The paper describes the recent progress of the understanding of fatigue crack propagation under complex loading. The behavior of fatigue crack propagation is dependent on the macroscopic mode of crack growth : tensile mode and shear mode growth. Tensile mode growth takes place on the plane perpendicular to the maximum tangential stress range, satisfying the local symmetry. Shear mode growth takes place on the plane of the maximum shear stress when a precrack is subjected to high cyclic shear stress. When the shear stress is low, a bent or branched crack is formed at the precrack tip, showing tensile crack growth. Under an intermediate stress, a crack first shows shear mode growth and turns to tensile mode after some extension. This transition is caused by the decrease in the mode II crack-tip stress intensity factor due to increasing crack surface contact. The criterion of the maximum crack growth rate may control the mode of fatigue crack growth. In notched components under complex loading, the life of crack initiation at the notch root is calculated from the critical plane model using the stress and strain at a critical distance from the notch root. The propagation of a fatigue crack from a notch is dependent on the geometry of notched component. The resistance-curve method is applicable for the prediction of fatigue thresholds of thin walled tubular specimens subjected to in-phase and out-of-phase combination of cyclic torsion and tension-compression.

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Treatment of stress multiaxiality in the fatigue assessment is of practical importance for many components subjected to multiaxial cyclic loading during operation. Based on the results of multiaxial fatigue tests on hollow and solid bar specimens of SUS316NG and SGV410, accuracy of fatigue life prediction based on the conventional parameters as well as a two parameter approach proposed here was studied. The conventional parameters tended to provide conservative life prediction for the case of shear-dominated cases especially for 316NG while the new approach was able to give more realistic prediction

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It is well known that approximately 70 to 80 percent of the failure accidents that occur in complex structures that have such holes are caused by the fatigue fracture. That is, we need to know fatigue fracture path to evaluate health. In this paper, we carried out fatigue fracture path prediction simulation containing multiple circular holes or inclusions in specimens. Furthermore the effect of their existence on crack growth was examined. The simulations were conducted using the moving finite element method based on Delaunay automatic triangulation, and the numerical result of the specimens containing multiple circular holes was compared with the experimental result. In the fatigue fracture path prediction simulation, Paris's law was used to decide crack propagation length Δa and local symmetry criterion was used to decide crack propagation direction.

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Fatigue crack growth under mixed mode loading conditions is simulated using Superimposed-FEM (S-FEM). By using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack growth. By combining with re-meshing technique, local mesh is re-meshed automatically, and curved crack path is modeled easily. At first, slant crack problem is solved. It is shown that results agree with conventional solutions, and verified the availability of this technique.Then two-crack problems are solved in several cases by changing each crack location. Stress intensity factors are evaluated and interaction effect between two cracks is discussed. Results are compared with crack coalescence criteria, and it is verified that these criteria are reasonable ones.

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Three dimensional crack propagation analysis will enable more realistic assessment of the structural integrity for aging structures such as aircrafts and power plants. Recently, as a linear-elastic fracture parameter evaluation method, VCCM (Virtual Crack Closure-Integral Method) is extended for 3-D tetrahedral element. With fully automatic generation capability of tetrahedral mesh, more realistic crack propagation of a 3-D arbitrary curved geometry can be handled efficiently. Here, we developed a new mesh generation technique for 3-D crack propagation analysis. A crack propagation analysis of an elliptical crack in a mixed-mode environment, which is initially flat and is gradually growing into a curved crack in 3-D space, is also demonstrated.

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In recent years, the object for numerical analysis becomes complicated and meshing is more and more difficult. The Free Mesh Method, which is one of the mesh-less method is, considered to make large-scale analysis easier without the so-called meshing process. However, mesh generation still needs much computation time. In order to reduce the computation time for meshing, we develop a new method using semi-circle with searching circle of nodes when employing the Free Mesh Method.

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This paper presents a virtual crack closure-integral method (VCCM) for a generalized four-node tetrahedral finite element that has drilling nodal degrees of freedoms (DOFs). The generalized tetrahedral finite element has superior accuracy over the liner tetrahedral element. VCCM computes the energy release rate and the stress intensity factor at the crack front. VCCM computes the fracture mechanics parameters accurately only from the nodal reaction forces and displacements. Furthermore, in mixed mode problem, it is easy to split energy release rate into each mode of crack deformation. In this paper, a VCCM formulation for the generalized tetrahedral finite element is developed for the case of mode I crack deformation. The contributions of nodal drilling DOFs on the computation of energy release rate are described in detail. Similar discussions can be extended to mixed mode problems. Finally, some numerical examples are presented and discussions on the accuracy of proposed VCCM are given.

