Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials

Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods for predicting small crack fatigue behavior. In this paper, small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and it was used to 1) determine the bridging stress profile for the material and 2) make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, is has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.Keywords: fracture, toughness, fatigue, crack bridgingKeywords: fracture, toughness, fatigue, crack bridgin

A direct comparison of non-destructive techniques for determining bridging stress distributions

Crack bridging is an important source of crack propagation resistance in many materials and the bridging stress distribution as a function of crack opening displacement is widely believed to represent a true material property uninfluenced by sample geometry, loading conditions, and other extrinsic factors. Accordingly, accurate measurement of the bridging stress distribution is needed and many non-destructive methods have been developed. However, there are many challenges to accurately determining bridging stresses. A comparison of bridging stresses measured using R-curve, crack opening displacement (COD), and spectroscopy methods has been made using two bridging ceramics, Y₂O₃ and MgO doped Si₃N₄ and 99.5% pure Al₂O₃. The COD method is surface sensitive and gives a lower peak bridging stress compared to the R-curve technique which samples through the entire material thickness. This is attributed to a more compliant near surface bridging zone. Conversely, when R-curves rise steeply over the first few micrometers of growth from a notch, an effect of negative T-stress is expected to raise the R-curve determined peak bridging stress. Spectroscopy methods were only found to yield reliable bridging stress results if a reasonable through thickness volume of material is sampled. It was found that 2.5% of the specimen thickness achieved using fluorescence spectroscopy appears adequate for Al₂O₃ while 0.1–0.2% of the sample thickness achieved using Raman spectroscopy for Si₃N₄ appears inadequate. Overall, it is concluded that in the absence of T-stresses a bridging distribution can be determined that is a true material property. Also, a new method is proposed for determining the bridging stresses of fatigue cracks from (1) the bridging stress distribution for monotonically loaded cracks and (2) experimental fatigue data.Keywords: R-curves, Fracture mechanisms, Bridging stress distribution, T-stress, Ceramic materialKeywords: R-curves, Fracture mechanisms, Bridging stress distribution, T-stress, Ceramic materia

Tensile Creep and Creep-Recovery Behavior of a SiC-Fiber Si 3 N 4 -Matrix Composite

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65802/1/j.1151-2916.1993.tb03753.x.pd

Finite element modelling of creep deformation in fibre-reinforced ceramic composites

The tensile creep and creep-recovery behaviour of a unidirectional SiC fibre-Si 3 N 4 matrix composite was analysed using finite element techniques. The analysis, based on the elastic and creep properties of each constituent, considered the influence of fibre-matrix bonding and processing-related residual stresses on creep and creep-recovery behaviour. Both two- and three-dimensional finite element models were used. Although both analyses predicted similar overall creep rates, three-dimensional stress analysis was required to obtain detailed information about the stress state in the vicinity of the fibre-matrix interface. The results of the analysis indicate that the tensile radial stress, which develops in the vicinity of the fibre-matrix interface after processing, rapidly decreases during the initial stages of creep. Both the predicted and experimental results for the composite show that 50% of the total creep strain which accumulated after 200 h at a stress of 200 MPa and temperature of 1200°C is recovered within 25 h of unloading.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44717/1/10853_2004_Article_BF00576283.pd

Copper as a signal for alginate sythesis in Pseudomonas syringae pv. Syringae

Plant-associated pseudomonads are commonly exposed to copper bactericides, which are applied to reduce the disease incidence caused by these bacteria. Consequently, many of these bacteria have acquired resistance or tolerance to copper salts. We recently conducted a survey of 37 copper-resistant (Cu^r) Pseudomonas spp., including P. cepacia, P. fluorescens, P. syringae, and P. viridiflava, and found that a subset of the P. syringae strains showed a dramatic increase in exopolysaccharide (EPS) production on mannitol-glutamate medium containing CuSO4 at 250 mg/ml. A modified carbazole assay indicated that the EPS produced on copperamended media contained high levels of uronic acids, suggesting that the EPS was primarily alginic acid. Uronic acids extracted from selected strains were further confirmed to be alginate by demonstrating their sensitivity to alginate lyase and by descending paper chromatography following acid hydrolysis. Subinhibitory levels of arsenate, cobalt, lithium, rubidium, molybdenum, and mercury did not induce EPS production, indicating that alginate biosynthesis is not induced in P. syringae cells exposed to these heavy metals. A 200-kb plasmid designated pPSR12 conferred a stably mucoid phenotype to several P. syringae recipients and also increased their resistance to cobalt and arsenate. A cosmid clone constructed from pPSR12 which conferred a stably mucoid phenotype to several P. syringae strains but not to Pseudomonas aeruginosa was obtained. Results obtained in this study indicate that some of the signals and regulatory genes for alginate production in P. syringae differ from those described for alginate production in P. aeruginosa.Peer reviewedPlant Patholog

