This paper demonstrates that the scanning probe microscopy (SPM; scanning tunneling microscopy (STM) and atomic force microscopy (AFM)) is capable of performing in situ nanoscopic observation of localized damage in an operating environment: initiation and growth processes of corrosion products, pitting corrosion and intergranular corrosion as well as stress corrosion cracking of metals. This technique is also applied to examine brittle fracture surfaces of various engineering materials including metals, ceramics and single crystal silicon, and the nanoscopic fracture features are discussed. We discuss the related issues in in situ SPM visualization, advantages and disadvantages of STM and AFM, and nanoscopic damage mechanisms based upon nanoscopic observation of damage process; we show that an AFM is more suitable for performing in situ, serial observation of corrosion processes.
In structural ceramics, a remarkable scatter of strength is observed due to brittle behavior in deformation and flaws unavoidably formed during material processing. Therefore, information on strength characteristics of ceramics is required for the mechanical design in their applications to high-performance components. In the present paper, the strength properties of typical ceramics are reviewed from statistical aspects, especially. As for the static strength, general features of ceramic strength are first outlined, and then several factors affecting strength, such as environments, specimen geometry, stress state and machining, are described. The fatigue strength is reviewed in two categories of the static and cyclic fatigue. As one of screening procedures to exclude components with a lower strength, the proof testing in ceramics is described and the problems in the testing are also mentioned.
The spin-lattice relaxation times (T1) of D and 17O nuclei of coordinated heavy water (D2O) molecules for Na+ and K+ in sodium bromide and potassium bromide dilute aqueous solutions were measured in the range of 5-50°C by means of NMR. The spin-lattice relaxation rates (R1=1/T1) varied linearly with the concentration up to 1mol/kg at a given temperature. The parallel (D) and the perpendicular (17O) rotational correlation times of coordinated D2O molecules for Na+ and K+ were determined. The ratio τ(D)/τ(17O) of the rotational correlation times of the coordinated D2O molecules was larger and smaller than unity for Na+ and K+, respectively, and was practically independent of temperature, which indicated that the rotational anisotropies of the coordinated D2O molecules was almost temperature independent.
Quantitative analysis for the shape irregularity of ceramic particles was performed by applying the concept of “Fractal” proposed by B.B. Mandelbrot. Two types of ceramic particles processed by different methods were employed as samples. Silhouette of each particle was repeatedly observed along 3-dimensional Cartesian axes, and then Richardson effect of the configuration of the respective silhouette was normalized by the perimeter. In this normalized Richardson effect, a characteristic aspect concerning the microscopic and macroscopic irregularities of particle shapes was found. The microscopic irregularity was successfully represented by a fractal dimension, whereas the macroscopic one was well evaluated by another parameter of macroscopic shape index introduced in this study.
The influences of superplasticizers, mixing time, mixing temperature and cement content on high-volume fly ash concrete were investigated in order to use the industrial by-product, fly ash from coal thermal power plant, more actively for concrete. The slump, air content, compressive strength and tensile strength of high-volume fly ash concrete were investigated using different mixtures. The results of these investigations indicate that the properties of fly ash concrete are influenced by the type of superplasticizer and the use of naphthalene-based type superplasticizer is most suitable for this concrete. The slump and the air content of high-volume fly ash concrete are varied by the mixing time and mixing temperature. A three minute mixing time is best to obtain workable high-volume fly ash concrete in a pan-type mixer.
The oxidation characteristics of Ti-14Al-21Nb has been studied with an emphasis on the influence of nitrogen. The oxidation curve in N2-21%O2 is significantly lower than in O2 or in Ar-21%O2 at 1300 K. At the scale/substrate interface, two thin continuous layers and an Al-enriched layer were formed. These three layers were identified as/TiN/Nb2Al/TiAl/ by XRD, EPMA, SEM, and optical observation. The oxidation rate is reduced by the formation of the TiN layer, although the oxidation of bulk material of TiN is as fast as Ti Alloy. This difference can be explained by such a low oxygen partial pressure at the interface that the TiN layer is kept intact.
The effect of carbon dioxide on corrosion rate, polarization curve and hydrogen content has been investigated for carbon steels in acetate solutions and sodium carbonate solution with a pH range of 2 to 9 at 313 K under carbon dioxide and nitrogen atmospheres. In acid solutions less than pH 6 both general corrosion and hydrogen absorption are accelerated by the existence of carbon dioxide, but not clear in the intermeadiate pHs 5 and 6. In the solutions more than pH 6 little corrosion rate is observed regardless of carbon dioxide, whereas hydrogen content is surely detected only at the existence of carbon dioxide. The anodic and cathodic polarization curves show that their reaction overpotentials are affected by carbon dioxide, whose effect changes depending upon pH. The results obtained are qualitatively explained in terms of the formation of iron carbonate and the adsorption of carbonate ions.
Crush behavior of the bare honeycomb structure with hexagonal cell (without the reinforcing plate members bonded on both edges) is studied experimentally and numerically. The dimensions of honeycomb cell used are 3/8in. (9.525mm) and 3/4in. (19.05mm) and the material of foil is an aluminum alloy, A5052. Their foil thicknesses are 0.020mm, 0.033mm and 0.046mm. In experiment, the impact velocity is ranged from 1.98 to 10m/s. As for 3/8in. honeycomb, irregular folding pattern appears just to a little extent in experiment. On the other hand, very irregular buckling pattern is observed in 3/4in. honeycomb. Crush strength becomes slightly larger as the impact velocity increases. The maximum value of acceleration measured by an acceleration sensor attached to the drop-hammer seems to be almost constant when the impact velocity is greater than a certain value, and increases rapidly as the foil thickness increases. Furthermore, numerical simulation is carried out by using the dynamic explicit nonlinear finite element code DYNA3D. In the computation, due to the geometrical symmetry of hexagonal honeycomb core, the 'Y' letter model is proposed. In computation the buckling with folding pattern appears, though, the irregularity in folding pattern is larger in computation than in experiment. Predicted dependence of the crush strength on the impact velocity and the foil thickness is similar to the corresponding experimental result.
Rotating bending fatigue tests were conducted at room temperature in laboratory air using specimens of a medium carbon steel (S45C) with sprayed coating of a cermet (WC-12%Co), and the fatigue strength and fracture mechanisms were studied. The fatigue strength was enhanced by the cermet spraying and the fatigue limits of the sprayed specimens were 30-40% higher than that of electro-polished specimens of the substrate steel. Fatigue cracks were initiated at defects between the substrate and sprayed coating. In specimens blasted by alumina grits before the cermet spraying, fatigue cracks were initiated at alumina grits remained on the substrate surface. Therefore, fatigue strength was increased when the substrate was blasted by steel balls or blasting was omitted. In these specimens, fatigue cracks were initiated at debonding defects at the interface between the substrate and sprayed coating. When a crack was initiated in the sprayed coating by the increase in localized strain due to crack initiation in the substrate, the crack growth rates in the substrate were accelerated by the stress concentration of the crack in the sprayed coating, leading to the decrease in the crack growth life of the sprayed specimens.