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A composite material was produced by compression treatment of Sitka spruce plate soaked into biodegradable poly (vinyl alcohol) under various treatment conditions. To enhance the wettability of specimen, corona discharge and cutting of holes were conducted as pre-treatments. The pre-treated specimen was soaked into the poly (vinyl alcohol), then it was compressed and fixed in a airtight atmosphere. For examining the influence of the treatments on the elastic properties of composite material, Young's modulus in the longitudinal direction and Poisson's ratios and shear moduli in the longitudinal-radial and longitudinal-tangential planes of the composite material were measured. The test results indicated that these elastic properties can be improved when the composite material is fabricated under the appropriate pre-treatment conditions.

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B-spline wavelet Galerkin method is adopted to the solid/structural mechanics analyses. B-spline scaling function and wavelet are used as the basis functions. These basis functions have the so-called multiresolution properties. The steep gradients of stresses or strains can be enhanced by superposing different length scale wavelet basis functions. In the authors' previous study, B-spline wavelet Galerkin method adopted an adaptive strategy based on the posterior error estimation. On the other hand, there are some difficulties in dealing with external boundaries for the analyses of complicated shaped structures. There are loss of linear independence of the basis functions. A technique to remove particular basis functions that can be expressed by the linear superposition of the other basis functions is presented. In this paper, some numerical tests are carried out to validate the technique and some numerical examples are shown.

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The radiated sound from the impacted body must have the information with respect to the impact force. We have proposed the method in order to identify the impact force by analyzing the radiated sound from the impacted body. In that method, the transfer function in between the impact force and the sound pressure should be obtained by the extra experiment. In this paper, the relationship between the impacted force and sound pressure is obtained by the FEM simulation. Thus, the impact force can be given one time measurement of the impacted sound without any experiment for the calibration. The efficiency of the present method is confirmed by using the experiment of a plate.

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A mismatch of material properties may cause stress singularity, which lead to failures of bonding part in joints. The stress singularity of thermal residual stresses occurs in a cooling process after bonding the joints at a high temperature. In the present paper, boundary element method and an eigen value analysis based on finite element method are used for evaluating the intensity of singularity for residual thermal stresses at a vertex in three-dimensional joints. Three-dimensional boundary element program based on the fundamental solution for two-phase isotropic body is used for calculating the stress distribution in a three-dimensional joint. Angular function in the singular stress field at the vertex in the three-dimensional joint is calculated using eigen vector determined from the eigen analysis. The distribution of angular function of stress components is well agreed with and the distribution of residual thermal stresses. Furthermore, the angular function for σ_θθhas a singularity at the cross points of the interface and side free surfaces. In the present paper, the intensity of singularity at the vertex in three-dimensional joints is determined using the characteristic of angular function and the coefficient of power law singularity of stress distribution against the distance from the origin of singularity.

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It is important to obtain the stress frequency distribution by service load in order to estimate the fatigue life in welds of vehicle truck frames. Although the stress frequency distribution by service load can be obtained by vehicle running test, it is difficult to estimate the distribution in the stage of design. Accordingly, the characteristics of stress frequency obtained by the past running test data were examined and analyzed together with its analysis of the effect of running conditions, which include distance, speed and payload. The study results will be used to estimate the stress frequency distribution in the stage of design or modification by operating conditions.

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Fatigue test system for thin sheets was constructed using an ultrasonic fatigue testing machine. Specimens were fixed at the location where the displacement amplitude became the maximum. The specimen was fatigued under full-reverse bending. Displacement at a fixed edge of the specimen and the deflection of the specimen were measured with a laser displacement gage during fatigue tests. Modal analysis was carried out to determine the dimensions of the specimen and to calculate the applied stress on the basis of the flexural displacement amplitude. The maximum stress was observed on the free side edge near the fixed edge. Two kinds of martensitic stainless steel sheets, SUS301-CSPH and SUS632J1-CSP, were fatigued with the fatigue test system. The thickness of the specimen was 83 and 106μm, respectively. The 3rd vibration mode was selected for fatigue tests. The fatigue cracks initiated from the free side edge of the specimen. For the case of shorter fatigue life, fatigue strength for both stainless steels were almost identical. On the other hand, for longer fatigue life, fatigue strength of SUS632J1 is higher than that of SUS301.

