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A method to evaluate the probability of failure of a failure path leading to fatigue collapse of an offshore structure is presented. The failure path is modeled by a number of failure elements, which represent the fatigue-sensitive joints in the structure, connecting in parallel and failing in sequence. The probability of failure of the parallel system is calculated by using first-order second-moment method. The formulation is further improved by considering the probability of occurrence of a specified sequence in which the failure elements fail. The method can be employed both in searching for important failure paths and in evaluating the probability of failure of the whole structure in systems fatigue reliability analysis of offshore structures.

Based on the KGD scheme, this paper investigates, with both analytical and numerical approaches, the propagation of a hydraulic fracture with a fluid lag in permeable rock. On the analytical aspect, the general form of normalized governing equations is first formulated to take into account both fluid lag and leak-off during the process of hydraulic fracturing. Then a new self-similar solution corresponding to the limiting case of zero dimensionless confining stress (T=0) and infinite dimensionless leak-off coefficient (L=∞) is obtained. A dimensionless parameter R is proposed to indicate the propagation regimes of hydraulic fracture in more general cases, where R is defined as the ratio of the two time-scales related to the dimensionless confining stress T and the dimensionless leak-off coefficient L. In addition, a robust finite element-based KGD model has been developed to simulate the transient process from L=0 to L=∞ under T=0, and the numerical solutions converge and agree well with the self-similar solution at T=0 and L=∞. More general processes from T=0 and L=0 to T=∞ and L=∞ for three different values of R are also simulated, which proves the effectiveness of the proposed dimensionless parameter R for indicating fracture regimes. [ABSTRACT FROM AUTHOR]

Quantifying interactions between motors and filaments is important for the understanding of intriguing emergent behaviors of motor-filament systems, which play critical roles in various biological processes. Recently, unusually high detachment rates of a myosin from actin were obtained with a force spectroscopy technique of an unprecedented spatial-temporal resolution. Here, we suggest that these high apparent detachment rates may be due to the inherent coupling between bond breaking and state transition, which can be common in protein-protein interactions. Based on a kinetic model for the chemomechanical cycle of single myosin, rates of bond breaking between myosin and actin at different nucleotide states are systematically calculated. These quantitative results indicate that myosins may adopt much higher transition rates than bond breaking rates at different nucleotide states under physiological conditions when applied forces are relatively low. This work also iridicates that accurate biophysical models considering both protein unbinding dynamics and protein state transitions are required in order to properly interpret the experimental data when the ultrafast force-clamp spectroscopy technique is employed to study, for example, the DNA-protein interactions. [ABSTRACT FROM AUTHOR]

When simulating or conducting land mine blast tests on armored vehicles to assess potential occupant injury, the preference is to use the Hybrid III anthropomorphic test device (ATD). In land blast events, neither the effect of body-borne equipment (BBE) on the ATD response nor the dynamic response index (DRI) is well understood. An experimental study was carried out using a drop tower test rig, with a rigid seat mounted on a carriage table undergoing average accelerations of 161 g and 232 g over 3 ms. A key aspect of the work looked at the various lumbar spine assemblies available for a Hybrid III ATD. These can result in different load cell orientations for the ATD which in turn can affect the load measurement in the vertical and horizontal planes. Thirty-two tests were carried out using two BBE mass conditions and three variations of ATDs. The latter were the Hybrid III with the curved (conventional) spine, the Hybrid III with the pedestrian (straight) spine, and the Federal Aviation Administration (FAA) Hybrid III which also has a straight spine. The results showed that the straight lumbar spine assemblies produced similar ATD responses in drop tower tests using a rigid seat. In contrast, the curved lumbar spine assembly generated a lower pelvis acceleration and a higher lumbar load than the straight lumbar spine assemblies. The maximum relative displacement of the lumbar spine occurred after the peak loading event, suggesting that the DRI is not suitable for assessing injury when the impact duration is short and an ATD is seated on a rigid seat on a drop tower. The peak vertical lumbar loads did not change with increasing BBE mass because the equipment mass effects did not become a factor during the peak loading event.

