Free-edge stress singularity usually develops near the edge of interface of bonded dissimilar materials. Because of the difference of the coefficient of the thermal expansion of the materials, the residual stress may develop when two materials are bonded. To defuse the residual stress, an interlayer may be inserted between the two materials. Stress distribution near the edge of the interface in the presence of the interlayer is very important to evaluate the strength of the bonded dissimilar materials. In this study, stress distribution on the interfaces of the bonded dissimilar materials with an interlayer under thermal stress loading was calculated by using the boundary element method. An effect of an interlayer on the stress distributions on the interface was examined numerically. It was found that the influence of the interlayer on the stress distribution was limited within a region where distance from interface edge is smaller than the interlayer thickness. Stress distribution around the edge of the interface of the bonded dissimilar materials with an interlayer can be normalized by the parameters of the singularity and the thermoelastic constant stress term of the bonded dissimilar materials without an interlayer.
The heterogeneous material properties affect on crack growth. For this reason, it is important to develop a simulation system to evaluate the crack growth in heterogeneous materials. This paper develops a fully automatic crack growth simu-lation system in heterogeneous materials based on Superposed FEM (S-FEM). The simulation should be treats mixed-mode loading condition. The virtual crack closure method (VCCM) is used for evaluation of the stress intensity factors (SIFs). The direction of crack growth is calculated by the maximum tangential stress (MTS) criterion, and the growth rate is calcu-lated through Paris law. First, in order to demonstrate the effectiveness, the SIFs evaluated by the developed system are compared with those of reference solution, the boundary element method (BEM) and body force method (BFM). Then, two-dimensional crack growth problems are simulated using the developed system. The first example is crack growth in a plate with an interface between hard and soft materials. The crack tends to grow into soft material through the interface. The crack path and the change of SIFs are studied. The second example is crack growth in a plate with a slant phase inter-face. Simulated crack paths are compared with that of experimental paths. The well agreement of these crack paths shows the usefulness of the developed system for crack growth simulation in heterogeneous materials.
The effects of fiber orientation and stress ratio on the crack propagation behavior were studied with single edge-notched specimens which were cut from an injection-molded plate (IMP) of short carbon-fiber reinforced polyphenylene sulphide, at five fiber angles relative to the loading axis, i.e. θ = 0° (MD), 22.5°, 45°, 67.5°, 90° (TD). Macroscopic crack propagation path was nearly perpendicular to the loading axis for the cases of MD and TD. For the other fiber angles, the crack path was inclined because the crack tended to propagate along inclined fibers. In the relation between the crack propagation rate, da/dN, and the stress intensity factor range, ΔK, the propagation rate of fatigue cracks was slowest for MD, and increased with increasing fiber angle. When da/dN was correlated to ΔK/E (E = Young's modulus), the relations for different orientations merged into a single relation. The core layer existing in IMP accelerated crack propagation in MD direction, and decelerated in TD direction. The da/dN vs ΔK/E relation of skin-layer plates is close to that for IMP. The effect of stress ratio becomes minimal when da/dN is correlated to the range of the energy release rate, ΔG = Gmax - Gmin (Gmax, Gmin = maximum, minimum energy release rates). The relation between da/dN and ΔG/E shows the least scatter for SFRP with different fiber orientations under different stress ratios.
Relationship between process parameter, such as tool rotation speed and plunging speed, and strength of FSSWed PVC joint compounded SiC particle in the welded region was investigated. In case of a joint compounded SiC particles, width of welded area increased compared with that of a joint without SiC particles due to increasing heat input during the process by friction between a rotating tool and SiC particles in all welding conditions tested. In case of a joint joined with the medium heat input condition, elastic modulus at the composite region was higher than that at stirred region in a joint without SiC particlse and the similar as that of the base material. On the other hand, in case of a joint joined with higher heat input condition, degradation of mechanical property at the composite region occurred significantly in the joint compounded SiC particles. The maximum tensile shear loads of the joints joined with the lower and medium heat input conditions were improved by fabricating composite material in welded region. However, the maximum tensile shear load obtained in a joint joined with the higher heat input condition was similar or lower than that of a joint without SiC particle. Effect of width of welded area and elastic modulus at the composite region on joining strength was evaluated with J-integral. Strength of the joint was affected by not only width of welded area but also mechanical property of the composite region. Heat input generated by fabricating composite material in the welded region leads to form larger width of welded area and results in higher the maximum tensile shear load. However, in the same time, mechanical property of composite region is also possibly degraded due to excessive heat input. In order to optimize the welding condition, both effect of changes in size and mechanical property of welded area should be taken into account.
