Yield anisotropy effects on buckling of circular tubes under bending

AbstractRelatively thin-walled tubes bent into the plastic range buckle by axial wrinkling. The wrinkles initially grow stably but eventually localize and cause catastrophic failure in the form of sharp local kinking. The onset of axial wrinkling was previously established by bifurcation analyses that use instantaneous deformation theory moduli. The curvatures at bifurcation were predicted accurately, but the wrinkle wavelengths were consistently longer than measured values. The subject is revisited with the aim of resolving this discrepancy. A set of new bending experiments is conducted on aluminum alloy tubes. The results are shown to be in line with previous ones. However, the tubes used were found to exhibit plastic anisotropy, which was measured and characterized through Hill’s quadratic anisotropic yield function. The anisotropy was incorporated in the flow theory used for prebuckling and postbuckling calculations as well as in the deformation theory used for bifurcation checks. With the anisotropy accounted for, calculated tube responses are found to be in excellent agreement with the measured ones while the predicted bifurcation curvatures and wrinkle wavelengths fall in line with the measurements also. The postbuckling response is established using a finite element model of a tube assigned an initial axisymmetric imperfection with the calculated wavelength. The response develops a limit moment that is followed by a sharp kink that grows while the overall moment drops. The curvature at the limit moment agrees well with the experimental onset of failure. From parametric studies of the various instabilities it is concluded that, for optimum predictions, anisotropy must be incorporated in both bifurcation buckling as well as in postbuckling calculations

PMU Placement for Power System Observability Using Binary Particle Swarm Optimization

A binary particle swarm optimization (BPSO) based methodology for the optimal placement of phasor measurement units (PMUs) for complete observability of a power system is presented in this paper. The objectives of the optimization problem are to minimize the total number of PMUs required, and to maximize the measurement redundancy at the power system buses. Simulation results on the IEEE 14-bus and 30-bus test systems are presented in this paper

Mestastable State Population in Laser Induced Plasmas

Laser induced plasma has been used as a source of neutrals and ions in the study of astrophysical plasmas. The purity of state of this source is essential in the determination of collision parameters such as the charge transfer rate coefficients between ions and neutrals. We will show that the temperature of the laser induced plasma is a rapidly decreasing function of time. The temperature is initially high but cools off rapidly through collisions with the expanding plasma electrons as the plasma recombines and streams into the vacuum. This rapid expansion of the plasma, similar to a supersonic jet, drastically lowers the internal energy of the neutrals and ions

Restenosis is not associated with stent length in a pig model of coronary stent implantation

Background: The aim of this study was to determine if stent length is by itself a risk factor for intimal proliferation and restenosis. Long lesions represent an independent risk factor for restenosis after coronary stent implantation. A longer stented segment might result in a higher probability of restenosis. Methods: Twenty-two 7-month-old male farm pigs underwent implantation of two steel stents, one short (8 mm length) and one long (16 mm length), in the right coronary artery. The pigs were sacrificed 28 days after stent implantation and histomorphometric analysis of the coronary arteries was performed for neointimal area proliferation and area stenosis evaluation. Results: Seventeen short stents and 19 long stents were finally implanted. There were no differences in neointimal proliferation (1.84 ± 0.64 mm2 vs. 1.81 ± 0.94 mm2, p = 0.84), area stenosis (40 ± 9% vs. 41 ± 19%, p = 0.86) and lumen area (2.96 ± 1.30 mm2 vs. 2.51 ± ± 1.18 mm2, p = 0.21) between the short stent group and the long stent group, respectively. Conclusions: These data suggest that stent length by itself does not influence restenosis extent in the porcine model

Evaluation of seismic demand for substandard reinforced concrete structures

Background: Reinforced Concrete (RC) buildings with no seismic design exhibit degrading behaviour under severe seismic loading due to non-ductile brittle failure modes. The seismic performance of such substandard structures can be predicted using existing capacity demand diagram methods through the idealization of the non-linear capacity curve of the degrading system, and its comparison with a reduced earthquake demand spectrum. Objective: Modern non-linear static methods for derivation of capacity curves incorporate idealization assumptions that are too simplistic and do not apply for sub-standard buildings. The conventional idealisation procedures cannot maintain the true strength degradation behaviour of such structures in the post-peak part, and thus may lead to significant errors in seismic performance prediction especially in the cases of brittle failure modes dominating the response. Method: In order to increase the accuracy of the prediction, an alternative idealisation procedure using equivalent elastic perfectly plastic systems is proposed herein that can be used in conjunction with any capacity demand diagram method. Results: Moreover, the performance of this improved equivalent linearization procedure in predicting the response of an RC frame is assessed herein. Conclusion: This improved idealization procedure has been proven to reduce the error in the seismic performance prediction as compared to seismic shaking table test results [1] and will be further investigated probabilistically herein

Systemic hypertension augments, whereas insulin-dependent diabetes down-regulates, endothelin A receptor expression in the mammary artery in coronary artery disease patients

Background: Endothelin (ET) A receptor antagonism causes decreased vasodilation in hypertensive coronary arteries and decreased effects on coronary artery compliance in diabetic patients. Methods: We investigate the mRNA expression of ET-1, ETA and ETB receptors, using real time RT-PCR, in biopsies from the internal mammary artery obtained from 49 patients, 18 diabetics and 34 hypertensives, all undergoing coronary artery bypass grafting. Results: Hypertensive patients had higher ET-1 mRNA expression (16438 [8417, 23917]), than normotensive patients (2974 [2283, 18055], p=0.008). Diabetic patients had significantly lower ETA receptor levels than non-diabetic patients (455 [167, 1496] vs. 1660 [700, 3190], respectively, p = 0.003). Conclusions: Multivariate analysis demonstrated that the presence of systemic hypertension was the only independent predictor of log ETA receptor expression and log ET-1 expression, while insulin-dependent diabetes was negatively correlated with ETA receptor expression. ETB receptor expression was not correlated with any predictor. Systemic hypertension is associated with increased ET-1 and ETA receptor mRNA expression, whereas insulin-dependent diabetes down-regulates ETA receptor mRNA expression in the internal mammary artery in patients with coronary artery disease undergoing bypass grafting

The role of thrombospondins in wound healing, ischemia, and the foreign body reaction

Thrombospondin (TSP) 1 and TSP2 have been implicated in the regulation of several processes during tissue repair. Due to their matricellular nature, these proteins are thought to modulate cell-matrix interactions through a variety of mechanisms specific to the spatio-temporal context of their expression. Most notably, TSP1 and TSP2 appear to play distinct, non-overlapping roles in the healing of skin wounds. In contrast, both proteins have been implicated as regulators of ischemia-induced angiogenesis. Moreover, TSP2 has been shown to be a critical regulator of angiogenesis in the foreign body response (FBR). In this review, we discuss the role of TSPs in tissue repair and examine the mechanistic data regarding the ability of the thrombospondins to modulate cell-matrix interactions in this context