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Mechanical conditioning of mesenchymal stem cells (MSCs) has been adopted widely as a biophysical signal to aid tissue engineering applications. The replication of in vivo mechanical signaling has been used in in vitro environments to regulate cell differentiation, and extracellular matrix synthesis, so that both the chemical and mechanical properties of the tissue-engineered construct are compatible with the implant site. While research in these areas contributes to tissue engineering, the effects of mechanical strain on MSC apoptosis remain poorly defined. To evaluate the effects of uniaxial cyclic tensile strain on MSC apoptosis and to investigate mechanotransduction associated with strain-mediated cell death, MSCs seeded on a 2D silicone membrane were stimulated by a range of strain magnitudes for 3 days. Mechanotransduction was investigated using the stretch-activated cation channel blocker gadolinium chloride, the L-type voltage-activated calcium channel blocker nicardipine, the c-jun N[H.sub.2]-terminal kinase (JNK) blocker D-JNK inhibitor I, and the calpain inhibitor MDL 28170. Apoptosis was assessed through DNA fragmentation using the terminal deoxynucleotidyl transferase mediated-UTP-end nick labeling method. Results demonstrated that tensile strains of 7.5% or greater induce apoptosis in MSCs. L-type voltage-activated calcium channels coupled mechanical stress to activation of calpain and JNK, which lead to apoptosis through DNA fragmentation. The definition of the in vitro boundary conditions for tensile strain and MSCs along with a proposed mechanism for apoptosis induced by mechanical events positively contributes to the development of MSC biology, bioreactor design for tissue engineering, and development of computational methods for mechanobiology. [DOI: l0.1115/1.2979870] Keywords: mesenchymal stem cells, mechanical strain, apoptosis, c-jun N-terminal kinase, calpain, calcium

The role of applied mechanical stress on space charge breakdown in electron beam irradiated polymethyl methacrylate (PMMA) is investigated. Space charge is implanted into PMMA sheets by electron beam. Space charge is measured by laser induced pressure pulse technique to investigate injection, trapping and detrapping of charge. Mechanical stress is applied on PMMA sheet to study its influence on space charge behavior. Experimental results show that applied electric field is not a necessary condition for breakdown of insulating materials. The breakdown can be triggered during space charge detrapping. The detrapping of space charge can result in tree-like breakdown, wherein this phenomenon is termed as space charge breakdown. Effect of applied mechanical stress on the initiation of breakdown and the development of tree-like breakdown is discussed in this paper. Index Terms--space charge, tree-like breakdown, applied mechanical stress.

This paper presents very recent results of an experimental program aimed at disclosing size and strain rate effects on the fracture behavior of reinforced concrete beams. Thirty-six reinforced beams made from two microconcretes with different Hillerborg's characteristic length [one is 66 mm (material A), the other is 105 mm (material B)], of three sizes (75, 150, and 300 mm in depth), were tested under four strain rates (1.05 X [10.sup.-5], 1.25 x [10.sup.-3], 1.25 x [10.sup.-2], and 3.75 x [10.sup.-2] [s.sup.-1]). The results show that the peak loads increase with an increase in the strain rate; the rate dependence of the peak load is stronger for larger specimens than for smaller ones. Moreover, size effect is only shown under the nominal strain rate 1.05 x [10.sup.-5] [s.sup.-1]; under the higher strain rates, it is inconspicuous. These results seem to reveal an apparent physical inconsistency, since there is no obvious reason that the size effect disappears when the strain rate increases. The explanation to this is sought numerically using an explicit cohesive model. DOI: 10.1061/(ASCE)0899-1561 (2008)20:8(544) CE Database subject headings: Cracking; Concrete, reinforced; Strain rate; Size effects; Experimentation.

An experimental study investigating the influence of lateral displacement on the vertical stiffness of elastomeric and lead-rubber seismic isolation bearings is summarized. Two identically constructed low-damping rubber and lead-rubber seismic isolation bearings were subjected to a series of tests with varying levels of combined lateral displacement and axial (compressive) loading to study this relationship. The results of these tests showed the vertical stiffness decreases with increasing lateral displacement for each bearing tested. Additionally, the vertical stiffness data are used to evaluate four formulations for the estimation of the vertical stiffness as a function of the lateral displacement. From this comparison, two formulations, one based on the Koh-Kelly two-spring model and the other on a piecewise linear relationship, showed good agreement with the experimental data over the wide range of lateral displacements considered in this study. DOI: 10.1061/(ASCE)0733-9445(2007) 133:9(1227) CE Database subject headings: Elastomer; Rubber; Stiffness; Lateral displacement; Experimentation.

