Key Engineering Materials Vols. 353-358

Abstract: The crack problem for a functionally graded orthotropic coating-substrate structure under an in-plane load is studied. The orthotropic coating is assumed to contain a crack perpendicular to the interface. Integral transform method is used to obtain singular integral equation. Stress intensity factors (SIFs) are evaluated. The influences of orthotropic material constants and the geometry parameters on SIFs are analyzed.

Abstract: The effect of plasma radical nitriding treatment on fatigue properties of SCM435 steel in super long life region was investigated. Fatigue tests were carried out using a dual-spindle rotating bending fatigue-testing machine at room temperature in air for the specimens nitrided at 773 K and 823 K for 3 hrs. The fatigue strength of nitrided specimen was greater than that of un-nitrided specimen and the crack initiation mode changed from the surface cracking of un-nitrided specimen to the subsurface cracking of nitrided specimen. Hardening layer and compressive residual stress were formed by nitriding, which resulted in the improvement of the fatigue strength. The stress intensity factor was calculated using facet area in Fish-eye fracture mode. As a result, the stress intensity factor indicated almost constant value, ~ 3-4 MPa·m1/2, regardless of the number of cycles to failure.

Abstract: A number of mechanisms have been proposed to understand stress corrosion cracking (SCC) of metals, e.g. (1) slip dissolution and active pass corrosion based on anodic dissolution of metals, (2) tarnish rupture and internal oxidation based on oxidation ahead the crack tip followed by cracking of the oxides, and (3) hydrogen cracking, etc. If dissolution of metals takes the essential role in the stress corrosion cracking concerned, cracking susceptibility is expected to be significantly affected by dielectric constant of water. Because dielectric constant represents a character of water as a solvent, which determines solubility of metal oxides, and therefore corrosion rate of metals is strongly dependent on dielectric constant of water. K-constant type SCC growth rate tests have been done as a function of physical property (dielectric constant) of water by either manipulating temperature under iso-pressure condition (15MPa) or manipulating pressure under iso-thermal condition (330oC). Intergranular cracking was more enhanced and the crack growth was significantly accelerated under the condition of higher dielectric constant, indicating that dissolution of metal plays important role in the cracking mechanism of 316L stainless steels under the present testing conditions.

Abstract: Three point bending fatigue tests were carried out to investigate the influence of bias voltage on fatigue behavior of medium carbon steel with chromium nitride film. The substrate material of the specimens was JIS S45C medium carbon steel. The chromium nitride films were coated at two different bias voltages (VB = -20V, -300V) for fatigue test by arc ion plating method. As a result, the fatigue limit was improved by coating at low bias voltage, while the fatigue limit of high bias voltage samples decreased by coating. In order to clarify the difference between high and low bias voltages, the tensile test was performed using the coated specimens. For this test, the chromium nitride films were deposited at four different bias voltages. It was found the cracking behavior clearly changed between -40V and -80V. For low bias voltage samples, short cracks were initiated in the films, while the long cracks were initiated at same strain for high bias voltage samples. These differences between low and high bias voltage sample may be due to the elastic modulus of the film and droplets distribution in the film.

Abstract: Recently, composite materials are used in many fields because their properties are high strength, high stiffness, and they have light weight, good corrosion resistance and good thermal conductivity. However, composite materials have relatively a lot of problems, especially delamination, compared with common materials such as a steel and aluminum, etc. Therefore, having the interlaminar fracture toughness for a laminate composite is important. In this study, the end notched flexure (ENF) specimens are employed in order to evaluate modeⅡ interlaminar fracture toughness for CFRP laminate composites. Three kinds of a/L ratio were applied to these specimens under the different pressure level. Also, we discuss the relation of crack growth and the interlaminar fracture toughness in terms of AE characteristics using ENF test.

Abstract: The fatigue properties of ferritic stainless steel containing deformation twins were investigated. Monotonic tensile tests and push-pull fatigue tests were conducted on the specimens both with and without twins. Fatigue lives of the twinned specimens were about four times shorter than those without the deformation twins, although yield stresses of both specimens were almost equal. It was found that the fatigue cracking along the deformation twin boundaries caused the reduction in fatigue life. Dislocation structure observation using the ECCI method revealed that no specific dislocation structure was formed near the cracked deformation twin boundary, although the ladder-like PSB structure was developed along an annealing twin boundary in an austenitic stainless steel.

Abstract: Shear-type fatigue crack behavior in a bearing steel, SAE52100, was investigated in a biaxial fatigue testing machine using cylindrical specimens. Either of the following two types of artificial defect with the total length of 400 ~ 440 %m and the depth of 200 ~ 300 %m was introduced into the specimen surfaces: (a) a semi-elliptical pre-crack emanating from 2 adjacent holes by a tension-compression loading, (b) 3 adjacent holes oriented in the axial direction which had slits made by the focused ion beam technique at the both ends. Fully reversed torsion with a shear stress amplitude at specimen surface, τa = ~ 600 MPa, was applied to the specimens under the static axial compressive stress σm = −1000 ~ −1200 MPa. In case of the defect (a), a shear-type crack propagated from the pre-crack in direction perpendicular to the specimen axis. At the specimen surface, the shear-type crack periodically branched from the crack tip and propagated 5~10 %m in Mode I directions. The shear-type crack growth was decelerated with an increase in the crack length and finally stopped at N ≅ 7.5×106 cycles and 2a ≅ 600 %m. On the other hand, in case of the defect (b), the shear-type crack propagated in axial direction, and the crack growth was accelerated with an increase in the crack length. In addition, the threshold stress in case without the compressive stress on crack-face was determined by a τa-decreasing test. The results revealed that the shear-type crack growth was strongly influenced by the crack-face friction and the existence of the Mode I blanched cracks.

Abstract: In the present study, fatigue experiments were conducted using two kinds of extruded magnesium alloys with different extrusion ratios. Effect of the extrusion ratio on fatigue lives and crack propagation behavior was studied in detail. There was no dramatic effect of the extrusion ratio on fatigue life, fatigue limit and crack propagation behavior.

Abstract: Microstructures of long-term serviced F12 steel exposed at 545 °C have been investigated by electron microscopes. The hardness of the material was measured to be correlated with the variation of the microstructures. Fatigue properties of the material with different running time were evaluated and analyzed. The experimental results show that the coarsening of the precipitated carbides along boundaries and the formation of subgrains accelerate the degradation of the long-term creep properties of the steel. Fatigue crack initiation threshold from a notch linearly deceases with increasing the running time due to the variation of the distribution and the shape of the precipitated carbides. The degradation mechanisms of the F12 steel during their long-term service at high temperature are discussed.

Abstract: This paper describes new structure of specimen easy to manipulate, align and grip a thin-film and test machine for a fatigue test. High cycle fatigue test has been performed on tensile type specimen of Al-3%Ti alloy using the newly developed fatigue test machine. The material used in this study was Al-3%Ti thin film, which was used in RF MEMS switch. The structure of the suggested specimen has two holes and several bridges. The holes at centre of grip end are able to make alignment and gripping easy. The bridges are to remove the side support strip easily and extract specimen from wafer without sawing. The test machine was developed using the voice coil of speaker. The new tensile loading system has a load cell with maximum capacity of 0.5N and a non-contact position measuring system based on the principle of capacitance micrometry with 0.1nm resolution for displacement measurement. Fatigue tests was performed on 7 specimens. The thickness and width of the thin film of specimen are 1.0μm and 150μm, respectively. The fatigue strength coefficient and the fatigue strength exponent of Al-3%Ti alloy micro-sized specimen are determined to be 164MPa and -0.01322, respectively