Experimental Investigations On Near-Threshold Events On Fatigue Crack Growth

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios

Experimental Investigations On Near-Threshold Events On Fatigue Crack Growth

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios

Isolation and amino acid sequence of the 30S ribosomal protein S19 from Mycobacterium bovis BCG

AbstractThe 30S ribosomal proteins from Mycobacterium bovis BCG were separated by reverse phase-high performance liquid chromatography (RP-HPLC). The isolated proteins were analyzed by SDS-PAGE, blotted on PVDF-membranes and subjected to sequence analyses using a gas-phase sequencer to correlate them to those of the well studied Escherichia coli and Bacillus stearothermophilus ribosomes. Moreover, the internal amino acid sequence of one ribosomal protein, MboS19, which is homologous to E. coli ribosomal protein S19 (EcoS19) and B. stearothermophilus ribosomal protein S19 (BstS19), was further analyzed by sequencing its internal peptides and two segments from the N- and C-termini of the protein were selected to deduce the sequence of two oligonucleotide primers which were used in a polymerase chain reaction. Using the amplified DNA fragment thus obtained as a hybridization probe, the gene encoding protein S19 was identified and cloned. Sequence analysis of the DNA fragment, together with peptide sequence analysis could determine the complete amino acid sequence of MboS19. This sequence proved to be 64% and 71% identical to those of the corresponding S19 proteins from the eubacteria E. coli, and B. stearothermophilus, respectively; 33% of the residues of MboS19 were identical to those in the archaebacteral ribosomal protein HmaS19

Synaptic-like microvesicles, synaptic vesicle counterparts in endocrine cells, are involved in a novel regulatory mechanism for the synthesis and secretion of hormones

Microvesicles in endocrine cells are the morphological and functional equivalent of neuronal synaptic vesicles. Microvesicles accumulate various neurotransmitters through a transmitter-specific vesicular transporter energized by vacuolar H+-ATPase. We found that mammalian pinealocytes, endocrine cells that synthesize and secrete melatonin, accumulate L-glutamate in their microvesicles and secrete it through exocytosis. Pinealocytes use L-glutamate as either a paracrine- or autocrine-like chemical transmitter in a receptor-mediated manner, resulting in inhibition of melatonin synthesis. In this article, we briefly describe the overall features of the microvesicle-mediated signal-transduction mechanism in the pineal gland and discuss the important role of acidic organelles in a novel regulatory mechanism for hormonal synthesis and secretion

Variations in amount of TSST-1 produced by clinical methicillin resistant Staphylococcus aureus (MRSA) isolates and allelic variation in accessory gene regulator (agr) locus

<p>Abstract</p> <p>Background</p> <p><it>Staphylococcus aureus </it>(S. aureus) is an important pathogen associated with both nosocomial and community-acquired infections and its pathogenicity is attributed to its potential to produce virulence factors. Since the amount of toxin produced is related to virulence, evaluating toxin production should be useful for controlling S. aureus infection. We previously found that some strains produce relatively large amounts of TSST-1; however, no reports have described the amount of TSST-1 produced by clinical isolates.</p> <p>Methods</p> <p>Amounts of TSST-1 produced by clinical methicillin resistant S. aureus (MRSA) isolates were measured by Western blotting. We determined their accessory gene regulator (<it>agr</it>) class by PCR and investigated whether TSST-1 production correlates with variations in the class and structure of the <it>agr</it>.</p> <p>Results</p> <p>We found that 75% of surveyed MRSA isolates (n = 152) possessed the <it>tst </it>gene and that 96.7% belonged to <it>agr </it>class 2. The concentrations of TSST-1 secreted into culture supernatants by 34 strains measured by Western blotting differed 170-fold. Sequencing the entire <it>agr </it>locus (n = 9) revealed that some had allelic variations regardless of the amount of TSST-1 produced whereas sequencing the <it>sar</it>, sigma factor B and the <it>tst </it>promoter region revealed no significant changes.</p> <p>Conclusion</p> <p>The amounts of TSST-1 produced by clinical MRSA isolates varied. The present results suggest that TSST-1 production is not directly associated with the <it>agr </it>structure, but is instead controlled by unknown transcriptional/translational regulatory systems, or synthesized by multiple regulatory mechanisms that are interlinked in a complex manner.</p

A New Microarray System to Detect Streptococcus pneumoniae Serotypes

Streptococcus pneumoniae, one of the most common gram-positive pathogens to colonize the human upper respiratory tract, is responsible for many severe infections, including meningitis and bacteremia. A 23-valent pneumococcal vaccine is available to protect against the 23 S. pneumoniae serotypes responsible for 90% of reported bacteremic infections. Unfortunately, current S. pneumoniae serotype testing requires a large panel of expensive antisera, assay results may be subjective, and serotype cross-reactions are common. For this study, we designed an oligonucleotide-based DNA microarray to identify glycosyltransferase gene sequences specific to each vaccine-related serotype. Out of 56 isolates representing different serotypes, only one isolate, representing serotype 23A, was not detected correctly as it could not be distinguished from serotype 23F. Our data suggest that the microarray provides a more cost-effective and reliable way of monitoring pneumococcal capsular types