In order to evaluate quantitatively the effects of casting defects on the fatigue properties of aluminum die-casting alloy, a fatigue limit estimation for aluminum die-casting alloy (JIS ADC12) containing large casting defects was proposed by means of √area method. Fatigue tests were performed in ambient air by means of a dual spindle type rotating bending fatigue testing machine for two types of the specimen; one is an hourglass type specimen and the other is a smooth specimen having four artificial drilled holes whose √area were ranged from 300 to 2000 μm on the surface. After fatigue tests, fracture surfaces and artificial defects were observed using an optical microscope and a scanning electron microscope (SEM). As results, all hourglass type specimens failed from casting defects. In the case of the smooth specimen having four artificial drilled holes, fatigue limit tended to decrease as artificial defects became large. According to the fatigue limits of the hourglass type specimen and of the straight type specimens having four artificial drilled holes, fatigue limit estimation for aluminum die-casting alloy was proposed by means of modified √area method. In addition, it was clarified that the proposed equations for JIS ADC12 could estimate fatigue limits of another type aluminum die-casting alloys, JIS ADC10-T6.
Fatigue limit estimation based on dissipated energy measurement was applied to the evaluation of shot peening effect on fatigue limit improvement. It was found that estimated fatigue limit based on dissipated energy for the shot peened material showed conservative value, compared with that obtained by conventional fatigue tests. In the constant stress amplitude fatigue test, dissipated energy for unbroken specimens with shot peening treatment was larger than that for broken as-received specimen. SEM observation for unbroken specimens with shot peening treatment showed that some micro cracks existed inside of dents. These facts indicated that dissipated energy caused by structure change leading to crack initiation was observed at stress amplitude above the estimated fatigue limit for unbroken specimens with shot peening treatment, however crack propagation was inhibited by compressive residual stress field around initiated micro cracks. Therefore, it was considered that fatigue limit estimated by dissipated energy measurement indicated crack initiation limit in total fatigue life.
In our previous studies, the changes in the magnetic flux density around fatigue cracks during their propagation process were observed, and a good correlation between the changes of the magnetic flux density distribution and stress intensity factor was recognized. This result suggested that non-destructive evaluation of the stress intensity factor of fatigue cracks would be possible using this relationship. In order to clarify the mechanisms of the changes in the magnetic flux density distribution around fatigue cracks, four-point bending fatigue tests for JIS SCM440 specimens were carried out, and the effects of the heat treatments to ease the plastic deformation around a crack tip, as well as the additional fatigue tests after the heat treatment were discussed. From the changes of the magnetic flux density distribution accompanying “the heat treatment” and “the fatigue testing after the heat treatment”, it was considered that the change of the magnetic flux density around the fatigue crack is caused by plastic deformation generated near the fatigue crack tip and accumulated along the crack path.
Although culms of bamboo are utilized for various materials, technologies for effective use of bamboo rhizomes have not been established yet. In this study, therefore we analyzed the rhizomes of moso bamboo (Phyllostachys pubescens) to determine their chemical components and to quantify various inorganic substances for the purpose of the investigation of their fundamental characteristics. The ash and extractable compounds contents were much higher in the rhizomes than in the culms. The lignin and cellulose contents were higher in the culms than in the rhizomes. The hemicellulose content was higher in the rhizomes than in the culms. The ash particles formed various shapes: needles, grains, and fluff. The ash particles contained high levels of O, Mg, Si, P, S, and K. We mapped the localization of various inorganic substances in the rhizome, and observed that Si was concentrated in the epidermis. The distributions of elements were different among various tissues in the rhizome such as metaphloem and tylosoid. The amount of extractives tended to increase in rhizomes from summer to autumn. The amount of free saccharides such as sucrose, glucose and fructose, and starch tended to decrease in summer and increase from winter to spring. The highest free saccharides and starch content (12%) was in rhizomes collected in April. Sucrose and starch were the main components of total free saccharides and starch in all seasons.