Papers by Keyword: Fractography

Abstract: In order to investigate the interior-induced fatigue crack propagation behavior of high cleanliness valve spring steel (JIS SWOSC-V), rotating bending fatigue tests were performed for various kinds of specimens with different hardness or surface finishings. The harder specimen with higher compressive residual stress showed longer fatigue life. The electrochemical polished specimen pre-treated with shot peening showed almost same fatigue life as the shot-peened specimen in spite of the difference in surface roughness. After fatigue tests, fracture surfaces were observed using a scanning electron microscope (SEM) to evaluate the fatigue fracture mechanism. Most specimens failed in surface-induced fracture mode due to high cleanliness; however, some specimens failed in interior-induced fracture mode in the very high cycle regime. Although non-metallic inclusions were not observed at interior fatigue crack initiation sites, 2 types of significant microstructures (with smooth surface or granular surface) were observed. EBSD analysis, profile analysis and computational simulation using a fracture surface topographic analysis (FRASTA) method were performed to investigate the mechanism of the interior-induced fatigue fracture caused by the microstructure at defect without any inclusion.

Abstract: Fatigue tests were carried out at the stress ratio R = -1 using a 20 kHz ultrasonic testing facility to investigate the effects of low temperature nitriding on the fatigue properties of Ti-6Al-4V alloy in the very high cycle fatigue (VHCF) regime in detail. The oscillation and fatigue behavior of the nitrided Ti-alloy were characterized by measuring parameters like the ultrasonic generator power, the displacement of the specimens and dissipated energy under ultrasonic cyclic load. Moreover, the surface microstructure of the nitrided Ti-alloy was characterized using a micro-Vickers hardness tester, an optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscatter diffraction technique (EBSD) to clarify the fatigue fracture mechanism. The Ti-alloy nitrided at the temperature of 873 K showed duplex S-N properties consisting of the respective fracture modes of the surface fracture and the subsurface fracture. The low temperature nitriding reduced the surface fatigue life of Ti-alloy in comparison to the un-nitrided one due to the formation of a brittle titanium nitride (T_i2N), whereas the subsurface fatigue life in the VHCF regime was increased by the low temperature nitriding. In addition, the fatigue fracture mechanisms of the low temperature nitrided Ti-alloy were discussed from viewpoints of fractography and fracture mechanics.

Abstract: In order to use a low flammability magnesium alloy as structural components, very high cycle fatigue properties of this alloy (AMCa602) were investigated. S-N properties obtained in both rotating bending and axial loading were compared with each other. It was found that S-N curve in the axial loading appeared a little lower than that in the rotating bending due to the differences of stress distributions and critical volumes for both loading types. Moreover, the statistical aspect on the fatigue property was analyzed as P-S-N characteristics in the rotating bending. After fatigue tests, fracture surfaces of failed specimens were observed by means of a scanning electron microscope (SEM) and the microstructures at the crack initiation site and the propagation path were also observed by combining FIB technique and EBSD analysis. Thus, it was found that some specimens failed from surface inclusions and their fatigue lives were lower in comparison to those of the specimens without surface inclusions. In addition, the fracture surfaces of this alloy revealed very rough in the usual life region, whereas a characteristic smooth area was observed on the fracture surfaces of specimens failed in the surface inclusion-initiated fracture and in very high cycle regime. A stress intensity factor range at the front of the smooth area (ΔK_smooth) tended to a definite value so that the fracture mechanism of this alloy was governed by a concept of ΔK.

Abstract: Four compositions of some usually commercial dental alloys were investigated in order to determine the mechanical characteristics and fractographic analysis of tensile and bending tests surfaces. A correlation between chemical composition (either molybdenum or molybdenum and chromium contents) and mechanical characteristics (longitudinal modulus, tensile strength and elongation) were finally done.

Abstract: The paper demonstrates that indirect testing performed with thermal microscopy facilities can be used successfully to study the properties of heat-resistant added metal.

Abstract: This contribution deals with the analysis of causes of a breakdown of a turbine rotor where stage 13 completely broke off. Microstructure and fractographic analyses showed that a dominant cause of the breakdown was stress corrosion cracking caused by impurities in the steam.

Abstract: This paper deals with an identification methodology of the interfacial fracture parameters to predict the lifetime of a metallic brazed joint. The methodology is based on an experimental-numerical study whereby the optimal parameters are obtained. The experimental data, using the scanning electron microscope analysis, allowed approving that failure of the assembly based AuGe solder seems first to appear near the interfaces. These results were confirmed by micrographs analysis of the solder/insert and solder/substrate interfaces. Then, using shear test results and parametric identification coupled with a finite elements model (FEM) simulation, the damage constitutive law of the interfacial fracture based on a bilinear cohesive zone model are identified. The agreement between the numerical results and the experimental data shows the applicability of the cohesive zone model to fatigue crack growth analysis and life estimation of brazed joints.

Abstract: In this work, nanostructured magnesium oxides were formed by PEO process on cast pure magnesium and the tensile properties of thin (5.8 μm) and thick (11.2 μm) coated samples were evaluated. The results obtained by uniaxial tensile testing show that the thin PEO coating had very little effect on the ultimate tensile strength (UTS) and elongation of the cast pure magnesium, while its yield strength was somewhat decreased. For the thick coated sample, both the yield strength and UTS decreased considerably, but the variation of the elongation was moderate, compared to the uncoated sample. The microstructures characterized by SEM revealed that the high porosity content and the large size of pores in the thick PEO coating should be responsible for the significant reduction in the properties of cast pure magnesium. The relationship between the tensile properties and microstructure of the PEO coated samples suggests that the thin PEO coating should be selected to protect the pure magnesium for biodegradable applications.

Abstract: The paper deals with the analysis of the crack detected in the Kaplan turbine blade. In order to keep the blade integrity the sample was removed by using small sample method. Fractographic and metallographic analyses were used to determine the cause of a detected crack appearance and propagation. The material of the turbine blade was made in 1937 and its structure containing numerous eutectic sulphide inclusions corresponded to the steelmaking technology of that time. Numerous occurrences of sulphide inclusions and sulphide eutectics were identified as the main cause of material failure. Regardless of these non-metallic inclusions the blade was repaired by welding and nowadays the repaired blade has been under safe operation for more than 9 years.

Abstract: The objectives of this in vitro study is to display the overall planar morphology of leakage at resin-dentin interface using ammoniacal silver nitrate as tracer. Twelve human extracted third molars were used and the occlusal enamel of each tooth was removed. All teeth were divided into four groups of three teeth each and bonded with one of four adhesives (Prime&Bond NT, Adper Prompt, Xeno III, Clearfil S³ Bond) and then a composite resin crown was built up. After storage in water (37°C) for 24 h, all teeth were vertically serially sectioned into stick-shaped specimens the bond interfaces. All specimens were immersed in ammoniacal silver nitrate solution, followed by developing solution and subjected to tensile test and the fracture surface were observed with a SEM. The overall planar morphology of leakage at resin-dentin interface appeared to be various tree-like figures, with character of stem-like portion in the periphery and extending from the periphery to the center of the fractured surface and stretching out a lot of branches. Prim&bond NT presented a couple of big tree-like silver deposition extending to center besides many short tree-like figures located along the periphery. Adper Prompt showed short tree-like figures with many branches. Xeno III displayed tree-like figures with thinner stem portion and more branches. Clearfil S³ Bond presented many short shrub-like figures with fewer branches.