Further Considerations Concerning a Mechanism of Fatigue Crack Propagation

X-ray microbeam diffraction technique is a useful one to investigate the features of the crystal deformation in a localized area. That is, this method have been adopted to examine the density and array of dislocation, microscopic lattice strain and macroscopic residual stress. And so, the informations obtained from the tip of the crack during the fatigue process have been correlated with the behaviours of the initiation and propagation of the crack. The authors, in the present paper, investigated a relation between the distributions of the microscopic lattice strains which are calculated and measured by the technique, and suggested the sort of dislocation at the tip of the crack that relate to the fatigue crack propagation. The crack initiated at the notch root of the specimen which was composed of the coarse grain and propagated along the grain-boundary in the early stage under fatigue process of the alternating stress 4.1 kg/mm(2). Thereafter, it changed the propagating direction toward the inside of the grain. The distributions of the micro lattice strain in each reflecting plane which were measuerd at the plastically deformed zone in the vicinity of the grainboundary and at the crack tip agreed well with modes of the strain distribution due to a screw and a edge dislocations by the calculation, respectively. From these results, the authors concluded that the fatigue crack propagation would relate closely to the changing in the sort of the dislocation from the screw to the edge

Yield Strength and Stacking Fault Energy on Fatigue Crack Propagation

It is very important to clarify the question whether fatigue crack propagation will be affected by mechanical propaties or other propaties of materials. In the present paper the authors studied in relation of yield strength and stacking fault energy to rate of fatigue crack propagation. αbrass were chosen for the investigation because they provided sufficient range in both quantities of interest that either could be varied independently of the other. Fatigue tests were carried out under full bending moment of flat specimens with V-shape notch. Chosen stress levels were 0.6 σy and 0.8 σy in which σy is yield strength, rate of fatigue crack propagation was evaluated from the second stage of the curve of fatigue crack propagation. The dependence of the rate on stacking fault energy γ was found to be dl/ dN=G・γ(n). But dl/dN did not systematically to change in yield strength. Thus, γ is concluded to be the controlling variable

X-ray Investigation on the Fatigue Damage of Metals Containing α- and β-Phases (On the Changes in Half-Value Breadth and ResidualStress of 6-4 Brass due to Stress Cycles)

In the field of mechanical engineering, the need for a simple but effective way of evaluating the fatigue strength and lifetime of structural materials is an important problem with which the design engineers have faced. Accordingly, a cunsiderable amount of investigations have been made in this field. However, the basic nature of fatigue damage and the conditions which lead to the initiation and propagation of fatigue cracks are not sufficiently understood. Nor any satisfactory method of assessing the exact state of fatigue damage has yet been found. X-ray diffraction technique is not a new as the method of experimental study, but has often been adopted for direct and non-destructive observations of change in the local structures of crystalline materials at fundamental research. X-ray technique is one of the most powerful means to investigate the changes of microscopic- structure due to external forces, considering the fatigue phenomena occur on the surface of materials. Accordingly, one of the authors have studied the relations of half-value breadth of X-ray diffraction lines, residual stresses and hardness and number of cycles of fatigue stresses in detail for various sorts of engineering metallic materials. In the results of a number of experiments, it has been found that the variation in half-value breadth showed very regular relation with number of stress cycles, the authors have reported that the lifetime of materials in fatigue could be predicted. Consequently, it needs to investigate whether or not this predicting method is fitted for alloy containg α and β phase, moreover, it is interesting to make clear the fatigue mechanism of such materials which are present two phases having the different yield stress and type of crystal structure

On the Development of X-Ray Stress Measurement TechniqueUsing X-Ray Diffraction by Crystal Oscillation Method. I,

In the present paper, to provide information on the stress measurement in coarse grained materials by X-ray micro-beam diffraction technique using a crystal oscillation method, the authors first examined experimentaly and theoretically the relation between the sizes of X-ray beam and crystal to obtain the particular diffraction ring in the case of use of crystal oscillation method. The specimen attachment of X-ray camera used in this experiment can be oscillated automatically around a horizontal and vertical axes with high accuracy centering around an illuminated position on the specimen surface. Accordingly it is possible to increase the number of the diffraction spots without changing the area and position of the specimen illuminated. Experiments were carried out for three kinds of annealed low careon steel with grain sizes of about 15, 30 and 50μ in diameter, and with X-ray beam collimated by pinhole slits of 0.12, 0.30 and 1.00 mm in diameter, using CrKα characteristic X-rays. On the other hand, a theoretical analysis was carried out according to the X-ray diffraction theory which have been proposed by P. B. Hirsch et al. As the conclusion, it is found that the crystal oscillation method is extremely useful for X-ray stress measurement of coarse grained materials. Moreover, the conditions of the crystal oscillating operation were clarified theoretically for any pair of the sizes of X-ray beam and crystal

Mechanism of Fatigue Fracture (On the Distribution of Plastic Strain around Fatigue Cracks)

Broadly speaking, there are three kinds of approaches to investigate the deformation and fracture of materials, that is, the microscopic (metallurgical), macroscopic(mechanical) and theoretical (mathematical) researches. It is necessary, however, to bridge the gap which persists among of them. Many investigations on the form of plastically deformed zone at a crack tip, the distribution of plastic strain in plastic zone and fracture criterion have been made for the purpose to clarify the mechanism on initiation and propagation of cracks. In this paper, the authors report the studies, from the standpoint of microscopic views, on the crystal deformation at the tip of crac's in notched specimen during fatigue process

