Key Engineering Materials Vols. 345-346

Abstract: Recent advances in nanotechnology have revealed numerous new methods of manufacturing functionally graded coatings and materials, but progress in this field is limited by the lack of knowledge about the mechanical behavior of such structures. Existing models of the mechanics of layered structures are not generally adequate for this purpose, since functionally graded structures can exhibit both qualitative and quantitative behavioral differences in comparison with homogeneous or layered structures, particularly if there is a significant gradient of elastic properties in the coating. In applications, interest is focused mainly on the deformation fields and stresses inside the inhomogeneous material caused by the contact tractions. Stresses at the interface between the functionally graded coating and the elastic half-space are of particular interest because of their influence on the propagation of cracks and other defects on this interface. Shear stresses at this interface associated with rapid variation in elastic properties with depth are particularly dangerous because of potential delaminations. In their work the authors: • develop a precise mathematical model and of the computational methods which makes it possible to achieve stable numerical results while analyzing the mechanical properties of functionally graded coatings; • study the variation effect in elastic properties on the maximum stresses in the surface layers of materials with functionally graded coatings caused by indentation.

Abstract: Thickness effect is studied experimentally. At free surface of the specimen, shear lip fracture pattern appears, though dimple fracture pattern is observed inside of the specimen. The area of shear-lip fracture changes due to the change of the specimen thickness. In this study, experimental study is conducted by changing specimen thicknesses. Fracture surfaces are precisely observed using SEM, and dimple patterns on them are observed. At the free surface, very narrow no-void area is observed. It is also found that many voids are nucleated in shear-lip fracture area. FEM simulation is carried out using Gurson’s constitutive equation. It is found that shear-lip type fracture is simulated near free surface area by this method. The results show similar tendency with the experimental observation.

Abstract: In this paper, a fast Fourier transforms (FFT)-based spectral analysis method (SAM) is proposed for the dynamic analysis of spectral element models subjected to the non-zero initial conditions. To evaluate the proposed SAM, the spectral element model for the simply supported Bernoulli-Euler beam is considered as an example problem. The accuracy of the proposed SAM is evaluated by comparing the dynamic responses obtained by SAM with the exact analytical solutions.

Abstract: Forging is applied for many industrial fields. Of course, there is no exception in nipple of automotive hose. Finding method of forging process is metallic stress analysis, and we can predict this possibility by finite element forging analysis. But there are many manufacturing procedure after forging, and additional heat treatment or coating can vary metal texture. So, in this research, we focus on the measuring and analysis of plastic residual stress distribution at overall manufacturing process. First step, we measured real residual stress at each forging process by X ray diffract meter from raw material to final product. Second step, we simulated parts-assembly process by nonlinear finite element analysis. In this step, we can prove how Zn–Ni coating is more contributable to metal strength than Zn coating. And we can conclude for robust design that manufacturing process analysis must be observed carefully from raw material to final manufacturing state.

Abstract: In this study, to determine incremental, perturbed displacement fields in periodic elastic-viscoplastic solids, an incremental homogenization problem is fully implicitly formulated using a linearized constitutive relation, a micro/macro-kinematic relation, and a stress balance equation. It is shown that the homogenization problem can be iteratively solved with quadratic convergences by successively updating strain increments in unit cells, and that the present formulation allows versatility in the initial setting of strain increments in contrast to Terada-Kikuchi (2001) and Miehe (2002). This homogenization algorithm is then examined by analyzing a holed plate, with an elastic-viscoplastic micro-structure, subjected to tensile loading. It is thus demonstrated that the convergence in iteratively solving the homogenization problem strongly depends on the initial setting of strain increments in unit cells, and that quick convergences can be attained if the initial setting of strain increments is appropriate.

Abstract: Formability and springback of the automotive aluminum alloy sheet, 6K21-T4, in the sheet forming process were numerically investigated utilizing the combined isotropic-kinematic hardening law based on the modified Chaboche model. To account for the anisotropic plastic behavior, the non-quadratic anisotropic yield stress potential, Yld2004-18p was considered. In order to characterize the mechanical properties, uni-axial tension tests were performed for the anisotropic yielding and hardening behavior, while uni-axial tension/compression tests were performed for the Bauschinger and transient behavior. The Erichsen test was carried out to partially obtain forming limit strains and FLD was also calculated based on the M-K theory to complete the FLD. The failure location during simulation was determined by comparing strains with FLD strains. For verification purposes, the automotive hood outer panel was stamped in real. After forming, the amount of draw-in, thinning and springback were measured and compared with numerical simulation results.

Abstract: A hybrid stress determination around circular and elliptical holes utilizing photoelastic phase-shifting and nonlinear least-squares methods is presented. The method was demonstrated by calculating fringe orders of distant points along straight lines using 8-step phase-shifting method. The data was used to evaluate the coefficients in the complex stress functions for hybrid analysis. Tangential stresses around the boundary of the holes were obtained using conformal mapping technique. Different number of terms in a power-series representation of the complex type stress function was tested to qualitatively observe the effects of varying stress field. Actual fringes were related with the reconstructed and sharpened fringes along with the change in the number of terms, m. Good agreement was obtained when m in stress functions was equal to nine. At high stress concentration, the result obtained from the hybrid method agrees with FEM by two and five percent for circular and elliptical hole, respectively. The results show that the established numericalexperimental method for stress analysis is considerably reliable.

Abstract: The evolution of hot rolling texture in FCC materials has been simulated numerically using a visco-plastic self-consistent (VPSC) polycrystal model. A finite element (FE) analysis with ABAQUS/StandardTM was conducted to evaluate the deformation gradients during hot rolling deformation. In order to capture crystallographic rotation during hot rolling deformation, an octahedral slip system was considered in a microscopic hardening model. The FE analysis with the VPSC polycrystal simulations successfully predicted the inhomogeneous texture development through the thickness direction in the hot-rolled Al-5wt%Mg alloy sheets.

Abstract: The pedometer, an objective assessment of measuring step counts, has often been used to motivate individuals to increase their ambulatory physical activity. Minimal contact pedometer-based intervention (MCPBI) is gaining in popularity because they are simple and inexpensive. MCPBI is based on self-monitoring by the participants; however, one limitation of using the self-monitoring approach was the participant attrition (i.e., dropout), which makes it difficult to achieve the successful intervention. A new algorithm for pedometer-based intervention, the systematic-monitoring based on conditional feedback, was designed to increase awareness and allow participants to more successfully attain their step goals. Thus, the purpose of this study was to examine the effect of the systematic-monitoring based on conditional feedback algorithm on 10,000 step goal attainments. The study result can be used to design more comprehensive pedometer-based physical activity interventions to increase individuals’ overall health status.