Dynamic Reciprocity Research Articles

Uniqueness and reciprocity theorems in generalized linear micropolar thermoviscoelasticity

A model of the equations of generalized linear micropolar thermoviscoelasticity is given. The formulation is applied to the coupled theory as well as to five generalizations, the Lord–Shulman theory with one relaxation time, the Green–Lindsay theory with two relaxation times, the Green–Naghdi theories of type II (without energy dissipation) and of type III, and the Chandrasekharaiah–Tzou theory with dual-phase-lag. Using Laplace transforms, a uniqueness theorem for this model is proved, restrictions on relaxation functions are deduced and the dynamic reciprocity theorem is derived. The cases of generalized linear micropolar thermoviscoelasticity of Kelvin–Voigt model, generalized linear micropolar thermoelasticity, generalized thermoviscoelasticity and generalized thermoelasticity can be obtained from the given general model.

Acoustic emission from strain-determined sources

Monitoring of structures can be based on observing signals emitted in the course of developing defects. Layered ductile structures are considered which react on overloads by the formation of (small scale) plastic zones. Within the linear elastic background concept such an event is considered by the formation of sources of eigenstress. Since a direct description of the source characteristics is rather cumbersome, alternatively, the time convolution of the imposed plastic strains with the complementary Green’s stress dyadic is proposed to describe the signal of the acoustic emission. Thus, the dynamic generalization of Maysel’s formula of thermo-elasticity, a special form of the dynamic reciprocity theorem, enters the field of computational “nondestructive evaluation”. In that context, the novel contribution of this paper to acoustic emission and monitoring of structures is the formulation of the full three-dimensional problems and, for layered structures, the construction of the generalized rays (plane wave expansions) in the background, considering an instantaneous oblique force point source at the transducer’s location. Consequently, all information on the wave guide is contained in the Green’s stress dyadic. The Laplace transformed solution is worked out for localized plastic sources. Deconvolution and transducer dynamics is not discussed here.

THE UNIQUENESS AND RECIPROCITY THEOREMS FOR GENERALIZED THERMOVISCOELASTICITY FOR ANISOTROPIC MEDIA

A new model of the equations of generalized thermoviscoelasticity for anisotropic media is given. The formulation is applied to both generalizations, the Lord-Shulman theory with one relaxation time and the Green-Lindsay theory with two relaxation times, as well as to the coupled theory. Using Laplace-Carson transforms, a uniqueness theorem for this model is proved, the dynamic reciprocity theorem is derived, and some applications are given. The cases of isotropic thermoviscoelasticity (with or without the volume rheological properties), anisotropic thermoelasticity, and isotropic thermoelasticity can be obtained from the given general model for the coupled and generalized theories.

An efficient formulation of Krylov’s prediction model for train induced vibrations based on the dynamic reciprocity theorem

In Krylov's analytical prediction model, the free field vibration response during the passage of a train is written as the superposition of the effect of all sleeper forces, using Lamb's approximate solution for the Green's function of a halfspace. When this formulation is extended with the Green's functions of a layered soil, considerable computational effort is required if these Green's functions are needed in a wide range of source-receiver distances and frequencies. It is demonstrated in this paper how the free field response can alternatively be computed, using the dynamic reciprocity theorem, applied to moving loads. The formulation is based on the response of the soil due to the moving load distribution for a single axle load. The equations are written in the wave-number-frequency domain, accounting for the invariance of the geometry in the direction of the track. The approach allows for a very efficient calculation of the free field vibration response, distinguishing the quasistatic contribution from the effect of the sleeper passage frequency and its higher harmonics. The methodology is validated by means of in situ vibration measurements during the passage of a Thalys high-speed train on the track between Brussels and Paris. It is shown that the model has good predictive capabilities in the near field at low and high frequencies, but underestimates the response in the midfrequency band.

Matricellular proteins: an overview.

Matricellular proteins: an overview.

