Wedge-shaped defects are frequently observed on the cervical region of the human tooth. Previously, most studies explained that improper toothbrushing causes such defects. However, recent clinical obsarvation suggested that the repeated stress due to occlusal force may induce the formation of these wedge-shaped defects. In this study, a two-dimensional human tooth model after a wedge-shaped defect is restored with the composite resin is analyzed by using the finite element method. To obtain the intensity of the singular stress field accurately, a method of analysis is discussed for calculating generalized stress intensity factors, which control the singular stress around the tip of the defect. Then, the relationships between the stress intensity and occlusion are discussed.
Linear elastic fracture mechanics (LEFM) parameter, such as mode I stress intensity factor required to crack propagation for rock under confining pressure is analyzed based on a cohesive crack model. In rocks, the LEFM parameter varies with the confining pressure. This study provides analytical solutions of relation between the LEFM parameter and the fracture toughness using a cohesive crack model, which is a model for the fracture process zone. The fracture toughness is defined by the cohesive crack model. The problem analyzed in this study is a fluid driven fracture of a two-dimensional crack with a cohesive zone under confining pressure. The size of the cohesive zone is assumed to be negligibly small in comparison to the crack length. The analyses are performed for two types of the cohesive stress distribution, namely the constant cohesive stress and the linearly decreasing cohesive stress. The analytical solutions are confirmed by comparing with the results of numerical computations performed using the body force method. The analytical solution suggests a substantial increase in the LEFM parameter due to increased confining pressures, even if the size of the fracture process zone is small.
A novel in situ TEM fracture testing method is developed to elucidate the mechanics dominating crack initiation at an interface edge in nano-structures. The testing system includes a special mechanical apparatus equipped with a piezo-actuated stage and a diamond loading tip attached to a MEMS load sensor, which enables us to control specimen position and perform quantitative load measurement with enough precision. Nano-scale cantilever specimens with interfaces are fabricated from a multi-layered material (Si/Cu/SiN) using FIB process technique. The validity of the testing method is demonstrated by conducting interfacial crack initiation experiments for the specimens with different size. The applied load and the specimen image have been successfully monitored and recorded during the experiments. For each specimen, the crack is initiated from the free-edge of interface between Si/Cu at a maximum load, Pc, and the specimen is instantly separated at the interface. Using the experimental results the stress fields along the Si/Cu interface at the crack initiation are analyzed for each specimen by finite element method. This reveals that the crack initiation of the Si/Cu interface is governed by the normal stress on the interface near the edge.
The aim of this paper is to investigate the effect of ultra-violet (UV) ray irradiation on the crack formation of brittle ceramic coating on polymer substrate. It is well known that ultra-violet ray irradiation degrades the mechanical properties of polymer and polymer-based coating films. We carried out the tensile tests of PET/ITO film specimen after UV irradiation under the microscope and observed the crack formation on ITO surface. Also, we carried out nano-indentation tests of PET substrate after removing ITO layer to characterize the change of the mechanical properties near the interface between PET and ITO. The results show that the number of cracks vertical to loading direction and the rate of vertical crack formation decreased after UV irradiation. Young's modulus and hardness of PET substrate increase due to the oxidation of PET after UV irradiation. To explain the relationship between the crack formation and mechanical property change, the energy release rates of the thin film on the substrate are considered. Also, the tensile stresses induced in cracking film are estimated by simple shear lag model.
