The singular stress distribution around an interface edge of bonded different power-law hardening materials has been investigated by elastoplastic finite element analysis. The theoretical results based on the displacement-matching method has shown that the stress singularity depends only on the material with larger hardening exponent. However, the numerical results has shown that both the size of the yield zone and the elastic-plastic stress distribution are dependent of the material combination. It is found that there is a region within which the theoretical elastic-plastic solution is valid, but the size of such a region is strongly effected by the material combination. If the difference of the hardening exponent is small, the elastic-plastic theoretical solution may lose its physical meaning because its domain region is too small, and the logarithmic stress distribution near the interface edge behaves no longer as linear.
To evaluate arbitrary shaped defects or cracks terminating at an interface, a formula is proposed in terms of Murakami's √area parameter for the maximum stress intensity factors. Here “area” is the projected area of the defects or cracks. First, the solution for a rectangular crack, which is perpendicular to and terminating at a bimaterial interface, is considered with varying the aspect ratio of the crack and combinations of materials constants systematically. Then, the maximum stress intensity factors at the other side of the interface are expressed as a function of the elastic ratio of the materials. On the other hand, the general stress intensity factors at the interface is expressed as a function of Dundurs parameters α and β. Those expressions are usefully evaluating the defects under any combinations of the materials.
In the reflow soldering process, the IC package is heated over 270°C by the infrared rays. In a delamination interface, the stress field near a crack tip will always be in a mixed-mode state. It was assumed that the initial defects were generated among the corner part of die pad and epoxy molding compound. The 3-D finite element analysis was carried out to evaluate the strain energy release rate of delamination interface in IC packages subjected to non-steady thermal stress. The strain energy release rate of mixed mode was evaluated by the modified crack-closure integral and virtual crack extension method. The maximum difference of strain energy release rate between the modified crack-closure integral and virtual crack extension method was about 8%. Therefore, it is considered that the modified crack-closure integral is useful as a simple evaluation method of the strain energy release rate. The stress intensity factor K was examined from 3-D and 2-D virtual crack extension method. The virtual crack extension method can evaluate stress intensity factors of the interface crack under a mixed-mode loading accurately.
Substrate specimens of mild steel (JIS: SS400) and tool steel (JIS: SKD5) were coated with WC-Co cermet by High-Pressure High-Velocity Oxygen Fuel (HP-HVOF) spraying. Repeating pressure was applied on the surface of the specimens, and tensile tests and edge-indent tests were carried out to evaluate the tensile strength and the delamination strength of the coating. The tensile strength σC of WC-Co coating increases first and then decreases with increasing surface pressure cycles N when the maximum repeating pressure σmax is 240MPa, while σC increases with increasing N for σmax=480MPa. Both the interfacial fracture toughness Gc1/2 and the delamination strength Ed for SS400 specimens decreases with increasing N. On the other hand, Ed for SKD5 specimens does not change with N in each σmax. The SEM micrographs of cross section of coating show the existence of cracks parallel to the interface for SS400 specimens and they are developed with increasing pressure cycles, while the crack extension is small for SKD5 specimens.
The effects of microstructure and triaxiality on ductility were investigated for α/β Titanium alloys with equiaxed α phase. The materials tested were Ti-6Al-4V, Ti-4.5Al-3V-2Fe-2Mo and Ti-10V-2Fe-3Al, and several different heat treatments were conducted for these materials to obtain the different grain size and morphology. It was shown that the most of micro-voids in α/β titanium alloys were nucleated at α/β interface, and the microstructural parameter defined as the average distance between α/β interfaces was closely related to the dimple size on the fracture surface. The refinement of microstructure leads to the reduction of ductility, though near-β titanium alloys with ultra-fine particle have exceptionally a different tendency from other materials. The dependency of triaxiality on fracture strain obtained in the round bar specimens were also investigated. Significant reduction of strain with the increase of triaxiality is observed, but the reduction due to the high triaxiality is different in each materials. It is shown that the void growth strain in higher strength materials is relatively small, and it results small dependency of the triaxiality on the ductility.
This paper dealt with fracture toughness and fracture behavior of ceramics-metal functionally graded material (FGM). A possibility of stable crack growth in a three-point-bending specimen was examined based on the crack driving force and the crack growth resistance of FGM, and the distribution of fracture toughness was evaluated on the FGM fabricated by powder metallurgy using partially stabilized zirconia (PSZ) and stainless steel (SUS 304). The materials had a functionally graded surface layer (FGM layer) with a thickness of 1mm or 2mm on SUS 304 substrate. Three-point-bending tests were carried out on a rectangular specimen with a very short crack in the ceramics surface. On the three-point-bending test, unstable crack growth occurred from a short pre-crack to some amount of crack length, and then the crack grew stably to the interface between FGM layer and substrate with increasing the applied load. Finally, the crack was arrested at the SUS 304 substrate, and the specimen deformed plastically. From the relationship between the applied load and crack length during the stable crack growth in the FGM layer, the fracture toughness was evaluated. The fracture toughness increases with an increase in a volume fraction of SUS 304. It is concluded that the three-point bending test of the rectangular specimen with a short pre-crack is useful to evaluate the distribution of fracture toughness in FGM layer.