Hirbet Hizah: Between Remembrance and Forgetting

The questions what, how, and when we remember remain a source of fascination for scholars. They remain among the most elusive and complex to define. Collective memory is situated at the divide between the conscious and the subliminal, between acknowledgment and denial, between history and psychology. Currently in vogue is the conception of the "usable past": collective memory as a construct, a product of national-cultural manipulation that seeks to entrench in memory those portions of the past useful for invigorating the imagery of the society's self-identity and fostering its vital current interests and agendas. That conception repudiates the notion that there are spontaneous, unintentional processes at work as collective memory crystallizes. If we attach conscious intent in shaping memory and its construction, then who are its agents? What are their tools? Moreover, in democratic states--and, arguably, in other states too-- there is never one single source of inspiration and guidance. How does the open arena of conflicting interests affect memory's configuration? When is a particular event stamped, indelibly or temporarily, in memory? What processes catalyze its fixing, what forces act to submerge its recollection? If the "usable past" ministers to interests in the present, then what is the fate of those segments of the past that do not serve the [End Page 1] society's current agendas? Are they doomed to oblivion? Or is there a constant dynamic reciprocity between the past and the present, manifested in memory's transformations, not necessarily due to some template imposed from above--but rather to changing circumstances as they affect public consciousness?

Teaching boundary element methods in acoustics

The boundary element method (BEM) provides a powerful tool for the numerical solution of sound fields, because (1) it maps the domain equation on the boundary and thus reduces the problem dimension by one, (2) it provides a nonreflecting boundary on an arbitrary radiating surface, and (3) pressure, velocity, and intensity can be calculated at every field point from the boundary data. The author’s concept of teaching BEM in a graduate engineering course is to provide a link with subjects taught in undergraduate engineering mechanics such as rod vibrations, reciprocity principle, influence functions, and finite elements (FE). These subjects are generalized to the field equations of acoustics and elastodynamics. The associated integral equations are derived from the dynamic reciprocity principle. Their kernels are fundamental solutions which turn out to be influence functions of a full space. Boundary formulations of rods and beams recall well-known FE matrices. The fluid–structure interaction of interior and exterior acoustics is formulated by adopting the unsymmetric allocation approach of BEM and a symmetric hybrid BEM which is based on a multifield varational principle. Waves and vibrations in a flexible pipe and the sound radiation from a vibrating tire are treated as examples.

Communication between the cell membrane and the nucleus: Role of protein compartmentalization.

Understanding how the information is conveyed from outside to inside the cell is a critical challenge for all biologists involved in signal transduction. The flow of information initiated by cell-cell and cell-extracellular matrix contacts is mediated by the formation of adhesion complexes involving multiple proteins. Inside adhesion complexes, connective membrane skeleton (CMS) proteins are signal transducers that bind to adhesion molecules, organize the cytoskeleton, and initiate biochemical cascades. Adhesion complex-mediated signal transduction ultimately directs the formation of supramolecular structures in the cell nucleus, as illustrated by the establishment of multi complexes of DNA-bound transcription factors, and the redistribution of nuclear structural proteins to form nuclear subdomains. Recently, several CMS proteins have been observed to travel to the cell nucleus, suggesting a distinctive role for these proteins in signal transduction. This review focuses on the nuclear translocation of structural signal transducers of the membrane skeleton and also extends our analysis to possible translocation of resident nuclear proteins to the membrane skeleton. This leads us to envision the communication between spatially distant cellular compartments (i.e., membrane skeleton and cell nucleus) as a bidirectional flow of information (a dynamic reciprocity) based on subtle multilevel structural and biochemical equilibria. At one level, it is mediated by the interaction between structural signal transducers and their binding partners, at another level it may be mediated by the balance and integration of signal transducers in different cellular compartments. J. Cell. Biochem. Suppls. 30/31:250-263, 1998. © 1998 Wiley-Liss, Inc.

Studies of health outcomes in transit operators: policy implications of the current scientific database.

Urban transit operators' medical symptoms and conditions exceed other occupational groups, resulting partly from working conditions. Medical outcomes among operators have an impact on the transit system, including on performance, work attendance, and medical costs. This is exacerbated by external economic and political pressures in which expected service often exceeds the system's capacity in a congested, chaotic urban environment. A vicious cycle of poor working environment, reduced health and well-being among operators, and lowered efficiency and increased costs often results. Operator-health policies focusing solely on individuals are not sufficient to address these problems. A broader approach is needed, acknowledging the relationship between health of the system and health of employees, including redesigning the interface between transit systems and the urban environment and organizational change within companies. This comprehensive approach recognizes the dynamic reciprocity among the transit operators, the transit system, and the larger community.

Calibrated Noncontact Exciters for Optical Modal Analysis

Two types of exciters were investigated experimentally One of the exciters uses a small permanent magnet fastened on the object. The force is introduced by the change in the electromagnetic field from a coil via an air gap. The second exciter is an eddy-current electromagnet one. The amplitude of the forces from these exciters are calibrated by using dynamic reciprocity in conjunction with electronic holography. These forces strongly depend upon the distance between the exciter and the object.

