Laser beam welding using two or twin laser beams has become an emerging welding technique. In addition to the experimental researches, the numerical modeling is helpful to reveal invisible flow phenomena in welding process and provide possible improvements to the welding technique. This paper proposed a two-dimensional thermal-fluid coupled model and the suitable boundary conditions for analyzing the flow behavior of molten pool. Using finite element analysis software MSC. Marc, the flow pattern and shape of molten pool for the single and twin beam welding were systematically investigated. The obtained numerical results could be successfully used to explain the experiment results.

View full abstract

In this paper the penetration depth of long-rod electromagnetic projectiles on semi-infinite targets are studied with numerical approach. And the role of nose profile was studied. The results obtained from this investigation were compared with the recent studies. For the targets and projectile were considered to be made of soft metals. All the results were compared with two analytical models: Tate and Vahedi. These two models don't consider the role of nose profile. The target is constrained to have no movement and the projectile doesn't pass through the target. The impact velocity of the projectile was considered as a variable and the results were compared together and they were in good agreement with other studies in this field.

View full abstract

Bending creep tests were conducted on alumina-alumina butt joints at temperatures of 788-888 K. The lifetimes in three-point bending tests under constant loads were measured and the specimens tested were investigated using scanning electron microscope. From the elastic stress-distribution at the joining layer calculated using the finite element method, it was confirmed that the thin metal film in the joining layer is under the conditions of triaxial tension and the hydrostatic tensile stress. The results of SEM observation of creep fractured surfaces showed numerous small and large cavities in the tension side of the joining layer. Under the assumption that the mechanism of creep deformation and fracture for the thin metal film in the joining layer were the nucleation, growth and aggregation of void with the aid of the hydrostatic tensile stress component, the lifetimes in creep bending test were predicted. Time-dependent change of stress distribution related to initial outer fiber stress in bending has been calculated with formulas based on linear elastic beam theory. Predicted lifetimes are in good agreement with the experimentally obtained lifetimes. The neutral-axis position measured from the photographs of the fracture surfaces can be explained by the predicted stress distribution in creep bending tests.

View full abstract

This study numerically compares the optimal solutions generated by element- and nodal-based material topology optimization of linear elastostatic structures. Both of these designs utilize a density distribution method for the design domain concept and produce optimal boundary representations on fixed grids. Since the nodal-based design utilizes material density redistributions for topologies and shapes which are based on nodal density averages, numerical instability is reduced. This instability often occurs as checkerboard patterns in classical material element-based topology optimization using design variables of element densities. Numerical examples are used to investigate numerical aspects of optimal solutions and convergences between element- and nodal-based designs, and demonstrate the efficiency of the nodal-based approach, which generates material boundaries with smooth shapes in the range of material topology optimization.

View full abstract

Threshold stress intensity factor ranges, ΔK_ths, of spheroidal cast iron, which are composed of ferritic, ferrite-pearlitic, or pearlitic structures, have been estimated by ΔK-increasing tests for small cracks and by ΔK-increasing and ΔK-decreasing tests for long cracks at several stress ratios. With respect to the effects of the hardness and stress ratio, ΔK_th estimated by the ΔK-decreasing tests for long cracks is evidently different from that for small cracks, while ΔK_th estimated by the ΔK-increasing tests for long cracks is similar to that for small cracks. Moreover, in the case of long cracks, the values of ΔK_th estimated by the ΔK-decreasing tests were larger than those estimated by the ΔK-increasing tests, irrespective of the type of matrix structures. These tendencies are attributed to the fact that the crack closure stress of the long cracks in the ΔK-decreasing tests is relatively higher than that of the long cracks in the ΔK-increasing tests. On the fracture surface of the long cracks in the ΔK-decreasing tests, oxide layers produced by fretting are observed at sites around the crack tip, although such layers are not observed on the fracture surface in the ΔK-increasing tests.

View full abstract

This paper deals with fatigue strength and damage behavior of glass-particle-reinforced Nylon 66 composites. Fatigue tests are carried out on seven kinds of glass-particle-reinforced Nylon 66 composites in which a volume fraction of glass particles and interface treatment between particles and matrix are changed. The fatigue strength is low in the interface-untreated composite and is high in the interface-treated composite as compared with that of the Nylon 66. Variation of stiffness during fatigue is determined from the equilibrium stress-strain relations obtained by multi-step relaxation tests after given stress cycles, which eliminate the influence of viscous deformation of the Nylon 66 matrix. The stiffness reduces significantly at the early stage of stress cycles and then becomes constant in the Nylon 66 and both composites. In the fatigue tests under the applied stress for the same fatigue life, the stiffness reduction is more remarkable in both composites than in Nylon 66 because of the debonding damage. From the variation of the equilibrium stress-strain relations during fatigue, it is found that the viscous component of deformation gradually disappears with increasing stress cycles.