Stimulus-Dependent State Transition between Synchronized Oscillation and Randomly Repetitive Burst in a Model Cerebellar Granular Layer

Information processing of the cerebellar granular layer composed of granule and Golgi cells is regarded as an important first step toward the cerebellar computation. Our previous theoretical studies have shown that granule cells can exhibit random alternation between burst and silent modes, which provides a basis of population representation of the passage-of-time (POT) from the onset of external input stimuli. On the other hand, another computational study has reported that granule cells can exhibit synchronized oscillation of activity, as consistent with observed oscillation in local field potential recorded from the granular layer while animals keep still. Here we have a question of whether an identical network model can explain these distinct dynamics. In the present study, we carried out computer simulations based on a spiking network model of the granular layer varying two parameters: the strength of a current injected to granule cells and the concentration of Mg2+ which controls the conductance of NMDA channels assumed on the Golgi cell dendrites. The simulations showed that cells in the granular layer can switch activity states between synchronized oscillation and random burst-silent alternation depending on the two parameters. For higher Mg2+ concentration and a weaker injected current, granule and Golgi cells elicited spikes synchronously (synchronized oscillation state). In contrast, for lower Mg2+ concentration and a stronger injected current, those cells showed the random burst-silent alternation (POT-representing state). It is suggested that NMDA channels on the Golgi cell dendrites play an important role for determining how the granular layer works in response to external input

Differential hepatitis C virus RNA target site selection and host factor activities of naturally occurring miR-122 3′ variants

In addition to suppressing cellular gene expression, certain miRNAs potently facilitate replication of specific positive-strand RNA viruses. miR-122, a pro-viral hepatitis C virus (HCV) host factor, binds and recruits Ago2 to tandem sites (S1 and S2) near the 5΄ end of the HCV genome, stabilizing it and promoting its synthesis. HCV target site selection follows canonical miRNA rules, but how non-templated 3΄ miR-122 modifications impact this unconventional miRNA action is unknown. High-throughput sequencing revealed that a 22 nt miRNA with 3΄G (‘22–3΄G’) comprised <63% of total miR-122 in human liver, whereas other variants (23–3΄A, 23–3΄U, 21–3΄U) represented 11–17%. All loaded equivalently into Ago2, and when tested individually functioned comparably in suppressing gene expression. In contrast, 23–3΄A and 23–3΄U were more active than 22–3΄G in stabilizing HCV RNA and promoting its replication, whereas 21–3΄U was almost completely inactive. This lack of 21–3΄U HCV host factor activity correlated with reduced recruitment of Ago2 to the HCV S1 site. Additional experiments demonstrated strong preference for guanosine at nt 22 of miR-122. Our findings reveal the importance of non-templated 3΄ miR-122 modifications to its HCV host factor activity, and identify unexpected differences in miRNA requirements for host gene suppression versus RNA virus replication

Juvenile hormone acid O-methyltransferase in Drosophila melanogaster

Juvenile hormone (JH) acid O-methyltransferase (JHAMT) is the enzyme that transfers a methyl groupfrom S-adenosyl-L-methionine (SAM) to the carboxyl group of JH acids to produce active JHs in thecorpora allata. While the JHAMT gene was originally identified and characterized in the silkwormBombyx mori, no orthologs from other insects have been studied until now. Here we report on thefunctional characterization of the CG17330/DmJHAMT gene in the fruit fly Drosophila melanogaster.Recombinant DmJHAMT protein expressed in Escherichia coli catalyzes the conversion of farnesoic acidand JH III acid to their cognate methyl esters in the presence of SAM. DmJHAMT is predominantlyexpressed in corpora allata, and its developmental expression profile correlates with changes in the JHtiter. While a transgenic RNA interference against DmJHAMT has no visible effect, overexpression ofDmJHAMT results in a pharate adult lethal phenotype, similar to that obtained with application of JHanalogs, suggesting that the temporal regulation of DmJHAMT is critical for Drosophila development

RecR forms a ring-like tetramer that encircles dsDNA by forming a complex with RecF

In the RecFOR pathway, the RecF and RecR proteins form a complex that binds to DNA and exerts multiple functions, including directing the loading of RecA onto single-stranded (ss) DNA regions near double-stranded (ds) DNA–ssDNA junctions and preventing it from forming a filament beyond the ssDNA region. However, neither the structure of the RecFR complex nor its DNA-binding mechanism was previously identified. Here, size-exclusion chromatography and small-angle X-ray scattering data indicate that Thermus thermophilus (tt) RecR binds to ttRecF to form a globular structure consisting of four ttRecR and two ttRecF monomers. In addition, a low resolution model shows a cavity in the central part of the complex, suggesting that ttRecR forms a ring-like tetramer inside the ttRecFR complex. Mutant ttRecR proteins lacking the N- or C-terminal interfaces that are required for tetramer formation are unable to form a complex with ttRecF. Furthermore, a ttRecFR complex containing the DNA-binding deficient ttRecR K23E/R27E double mutant, which contains mutations lying inside the ring, exhibits significantly reduced dsDNA binding. Thus, we propose that the ring-like ttRecR tetramer has a key role in tethering the ttRecFR complex onto dsDNA and that the ring structure may function as a clamp protein