Thermal spraying of WC-Co cermet was applied to annealed or quench and tempered tool steel specimens by a high-pressure high-velocity oxygen fuel method. Repeated sliding friction tests were subsequently carried out using a contact load of 1.0 or 1.5kN (the contact area was 6×12mm), and the delamination energy Ed was evaluated by means of an edge-indent test. Many parallel cracks were observed on the coating surface after a certain number of sliding friction cycles and partial shallow delamination of the coating began at 2.5×105cycles. The delamination energy Ed decreased with increasing number of cycles for both types of substrate. Although the Ed was higher for the annealed than the quench and tempered substrate before the sliding friction tests, the decrease in Ed by repeated friction was greater for the annealed substrate. Two-dimensional finite element analyses revealed that large maximum principal and shear stresses existed in the direction parallel to the coating surface near the contact edge of the sliding block and this was considered to be the reason for crack initiation and the partial shallow delamination of the coating observed. Although the range of shear stress and normal stress were the greatest at the surface, they were still large near the interface and the range of shear stress near the interface was the greatest in the direction parallel to the interface. The reason for the decrease in the delamination energy with increasing number of friction cycles seemed to be the accumulation of interfacial damage resulting from the combination of a range of normal stress and the shear stress along the interface.
In this paper, quasi-static loading tests and fatigue tests are carried out on spot welded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) sheets in order to investigate the influence of strength level of base steels on fatigue strength and fracture mechanism of spot welded joints. Under quasi-static loading, a spot welded part is more liable to be a fracture-initiation site in the ultra-high strength steel than in the mild steel, and as a result fracture behavior of the ultra-high strength steel is considerably affected by a spot welded part as compared with that of the mild steel. Although the fatigue strength of spot welded joints is higher in the ultra-high strength steel than in the mild steel in low cycle fatigue regime, the difference in fatigue strength decreases in high cycle fatigue regime and the fatigue limit is almost the same in both steels. Fracture morphology of spot welded joints under cyclic loading depends on the load level in the ultra-high strength steel, but not in the mild steel. From the comparison between S-N curves for spot welded joints and notched base steels, the dependency of fatigue strength of spot welded joints on base steels is attributed to the notch-sensitivity to fatigue strength of the base steels.
Fretting fatigue fractures of industrial machines often occur at the point where high contact pressure occurs due to an uneven contact. In this study, fretting fatigue tests were performed under high contact pressure applied by line-contact conditions using 12-Cr steel with parameters of the mean stress and the material strength. It was shown that the fretting fatigue strength decreased slightly with increasing the contact pressure and became minimum when Hertz's average contact pressure is about 1.5 times of 0.2% proof stress, σ0.2. I also showed that test results concerning the fretting fatigue strength and the non-propagating crack length can be successfully explained by the micro-crack propagation model in which a micro-crack can propagate when its stress intensity factor range, ΔK is greater than the threshold value, ΔKth considering small crack size effects and mean stress effects.
This paper presents an image-based multi-scale analysis in order to evaluate macroscopic properties of heterogeneous porous piezoelectric materials by finite element calculations. The multi-scale analysis is based on the mathematical homogenization theory. Material heterogeneity is represented as a micro structure FE model that is automatically generated from the cross-sectioned images by high resolution X-ray CT. The high resolution X-ray measurement for porous PZT is compared with the digital image of SEM. Radius of pore is quantified from their images to compare these images. Nodal blocking preconditioned iterative solver is developed for piezoelectric FEM with high degree of freedom. In the numerical examples, material properties of porous PZT materials with different porosity are evaluated by the image-based multi-scale analysis to show the applicability and efficiency. Image-based multi-scale analysis with one million elements can be conducted by conventional PC with help of the proposed iterative solver.
Numerical analysis method considering microstructure and deformation near the grain boundary is proposed in order to fundamentally evaluate strength properties of steels. The parameters that characterize grain shape, such as grain diameter, aspect ratio and grain orientation, are quantified for ferritic-phase steel. Two dimensional grain shapes reflecting the distribution of those parameters are modeled by the use of Voronoi tessellation that is the method to divide the region into arbitrary polygons. Stress distribution in the case of tensile load applied to such a set of grains is calculated by Finite Element Method (FEM). Mismatch of displacement near the grain boundary is also considered in the calculation as a boundary slip. Discontinuity of stress distribution on grain boundary is seen even though the single-phase steel is used and the same stress-strain property is given to all grains. Stress concentration around triple junction is also observed due to results of considering the mismatch of displacement near the grain boundary. The effects of distribution of grain shape parameters on stress distribution are investigated for the fundamental application of this analysis model, and the difference of stress distribution is estimated. It can be mentioned that numerical analysis model in this paper is available for the evaluation of strength characteristics of steels.