View full abstract

The aim of this research is to study the effect of steel sheets produced by different manufacturers upon the forming limit. This work used forming limit diagram (FLD) to determine the formability of different trademarks steel sheets SPCC and SPCE, which have the same thickness of 0.8 mm. These two steel sheets are commonly used to produce automotive parts in Thai automotive industry. In order to produce FLD, the circular grid pattern was marked onto the surface of the blank sheet using electrochemical-etching technique. Subsequently, the blank sheet was continuously formed into a semi-dome shape by hydraulic press machine with a 100-mm punch. The pressing process was stopped when local necking and cracks were observed on the surface. The deformations of circular grids were accurately measured to obtain the major and minor strains. Afterward, this process was repeated again for different semi-dome's radii in order to produce FLD. The results were found that FLDs of SPCC and SPCE are slightly different. It has been also found that SPCE has the values of major and minor strains higher than SPCC. In other words, it indicates that SPCE has better formability than SPCC. The FLDs of both steel sheets have predicted failure in forming process consistent with the real experiment and finite element result.

View full abstract

S-version finite element method (s-FEM) is applied to two-dimensional linear/nonlinear fracture mechanics problems. s-FEM in crack problems is such that local detailed finite element mesh (local mesh) is superposed on cores finite element model (global mesh) representing the global structure. Therefore, the local and the global meshes are generated independently. This methodology is considered to be one of global-local type analysis strategies. In this paper, the formulation of s-FEM is reviewed and applied to various linear/nonlinear fracture mechanics problems. Stress intensity factors for linear fracture problems are computed by using virtual crack closure method (VCCM). For nonlinear fracture problems, J-integral is evaluated. Calculations for the fracture parameters are carried out based on the local mesh only.

View full abstract

In this study, the monosemous effect of thermal plastic strain on the thermal fatigue life is newly found on ferrite ductile cast iron around the alpha phase field. At first, the monosemousness is defined and its meaning described. Next, the monosemousness of thermal fatigue is demonstrated by its conditional equation and its existence is verified by the thermal fatigue test on ferrite ductile cast iron. By doing so, the feature on the thermal fatigue of ferrite ductile cast iron is clarified. Generally, it is considered that fatigue life in ferrite-matrix temperature range can be expressed at least by two or more different Arrhenius equations, namely there are two or more different activation mechanisms to govern the thermal fatigue life corresponding to various ferrite temperature ranges. In this case, for determining the life in any various ferrite temperature ranges, it must have at least four or more unknown quantities. If there is the presence of a general equation which is able to replace above described plural equations, then the life can be determined by simple one variable. Here, by introducing conditional equations, it is verified that the general equation is a Coffin and Manson's equation of low cycle fatigue and whole thermal fatigue life can be determined by a variable of thermal plastic strain occurred in thermal cycle. As a result, the law can apply to describe thermal fatigue phenomenon and predict thermal fatigue life monosemously from cyclic thermal plastic strain on ductile cast iron with ferrite matrix.

View full abstract

In the study, a formula for predicting fatigue crack tip opening displacement is deduced firstly. And then, due to comparing actual crack growth rate with the deduced formula, the crack tip configuration factor is defined to figure out the crack tip opening configuration that is useful to clarify the behavior of fatigue crack tip formation apparently. Applying the concept, the crack growth of 7/3 brass and 6/4 brass is predicted from the formula, by replacing material properties such as plastic flow resistance, Young modulus, the Poisson ratio, and fatigue toughness, and fatigue test conditions such as the stress intensity factor range, the load ratio, and cycle frequency. Furthermore, the theoretically expected results are verified with the fatigue tests which were carried out on CT specimens under different load conditions of load ratio, cycle frequency, and cyclic peak load, in different environments of air or corrosive ammonia atmosphere, for various brasses. And by comparing and discussing the calculated crack growth rate with attained experimental results, the apparent configuration factor at the crack tip is determined. And through the attained factor which changes along with crack growth, the behaviors of fatigue crack tip formation under different test conditions have been found out.

View full abstract