Resilient modulus ([M.sub.R]) value of a subgrade soil is the primary property needed for pavement design and analysis. It is the elastic modulus based on the recoverable strain under repeated loads, and depends on several factors including soil properties, soil type, and state of stresses. This paper presents prediction equations developed using regression analysis for six AASHTO soil types (A-1-b, A-3, A-2-4, A-4, A-6, and A-7-6) for estimating [M.sub.R]. Data extracted from the Long-Term Pavement Performance Information Management System (LTPP IMS) database for 258 test specimens (approximately 3,870 [M.sub.R] values) collected in 19 states in New England and the nearby regions in the United States and two provinces in Canada were used in this study. A generalized constitutive model that captures the effect of both bulk stress and octahedral shear stress was used to predict the MR of subgrade soils by developing regression equations that relate the k coefficients to the soil physical properties. The prediction models developed were verified for four types of soils collected in the New England region by independent laboratory [M.sub.R] tests. It was observed that MR values from the prediction models compared well with the laboratory values for the majority of the soil types. CE Database subject headings: Subgrades; Flexible pavements; Regression models; Model verification; Soil properties; Predictions: Stress.

Almandine garnet ([Alm.sub.62.6][Pyr.sub.11.4][Sps.sub.8.4][And.sub.65][Grs.sub.4.0]) exceptionally rich in [Y.sub.2][O.sub.3] (1.8-2.3 wt%), [Sc.sub.2][O.sub.3] (0.11-0.23 wt%), HRE[E.sub.2][O.sub.3] (1.9-2.3 wt%), and [Na.sub.2]O (~0.3 wt%) occurs in Precambrian quartzofeldspatic granulites (T ~ 860 [degrees]C) on Hisaroya, Western Gneiss Region, Norway. The granulites were partially reequilibrated to eclogite-facies (P = 14.9 [+ or -] 1.3 kbar, T= 649 [+ or -] 67 [degrees]C) and amphibolite-facies assemblages during the Caledonian Orogeny. The granulite-facies garnet is reequilibrated in ah outer zone, typically ~4 [micro]m thick, where both [Y.sub.2][O.sub.3] and HRE[E.sub.2][O.sub.3] concentrations decrease to 1.2 wt%, and overgrown by Caledonian garnet with [Y.sub.2][O.sub.3] and HRE[E.sub.2][O.sub.3] both below 0.1 wt%. The granulitefacies cores are also reequilibrated to lower [Y.sub.2][O.sub.3] and HRE[E.sub.2][O.sub.3] (both ~1.8 wt%) along deformation structures including microfaults with horsetails, en-echelon bands, and splaying features around the tip of fractures. Locally, these reequilibrated zones, typically 10-20 [micro]m across, have shoulders with higher Y and HREE (~2.9 wt%) than in the original garnet, suggesting closed-system behavior for these elements. In addition, the garnet locally displays a complex mesh-like pattern with high and low HREE and Y bands, possibly related to strain. Two charge-balancing mechanisms account for Y and HREE incorporation: (1) the yttrium aluminum garnet ([Y.sub.2][Al.sub.2][O.sub.6], YAG) substitution involving incorporation of Al into the tetrahedral site is dominant in the granulite-facies garnet, and (2) coupled substitution with Na into the dodecahedral site is increasingly important during high-P reequilibration. The internal closed-system reequilibration of the granulite-facies garnet may be related to stress concentrations build up in garnet during the eclogite-facies event. Local stress concentrations during propagation of fractures may be the driving force behind the redistribution of elements seen within cores of granulite-facies garnet. Keywords: Garnet, almandine, HREE, heavy rare earth elements, yttrium, sodium, Lu, Yb, Er, Tm, Dy, YAG, WGC, granulite, eclogite, deformation, fracturing, fracture propagation, diffusion

This paper investigates the in-plane elastic stability of a shallow parabolic arch with horizontal spring supports subjected to a uniformly distributed vertical load. A virtual work formulation is used to establish both the nonlinear equilibrium equations and the buckling equilibrium equation for shallow arches. Analytical solutions for the in-plane buckling loads of shallow arches subjected to this loading regime are obtained, and a closed-form solution for the in-plane antisymmetric bifurcation buckling load and an approximation to the symmetric snap-through buckling load of shallow arches are proposed. Comparisons with the finite-element predictions of these buckling loads, together with the measured buckling loads of arches tested in the laboratory and reported in the companion paper, demonstrate that the analytical solutions are accurate. It is found that the effects of the stiffness of the horizontal springs on the buckling load and buckling behavior of arches are significant, and that the buckling load of an arch increases with an increase in the flexibility of the horizontal springs at the supports. Values of the slenderness that delineate the buckling modes increase with an increase of the flexibility of the horizontal springs. Values of the slenderness that distinguish between an arch (for which buckling is possible) and a beam curved in elevation (that cannot buckle in plane) also increase with an increase of the flexibility of the horizontal springs. DOI: 10.1061/(ASCE)0733-9445(2007)133:8(1130) CE Database subject headings: Arches; Buckling; Elasticity; Nonlinear analysis; Structural stability; Supports.

