Cylinders and rings fabricated from AerMet® 100 alloy and AISI 1018 steel have been explosively driven to fragmentation in order to determine the fracture strains for these materials under ...plane-strain and uniaxial-stress conditions. The phenomena associated with the dynamic expansion and subsequent break up of the cylinders are monitored with high-speed diagnostics. In addition, complementary experiments are performed in which fragments from the explosively driven cylinders are recovered and analyzed to determine the statistical distribution associated with the fragmentation process as well as to determine failure mechanisms. The data are used to determine relevant coefficients for the Hancock–McKenzie (Johnson–Cook) fracture model. Metallurgical analysis of the fragments provides information on damage and failure mechanisms.
Aims Maternal leptin affects placental growth hormone (GH), whereas ghrelin, a natural ligand of the growth‐hormone‐secretagogue receptor, modulates GH action. Both hormones may affect fetal growth, ...and dysregulation in diabetes may lead to fetal growth disturbances. The aim was to investigate changes in maternal ghrelin during pregnancy with diabetes and to establish reference leptin levels.
Methods Twelve healthy non‐diabetic (ND) and 12 pregnant women with Type 1 diabetes (T1DM) were recruited. Age and body mass index (BMI) ND: age 29.9 ± 4.7 years (mean ± sd), BMI 25.2 ± 3.7 kg/m2; T1DM: age 31 ± 5.5 years, BMI 27 ± 3.1 kg/m2 were similar in the groups. HbA1c in T1DM was 6.2 ± 1.1% at 20 weeks, 6.3 ± 1.1% at 30 weeks’ gestation and 7.8 ± 2.1% postpartum. Fasting plasma ghrelin, total leptin, free leptin (FL) and soluble leptin receptor (sOB‐R) levels were measured at 20 and 30 weeks’ gestation and postpartum and determined by radioimmunoassay.
Results All pregnancies resulted in full‐term singleton births with no differences in birth weight between groups T1DM: 3.4 ± 0.56 kg (mean ± SE); ND: 3.6 ± 0.3 kg, P = NS. Ghrelin levels were lower in T1DM when corrected for age and mothers’ weight (T1DM: 458 ± 36 pg/ml and 432.9 ± 26.6 pg/ml; ND: 562 ± 52 pg/ml and 515.8 ± 63 pg/ml at 20 and 30 weeks, respectively, P < 0.05). T1DM mothers had higher levels of sOB‐R and FL levels declined at 30 weeks’ gestation in T1DM (P = 0.04) but not in ND.
Conclusion In a population of pregnant women with expected changes in leptin levels as previously reported, ghrelin levels were lower in T1DM pregnancies at 20 and 30 weeks. This may have implications for fetal development and requires further study in diabetes, particularly in pregnancies that result in macrosomia.
The Engel-Brewer (E-B) electron theory of crystal structures has been used to develop a model of lath martensite formation in quenched Fe-C steels. Lath martensite is described as a cluster of eight ...iron atoms surrounding each carbon atom in a matrix of fine subgrained body centred cubic iron. The E-B theory is used to describe the valence state of the cluster and the matrix of iron. Three sequential transformations take place during quenching, involving face centred cubic austenite with valence of 3, hexagonal close packed hexagonite with valence of 2 and body centred cubic ferrite with valence of 1. The E-B theory predicts the maximum solubility of carbon in hexagonal close packed iron, i.e. hexagonite, to be 2·625 at-%C, supporting the phenomenological value of 2·75 at-%C (0·6 wt-%C). The electron binding energies associated with the creation of hexagonite are consistent with experimental observations. The E-B theory also predicts the volume change from austenite to hexagonite and from hexagonite to ferrite that is in agreement with the values obtained from pressure-temperature studies. The exceptional hardness of lath martensite is attributed to the presence of iron-carbon clusters and subgrains and is proportional to its specific volume.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Nano-scale strengthening has been studied for Fe and Fe-based alloys and found to be a function of grains, subgrains, and particles. In Fe–C/O alloys, these microstructural features can result from ...phase transformations and deformation processes. The relationships between these structural features and strength have been quantified and the limitations established. The relationships were shown to apply equally well to structures produced by deformation and quenching. The major contributors to strength were carbide and oxide particles. Particles, grains, and subgrains were shown to provide significantly higher strengthening than contributions from solid solution effects.
Severe plastic deformation is observed within adiabatic shear bands in iron–carbon steels. These shear bands form under high strain rate conditions, in excess of 1000
s
−1, and strains in the order 5 ...or greater are commonly observed. Studies on shear band formation in a ultra-high carbon steel (1.3% C) are described in the pearlitic condition. A hardness of 11.5
GPa (4600
MPa) is obtained within the band. A mechanism is described to explain the high strength based on phase transformation to austenite from severe plastic deformation and adiabatic heating. Rapid re-transformation leads to an ultra-fine ferrite grain size containing carbon principally in the form of nano-size carbides. It is proposed that the same mechanism explains the ultra-high strength of iron–carbon steels observed in ball-milling, ball drop tests and in severely deformed wires.