Calibrated Noncontact Exciters for Optical Modal Analysis

Two types of exciters were investigated experimentally One of the exciters uses a small permanent magnet fastened on the object. The force is introduced by the change in the electromagnetic field from a coil via an air gap. The second exciter is an eddy-current electromagnet one. The amplitude of the forces from these exciters are calibrated by using dynamic reciprocity in conjunction with electronic holography. These forces strongly depend upon the distance between the exciter and the object.

Dynamic reciprocity revisited: a continuous, bidirectional flow of information between cells and the extracellular matrix regulates mammary epithelial cell function.

Interactions between cells and the extracellular matrix (ECM) generate two classes of signals, mechanical and biochemical. In the case of the mammary epithelial cell, both are required to initiate ECM-dependent expression of the abundant milk protein beta-casein. Mechanical signals induce a cellular rounding, while functional biochemical signals are associated with an increase in tyrosine phosphorylation. These individual components are part of a complex signalling hierarchy that leads to the emergence of the fully functional lactational phenotype. Interestingly, both the assembly and disassembly of this hierarchy, which occur cyclically in vivo, are constantly modulated by dynamic and reciprocal interactions that take place within a functional unit composed of both the cell and the ECM.

Basics of Cutaneous Wound Repair

Cutaneous wound repair consists of multiple integrated networks of cell-matrix-cytokine interactions. It is generally believed that a better understanding of these networks will lead to improved care of cutaneous wounds, whether freshly made by the surgeon's scalpel or previously existing and not healing secondary to underlying abnormalities. This review is intended to update the readership in some of the salient aspects of wound repair networks. To facilitate the review of multiple integrated networks, cutaneous wound repair was arbitrarily divided into three phases: inflammation, tissue regeneration including re-epithelialization and granulation tissue formation, and tissue reorganization. Throughout the entire process of wound repair it is clear that cells produce or alter various cytokines and extracellular matrix. The cytokines and matrix in turn alter the behavior of the producer cells (autocrine response) or neighbor cells (paracrine response). The dynamic reciprocity among cells, cytokines, and matrix material helps explain how integrated wound healing networks are sequential as well as tightly controlled.

The Dynamic Rybczynski Theorem and Its Dual

In the context of capital accumulation, it is shown that there exists a dynamic version of the Rybczynski Theorem, and of the Stolper-Samuelson Theorem. If, in addition, the investment good is labour intensive, then a dynamic reciprocity relation is also obtained. In the case where the investment good is capital intensive, only a weak form of duality between the two theorems can be established. The paper makes use of the dynamic envelope results of Caputo, and of Lafrance and Barney.

Regulation of Milk Protein and Basement Membrane Gene Expression: The Influence of the Extracellular Matrix

Synthesis and secretion of milk proteins (α-casein, β-casein, γ-casein, and transferrin) by cultured primary mouse mammary epithelial cells is modulated by the extracellular matrix. In cells grown on released or “floating” type I collagen gels, mRNA for β-casein and transferrin is increased as much as 30-fold over cells grown on plastic. Induction of β-casein expression depends strongly on the presence of lactogenic hormones, especially prolactin, in the culture. When cells are plated onto partially purified reconstituted basement membrane, dramatic changes in morphology and milk protein gene expression are observed. Cells cultured on the matrix for 6 to 8 d in the presence of prolactin, insulin, and hydrocortisone form hollow spheres and duct-like structures that are completely surrounded by matrix. The cells lining these spheres appear actively secretory and are oriented with their apices facing the lumen. Hybridization experiments indicate that mRNA for β-casein can be increased as much as 70-fold in these cultures. Because >90% of the cultured cells synthesize immunoreactive β-casein, as compared with only 40% of cells in the late pregnant gland, the matrix appears to be able to induce protein expression in previously silent cells. Synthesis of laminin and assembly of a mammary-specific basal lamina by cells cultured on different extracellular matrices also appears to depend on the presence of lactogenic hormones. These studies provide support for the concept of “dynamic reciprocity” in which complex interactions between extracellular matrix and the cellular cytoskeleton contribute to the induction and maintenance of tissue-specific gene expression in the mammary gland.

How does the extracellular matrix direct gene expression?

