In advanced material such as composite and coatings, interfacial crack initiation and propagation, which are caused by mechanical and thermal loadings, are serious problem. A lot of interfacial fracture testing methods have been proposed to evaluate the adhesive strength of those advanced materials. However, there are some difficulties when those proposed testing methods are utilized because of restriction of the specimen geometry, of fracture loading direction, and so on. Especially, the stress field at the interface under an actual loading is known to be in mode-II load condition, which gives us motivation to do this study. In this study, simple interfacial fracture testing method is proposed. In the proposed method, the ridged indenter is just indented on the sample surface, which leads easily to the interfacial crack under mode-II condition, and the interfacial fracture toughness can be easily estimated from both indentation depth and interfacial crack size. The closed formula to estimate interfacial elastic J integral as the interfacial fracture toughness is obtained based on stress function and variation of energy released by crack propagation with a help of the FEM results by using domain integral technique installed in commercialized finite element code MARC. The indentation tests are conducted to check verification of the interfacial fracture toughness by the proposed test method for Aluminum alloy / PMMA combination samples.
On 8 grades of porous ceramics which have various pore sizes and their distributions, bending tests were carried out to clarify correlation between strength and microstructure. To identify fracture origin, in-situ observation technique using high speed camera was developed. In most cases, the linking pore was observed at fracture origin on surface or nearby surface of specimen. Correlation between microstructure of porous ceramics, linking pore size and strength was formulated on the basis of point stress model. Strength was well correlated with a parameter which is the linking pore size non-dimensionalized by average diameter of skeleton structure. According to the relational expression, it was predicted that the larger pore size leads to higher strength in the case of comparable porosity.
The influence of fiber orientation on the crack propagation behavior was studied with single edge-notched specimens which were cut from an injection-molded plate (IMP) of short carbon-fiber reinforced polyphenylene sulfide (PPS) at two fiber angles relative to the molding direction, i.e. θ = 0° (MD) and 90° (TD). Specimens made of only shell layer of IMP and of only PPS were also produced. The finite element method based on anisotropic elasticity was used to determine the stress intensity factor, energy release rate, and crack-tip opening parameter as crack driving forces. The macroscopic crack propagation path was perpendicular to the loading axis in MD and TD, showing mode I propagation. Microscopically, for MD, the crack was blocked by fibers and circumvented fibers along interfaces, showing a zigzag path. For TD, the crack path was less tortuous following the fiber direction. When the crack propagation rate, da/dN, was correlated to the range of stress intensity factor ΔK, da/dN was lowest for MD and highest for PPS. The core layer in TD of IMP retarded crack propagation in the shell layer. Difference among MD, TD and PPS became small when da/dN was correlated to a parameter corresponding the crack-tip-opening radius, HΔG, where H was a compliance parameter. Strictly speaking, da/dN was lowest for MD, highest for TD, and PPS in between. Fibers perpendicular to the crack direction block crack propagation, while parallel fibers provide preferential crack paths.
Borated austenitic stainless steel, B-SUS304P-1, has been used for storage and transportation cask in the nuclear industry, because it has enough structural strength, heat dissipation function and neutron absorption ability. In the present study, fatigue crack growth (FCG) tests were conducted under stress ratio, R, and maximum stress intensity factor, Kmax, constant conditions and the FCG rates of B-SUS304P-1 were compared with those of SUS316. In order to clarify the upper bound characteristics of FCG, fracture toughness tests were performed for both materials according to the ASTM standard E1820 and revealed the crack growth characteristics under static loading condition. Detailed observations of fracture surfaces indicated that the boride consisted of iron, chromium and boron and produced locally flat fracture surface, which suggested brittle fracture. However, the FCG rates were insensitive to the boride presence, while the fracture toughness was reduced. Based on the results, the effect of boron addition on the FCG characteristics were discussed over the wide range of FCG rate.
In order to study influencing factors in interfacial structure on characteristic of macroscopic strength of A1100/Zn-plated steel friction stir welded dissimilar materials lap joint, fracture process under tensile shear loading was investigated by in-situ observation test conducted inside a scanning electron microscope. Intermetallic phase between Fe and Al was formed at the interface in all joints obtained in the present study. Moreover, in the joint with lower strength, Fe-Zn alloy was also formed interface between the intermetallic layer and Zn-plated steel. Those intermetallic phase and Fe-Zn alloy influenced on the fracture process and strength. In case of the joint with higher strength, change in mechanical property of A1100 due to penetration of Zn into A1100 might enhance plastic deformation and crack deflection. Based on results of the in-situ observation test, appropriate condition for friction stir welding of the dissimilar material joint was proposed by taking into account influencing factors shown.
Experimental technique of in situ FESEM fatigue/creep experiments has been developed to investigate the mechanisms of fatigue/creep crack propagation in freestanding metallic nano-films. We developed an in situ FESEM fatigue/creep dual-mode testing machine which was able to conduct the fatigue or creep experiments inside the FESEM chamber. The testing machine has the capability to apply high-frequency cyclic load or constant load under load-control conditions to the freestanding metallic nano-film specimens. In situ FESEM observations of the fatigue crack propagation in 523-nm-thick freestanding copper (Cu) films confirmed that intrusions/extrusions were formed ahead of the fatigue crack tip in the lower stress intensity factor range (ΔK), and the size of the intrusions/extrusions increased as the number of stress cycles increased. The fatigue crack then propagated preferentially through these intrusions/extrusions. In the higher ΔK, the fatigue crack propagated in tensile fracture mode. In addition, in situ FESEM observations of the creep crack propagation in 391-nm-thick freestanding gold (Au) films confirmed that voids were formed ahead of the creep crack tip, and the crack then propagated by coalescence of the voids and the crack. These results indicate that this experimental technique is effective to clarify the mechanisms and mechanics of fatigue/creep crack propagation in freestanding nano-films.
There is historical evidence that Japanese coins were imported from China from the 12th century for about 600 years. These coins stayed in circulation until the 18th century. During the period, daimyos and influential persons such as merchants energetically minted imitations of inferior quality in order to make higher profits as well as to supplement the lack. Some of these imitations had the marks of rulers or technicians minting these coins and, ironically, are valuable in their own right due to their historical value. The market for these marked original imitations has resulted in original, unmarked coins being marked and sold as original imitation coins. Unfortunately the conventional X-ray fluorescence analysis cannot tell whether the mark is authentic or made after minting the imitation coin. In this study, we tried to apply X-ray stress measurement to the analysis of differences between an authentic coin and a counterfeit coin. The primary elemental composition of two kinds of old coins of “Shofu Tsuho” originating from the Shofu Era of the Chinese Northern Sung Dynasty were determined by the X-ray fluorescence analysis. For Cu element of the major elements, the difference between the authentic coin with the mark and the counterfeit coin with the same mark was evaluated by the residual stress and the work hardening of the marked area using X-ray residual stress measurement. Herein we report the results of the analysis.