A testing method for investigating fatigue strength under equi-biaxial stress/strain condition was developed. In this method, the equi-biaxial stress condition was achieved by applying pressure on the surfaces of a disc-type specimen, for which the disc edge was constrained by supporting jigs. Air pressure was used to apply the cyclic loading and the failure of the specimen was determined by detecting the crack penetration of the specimen thickness. This method allows application of the cyclic equi-biaxial stress without a complex testing apparatus or a complex controlling system such as the testing system using the cruciform or tubular specimens, conventionally used for the fatigue test under the equi-biaxial stress condition. After developing the testing system, the configuration of the disc-type specimen was designed by finite element analysis so that a crack would be initiated at the center of the specimen. Then, carbon steel (SS400 in JIS) specimens were subjected to the fatigue test. The developed system was demonstrated to be able to initiate a fatigue crack at the center of the specimen and to detect the specimen failure successfully. The test results showed that the fatigue lifetime under equi-biaxial stress was longer than that under uniaxial stress for the same Von Mises equivalent strain range.
The purpose of present study was to examine the corrosion fatigue strength and fracture mechanism of Ti-6Al-4V alloy coated with cold-sprayed pure titanium. Both grit blasting and electrolytic polishing were used as the treatments of substrate surface before coating. Rod Specimens of Ti-6Al-4V alloy were coated with cold-splayed pure titanium after the surface treatment. Then, hydroxyapatite was coated using plasma spraying as top coating. Adhesive strength of coating was 27.2MPa for the specimen coated with pure titanium after grit blasting (GBC) and 20.2MPa for that after electrolytic polishing (EPC), respectively. The corrosion fatigue tests were conducted using rotating bending fatigue machine at a frequency of 10 Hz in a physiological saline solution at 310K. The fatigue life of the coated specimens decreased in comparison with the electrolytic polished substrate (EP). The reduction in fatigue strength at 107 cycles was 18% for GBC and 15% for EPC in comparison with EP. The corrosion damage was not found in titanium coating and substrate. In addition, the residual stress measured by X-ray diffraction method was compressive mode in titanium coating and substrate subsurface for both GBC and EPC. From SEM examination, it was revealed that the reduction in fatigue strength was due to the stress concentration at dents formed by grit blasting in GBC and at non-bonded regions of titanium coating in EPC.
Fatigue tests of friction stir welded (FSWed) joints of A6061-T6 aluminum alloy have been conducted with fully reversed plane bending conditions (R= -1) to investigate the effect of shot peening on the high cycle fatigue property. Shot peening was performed with two types of fine zirconia shot grid. The results showed that the fatigue strength at 107 cycles of unwelded specimens (base material) was 130 MPa and fine zirconia shot peening enhanced the strength by 10 MPa with B120 shot grid type. The fatigue lives were dramatically improved in all stress levels by the peening. On the other hand, the fatigue strength at 107 cycles of the SP treated FSWed specimens was nearly the same as that of the base material, and the fatigue lives were improved at only lower stress levels. To discuss these fatigue properties, fracture surfaces were observed and peening effects such as surface roughness, hardness and residual stress distributions were examined.
Residual stress remarkably affects the fatigue strength of welded socket joint. In carbon and austenitic stainless steel the fatigue strength of socket welded joint is about half of base metal. This is caused by the high tensile residual stress at the root of socked weld. In this study the variation of residual stress distribution near the weld start and end points, effects of miss-alignment on the residual stress distribution and the principal strain and stress were examined using the neutron diffraction method. As a result, the effects of miss-alignment are negligibly small and the residual stress at the weld root is almost the same with that of normally welded joint. The deviations of principal axes from the specimen directions are not so much large and the principal stresses almost agree with those of specimen axes.
Statistical analyses of the field inspection database were conducted for steam turbine high pressure inner casings. The apparent reducing trends of crack amounts at nozzle box fit radius were exhibited as repair numbers increased. This trend may be caused by the blunting of the corner root radius due to the grind-off actions for cracks. The average and upper trends of maximum crack length with repeated inspection/repair numbers were expressed by Green function successfully, indicating initial increasing and subsequent decreasing trends. To estimate total crack length, log-normal distribution was applied to predict the individual crack length from the maximum crack length and the total crack length was obtained from the summation of distributed crack length over the total number of cracks. The average growth rates of maximum crack length against start-up cycles showed similar reducing trends as maximum crack length. Using the average crack growth rates of maximum crack length and total crack length as the function of the numbers of repeated repairs, optimum repair intervals were demonstrated as the combination of initial frequent repair followed by long intervals after 3rd or 4th repair. The expression of crack growth rate as the function of the number of repeated repairs could be also very useful for maintenance scheduling.
The influence of groundwater rising for the liquefaction potential in Osaka and Wakayama plains was examined using GIS in which a digital database of underground information provided by Kansai Geo-informatics Database was introduced. The detailed geotechnical properties were calculated and given to Holocene sandy and clayey deposits that are representative value in each digital mesh of 250m × 250m areas. Then, liquefaction safety assessments were carried out by estimating pumping possible quantity of Holocene sandy deposits and considering negative effect of them against consolidation settlement problem that will occur in Holocene clayey deposits underneath. The efficacy of groundwater lowering method against liquefaction of Holocene sandy deposits was verified.
We investigated the bilayer interdigitation of diacylphosphatidylcholines (CnPCs) with long acyl chains (n = 19 - 22) under high pressure. The temperature-pressure phase diagrams of the CnPC bilayers were constructed by the methods of differential scanning calorimetry and high-pressure light transmittance. We confirmed the bilayer interdigitation for CnPCs (n = 19 - 21) under high pressure. On the other hand, the C22PC bilayer showed peculiar behavior: two transitions were observed under atmospheric pressure while one transition under high pressure. To identify the low-temperature phase of the C22PC bilayer, we performed small angle neutron scattering measurements under atmospheric pressure and fluorescence measurements under high pressure. From these results, the phase was identified as the interdigitated gel (LβI) phase. Considering our previous results, it turned out that the acyl-chain length of CnPC that exhibits the pressure-induced interdigitation is limited within n = 14 - 21. The present results definitely indicate that the bilayer interdigitation is governed by the hydrophobic interaction as well as the hydrophilic interaction of the PC molecule in the bilayer.