Key Engineering Materials Vols. 452-453

Abstract: Multiple circular inclusions exists widely in natural media, engineering materials and modern municipal construction, and defects are usually found around the inclusions. When composite material with multiple circular inclusions and a crack is impacted by dynamic load, the scattering field will be produced. The problem of scattering of SH waves by multiple circular inclusions and a linear crack is one of the important and interesting questions in mechanical engineering and civil engineering for the latest decade. It is hard to obtain analytic solutions except for several simple conditions. In this paper, the method of Green’s function is used to investigate the problem of dynamic stress concentration of multiple circular inclusions and a linear crack for incident SH wave. The train of thoughts for this problem is that: Firstly, a Green’s function is constructed for the problem, which is a fundamental solution of displacement field for an elastic space possessing multiple circular inclusions while bearing out-of-plane harmonic line source force at any point: Secondly, in terms of the solution of SH-wave’s scattering by an elastic space with multiple circular inclusions, anti-plane stresses which are the same in quantity but opposite in direction to those mentioned before, are loaded at the region where the crack is in existent actually; Finally, the expressions of the displacement and stress are given when multiple circular inclusions and a linear crack exist at the same time. Then, by using the expression, an example is provided to show the effect of multiple circular inclusions and crack on the dynamic stress concentration.

Abstract: Cracks in piezoelectric solids have been subject to fracture mechanical investigations for more than 30 years. In the early years of research, boundary conditions at crack faces have been adopted from pure mechanical systems assuming traction free boundaries. From the electrostatic point of view, cracks have been assumed to be either free of surface charge or fully permeable. Later, limited permeable crack boundary conditions have become popular among the community, however still assuming traction-free crack faces. Recently, the theoretical framework has been extended including electrostatically induced mechanical tractions into crack models yielding a significant crack closure effect. However, these models are still simple neglecting e.g. piezoelectric field coupling. As one consequence, the tractions do not depend on the direction of the electric field with respect to the direction of material polarization. Here, an extended model is presented, entirely accounting for the piezoelectric coupling effect. In this case, the crack closure effect depends on the electric field direction. Additionally, the model yields a new shear effect.

Abstract: The solution of heat transfer problems for functional graded materials (FGMs) by smoothed particle hydrodynamics, in which the thermal conductivity is a function of the spatial coordinates and the temperature, is discussed for both steady and non-steady problems under various boundary conditions. The boundary is treated using the corrective smoothed particle method to heighten the accuracy. Several calculations are performed to test the validity of the formulation. As an example of practical application, the problem of FGM cylindrical plates subjected to thermal shock is calculated, in which the thermal conductivity is temperature dependent and the heat transfer coefficient is varied in radial direction.

Abstract: In this study, we propose a simplification scheme for modeling a complex bellows structure. Using 3-dimensional finite element analysis, vibration modes and natural frequencies are analyzed. The analysis results are compared with those measured by telemetry system of acceleration. It is found that bending mode of vibration can be activated even a low operation frequency and this leads to uneven distribution of stress. The uneven distribution of stress can be a possible cause for the early failure of a bellows with a large diameter.

Abstract: Fiber reinforced concrete (FRC) exposed to extremely elevated temperature is of great concern in tunnel structures, for example. In the paper a study on spalling on the face of FRC block being subjected to fire of 12000C in duration of 2 hours is to be presented from both theoretical and experimental views. The experiments have been conducted in a furnace its dimensions are to be specified in the paper and the theoretical considerations start with a discrete element method based on free hexagons. The latter method assumes a “soft contact” between hexagonal particles, i.e. springs simulate either tensile and shear contacts in the normal concrete or FRC, where fibers over-bridge possible cracks appearing during cooling process after removing the source of fire. Since the experimental blocks are loaded only by change of temperature and no other type of loading is considered (volume weight of the block is neglected), the “soft contact” is fully sufficient to describe the mechanical behavior of the material. The results from numerical procedure and experimental are compared and the mechanical properties of the material are tuned accordingly.

Abstract: A variation of temperature by sunlight acting on a RC roof slab causes a change of stress in concrete since it expands during summer and is compressed during winter. This behavior repeats annually and affects structural capacity of member for both serviceability and ultimate level. In this paper, a variation of cyclic temperature loading is calculated by analyzing the weather data of Korea for 20 years. In addition, an experimental work is planned to find the long term effect of temperature change. Four RC slabs are made with same dimension. Test parameters are loading duration (10, 20, 30 years). Observation of stiffness variations according to cyclic loading duration shows that the serious stiffness drop happens after 10 year's cyclic loading at summer while after 30 year's loading at winter. From the failure test, maximum strength of specimen that experienced repeated preloading was approximately 12% less than standard specimen without any repeated preloading.

Abstract: The finite elements are extensively utilized to solve various problems in engineering fields with the growth of computing technologies. However, there is a lack of methodology for analyses of huge assembled structures. The mechanics on the interface of each components, for instance, contact, bolt joint and welding in assembly is a key issue for important huge structure such as nuclear power plants. On the other hand, it is well known that as finite element models become large and complex, construction of detailed mesh becomes a bottleneck in the CAE procedures. To solve these problems, the authors would like to introduce component-wise meshing approach and bonding strategy on the interface of components. In order to assemble component-wise meshes, the penalty method is introduced not only to constrain the displacements, but also to introduce classical spring connection on the joint interface, although penalty method is claimed that it is not suitable for iterative solver. In this paper, the convergence performance of an iterative solver with penalty method is investigated and the detailed component-wise distributed computation scheme is described with numerical examples.

Abstract: It is well known that a stress concentration of a crack can be relaxed by drilling a hole at the crack tip. The repair method is called a stop drilling procedure and often used to repair aircrafts, machines and so on. Then, when two additional holes are drilled near the stop drilling hole so as to face each other symmetrically, the stress concentration of the stop drilling hole can be relaxed further, and a fatigue life is extended. However, the size and the location of the additional hole suitable for repairing the fatigue damage are not clarified yet. In this paper, finite element simulations were performed on various plates containing the small center crack at which the stop drilling holes and the additional holes were drilled, and influences of the additional hole on the relaxation effect of the stress concentration of the stop drilling hole were investigated. Then, the size and the location of such additional hole that the stress concentration of the stop drilling hole is minimized were examined.

Abstract: For the medium-sized low-floor bus, backbone structure of chassis was designed to have light-weight structure with SAPH (Steel Automobile Press Hot rolled) 440. Strength for the designed backbone structure was also analyzed by finite element method under various dynamic load cases considered in this paper. On the basis of the analysis results, the structural safety for the designed backbone structure was evaluated and discussed in this paper.

Abstract: In this paper, the shear response of three-dimensional micro-lattice structures was investigated based on numerical stress analysis, FEM. The mechanical properties strongly depend on the number of unit cell in three directions x,y,z, and for a flat structure (number of cells in y-direction Ny=1), the deformation pattern observed in the structure can be classified into two types. The shear modulus G*for a flat structure obtained by FEM can be estimated by the elementary beam theory with a good accuracy. Also, for a flat structure with slender struts, the collapse is occurred by elastic buckling, and that with relatively thicker struts, the collapse strength agrees well with the theoretical result. Moreover, for the case of the cubic structure, if the structure has the same number of unit cell in x- and z- directions (numbers of cells in two directions Nx=Nz=M), the shear modulus G* shows a unit curve regardless of the number M, so that the modulus can be estimated by using the curve for various cubic structures.