In situ observation of the mechanical failure behavior was conducted for different kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of an optical microscopy under the static loadings at ambient and elevated temperatures; as the fundamental aspect, in order to clarify the thermo-mechanical failure mechanism in connection with various coating characteristics. Mechanical tensile or compressive loading was applied progressively to the TBC specimen by an axial loading manner. It was found that the failure behavior of TBC system depends strongly on the testing temperature under both the tensile and compressive loadings. Namely, at the elevated temperature which is higher than the ductile-brittle transition temperature (DBTT) of metallic CoNiCrAlY bond-coat (BC), the ceramic yttria-stabilized zirconia (YSZ) top-coat (TC) spalling can be prevented effectively by virtue of the strain accommodation due to the extensive plastic flow in the BC layer. At the ambient temperature which is lower than the DBTT of BC, on the contrary, the TC spalling is more pronounced as a strain increases regardless of stress direction. Such an initiation site of TC spalling is closely related with the magnitude of local plastic deformation in the alloy substrate. Furthermore, the influence of coating microstructures on the crack initiation and propagation behavior was investigated in detail.
The heat-resisting temperature of a conventional bonded magnet produced by combining a magnetic powder and a resin or a rubber is limited to about 150°C. A new type of metal bonded magnet was fabricated using AZ31 magnesium alloy powder and Nd-Fe-B -base magnetic powder to improve the heat-resisting temperature of the bonded magnet. The AZ31 magnesium powder was produced by pulverizing scrap generated by machining. An isotropic Fe3B/Nd2Fe14B nano-composite powder was used as the magnetic material. The mixed powder was formed by closed-die compaction and extrusion at 240∼350°C. The magnetic properties and mechanical strength of the formed metal bonded magnet were investigated. Also, bonded magnets were fabricated using bonding materials such as brazing and conventional resin powders, and the magnetic properties were compared with that of the magnet produced using AZ31 powder. It was found that the magnetic properties of the bonded magnet could be determined by the product of the density of the bonded magnet and the mass ratio of the magnetic powder, namely, the mass content of the magnetic powder per unit volume (dimensions Mg·m-3) regardless of the kind of the bonding material that was used. The maximum mixing ratio of the magnetic powder that satisfied the criterion of 15MPa strength for the splitting tensile strength test was 85 mass%, and the mass content of the magnetic powder per unit volume was 3.47Mg·m-3.
The X-ray stress measurement based on sin2ψ method has been widely adopted. This technique is not entirely efficient, because it is necessary to measure each diffraction profile one after other at five and more incident X-ray angles. On the other hand, stress measurement based on cosα method was proposed using X-ray film as detector. In this method, the stresses are determined from Debye-ring obtained at a single X-ray incident angle with high efficiency. However, it has the problem in this accuracy on stress determination. In this study, a new stress analyzer with sixteen position sensitive proportional counters (PSPC) radially arranged for incidence X-ray was developed. We examined to determine the plane stress components by multi-regression analysis using sixteen diffraction angles on a Debye-ring. As a result, stress components of σx and τxy were measured with sufficient accuracy. However, a stress of σy had large dispersion, because of low strain sensitivity in the y direction. Moreover, it was clarified that σy had pseud-multicollinearity on multi-regression analysis. The accuracy of σy value itself could not be improved, though an analysis to eliminate an effect of pseud-multicollinearity was carried out. In this paper, we proposed the new method to determine the components using two Debye rings obtained by X-ray incidences in both x and y directions. We tried to measure the residual stress components in the butt welded joint using the method. As a result, it was confirmed that the stresses, σx and σy, were obtained in the sufficient accuracy, and that the shear stress, τxy, was arising near the bond in the welds.