The behavior of air bubble clusters rising in water and the induced flow field are numerically studied using a three-dimensional two-way coupling algorithm based on a vortex-in-cell (VIC) method. In this method, vortex elements are convected in the Lagrangian frame and the liquid velocity field is solved from the Poisson equation of potential on the Eulerian grid. Two-way coupling is implemented by introducing a vorticity source term induced by the gradient of void fraction. Present simulation results are favorably compared with the measured results of bubble plume, which verifies the validity of the proposed VIC method. The rising of a single bubble cluster as well as two tandem bubble clusters are simulated. The mechanism of the aggregation effect in the rising process of bubble cluster is revealed and the transient processes of the generation, rising, strengthening, and separation of a vortex ring structure with bubble clusters are illustrated and analyzed in detail. Due to the aggregation, the average rising velocity increases with void fraction and is larger than the terminal rising velocity of single bubble. For the two tandem bubble cluster cases, the aggregation effect is stronger for smaller initial cluster distance, and both the strength of the induced vortex structure and the average bubble rising velocity are larger. For the 20 mm cluster distance case, the peak velocity of the lower cluster is about 2.7 times that of the terminal velocity of the single bubble and the peak average velocity of two clusters is about 2 times larger. While for the 30 mm cluster distance case, both the peak velocity of the lower cluster and two clusters are about 1.7 times that of the terminal velocity of the single bubble.

Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is attributed to low diffusivity of water through the glass. In addition, the water transport from the glassy matrix could be either diffusion or Lp limited. For diffusion-limited transport, lowering the film thickness at the beginning of drying by half almost lowers the drying time by an order of magnitude. In summary, the optimal design of convective desiccation protocols requires accounting for the size of the cell, their membrane permeability (Lp) and the starting thickness of the solution.

To investigate the structural mechanisms by which elevation of the intraendothelial cAMP levels abolishes or attenuates the transient increase in microvascular permeability by vascular endothelial growth factor (VEGF), we examined cAMP effect on VEGF-induced hyperpermeability to small solute sodium fluorescein (Stokes radius = 0.45 nm) P(sodium fluorescein), intermediate-sized solute alpha-lactalbumin (Stokes radius = 2.01 nm) P(alpha-lactalbumin), and large solute albumin (BSA, Stokes radius = 3.5 nm) P(BSA) on individually perfused microvessels of frog mesenteries. After 20 min pretreatment of 2 mM cAMP analog, 8-bromo-cAMP, the initial increase by 1 nM VEGF was completely abolished in P(sodium fluorescein) (from a peak increase of 2.6+/-0.37 times control with VEGF alone to 0.96+/-0.07 times control with VEGF and cAMP), in P(alpha-lactalbumin) (from a peak increase of 2.7+/-0.33 times control with VEGF alone to 0.76+/-0.07 times control with VEGF and cAMP), and in P(BSA) (from a peak increase of 6.5+/-1.0 times control with VEGF alone to 0.97+/-0.08 times control with VEGF and cAMP). Based on these measured data, the prediction from our mathematical models suggested that the increase in the number of tight junction strands in the cleft between endothelial cells forming the microvessel wall is one of the mechanisms for the abolishment of VEGF-induced hyperpermeability by cAMP.

Endothelial surface glycocalyx plays an important role in the regulation of microvessel permeability by possibly changing its charge and configuration. To investigate the mechanisms by which surface properties of the endothelial cells control the changes in microvessel permeability, we extended the electrodiffusion model developed by Fu et al. [Am. J. Physiol. 284, H1240-1250 (2003)], which is for the interendothelial cleft with a negatively charged surface glycocalyx layer, to include the filtration due to hydrostatic and oncotic pressures across the microvessel wall as well as the electrical potential across the glycocalyx layer On the basis of the hypotheses proposed by Curry [Microcirculation 1(1): 11-26 (1994)], the predictions from this electrodiffusion-filtration model provide a good agreement with experimental data for permeability of negatively charged a-lactalbumin summarized in Curry [Microcirculation 1(1), 11-26 (1994)] under various conditions. In addition, we applied this new model to describe the transport of negatively charged macromolecules, bovine serum albumin (BSA), across venular microvessels in frog mesentery. According to the model, the convective component of the albumin transport is greatly diminished by the presence of a negatively charged glycocalyx under both normal and increased permeability conditions.

To investigate the effect of junction strands on microvessel permeability, we extend the previous analytical model developed by Fu et al. (1994, J. Biomech. Eng., 116, pp. 502-513), for the interendothelial cleft to include multiple junction strands in the cleft and an interface between the surface glycocalyx layer and the cleft entrance. Based on the electron microscopic observations by Adamson et al. (1998, Am. J. Physiol., 274(43), pp. H1885-H1894), that elevation of intracellular cAMP levels would increase number of tight junction strands, this two-junction-strand and two-pore model can successfully account for the experimental data for the decreased permeability to water, small and intermediate-sized solutes by cAMP.