The stress concentration factor at a weld toe in a welded joint is a very important factor of fatigue strength in welded structures. Since the shapes of weld toes are individually different, it is not appropriate to evaluate stress concentration factors at weld toes using numerical analysis such as FEM. In order to evaluate practically and accurately stress concentration factors at weld toes, thermoelastic stress analysis method (TSA), which is a stress analysis technique based on the measurement of infrared emission from the surface of a body subjected to cyclic loading, was applied to the measurement of stress distributions near weld toes. In this study, stress concentration factors evaluated by TSA were extremely lower than comparable numerical results due to the resolution limit of the infrared camera used. When stress distributions near weld toes measured by TSA were corrected by the empirical equation using toe radius, stress concentration factors evaluated by TSA agreed with numerical values within 25%.

View full abstract

A new full-field digital image correlation method is proposed for identification of three-dimensional displacement field within a soft body. Two sets of three-dimensional image data, which consist of intensity values with discretely sampled positions, are obtained by X-ray CT images for undeformed and deformed states of the body. A displacement field is tentatively represented by B-spline basis functions with unknown coefficients on the overall identification domain. Following the conventional digital image correlation method, the error function of displacement field is defined by the sum of error squares of intensity values on the corresponding material points in the two sets of image data, where the relationship between deformed and undeformed positions of the material point is described by the tentative displacement field. The iterative solution search is employed for modification of the unknown coefficients to identify the actual displacement field. Use is made of the penalty function method with weak constraint of incompressibility to cope with the indefiniteness of solution caused by the intensity flatness and measurement errors of CT images. An efficient parallel algorithm for the solution search is devised by utilizing the locality of B-spline function. The decomposed coefficient matrix for each decomposed domain, derived from the linearized equation of the Levenberg-Marquardt method, is constituted parallel at computational nodes. The modified conjugate gradient method, where the sequence of a vector variable calculation is modified, efficiently solves the linear equation, using the decomposed coefficient matrices. A displacement field within a specimen under compressive load is identified for the validation of the proposed method.

View full abstract

In the authors' previous study, fatigue crack propagation tests were performed using four-point bending specimens with multiple parallel edge notches at regular intervals of 10 mm. In this study, similar tests were carried out using the specimens with crack intervals of 5 mm or 2.5 mm. It was found that multiple crack propagation behavior was affected by the crack interval. A small number of cracks grew selectively in the specimen with narrow crack intervals while many cracks grew to long lengths in the specimen with broad crack intervals. Experimental results were first simulated by a previously proposed method using stress intensity factor solution for multiple cracks with alternately different lengths. Simulation results showed a good agreement with experimental results in case of specimens with broad crack intervals but showed different tendency from test results in case of specimens with narrow crack intervals. To simulate the experimental results a new method was proposed, in which the stress intensity factor solution was modified to include an effect of cracks outside the neighboring two cracks. The method gave a good simulation result for the specimens with narrow crack intervals but gave higher propagation rate than experiment for the specimens with broad crack intervals, resulting in a conservative evaluation.

View full abstract

In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at the crack tip. The partial elastic contact has a significant effect on the lives of fatigue cracks. However there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of semi-elliptical slant surface fatigue cracks are predicted. The finite element method was employed for these simulations. The residual stress was introduced by applying an equivalent nodal force in correspondence to an initial strain to each finite element. The results of the propagation paths and lives of simulations considering the partial elastic contact of crack surfaces differed from those without the consideration of the elastic contact. Especially the estimated results of crack propagation rate sometimes brought one-order difference. These results demonstrate the necessity of simulation considering the partial elastic contact of crack surfaces.

View full abstract

Loosening of bolt-nut tightening system is still unsolved problem even in recent days. Recently, we have investigated the loosening mechanism of bolt-nut tightening system subjected to shear loading and have succeeded the reproduction of experimental result in the framework of three-dimensional finite element method. In this paper, we have applied the scheme to the loosening problems induced by axial loading. As a result, the loosening mechanism has been clarified. The resulting loosening rate and its dependence on friction coefficients show qualitative agreement with the Kumakura's experiments. Critical loading range of loosening also shows quantitative agreement with the experimental result. It is found that the evolution of bolt-nut relative rotations in the loading Process and unloading Process are different each other, that is, the relative rotation is enhanced in the loading process and is inhibited in the unloading process. The analysis involving different Young modulus of bolt, nut and moving plate also indicates that loosening is originated by the slippage at bearing surface induced by nut deformation. As an application to actual tightening system, the loosening of the piston nut in hydraulic cylinder has been investigated. It is found that the accumulation of rod torsion in the loading process and the simultaneous rotation of rod and nut induce the loosening.

View full abstract

Abrupt growth of displacement observed in the relationship between indent load and indent depth in nanoindentation of crystalline materials, so-called displacement burst, has been recognized as one of the representative examples for the nanoplastic behavior. This behavior corresponds to the early stage of the plastic deformation and has greatly been influenced by the collective dislocation emission. In the present paper, we construct the simplified model of the first displacement burst by using the elastic theory based on both the Hertzian contact theory and the classical theory of dislocations to evaluate displacement burst under nanoindentation. As the result of this energetic model, it is found that there is strong correlation between burst width and critical indent depth where dislocation emission occur. Finally, it is shown that more than one hundred high-density dislocations are emitted simultaneously and surface step corresponding to each dislocation causes significant displacement burst.

