A developed crack propagation analysis system using the finite element method for components like vessels and pipings in nuclear power plants is described. One of the characteristic features of the system is that its input data include welding residual stresses and the stresses produced by applied forces in the components which have been obtained from previous analyses. In the system, the nodal forces on the crack surface are calculated from these input stresses. The authors extended the Virtual Crack Closure-Integral Method so as to calculate stress intensity factors when nodal forces on crack surface exist and applied the method to the system. In order to set test analysis problems for the system, conditions were found in which an elliptical crack holds the elliptical shape in fatigue crack propagation, and accordingly the crack size changes can be predicted using the theoretical stress intensity factors for an elliptical crack in infinite bodies subjected uniform tensile stresses. Under these conditions, a test analysis was carried out using the system, and the obtained crack size changes were shown to be in good agreement with those obtained from the theoretical stress intensity factors. As an example problem for practical structures, crack propagation due to SCC in a cylinder with a residual stress distribution solved by the system are presented and the results are compared with reliable reference values calculated using stress intensity factor data in a literature.
This paper describes an Eulerian formulation for solid-fluid interaction dynamics. A computational mesh control can be divided into Lagrangian and Eulerian solutions. Although the Lagrangian and Eulerian solution has been generally adopted for deformation of solid and flow of fluid respectively, the highly distorted Lagrangian mesh cannot retain numerical accuracy for flexible solid material. The present approach establishes one governing equation for both solid and fluid models using mixture theory assuming incompressibility in the full Eulerian framework. Hyperelasticity for solid and Newtonian fluid are employed in the constitutive equations. A discretization of the proposed formulation for solid-fluid interaction dynamics is based on an explicit finite element method. The explicit finite element method reduces computational cost, except that the finite different method instead of the finite element method is used to solve Poisson and advective equations. We test the validity of the established formulation in the two repressentative solid-fluid interaction examples including flexible solid.
Passive infrared thermography is an effective technique for detecting delamination in concrete structures. However, the method is dependent on weather conditions, when solar radiation is used. This research investigates the possibility of quantitative estimation of delamination depth in concrete by using the passive infrared thermography under solar radiation. An inverse approach is introduced for the quantitative estimation from transient temperature distribution on concrete surface under various weather conditions. In the inverse method the time series thermographs of observed surface are compared with those obtained by the FEM analyses. The inverse analysis is applied to experimental data obtained by infrared camera. It is found that delamination depth can be quantitatively estimated by using the inverse analysis.
Annealed ultrafine-grained metals contain some grains with extremely low dislocation density, so that the critical resolved shear stress increases at the first stage of deformation due to the exhaustion of dislocation sources in a grain. In this paper, in order to express the increase of critical resolved shear stress, the conventional Bailey-Hirsh's relationship is extended on the basis of physical consideration for grain boundary that plays a role of dislocation source. A triple-scale dislocation-crystal plasticity FE simulation based on the above model, geometrically necessary crystal defects and the homogenization method is carried out for annealed FCC polycrystals with different initial grain size and initial dislocation density. Yield point drop and propagation of Luders bands observed in macroscopic specimen with annealed FCC fine-grains are numerically reproduced. Moreover, macroscopic yielding of specimen and microscopic grain yielding are investigated in detail so as to clarify the initial yield behavior of annealed ultrafine-grained metals. It is also shown that plastic deformation is easy to be localized and the tensile ductility decreases as the grain size reduces.
In this paper, the shear response of three-dimensional lattice structure was investigated based on numerical stress analysis, FEM. In particular, effects of number of unit cells in three directions on the mechanical properties (shear modulus G^* and collapse strength τ^*) of lattice structures were discussed based on theoretical analysis and FEM. It is found that the mechanical properties strongly depend on the number of unit cell in three directions x, y, z, and for a flat structure (N_y=1), the deformation pattern 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, the cubic structure having the same number of unit cell in x and z directions (N_x=N_z=N) shows a unit curve for the shear modulus G^*, so that the modulus can be estimated by the curve for various cubic structures.
