AN LMI APPROACH TO ROBUST H∞ CONTROL FOR UNCERTAIN CONTINUOUS‐TIME SYSTEMS

This paper investigates the problems of robust H∞ control for uncertain continuous‐time systems with time‐varying, norm‐bounded uncertainties in all system matrices. Necessary and sufficient conditions for the above problems are proposed. All conditions are represented in the form of linear matrix inequalities (LMIs). The robust H∞ controller can be easily designed from the solutions of the LMI conditions. Unlike earlier works, the proposed method does not involve any parameter tuning. Thus the robust H∞ optimization control problem, which has not been discussed in earlier reports, can be dealt with using this newly proposed method.     

The influence of mineral admixtures on sulfate resistance of limestone cement pastes aged in cold MgSO4 solution

The influence of slag (S), fly ash (FA) and silica fume (SF) on the sulfate resistance of limestone cements was evaluated. Hardened pastes were exposed to MgSO₄ solution at 5 °C. Visible changes of the samples during the exposure were followed. Absorption of sulfate was measured and changes in mineralogical composition were evaluated by thermogravimetric analysis and X-ray diffraction (XRD). It was found that among admixtures used, only the addition of silica fume to limestone cement significantly improved its sulfate resistance. Cement with lower contents of C₃A and C₃S also showed favorable performance compared to cement having higher contents of these minerals.     

Damage evolution in flexible polyurethane foams

Microcrazing in the struts of flexible polyurethane foams was discovered during compressive deformation and observed directly in the scanning electron microscope. Attributed to this phenomena was the decrease in stress at maximum compression and the intensity of acoustic emission during compressive cycling. The higher content of styrene–acrylonitrile (SAN) copolymer in these foams resulted in higher modulus, more severe microcrazing, an increase in acoustic emission activity, and a decrease in the stress at maximum compression as cycling progressed.     

Dimensional stability and property gradients in thick sheet molding compound (SMC) sections

The dimensional stability of sample cylinders cured with sheet molding compound pastes was investigated. A significant amount of dimensional change was found for these samples when they were annealed. Furthermore, the amount of change varied with location from the center to the wall along the radial direction of the sample cylinder to form a strain gradient. A series of experiments were then carried out to determine property gradients along the same direction in search of the source of the dimensional instability. It was found that the sample also had a gradient in cure and a gradient in dynamic mechanical properties. But these gradients are not in full agreement. In particular, the gradient of cure appears to be opposite to the direction of the strain gradient, while the gradient of the dynamic mechanical properties coincides with it. These results, therefore, suggest that the dimensional stability may be predominantly governed by the viscoelastic behavior of the material.     

JNK and p38 Inhibitors Prevent Transforming Growth Factor-β1-Induced Myofibroblast Transdifferentiation in Human Graves’ Orbital Fibroblasts

Transforming growth factor-β1 (TGF-β1)-induced myofibroblast transdifferentiation from orbital fibroblasts is known to dominate tissue remodeling and fibrosis in Graves’ ophthalmopathy (GO). However, the signaling pathways through which TGF-β1 activates Graves’ orbital fibroblasts remain unclear. This study investigated the role of the mitogen-activated protein kinase (MAPK) pathway in TGF-β1-induced myofibroblast transdifferentiation in human Graves’ orbital fibroblasts. The MAPK pathway was assessed by measuring the phosphorylation of p38, c-Jun N-terminal kinase (JNK), and extracellular-signal-regulated kinase (ERK) by Western blots. The expression of connective tissue growth factor (CTGF), α-smooth muscle actin (α-SMA), and fibronectin representing fibrogenesis was estimated. The activities of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) responsible for extracellular matrix (ECM) metabolism were analyzed. Specific pharmacologic kinase inhibitors were used to confirm the involvement of the MAPK pathway. After treatment with TGF-β1, the phosphorylation levels of p38 and JNK, but not ERK, were increased. CTGF, α-SMA, and fibronectin, as well as TIMP-1 and TIMP-3, were upregulated, whereas the activities of MMP-2/-9 were inhibited. The effects of TGF-β1 on the expression of these factors were eliminated by p38 and JNK inhibitors. The results suggested that TGF-β1 could induce myofibroblast transdifferentiation in human Graves’ orbital fibroblasts through the p38 and JNK pathways.     

