Maximum likelihood estimation of Gaussian models with missing data—Eight equivalent formulations

In this paper we derive the maximum likelihood problem for missing data from a Gaussian model. We present in total eight different equivalent formulations of the resulting optimization problem, four out of which are nonlinear least squares formulations. Among these formulations are also formulations based on the expectation–maximization algorithm. Expressions for the derivatives needed in order to solve the optimization problems are presented. We also present numerical comparisons for two of the formulations for an ARMAX model.     

Computer-assisted simulation of high-voltage electron microscopy using serial images recorded by conventional transmission electron microscopy

Here we describe a computer-assisted method which, based on conventional transmission electron microscopy, renders simulated high-voltage electron micrographs. We perform arithmetic minimum filtering on stacks of aligned serial transmission electron microscopic images. In this way, the structural information of the separate images is fused into one compound image that highlights organization patterns otherwise easily overlooked or impossible to comprehend. Our method, like high-voltage electron microscopy, offers the possibility to build stereo-pairs for high-resolution three-dimensional analysis of tissue layers 1-2 microm thick. The use of background elimination and the development of a depth enhancement routine improved the three-dimensional effect and facilitated the analysis of the interior of objects. As an example, we use the method to display the distribution of axonal organelles at nodes of Ranvier and the shape and contents of a highly branched hippocampal dendritic spine.     

Cohesive crack models for semi-brittle materials derived from localization of damage coupled to plasticity

Based on discontinuous displacement approximation of the continuum and shear band kinematics, two cohesive crack models are derived within the constitutive framework of coupled damage and plasticity. The models employ the Rankine fracture criterion, and the model parameters are determined from a uniaxial tension test (mode I cracking). Bifurcation analysis is used in order to diagnose critical directions along which the crack will gradually develop and propagate. These directions depend on the actual stress state and are kept fixed after fracture has initiated, whereby a fixed crack model is obtained. A discrete crack strategy is employed at the finite element implementation in the sense that interfaces (that represent the cohesive crack) are introduced along inter-element boundaries. This implementation strategy calls for gradual realignment of the mesh as a key feature of the algorithm. Numerical results from the analysis of mixed mode fracture in a notched concrete plate are presented.     

UV aging of poly(propylene)/wood-fiber composites

The effect of accelerated UV-aging on properties of PP/wood-fiber composites has been studied. Plates containing 0, 25 and 50 wt% wood-fibers were subjected to accelerated UV-aging in a QUV weatherometer for up to 8 weeks. Stabilization against UV-degradation was made by adding 3% of a UV stabilizer. The results showed that both the unfilled PP and the PP/wood-fiber composites displayed good UV resistance with regard to mechanical properties. The color of the PP/wood-fiber composite plates changed from brown to chalky white at the exposed area because of the formation of a thin, strongly degraded surface layer. Physical and chemical analysis of these surface layers using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy showed that degradation of the PP matrix had occurred, leading to chemicrystallization and extensive surface cracking. A decrease in PP melting temperature was also noted, due to molecular chain scission and the formation of extraneous groups, such as carbonyls and hydroperoxides. These results indicate that the wood-fibers act as chromophores when incorporated in the PP matrix. Increasing the fiber content from 25 to 50 wt% increases the rate of degradation of the PP matrix by approximately a factor of two. Polym. Compos. 25:543–553, 2004. © 2004 Society of Plastics Engineers.     

Effect of marine oils supplementation on coagulation and cellular activation in whole blood

A study was performed to explore the effects of supplemental intake of various marine oils known to be part of the Eskimo diet. Healthy men and women (134) were randomly selected to consume 15 mL/d of oil from blubber of seal, cod liver, seal/cod liver, blubber of Minke whale, or no oil for ten weeks. Total cholesterol was unchanged in the oil groups, whereas high density lipoprotein cholesterol increased 7% in the seal/cod liver oil (CLO) group (P<0.05) and 11% in the whale oil group (P<0.005). Triacylglycerol was significantly reduced in the CLO group only. The concentration of prothrombin fragment 1+2 was reduced 25% (P<0.05) after whale oil supplementation. No change in fibrinogen or factor VIIc was detected. Tumor necrosis factor generation in lipopolysaccharide (LPS)-stimulated blood was 30% reduced after whale oil (P<0.05), but was unaffected by intake of seal or CLO. The LPS-induced tissue factor activity in monocytes was reduced to a significant degree only in the seal/CLO group (34%) and whale oil group (35%) (P<0.05). The most dramatic change in thromboxane B₂ in LPS-stimulated blood was seen after whale oil intake with 44% reduction (P<0.01). Supplementation of a regular diet with a combination of seal oil and CLO and especially with whale oil seems to have beneficial effects on several products thought to be associated with cardiovascular and thrombotic diseases.     

PVC powder extrusion. Melting properties and particle morphology

The extrusion of unplasticized PVC powder has been studied. Powder blends with low amounts of lubricants have been used in order to observe differences between structurally different PVC powders. Experiments with varying screw speeds indicate that resins consisting of agglomerated, small, globular primary particles are more sensitive to shear and deformation in the extruder channel than more compact particles. Frictional heat seems to be dissipated inside loosely agglomerated resin particles, possibly because of a larger internal surface. This additional heat, which depends on the shear rate, influences the melting behavior considerably. Scanning electron microscopy shows that PVC particles seem to melt without first breaking into smaller particles. A comparison between bulk-polymerized and suspension-polymerized PVC with nearly the same particle structure reveals a close similrity in melting properties. The results obtained in this work indicate that structural differences may explain the differences in melting behavior often observed when rigid PVC blends are extruded.     

Risk Influence Analysis A methodology for identification and assessment of risk reduction strategies

This article describes Risk Influence Analysis, a methodology for the identification and assessment of risk reduction strategies in large-scale distributed systems such as transportation systems. The approach focuses on the identification of risk-influencing factors, i.e. sets of relatively stable conditions influencing the risk level. Risk reduction strategies are defined by sets of actions designed to change the state of risk-influencing factors. The expected effects of risk reduction strategies are evaluated by expert judgement supported by available data for the analysis object.Quality issues concerning the relevance and trustworthiness of the Risk Influence Analysis methodology are identified. The most important threats to the trustworthiness of the analysis are believed to be: (1) inadequate validity of expert judgements in estimating the effects of risk reduction strategies; and (2) incompleteness of the qualitative analysis results.     

Matrix theory of the motion of a charged particle beam in curvilinear space-time Part I. General theory

A general relativistic theory of charged particle beam motion along a curved optical axis, including the gravitational field, is important for designers of optimal beam control systems. In this paper, which is the first of two, a new matrix approach is presented. This allows the development of a relativistic matrix theory for charged particle beam motion in the most general case of a curved reference trajectory, including the gravitational force. This approach is based on three basic matrices: the reference frame matrix, the curvature matrix and the electromagnetic matrix. The equations of the particle beam motion and of the electromagnetic field, including the space charge, are given. The notations used is independent of the units of the measured fields and energies. In a second paper, also published in this issue, the matrix and recursive methods for solving the nonlinear equations of motion will be presented.