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In order to study the fatigue properties of low alloy steel, JIS SNCM439, under axial loading condition in very high cycle regime, fatigue test under the stress ratio of -1 was carried out in air at room temperature using the hourglass shaped specimen. From the experimental results, clear duplex S-N curve existed in this material ; that is, there were two curves composed of a surface inclusion-induced fracture mode and a subsurface inclusion-induced fracture mode with granular bright facet (GBF) area in the vicinity of inclusion. Roughness of the GBF area was about 2 times larger than the area outside the GBF in the fish-eye zone relating to the size and distribution of carbide particle in the matrix. An appearance of the dual-stage S-N behavior was related to the difference of threshold for crack initiation and crack growth rate between surface inclusion and subsurface inclusion. Stress amplitude for transition of the fracture mode was explained by considering an effect of compressive residual stress on the surface layer. From the evaluation of fatigue life based on the estimated subsurface crack growth rate from the S-Ndata, effect of inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.

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Bone continuously adapts to mechanical environment by reconstructing its structure. This property is called bone remodeling. One of the mechanical factors of remodeling is internal stress. Since internal stress remains in bone tissue for a relatively long period of time, it is a kind of residual stress. Bone tissue is a composite material composed of hydroxyapatite (HAp) and collagen fiber. The interplanar spacing of HAp crystals can be noninvasively measured using the X-ray diffraction method because HAp is a crystalline structure. The residual stress of bone tissue can then be calculated from the interplanar spacing. In this work, the sine squared psi method was applied to bone tissue and the residual stresses on eight regions of bone axial and circumferential direction in a specimen were calculated. The cortical bone specimen was cut out from the diaphysis of bovine femur. As a result, it was confirmed that the residual stress existed in bone. The residual stresses of the bone axial direction were different according to the circumferential region.

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Peening is a technique of surface modification to improve fatigue strength of metallic materials. Cavitation shotless peening (CSP) is the peening method using cavitation impacts and shots are not required. Residual stress and hardness of materials are also changed by CSP. Similarly, Young's modulus of surface improvement layer might be changed by CSP. Young's modulus is one of the main mechanical properties of materials, and indicates material structure, and it is an important parameter for stress measurement by X-ray diffraction. In the present paper, Young's modulus of type 316 L stainless steel peened by CSP was evaluated by measuring natural frequency of cantilever. And, Young's modulus of surface modification layer is evaluated by assuming composite cantilever. It was revealed that Young's modulus of surface modification layer by CSP was decreased about 11%, although Vickers hardness was increased about 40%. Namely, elastically hardness is decreased, and plastically hardness is increased by CSP.

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Plastic region tightening has been applied to the flange joint assembly in order to downsize the bolt and flange. The tightening procedure follows the modified HPIS flange tightening procedure, which employs the angle control method instead of the torque control method. Tightening tests is conducted using JPI 4 inch flanges and spiral wound gaskets. Bolts of M 16 with the reduced shank are used to adjust the tightening force. The estimation method is proposed for the yield tightening force of the bolt with reduced shank of hollow cylinder. The scatter of the axial bolt force is 10% or less after the tightening process is completed. The uniformity of the axial bolt forces obtained by plastic region tightening is equivalent or superior to that of elastic region tightening by HPIS procedure. In the tightening process, the increase in axial bolt force is very small in the plastic region of the bolt, so that the influence of the elastic interaction is also small, which is advantageous with respect to the uniformity of the axial bolt force. Application of plastic region tightening to the flange joint can downsize the bolts from M 16 to M 8 due to higher tightening bolt forces and their uniformity.

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An eruption of porosities which degrade strength reliability of products, is inevitable in the die casting process. We have investigated PRPC (Porosity Reducing Post Compression) process of ADC 12 aluminum alloy die castings to reduce inside porosity. In this study, a validity of PRPC process is demonstrated in terms of the Charpy impact strength. We ensure that the porosity ratio was considerably reduced by the PRPC process by means of X-ray CT observation. We conclude that the Charpy impact strength improves since the effective cross-sectional area of the specimen concerning fracture increases after PRPC process without any harmful influence. This conclusion is exemplified by the flat fracture surface of the specimen after PRPC process.

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