Comparison of shear-yielding modes in polycarbonate and a thermoplastic polyurethane

A thermoplastic polyurethane (PU) introduced by The Dow Chemical Company under the tradename Isoplast® possesses certain engineering properties that resemble those of polycarbonate (PC), including optical clarity, a glass transition of 140 °C, and comparable modulus and ductility. In this study, the shear-yielding behaviour at a circular hole was compared. Strain birefringence was used to analyse deformation in the linear elastic region. At higher stresses, optical microscope techniques were used to characterize the flow lines through the thickness of the plastically deformed region. Differences in shear-yielding behaviour were attributed to differences in the true stress-true strain relationship of the two materials. The true stress-true strain curve of PC exhibited a yield instability marked by a drop in the true stress. Shear yielding was described in terms of idealized elastic-plastic behaviour, and large-scale flow that accompanied yielding was responsible for the strong thickness dependence of shear yielding. The true stress-true strain curve of PU did not exhibit a true stress drop at the yield point. The primary features of shear yielding were consistent with elastic-plastic behaviour with work hardening; small local plastic strains resulted in more diffuse yielding that was not strongly dependent on thickness.     

Experimental Investigation of the Reinstallation of Spudcan Footings Close to Existing Footprints

The large footprints that remain on the seabed after offshore mobile jack-up platforms have completed operations provide hazardous conditions for any future jack-up installation at that site. The slope of the footprint and varying soil strengths below the surface cause detrimental horizontal and moment loads to be induced on the spudcan during the preloading process where only vertical loads are expected. Experimental data from 12 tests investigating the reinstallation of a spudcan footing close to an existing footprint is presented in this paper. The experiments were carried out using a geotechnical drum centrifuge at a radial acceleration level equivalent to 250 times that of Earth’s gravity. The stiffness of the loading leg and model spudcan shape were scaled to ensure conditions of stress similitude between the model and prototype. In all of the experiments, an initial footprint was created. The spudcan was then offset and reinstalled with the combined vertical, horizontal, and moment loads on the spudcan recorded. The effects of reinstallation location, preloading levels, and change in leg stiffness were investigated. The worst location for reinstallation was found to be at an offset half a spudcan diameter from the initial spudcan installation. The horizontal and moment loads were also greater when a more extensive footprint was created by the initial spudcan being embedded deeper and with a higher preload. For the range of conditions tested, changing the leg stiffness did not affect the results.     

Effects of foot placement on postural stability of construction workers on stilts

Stilts are elevated tools that are frequently used by construction workers to raise workers 18-40 inches above the ground. The objective of this laboratory study was to evaluate the potential loss of postural stability associated with the use of stilts in various foot placements. Twenty construction workers with at least 1 year of experience in the use of stilts participated in this study. One Kistler™ force platform was used to collect kinetic data. Participants were tested under six-foot-placement conditions. These 6 experimental conditions were statically tested under all combinations of 3 levels of elevation: 0″ (no stilts), 24″ stilt height and 40″ stilt height. SAS mixed procedure was used to evaluate the effect of different experimental conditions. The results of the multivariate analysis of variance (MANOVA) and repeated measures of univariate analyses of variance (ANOVAs) demonstrated that stilt height, foot-placement direction, and foot-placement width all had significant effects on the whole-body postural stability. This study found that the higher the stilts were elevated, the greater the postural instability. A stance position with one foot placed forward of the other foot produced greater postural instability than a position with the feet parallel and directly beneath the body. This study found that placement of the feet parallel and directly beneath the body, with the feet positioned a half shoulder width apart, caused a greater amount of postural sway and instability than one and one-and-half shoulder width. This study also found that construction workers using the stilts could perceive the likely postural instability due to the change in foot placements.