Characterization of crosslinked,macroporous, monosized polymer particles

Macroporous, monosized poly(meta‐divinylbenzene) and poly(para‐divinylbenzene) beads have been prepared by the two‐step activated swelling method with toluene or 2‐ethylhexanoic acid as pore‐forming agents. The type of divinylbenzene isomer as well as the type of porogen has a large effect on both physical and chemical properties of the monodisperse beads. Large pores are obtained with 2‐ethylhexanoic acid as porogen while beads prepared in the presence of toluene consist of only small pores and exhibit a shrinking behavior upon drying. The beads have considerable amounts of residual vinyl groups at the end of polymerization, as determined by bromination and Fourier transform IR analysis. The morphology and texture of the particles have been investigated by scanning electron microscopy and atomic force microscopy. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 152–169, 2000     

Effect of plastic prestrain on the crack tip constraint of pipeline steels

Before and during operation, pipelines may suffer from plastic pre-deformation due to accidental loading, cold bending and ground movement. Plastic prestrain not only modifies steel's yield and flow properties but also influences its fracture performance. This paper focuses on the effect of prestrain history on crack driving force and crack tip constraint. A single-edge notched tension specimen has been selected for the study and the crack is assumed to exist before a prestrain history was applied. The results show that prestrain history has a strong effect on the crack tip stress field. A new parameter has been proposed to characterize the prestrain-induced crack tip constraint. For the same crack tip opening displacement level, prestrain history will elevate the crack tip stress field. The prestrain-induced constraint decreases with the increase of loading.     

Residual stress induced crack tip constraint

This paper studies the effect of welding residual stresses on the near tip stress field in single edge notched bending and tensile specimens. A combined effect of mechanical stresses by the applied load and residual stress on the crack tip constraint is analyzed. Three initial residual stress distributions were considered. It has been shown that the crack tip stress field is strongly influenced by the residual stresses and a new parameter, R, is proposed to characterize the residual stress induced crack tip constraint. The results therefore suggest a three-parameter approach (CTOD, Q and R) to characterize the crack tip stress field in the presence of residual stress where CTOD sets the size scale over which large stresses and large strains develop, and the geometry constraint parameter Q and the new residual stress induced constraint parameter R control the actual crack tip constraint level. For the cases analyzed, R is in general positive, which indicates that residual stress can enhance the crack tip constraint. However, the results also indicate that the R decreases towards zero and the effect of residual stress on crack tip constraint can be neglected when a full plastic condition is approached in the specimen.     

Effects of crack depth and specimen size on ductile crack growth of SENT and SENB specimens for fracture mechanics evaluation of pipeline steels

A strong geometry dependence of ductile crack growth resistance emerges under large scale yielding. The geometry dependence is associated with different levels of crack tip constraint conditions. However, in a recent attempt to identify appropriate fracture mechanics specimens for pipeline steels, an “independent” relationship between the crack growth resistance curves and crack depths for SENT specimens has been observed experimentally. In this paper, we use the complete Gurson model to study the effects of crack depth and specimen size on ductile crack growth behavior. Crack growth resistance curves for plane strain, mode I crack growth under large scale yielding conditions have been computed. SENB and SENT specimens with three different specimen sizes, each specimen size with three different crack depths, have been selected. It has been found that crack tip constraint (Q-parameter) has a weak dependence on the crack depth for specimens in the low constraint regime.     

Bi2Sr2CaCu2Ox bulk superconductor with MgO particles embedded

MgO nano-size particles in various concentrations were added to the Bi-2212 superconductor precursor before partial melting. The resulting ceramic superconductors had a transition sharpness and volume fraction which increased with MgO addition up to 9 wt.%. Compositional variations in the superconducting matrix with varying MgO addition were found, with possible important consequences for the pinning mechanism, since compositional variations are expected to be accompanied by variations in T_c. By TEM it was directly documented that MgO particles could be embedded within the superconducting Bi-2212 grains. Hence wetting occurs. Thus, the requirements for MgO to play an effective role in pinning are satisfied. It is suggested that two distinct pinning mechanisms, both related to MgO addition, are likely to be present. Magneto-optic images clearly confirm the improvement of pinning, and screening, with increasing addition of MgO to Bi-2212.     

Effect of residual stresses on ductile crack growth resistance

It is well known that residual stresses influence the ductile fracture behaviour. In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. A sharp crack was embedded in the center of the weld region. The complete Gurson model has been applied to simulate the ductile fracture by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. The effect of residual stress on ductile crack growth resistance seems independent of the size of geometrically similar welds. When normalized by the weld zone size, the ductile crack growth resistance collapses into one curve, which can be used to assess the structural integrity and evaluate the effect of residual stresses. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f₀, hardening exponent n and T-stress.     

Effects of afferent and efferent celiac nerves on acute gastric lesions: due to changes in blood flow?

Since ablation of afferent nerves prior to stress results in increased severity of acute gastric mucosal lesions, afferent nerves are thought to mediate protective mechanisms in the stomach. These mechanisms are known to include vasodilation of gastric mucosal vessels; vasodilation is thought to allow the gastric mucosa to respond to injurious substances. However, it is not known whether other aspects of mucosal health, independent of those caused by increased blood blow, are affected by afferent blockade. This study compared gastric blood flow and acute gastric mucosal lesions during stress in rats with either chemical sympathectomy or afferent blockade. The purpose of the study was to compare the lesion index and blood flow in each treatment group. The lesion index was highest in rats with afferent blockade and lowest after sympathectomy. Gastric blood flow was partially preserved after sympathectomy, but was not greatly increased, suggesting that some of the effects observed after afferent blockade are unrelated to changes in blood flow.     

Identification of a parathyroid adenoma by operative ultrasonography

During surgery, real-time ultrasound scanning accurately localized a parathyroid adenoma posterior to be the superior pole of the right thyroid lobe. This was made feasible because of the ultrasound features of parathyroid tissue and current developments in ultrasound instrumentation.