, 1996.     

Determination of the energy of electroplastic deformation of metals

On the basis of the well-known mechanism of electroplastic deformation, we suggest a method for determination of the fraction of energy of pulses of the electric current directly spent on the work of plastic deformation. The experimental data presented in the work confirm the validity of the proposed approach. The present work was partially financially supported by the International Science Foundation and the Ukrainian Government (Grant UBW 200). Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 38–43, July–August, 1997.     

Determination of optical constants and inhomogeneity of optical films by two-step film envelope method

A simple two-step film envelope method has been proposed to determine the optical constants and small inhomogeneity of the optical films, which uses maximum envelopes and minimum envelope of the normal incidence transmittance of the two-step film. The two-step films were prepared by stopping the deposition process in the middle of the designed sputtering time, and then, after a full cooling down to room temperature, repeating the same deposition process again to complete the whole preparation of the films. The optical constants of Nb₂O₅-TiO₂ mixed films were calculated by two-step film envelope method and traditional envelope method. The experimental results demonstrate that the average refractive index and extinction coefficient calculated by two-step film envelope method are more accurate than those calculated by the traditional envelope method.     

基于拉普拉斯网格变形的三维植物叶片建模

论文提出一种基于拉普拉斯网格变形的三维植物叶片交互式设计方法。该方法以叶片轮廓及叶片主脉中轴点数据输入并生成网格曲面,通过拉普拉斯网格变形技术对叶片的曲面网格进行交互式编辑。轮廓中轴数据点既可以通过三维数字化获得,也可以根据叶脉形态计算得到。实验证明,该方法具有较好的普适性,变形计算快速,能够达到实时交互设计的需要,所生成的叶片不仅能够很好地保持叶片的面积特征,同时在形态上具有较强的真实感。     

Fatigue fracture of SnAgCu solder joints by microstructural modeling

The ongoing miniaturization trend in the microelectronic industry enforces component sizes to approach the micron, or even the nano scale. At these scales, the underlying microstructural sizes and the geometrical dimensions are comparable. The increasing influence of microscopic entities on the overall mechanical properties makes conventional continuum material models more and more questionable. In this study, the thermomechanical reliability of lead-free BGA solder balls is investigated by microstructural modeling. Microstructural input is provided by orientation imaging microscopy (OIM), converted into a finite element framework. Blowholes in BGA solder balls are examined by optical microscopy and a statistical analysis on their size, position and frequency is conducted. Combining the microstructural data with the appropriate material models, three dimensional local models are created. The fatigue life of the package is determined through a critical solder ball. The thermomechanical reliability of the local models are predicted using cohesive zone based fatigue damage models. The simulation results are validated by statistical analyses provided by the industry.     

Analytical modeling of fatigue crack propagation in metals coupled with elasto-plastic deformation

In the present investigation, the G _max criterion, which is based on the elastic strain energy principle, is extended to study the fatigue crack growth characteristics of mixed mode cracks. A modification has been made to this criterion to implement the plastic strain energy and, hence, a new elasto-plastic energy-based model is presented. Subsequently, the proposed model is employed to predict fatigue crack growth in rectangular steel plates under complex stress states. The results obtained using the elasto-plastic energy model proposed are compared with those obtained using the commonly used Paris law and our experimental data.     

Coupled simulation of microstructural formation and deformation behavior of ferrite–pearlite steel by phase-field method and homogenization method

A coupled simulation by the phase-field (PF) method and the finite element method based on the homogenization theory (FEH) is developed to predict the microstructure formations and mechanical properties of ferrite–pearlite steels. The formation of the phase during the isothermal transformation is simulated by the PF method. Furthermore, the FEH analysis is performed to clarify the effects of the predicted microstructure on the deformation behavior of the steels. In order to link to the FEH analysis, the microstructure in the steel is described by the representative volume element (RVE) based on the results of the PF simulation. The results reveal that although the macroscopic stress–strain relationship is mainly characterized by the volume fraction of the constituent phase, the localization of plastic strain is reduced due to the fine-grained phase. This numerical model provides a systematic way of predicting the mechanical properties of steel depending on the microstructure.     