The laboratory and field test data on the response of piles under the combined action of vertical and lateral loads is rather limited. The current practice for design of piles is to consider the vertical and lateral loads independent of each other. This paper presents some results from three-dimensional finite-element analyses that show the significant influence of vertical loads on a pile's lateral response. The analyses were performed in both homogeneous clayey soils and homogeneous sandy soils. The results have shown that the influence of vertical loads on the lateral response of piles is to significantly increase the capacity in sandy soils and marginally decrease the capacity in clayey soils. In general, it was found that the effect of vertical loads in sandy soils is significant even for long piles, which are as long as 30 times the pile width, while in the case of clayey soils, the effect is not significant for piles beyond a length of 15 times the width of the pile. The design bending moments in the laterally loaded piles were also found to be dependent on the level of vertical load on the piles. DOI: 10.1061/(ASCE)1090-0241(2007)133:5(512) CE Database subject headings: Piles; Lateral loads; Finite element method; Combined loads; Numerical models.

The flow rate of water flowing on a steel surface is considered to be one of the important factors strongly influencing the fatigue life of the steel, because the water flow produces difference in the local environmental conditions. The effect of the water flow rate on the fatigue life of a carbon steel was thus investigated experimentally. Fatigue testing of the carbon steel was performed at 289[degrees]C for various dissolved oxygen contents (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of O.4, 0.01, and 0.001 %/s were used in the fatigue tests. At the strain rate of O.4 %/s, no significant difference in fatigue life was observed under the various flow rate conditions. On the other hand, at 0.01 %/s, the fatigue life increased with increasing water flow rate under all DO conditions, such that the fatigue life at a 7 m/s flow rate was about three times longer than that at a 0.3 m/s flow rate. This increase in fatigue life is attributed to increases in the crack initiation life and small-crack propagation life. The major mechanism producing these increases is considered to be the flushing effect on locally corrosive environments at the surface of the metal and in the cracks. At the strain rate of 0.001 %/s, the environmental effect seems to be diminished at flow rates higher than 0.1 m/s. This behavior does not seem to be explained by the flushing effect alone. Based on this experimental evidence, it was concluded that the existing fatigue data obtained for carbon steel under stagnant or relatively low flow rate conditions may provide a conservative basis for fatigue life evaluation. This approach seems useful for characterizing fatigue life evaluation by expressing increasing fatigue life in terms of increasing water flow rate.

Stresses, microbending loss, and refractive-index changes induced simultaneously by axial strain and hydrostatic pressure in double-coated optical fibers are analyzed. The lateral pressure and normal stresses in the optical fiber, primary coating, and secondary coating are derived. Also presented are the microbending loss and refractive-index changes in the glass fiber. The normal stresses are affected by axial strain, hydrostatic pressure, material properties, and thickness of the primary and secondary coatings. It is found that microbending loss decreases with increasing thickness, the Young's modulus, and the Poisson's ratio of the secondary coating but increases with the increasing Young's modulus and Poisson's ratio of the primary coating. Similarly, changes in refractive index in the glass fiber decrease with the increasing Young's modulus and Poisson's ratio of the secondary coating but increase with the increasing Young's modulus and Poisson's ratio of the primary coating. Therefore, to minimize microbending loss induced simultaneously by axial strain and hydrostatic pressure in the glass fiber, the polymeric coatings should be suitably selected. An optimal design procedure is also indicated. OCIS codes: 160.4760, 160.0160.

The permanent strain behavior of unbound aggregates under repeated loading has been studied by many researchers over the years. A summarized review of the research into this area is given based on the literature available to date. Different views from various sources are outlined and discussed. A state of the art is presented regarding the factors affecting the permanent strain development in these materials and the modeling techniques available. This paper is the second of two companion papers discussing the current state of knowledge on the structural behavior of unbound aggregates. The resilient properties of these materials are reviewed in detail in the first paper.

Long-wavelength quantum well lasers have a minimum threshold current approximately between 0.6 and 0.7 micrometer of its stripe width and a quantum well (QW) thickness between 70 and 100 angstroms. An increase in compressive strain from 0 to 1.8% causes intervalence band absorption and an elevated peak differential model gain in internal transparency current density. BH lasers with low-thresholds can be obtained by the use of strained QW active regions.

Layer composition of separate-confinement heterostructure (SCH), when optimized, yielded zero-bias operations of high speed of compressive-strained multiple quantum well (MQW) lasers. A 60 milliampere drive current in zero-bias at 70 degrees centigrade and a lasing delay time of 120 picoseconds was obtained at 1.2 micrometer optimization of SCH layer composition. SCH composition is determined by the carrier life time of the laser. Interconnection systems and optical networks can effectively use the MQW laser.

Quaternary quantum wells that were compressively strained and lattice matched InGaAs-InP were analyzed for the influence of Auger recombination, and revealed that the response to temperature of both devices depend on variations in differential gain in noise measurement in relation to temperature. Strained devices show a decline in Auger recombination. The T0 value of both devices is the same.

Changes in the critical current of multifilamentary Nb3A1 superconducting wires due to transverse compressive stress and axial tensile stress were examined. Measurements of the two stress effects on Nb3Al were compared to to the commercially available Nb3Sn wires by using the estimates of the first elastic-modulus of Nb3Al exposed to cryogenic temperature. Relative insensitivity to stress was exhibited by the Nb3Al conductors.