In this paper, the collapse behaviors of thin plate with corrugated cross-section subjected to three-point bending are studied by using the finite element method. In order to estimate the energy-absorption characteristics of the beam, it is vital to understand the relation of load and displacement. It is found that the load decreases by flattening of the cross-section of the beam, and the flattening shape can be quantitatively expressed by using curvature radius of the plane of the top and bottom in the cross-section. Based on an idea that the external work is mainly expended by the flattening deformation of the cross-section, a new prediction method is proposed for estimating the relation of load and displacement. Its validity is verified by comparing with the numerical results by FEM under various conditions.
In this paper, the equivalent elastic constants for the out-of-plane deformation of three kinds of honeycombs, consisting of respectively triangle, rectangle and hexagon cells, were studied theoretically based on the equivalence of deformation energy. The out-of-plane deformation of honeycomb consists of bending deformation and twisting deformation of cell plates. For the twisting deformation of cell plates, a restraint condition must be satisfied at both ends, because each cell plate is in contact with adjacent plates at both ends. When not taking such a restriction condition into consideration, a large analysis error would arise. The validity of the analysis was checked as compared with results of numerical analysis of FEM.
Soft cellular solids like sponge and muscle have the nonlinear viscoelasticity due to the internal gas or liquid in cells and the behavior of the matrix of the solids. Their stress-time curves are different by the variation of their internal structures, and it causes difficulties in the evaluation of the mechanical characteristics of the solids. In this study, we adopted the multiplication form of power and exponent functions that is able to approximate the nonlinear and fluctuated behavior of soft cellular solids. Then, we examine the adequacy of this method by its application to the experimental results of tensile testings of SBR sponge and biological soft tissue. The results of the examination show that the adopted form of the functions have good ability to fit the complex results of the stress-time curves and it can realize good evaluation of viscoelasticity of soft cellular solid by using three-element solid model.
Although a lot of interface crack problems were previously treated, few solutions are available under arbitrary material combinations. This paper deals with a central interface crack in a bonded infinite plate and finite plate. Then, the effects of material combinations on the stress intensity factors are discussed. A useful method to calculate the stress intensity factors of an interface crack is presented with focusing on the stress at the crack tip calculated by the finite element method. For the central interface crack, it is found that the results of bonded infinite plate under remote uni-axial tension are always depending on the Dundurs' parameters α, β and different from the well-known solution of the central interface crack under internal pressure that is only depending on β. Besides, it is shown that the stress intensity factor F_1 of bonded infinite plate can be estimated from the stress of crack tip in the bonded plate when there is no crack. It is also found that dimensionless stress intensity factor F_1<1 when (α+2β)(α-2β)>0, F_1>1 when (α+2β)(α-2β)<0, and F_1=1 when (α +2β) (α-2β)=0.
The split Hopkinson pressure bar technique has widely been used for impact testing of materials in the strain rate range from 10^2 to 10^4s^<-1>. However, some problems still remain in obtaining precise stress-strain curves of a sample. In these problems, a radial inertia and a friction during the impact test affect a determination of a size in the sample. In this paper, the theory on the basis of the energy conservation, for this technique proposed in the past is examined and some modifications derived from the radial momentum conservation are applied. Then, it is shown that the inertia and friction effects are coupled to each other. A computational simulation by using the commercial FEM code ABA-QUS/Explicit ver. 6.8 is conducted to check a validity of these modifications. Simulations are performed by changing a friction coefficient and a ratio between the diameter and height of the specimen.
The contact stress distributions and permanent sets at the bearing surfaces in bolted joints under initial clamping load are analyzed using an elasto-plastic FEM calculation when hexagon bolts with flanges are used. It is found that the difference of the contact stress distributions between elastic analyses and elasto-plastic analyses is large when the bolt preload is high. The effects of the flange slope angle θ and the flange thickness t in the bolts with flanges on the contact stress distributions at the bearing surfaces are examined. It is seen that the flange slope angle θ and the flange thickness t substantially effect on the contact stress distribution and permanent sets at the bearing surface. In the case where θ is 0.25°, the thicker flange of the bolt head and the slope angle of θ=0.25° cause smaller permanent set. In addition, the equivalent length for the bolt with flange is proposed. Furthermore, in the experiments, hollow cylindrical specimens fabricated with mild steel and aluminum alloy were compressed by the bolts with flanges and the permanent sets at the bearing surfaces were measured. The permanent sets at the bearing surfaces obtained from the FEM calculations are in a fairly good agreement with the measured results. It is shown that the permanent set can be estimated by elasto-plastic FEM calculations. Discussion is made on the critical stress at the bearing surfaces. Taking into account the deformation of bolt head with flange, its equivalent length in the spring constant of the bolt is proposed.