Investigation of deformation and failure of the laminated composite Cr-V by the method of computer modeling

The article presents a further development of the mathematical method [N. V. Shadchina et al., Probl. Prochn., No. 2, 88–91 (1990)] that makes it possible to model the processes of deformation and failure of composite materials on a personal computer. The strength characteristics of laminated materials Cr-V are calculated on the basis of the physicomechanical and structural properties and with a view to their statistical scatter and the properties of the interlay er forming during the production of the composite. It was established that when the layers are thinner, the dependences of ultimate strength and yield strength of the composite material on the mean thickness of the layers are transformed into dependences Petch—Hall-type.Translated from Problemy Prochnosti, No. 2, pp. 47–54, February, 1996.     

Nonlinear microstructural constitutive equation of nanocrystalline metals

Summary. Based on experimental observations, nanocrystalline materials are modeled as composite systems in which the amorphous interfacial phase is treated as the matrix, whereas the nano-scale single crystals are modeled as inclusions. Generally speaking, the elastic moduli of nanoscale crystals are higher than those of the amorphous matrix phase, and the deformation mechanism of nanocrystalline materials depends heavily on the size of the crystals. For conventional macro size crystal materials, such as coarse-grained polycrystalline materials, the deformation mechanism due to dislocation movement is dominant. When the crystal size is reduced to a certain critical value, plastic deformation is caused by shear banding in the amorphous matrix. In order to model such a deformation mechanism in nanocrystalline materials, constitutive equations are established based on internal variable theory. The proposed model reveals the relation between the yield strength and the grain size of the material.     

Processes of microstructural evolution during superplastic deformation

Microstructural changes occurring at the surface and in the bulk of superplastically deformed materials have been considered. Surface studies showed formation of macroscopic surface steps and fibers, which determine surface roughness and can affect corrosion properties, respectively. Bulk microstructural changes include morphological and crystallographic changes, as well as defect accumulation. Phase/grain growth, phase spatial distribution, and phase/grain shape changes control the morphology of the phase constituents. Weakening of preexisting texture and an increase in the ratio of high angle grain boundaries determine crystallographic changes. Defect accumulation is related to cavity formation; density of lattice dislocation is superplastically deformed materials is low. Various explanations proposed for these processes of microstructural evolution in superplastic materials are considered. It is shown that these processes are closely related to cooperative grain boundary sliding—that is, the sliding of grain groups.     

Determining the deformation resistance of metals in nonisothermal deformation

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 120–123, May–June, 1992.     

Characterization of nanostructured metals produced by plastic deformation

Nanostructured metals produced by plastic deformation often exhibit characteristic structural features such as elongated morphology, a bimodal misorientation distribution and the presence of interior dislocations. The characterization of these parameters is demonstrated by results of TEM and EBSD analyses of pure Ni processed by high pressure torsion (HPT) and pure Al processed by accumulative roll bonding (ARB). Care needed in selecting sample plane and characterization technique is discussed.     

The effect of microstructural inhomogeneity on the growth paths of surface-cracks in copper processed by equal channel angular pressing

The growth behavior of cracks is monitored on specimens of ultrafine grained copper produced by equal channel angular pressing. Temporary retardation of crack growth under low stress amplitudes occurs when the crack length reaches about 0.1 mm, but there is no similar retardation at high stress amplitudes. Dependent on stress amplitude, different crack growth path morphologies develop. Analysis of the fracture surfaces is conducted by scanning electron microscopy, showing planer, granular and striated surfaces. The physical background of growth path and fracture surface formation is discussed by considering crack growth mechanism and microstructural inhomogeneity.     

Determination of the state parameters of explosive detonation products by computational inverse method

A computational inverse method is presented to determine the state parameters of Jones-Wilkins-Lee (JWL) equation of explosive detonation products based on cylinder test. In this method, the inverse problem of identifying the JWL parameters is formulated through minimizing the errors of the measured radial displacements on the cylinder surface and those computed by the forward solver. An available numerical model simulated by finite element method is built for the sake of obtaining results using the given JWL parameters. Because of the difference of coordinate systems between experiment and numerical model, it is necessary to conduct the transformation between the two coordinate systems. The sensitivity analysis method is adopted to evaluate the influence of the JWL parameters on the responses and find out the parameters those are suitable to be identified. In order to improve the computational efficiency, radial basis function approximate model is constructed to replace the time-consuming numerical model. In the process of constructing approximate model, the truncated singular value decomposition method is used to deal with the ill-condition of the model. At last, the intergeneration projection genetic algorithm is adopted to identify the parameters. The numerical results demonstrate that the proposed inverse method is potentially available to effectively identify the JWL parameters.