View full abstract

A mismatch of material properties may cause stress singularity, which lead to the failure of bonding part in joints. It is very important to analyze a stress singularity field for evaluating the strength of interface in three-dimensional joints. Thermal residual stresses occur in a cooling process after bonding the joints at a high temperature, and the stress singularity for thermal stresses also occurs. In the present paper, a boundary element method and an eigen value analysis based on a finite element method are used for envaluating the intensity of singularity of residual thermal stresses at a vertex in joints. Three-dimensional boundary element program based on the fundamental solution for two-phase isotoropic body is used. The singular stress field for residual thermal stresses at the vertex in three-dimensional bonded structures with an interlayer is calculated with varying material properties of interlayer. Additionally, a relationship between Dundurs' parameterα_3D for three-dimensional stress states and the intensity of sigularity K_θθ.Stress singularity field will be able to estimate for an arbitrary material combination in the present study using this relationship under thermal loadings.

View full abstract

This paper is concerned with the impact tensile strength of laser welded butt joints made by combining different steel sheets. Three classes of steel sheets for automobiles, mild steel (HR 270), high tensile strength steels (HR 590 and HR 780), were used in this study. Two kinds of CO₂ laser welded butt joints specimens made of their different steel sheets, abbreviated as HR 270 & HR 590 joint and HR 270 & HR 780 joint, were investigated in static and impact tests, together with the three kinds of laser welded joints made of the same steel sheets, that is, HR 270 & HR 270, HR 590 & HR 590 and HR 780 & HR 780 joint. The measurement of the impact tensile strength was performed by the apparatus of split Hopkinson tension bar. The static tensile strength of HR 270 & HR 590 joint and HR 270 & HR 780 joint were similar to that of HR 270 & HR 270 joint, but their impact tensile strengths significantly incresed in comparison with the static ones due to the effect of strain rate, especially in part of HR 270 base metal.

View full abstract

Recently, the eutectic solder alloys have been replaced by the lead-free solder alloys because of the environmental pollution problems. In this study, temperature dependency and strain rate dependency on inelastic deformation behavior of Sn-37 Pb and Sn-3.5 Ag-0.75 Cu solder alloys under tensile loading with sudden change in strain rate are investigated experimentally and theoretically based on the overstress theory. It is found that the Sn-3.5 Ag-0.75 Cu solder alloy shows higher yield stress and the dynamic recovery in work hardening. The overstress constitutive model is found to be useful to predict the inelastic deformation behaviors of eutectic and lead-free solder alloys precisely, including relaxation behavior.

View full abstract

In this study, cavitation erosion tests were carried out by using thermal spraying and deposition of Ti-Ni shape memory alloy for the surface coating. The results show the test speciment of Ti-Ni thermal spraying has many initial defects, so that the erosion resistance is very low. The erosion resistance of Ti-Ni deposit is about 5-10 times higher than that of SUS 304, thus erosion resistance of Ti-Ni deposit is better than that of Ti-Ni thermal spraying. The cavitation erosion tests were carried out by using Fe-Mn-Si with shape memory and gummetal with low elastic modulus. The erosion resistance of Fe-Mn-Si shape memory alloy solid is about 9 times higher than that of SUS 304. The erosion resistance of gummetal is almost the same as SUS 304, because the test specimen of gummetal has many small defects on the original surface.

View full abstract

Cavitation erosion often causes a leak of water in piping systems of industrial plants. Cavitation erosion tests were carried out for the S 15 C carbon steel equivalent to pipe steel STPG 370 under the condition of the stationary specimen test using the vibratory apparatus specified by ASTM G 32-03. Another test was performed using the cavitating liquid jet method by ASTM G 134-95 to simulate a flow condition. It was found that the maximum depth increases with erosion time with a power of about 0.5 and the erosion pit was formed locally with time by statistical analysis made on a Gumbel probability paper.

View full abstract

Long-term durability of FRP under hostile environment is strongly influenced by fiber strength degradation. In this paper, the fiber strength degradation in the FRP in water under constant loading has been investigated using a single fiber composite (SFC). In order to estimate the fiber degradation due to aging in water, distributions of remaining fiber strength were obtained by fragmentation test. The remaining strength distributions have been quantified on the basis of microscopic stress corrosion cracking of initial defects on the fiber surface. Because the initial defect growth depends on the fiber stress history, increase of SFC strain in water has been also formulated. The remaining strength distribution obtained by the proposed model shows a good agreement with the experimental results. Additionally, it has been exhibited that the proposed model is available to predict increase of fiber failure caused by the fiber degradation in the SFC under constant loading in water.