The stop hole is one of the crack arrester systems, which is used to prevent the growth of fatigue cracks. However, the stop hole is not expected to prevent high-speed crack propagation. Because the mechanical condition of fast crack propagation behavior near the stopping hole is not cleared, the criterion of the stop for brittle fracture is not established. In this study, authors focus re-propagation start (restart) condition of crack from the stop hole. To discuss the restart condition of high-speed crack propagation, experiment observation and finite element analysis result are compared. In the numerical simulation, dynamic crack propagation simulations are carried out, using the moving finite elememt method based on Delaunay automatic triangulation. The various parameters are calculated from the numerical results.
In general, cracks are initiated from stress concentration sites: notch, hole and so on. The stress intensity factors of the cracks which are initiated from typical notches were computed numerically in the earlier studies. Then, as for the stress intensity factor of the short crack from the notch root, some simple formulae were proposed by some researchers. However, there are few data on the stress intensity factors of the cracks from the complex notches. In this paper, the stress intensity factors of the cracks from a center double U-shaped notch in a finite plate under a tensile stress were computed by the body force method. Then, the influences of the double notch and the prate edges on the stress intensity factors were examined, and the stress intensity factors were expressed with the simple formulae approximately.
The non-combustible Mg alloy is useful for structural material because of the high specific strength and the high ignition point, but the suitable welding condition was not established for manufacturing floor. In this study, a simple evaluation method for fatigue limit characteristics of welded joint was proposed, using the edge shape of welding bead geometry, the inner defect size and the material characteristics. Moreover, the welding condition ranges to ensure a stable fatigue limit for TIG butt joint of non-combustible Mg alloy were proposed by using this method.
There are several factors of hydrogen gas environment effects on strength of high strength steel SCM435 with a sharp notched specimen. In this paper, tensile tests were carried out in several hydrogen and helium gas environments. The examined factors were the gas pressure, the gas temperature, the crosshead speed and the notch root radius. The result of tensile tests in hydrogen gas environments showed the degradation of tensile strengths at any given environment factors, which were not occurred in helium gas environments. Additionally, as the result of investigating the area of intergranular fracture, it was found that the tensile strength had an inverse proportion with the area of intergranular fracture regardless of several environment factors.
High and low cycle fatigue tests were conducted for extruded magnesium alloys, AZ31 and AZ61. In the high cycle fatigue properties, AZ61 showed higher fatigue strength than AZ31, while both alloys did not show fatigue limits. In this case, the rotating bending tests revealed higher fatigue strength than uniaxial loading tests. In the low cycle fatigue properties, AZ31 and AZ61 revealed almost equal fatigue strength under constant strain amplitude tests. However, the hysteresis loops were very unique, showing anisotropy between tension and compression sides.
In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075-T6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%〜70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.
In transverse fatigue of a bolted joint, althogh the real fatigue limit (the highest nominal stress at the root of the first thread of bolt without generating fatigue failure) is the same for bolts with the same size and property class, the apparent fatigue limits (the highest amplitude of transverse vibration force which can be applied to the bolted joint without generating fatigue failure) vary according to the bolt tightening conditions. Hence, it is necessary to develop a guideline for safety and design of bolted joints under transverse vibration. In this study, the relationship between the apparent fatigue limit and the real fatigue limit has been experimentally revealed. The method to predict the apparent fatigue limit using the real fatigue limit has been developed.
This paper describes fracture behavior evaluation of wall-thinned pipes by image processing strain measurement system. Regular grids with nominal size of 10×10mm were marked on the 100 A carbon steel pipes and the images taken with 6 CCD cameras of 15 million pixels were correlated to realize resolution of 0.3% strain. Strain of the cylinder outer surface was evaluated by 1) modeling the grids as a cylindrical shell, 2) measuring deformation of the grid on a projected plane, and 3) by applying updated Lagrangian method. The results indicate that the method is effective for analyzing fracture modes and fracture criterions.