The present paper describes the effect of humidity on the fatigue behavior of a high carbon chromium steel having very high strength. Cantilever-type rotary bending fatigue tests were performed in controlled relative humidity of 5, 45, 55 and 85%. It was found that step-wise S-N curves were seen in relatively lower humidity up to 55%. The transition stress at which the crack initiation site changed from the surface to the subsurface decreased with increasing humidity. In high humidity of 85%, however, cracks were always generated at the specimen surface regardless of stress level, thus step-wise S-N curve was not observed. The decrease of the transition stress and the disappearance of step-wise S-N curve in the high humidity were attributed to humidity-assisted surface crack initiation. At stress levels where surface or subsurface crack initiation occurred, fatigue lives were nearly the same regardless of humidity because of shorter time to fracture in the former and crack initiation from an inclusion inside the specimens in the latter. In subsurface fracture, a fish-eye was always seen with an inclusion at its center from which cracks initiated, and the morphology of fish-eyes was independent of humidity. Fatigue life increased with decreasing stress intensity factor range for inclusion, which was not influenced by humidity.
In order to measure the Mode II fatigue crack growth resistance of various materials, a particular chevron notched double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to the ΔKII-decreasing test with increase in crack length. The crack length was measured by the AC potential method. The particular morphology of Mode II crack fracture surface was investigated. The distribution of shear stress in crack tip and Mode II stress intensity factor were analyzed using a FEM software, ABAQUS. The relationships between Mode II crack growth rate (da/dN) and ΔKII for various materials were obtained, and the values of ΔKIIth were determined. It was found that ΔKIIth is larger than ΔKIth for all materials tested in this study.
When the piezoelectric film is glued on the surface of a cracked material subjected to mechanical load, change in distribution of electric potential is observed on the surface of piezoelectric film. Based on this phenomenon, the passive electric potential CT (computed tomography) method was proposed for identifying two- and three-dimensional cracks. This method uses passively observed electric potential distribution on piezoelectric film induced by the strain distribution of the objective body without applying electric current. Therefore, this method can be applied to develop an intelligent structure with a function of self-monitoring of flaws and defects. In this paper, the passive electric potential CT method using a piezoelectric material is applied to the identification of a through-thickness transverse crack. The effects of crack size and location on electric potential distribution were examined by using the finite element method. For the identification of cracks from electric potential distribution, an inverse method based on the least residual method is applied, in which square sum of residuals are evaluated between the measured electric potential data and those computed by using the finite element method. The crack was identified from experimentally observed electric potential distribution. It was found that the location and size of the crack can be quantitatively identified by the passive electric potential CT method.
Poly (vinyl chloride) (PVC) is a general-purposed plastic material widely used in the industry and daily life. It has quite good compatibility with some other plastics and plasticizers. However PVC is known to be fairly unstable at higher temperature. Hydrochloride acid (HCl) gas is generated due to the thermal decomposition during molding process and when it is thrown into fire. This may cause serious problems such as the corrosion of molding machine and the environmental pollution. Shell is a natural material, which contains 99wt% calcium carbonates. Since there are piles of wasted shells on the coastline, utilization of these shells has been expected in those areas. In this study, the role of shell powder mixed in PVC as an entrapping agent of HCl generated at the processing and burning of PVC was investigated. Two kinds of waste shells were ground into powder and mixed in PVC. HCl entrapping efficiency during burning process of PVC/shell powder composites were investigated and the change in the mechanical properties were also measured. The results obtained clearly show the excellent efficiency of shell powder to entrap HCl generated and the efficiency depends on the type and size of the powder. It is concluded that the shell powder in PVC composite materials is effective to separate generated chlorine from PVC and therefore, is benefit to the environment.
RC structures using the recycled aggregate concrete composed of recycled aggregate will evidently become concrete waste in due time. Such concrete waste has to be recycled as well in order to reduce concrete waste. In other words, the ideal goal is to use the recycled aggregate continuously. The purpose of this study is to determine whether the permanently recycled concrete is possible to use for reinforced concrete structures in terms of structural properties. For a structural property, aseismatic property was studied and discussed, taking into consideration the susceptibility of Japan to frequent earthquakes. The beam specimens of the flexural, shear and bond failure were tested on two types of concrete made of different concrete aggregate: recycled coarse aggregate made of original concrete, and mortar aggregate which was the final product of continually recycled concrete coarse aggregate. The test results showed that the beams using mortar aggregate concrete were equal in aseismatic property to normal concrete beams of the same concrete strength. From the above mention, it was verified that the permanently recycled concrete is possible to use for reinforced concrete structures.