Inhibition of Progenitor Dendritic Cell Maturation by Plasma from Patients with Peripartum Cardiomyopathy: Role in Pregnancy-associated Heart Disease

Dendritic cells (DCs) play dual roles in innate and adaptive immunity based on their functional maturity, and both innate and adaptive immune responses have been implicated in myocardial tissue remodeling associated with cardiomyopathies. Peripartum cardiomyopathy (PPCM) is a rare disorder which affects women within one month antepartum to five months postpartum. A high occurrence of PPCM in central Haiti (1 in 300 live births) provided the unique opportunity to study the relationship of immune activation and DC maturation to the etiology of this disorder. Plasma samples from two groups (n = 12) of age- and parity-matched Haitian women with or without evidence of PPCM were tested for levels of biomarkers of cardiac tissue remodeling and immune activation. Significantly elevated levels of GM-CSF, endothelin-1, proBNP and CRP and decreased levels of TGF- were measured in PPCM subjects relative to controls. Yet despite these findings, in vitro maturation of normal human cord blood derived progenitor dendritic cells (CBDCs) was significantly reduced (p < 0.001) in the presence of plasma from PPCM patients relative to plasma from post-partum control subjects as determined by expression of CD80, CD86, CD83, CCR7, MHC class II and the ability of these matured CBDCs to induce allo-responses in PBMCs. These results represent the first findings linking inhibition of DC maturation to the dysregulation of normal physiologic cardiac tissue remodeling during pregnancy and the pathogenesis of PPCM

Characterization of the alginate biosynthetic gene cluster in Pseudomonas syringae pv. syringae

Alginate, a copolymer of D-mannuronic acid and L-guluronic acid, is produced by a variety of pseudomonads, including Pseudomonas syringae. Alginate biosynthesis has been most extensively studied in P. aeruginosa, and a number of structural and regulatory genes from this species have been cloned and characterized. In the present study, an alginate-defective (Alg2) mutant of P. syringae pv. syringae FF5 was shown to contain a Tn5 insertion in algL, a gene encoding alginate lyase. A cosmid clone designated pSK2 restored alginate production to the algL mutant and was shown to contain homologs of algD, alg8, alg44, algG, algX (alg60), algL, algF, and algA. The order and arrangement of the structural gene cluster were virtually identical to those previously described for P. aeruginosa. Complementation analyses, however, indicated that the structural gene clusters in P. aeruginosa and P. syringae were not functionally interchangeable when expressed from their native promoters. A region upstream of the algD gene in P. syringae pv. syringae was shown to activate the transcription of a promoterless glucuronidase (uidA) gene and indicated that transcription initiated upstream of algD as described for P. aeruginosa. Transcription of the algD promoter from P. syringae FF5 was significantly higher at 32°C than at 18 or 26°C and was stimulated when copper sulfate or sodium chloride was added to the medium. Alginate gene expression was also stimulated by the addition of the nonionic solute sorbitol, indicating that osmolarity is a signal for algD expression in P. syringae FF5.Peer reviewedPlant Patholog

Vietnam military service history and prostate cancer

BACKGROUND: Three decades after US and Australian forces withdrew from Vietnam, there has been much public interest in the health consequences of service in Vietnam. One controversial question is whether the risk of prostate cancer amongst Vietnam veterans is increased. This paper examines relationships between military history, family history and risk of prostate cancer in a population-based case control study. METHODS: Cases were selected from the Cancer Registry of Western Australia as incident cases of histologically-confirmed prostate cancer, and controls were age-matched and selected from the Western Australian electoral roll. Study participants were asked to report any military service history and details about that service. RESULTS: Between January 2001 and September 2002, 606 cases and 471 controls aged between 40–75 years were recruited. An increased prostate cancer risk was observed in men reporting they were deployed in Vietnam although this was not statistically significant (OR = 2.12; 95% CI 0.88–5.06). An increased risk was also observed in men reporting prostate cancer in fathers (OR = 1.90; 95% CI 1.20–3.00) or brothers (OR = 2.05; 95% CI 1.20–3.50) diagnosed with prostate cancer. CONCLUSION: These findings support a positive association between prostate cancer and military service history in the Vietnam war and a first degree relative family history of prostate cancer