View full abstract

Advanced pore-free SiC (APF-SiC) with uniformly dispersing metal Si particle was developed by new process of manufacture. This ceramic material was investigated for mechanical properties and fracture behavior by evaluating tests such as 4-point bending, static fatigue and indentation at room temperature. It was found that bending strength of APF-SiC indicates about two times of strength for usual reaction-sintering SiC (RS-SiC). In addition, their mechanical properties (such as Poisson's ratio and Fracture toughness) are nearly equal. However, as for Weibull modulus and Young's modulus APF-SiC became smaller than RS-SiC. It is considered to be due to difference of their fracture mechanics. APF-SiC has the dispersing metal silicon particles of about 100 nm instead of the potential defects. As the load given to a specimen increases, a lot of cracks continues to occur in the boundary between metal silicon and silicon carbide and so the strain energy in and around them is released. Some clusters are formed by uniting some cracks. When maximum stress intensity factor around the cluster reached to a critical value, macroscopic fracture occurred with fracturing from some places at same time. The scatter as well as the fracture strength depends on the size of a cluster. In the static fatigue test of APF-SiC, the time dependence was not recognized in fracture strength. In addition, all experimental values obtained were more than the average of bending strength. This fracture strength property can be explained by a conceptual model that the cracks due to a constant bending load had stopped initiating and started uniting on releasing the energy around the cluster of many cracks.

View full abstract

Three point bend specimen with different thicknesses are used for ductile fracture tests. By the change of the specimen thickness, size of the shear-lip fracture zone near free surface also changes. Detailed observation of fracture surface is conducted, and number of voids and their sizes are measured at many locations along the crack affront. The differences of microscopic fracture patterns between dimple fracture zone and shear-lip fracture zone are studied. Numerical simulation is also conducted and stress distributions around the crack tip are obtained. Distribution patterns of stress triaxiality and principal shear stress correspond well with the dimple fracture zone and shear-lip zone, respectively.

View full abstract

The fatigue cracks commonly initiate at the notched components under repeated loadings. So, the estimation of low-cycle fatigue life for the notched components is significantly important in the design of structures. In this study, the low-cycle fatigue tests were carried out using the smooth round bar specimens and the notched plate ones with the stress concentration factors K_t=2.43, 3.26 and 4.25. The strains at the notch root were measured over the fatigue tests. The materials used are the copper alloy C 6161 P-O and the aluminum alloy A 7075-T 651. On the basis of test results, the relationships between the fatigue strength reduction factor Kf and the elastic-plastic strain concentration factor K_ε were obtained as K_ε <2 : K_f=K_ε and K_ε≥2 : K_f=1.25K_ε ^0.68. The relationship among the elastic stress concentration factor K_t, the elastic-plastic strain concentration factor K_ε and elastic-plastic stress concentration factor K_ε in the plastic region containing the tensile strength σ_B and the yield stress σ_y was proposed as (K_t-1) σ_y/σ_B=0.34 [(K_ε-1) (K_σ-1)] ^0.72. Then, the fatigue life on the notched components was considered using these equations.

View full abstract

Metal injection molding (MIM) is a suitable production technique for the mass production of the products of the complicated shape. The MIM materials produced by the technique have the properties that are excellent in the static mechanical properties such as the tensile strength. However, the studies on the dynamic properties such as the fatigue strength that is important for the design of the reliable structural sintered steels are not enough. Our aim is to evaluate the fatigue damage of MIM materials with AE method. In this study, the detected AE signals during the fatigue testing of the material were analyzed with weight mean frequency distribution, chaos time series analysis, et al. As the result, the possibility that the damage will be evaluated by the parameters that were proposed in this study was shown.

View full abstract

Dumbbell-type specimens having a straight part was used in ultrasonic fatigue testing for high-strength steel in order to enlarge risk volumes. In this case, the stress distribution in the straight part was almost uniform in the axial direction. The steel used was low-temperature-tempered SCM440 low-alloy steel and the steel showed fish-eye fracture originating mostly from an Al₂O₃ inclusion. The ultrasonic fatigue testing results were compared with the conventional servo-hydraulic fatigue testing. The two kinds of fatigue testing results showed good agreements in case of fish-eye fracture. This agreements supported the validity of the ultrasonic fatigue testing with specimens having straight part in the gigacycle fatigue tests ending in fish-eye fracture. Moreover, the ultrasonic fatigue testing were conducted also with specimens without a straight part to confirm the effects of the risk volumes. In comparing those ultrasonic fatigue testing results, the effects of the risk volume were cleary observed, i.e. the result with larger risk volumes showed lower fatigue strengths due to presence of a larger inclusion at the fish-eye fracture origin.

View full abstract

The effect of compressive or tensile plane-stress on formation energies and electronic properties of point defects in Si crystal was studied by first principles approach for in-plane strain up to 5.0%. It was found that the formation energy of interstitial Si (I) decreased under tensile in-plane strain. On the other hand, the formation energy of vacancy (V) decreased under compressive in-plane strain. The most stable states of I and V in intrinsic Si were I²+ at T site and V⁰ respectively, independent of type and value of the in-plane strain.

View full abstract