Nano-subgrain strengthening in ball-milled iron Lesuer, D.R.; Syn, C.K.; Sherby, O.D.
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
08/2007, Letnik:
463, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The strength and deformation behavior of ball-milled, iron-base materials containing nano-scale subgrains have been evaluated. As reported by several authors, nano-subgrains form during the early ...stages of ball milling as a result of severe plastic deformation inherent in the ball milling process. The strength for these nano-scale subgrains are compared with the strength of larger-scale subgrains in iron and iron-base alloys produced by traditional mechanical working. The data covers over two orders of magnitude in subgrain size (from 30
nm to 6
μm) and shows a continuous pattern of behavior. For all materials studied, the strength varied as
λ
−1, where
λ is the subgrain size. Strengthening from subgrains was found to breakdown at a much smaller subgrain size than strengthening from grains. In addition, the ball-milled materials showed significant strengthening contributions from nano-scale oxide particles. Shear bands are developed during compression testing of ball-milled materials containing ultra-fine subgrains. A model for shear band development in nano-scale subgrains during deformation has also been developed. The model predicts a strain state of uniaxial compression in the shear band with a strain of −1.24. Subgrains are shown to offer the opportunity for high strength and good work hardening with the absence of yield point behavior.
Laminated metal composites (LMCs) are a unique form of composite material in which alternating metal or metal containing layers are bonded together with discrete interfaces. The mechanical properties ...of these materials are reviewed. The tensile properties at low and high temperatures are described. At low temperature, very high tensile strengths can be achieved in deposition processed laminates and very high tensile ductilities can be achieved in roll bonded laminates by layer thickness refinement. At high temperature, superplasticity has been observed and agrees with predictions from constitutive creep relations. Damage critical properties (such as fracture toughness, fatigue, and impact behaviour) and damping can be superior to those exhibited by the component materials. The damage critical properties are strongly influenced by local delaminations at layer interfaces. Mechanisms responsible for many of the unique properties of LMCs have been proposed. The influence of processing, laminate architecture, and microstructure on these mechanisms and properties are described.
AerMet-100 alloy cylinders were explosively driven to fragmentation. Soft-captured fragments were studied to characterize the deformation induced by high explosive loading. The characterization of ...the fragments reveals that the dominant failure mechanism appears to be dynamic fracture along adiabatic shear bands. These shear bands differ in size and morphology depending on the heat-treated conditions. Nanoindentation measurements of the adiabatic shear bands in either material condition indicate higher hardness in the bands compared to the matrix regions of the fragments.
Abstract Background Mitogen-activated protein kinases (MAP kinases) participate in signal transduction pathways that control embryogenesis, cell differentiation, cell proliferation and cell death. ...The roles of extracellular signal-regulated kinase1/2 (ERK1/2) and p38 MAP kinase in the differentiation and invasion of human trophoblasts have been studied. However, the in vivo expression and activation of ERK1/2 and p38 at the placental bed have not been elucidated. Methods The study group consisted of placental bed biopsy tissues obtained from the pregnancies without preeclampsia ( n = 24) and with preeclampsia ( n = 8) between 31 and 40 weeks of gestation. We evaluated the expressions and phosphorylations of ERK1/2 and p38 MAP kinase in the invasive trophoblasts in the placental bed tissues using immunohistochemistry. Results p38 and phospho-p38 MAP kinase were not detected in invasive trophoblasts in cases or controls. ERK1/2 and phospho-ERK1/2 were positive in invasive trophoblasts albeit with variable staining. Phosphorylation of ERK1/2 was significantly less frequent in invasive trophoblasts in placental bed biopsies from women with preeclampsia compared with normotensive controls. Conclusion These findings suggest that preeclampsia is associated with decreased activation of ERK1/2 in invasive trophoblasts in vivo.
The low bioavailability of nutrients and oxygen in the soil environment has hampered successful expression of biodegradation and biocontrol genes that are driven by promoters highly active during ...routine laboratory conditions of high availability of nutrients and oxygen. Hence, in the present study, expression of the gus-tagged genes in 12 Tn5-gus mutants of the soil microbe Pseudomonas putida PNL-MK25 were examined under various conditions chosen to mimic the soil environment: low carbon, phosphate, nitrate or oxygen, and in the rhizosphere. Based on their expression profiles, three nutrient-responsive mutant (NRM) strains, NRM5, NRM7 and NRM17, were selected for identification of the tagged genes. In strain NRM5, expression of the glutamate dehydrogenase (gdhA) gene was increased 4.9-26.4-fold under various low-nutrient conditions. In NRM7, expression of the novel NADPH : quinone oxidoreductase-like (nql) gene was consistently amongst the highest and was synergistically upregulated by low-nutrient and anoxic conditions. The cyoD gene in NRM17, which encodes the fourth subunit of the cytochrome o ubiquinol oxidase complex, had decreased expression in low-nutrient conditions but its absolute expression level was still amongst the highest. Additionally, it was independent of oxygen availability, in contrast to that in Escherichia coli.
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