Based on the existing literature, a model is presented that postulates a “dynamic reciprocity” between the extracellular matrix (ECM) on the one hand and the cytoskeleton and the nuclear matrix on the other hand. The ECM is postulated to exert physical and chemical influences on the geometry and the biochemistry of the cell via transmembrane receptors so as to alter the pattern of gene expression by changing the association of the cytoskeleton with the mRNA and the interaction of the chromatin with the nuclear matrix. This, in turn, would affect the ECM, which would affect the cell, which…

Collagens of basement membranes.

Recent biochemical and immunohistochemical studies have described several components of basement membranes including heparan sulfate proteoglycan, 2 high molecular weight glycoproteins (fibronectin and laminin), and 2 collagen types (IV and V). These collagens have several properties which distinguish them from other types that are located in the interstitium: (a) type IV forms an amorphous, felt-like matrix, and neither IV nor V is found in large, cross-banded fibrils, (b) both have an increased content of hydrophobic amino acids, (c) the precursor (pro) forms are larger than those of interstitial collagens, (d) type IV contains interruptions within the triple helix, and e) both IV and V are resistant to human skin collagenase but are substrates for selected neutral proteases derived from mast cells, macrophages, and granulocytes. By immunofluorescence staining, type IV collagen has been localized to basement membranes at the dermal-epidermal junction, in capillaries, and beneath endothelial cells in larger vessels. Ultrastructurally it has been shown to be a specific component of the lamina densa. Type V collagen has been localized to the pericellular matrices of several cells types and may be specific for extramembranous structures which are closely associated with basal laminae. Other collagenous proteins have been described which may be associated with the extracellular matrix. One of these is secreted by endothelial cells in culture and by peptide mapping represents a novel collagen type. It is secreted under ascorbate-free conditions and is highly sensitive to proteolytic degradation. It has been proposed that a dynamic reciprocity exists between cells and their extracellular matrix which partially determines cell shape, biosynthesis, migration, and attachment. Examples of phenotypic modulation in several of these phenomena have been shown with endothelial cells grown on different substrates and isolated from different vascular environments.

Collagens of Basement Membranes

Recent biochemical and immunohistochemical studies have described several components of basement membranes including heparan sulfate proteoglycan, 2 high molecular weight glycoproteins (fibronectin and laminin), and 2 collagen types (IV and V). These collagens have several properties which distinguish them from other types that are located in the interstitium: (a) type IV forms an amorphous, felt-like matrix, and neither IV nor V is found in large, cross-banded fibrils, (b) both have an increased content of hydrophobic amino acids, (c) the precursor (pro) forms are larger than those of interstitial collagens, (d) type IV contains interruptions within the triple helix, and e) both IV and V are resistant to human skin collagenase but are substrates for selected neutral proteases derived from mast cells, macrophages, and granulocytes. By immunofluorescence staining, type IV collagen has been localized to basement membranes at the dermal-epidermal junction, in capillaries, and beneath endothelial cells in larger vessels. Ultrastructurally it has been shown to be a specific component of the lamina densa. Type V collagen has been localized to the pericellular matrices of several cells types and may be specific for extramembranous structures which are closely associated with basal laminae. Other collagenous proteins have been described which may be associated with the extracellular matrix. One of these is secreted by endothelial cells in culture and by peptide mapping represents a novel collagen type. It is secreted under ascorbate-free conditions and is highly sensitive to proteolytic degradation. It has been proposed that a dynamic reciprocity exists between cells and their extracellular matrix which partially determines cell shape, biosynthesis, migration, and attachment. Examples of phenotypic modulation in several of these phenomena have been shown with endothelial cells grown on different substrates and isolated from different vascular environments.

A Reciprocal Theorem in the Linear Theory of Directed Continuous Media

AbstractA linear theory of directed viscoelastic media is developed using the Stieltjes convolution. A dynamic reciprocity relationship is proved.

On recent advances in wireless propagation both in theory and in practice

Cutaneous wound healing ultimately functions to facilitate barrier restoration following injury-induced loss of skin integrity. It is an evolutionarily conserved, multi-cellular, multi-molecular process involving co-ordinated inter-play between complex signalling networks. Cellular proliferation is recognised as the third stage of this sequence. Within this phase, fibroplasia and angiogenesis are co-dependent processes which must be successfully completed in order to form an evolving extracellular matrix and granulation tissue. The resultant structures guide cellular infiltration, differentiation and secretory profile within the wound environment and consequently have major influence on the success or failure of wound healing. This review integrates in vitro, animal and human in vivo studies, to provide up to date descriptions of molecular and cellular interactions involved in fibroplasia and angiogenesis. Significant molecular networks include adhesion molecules, proteinases, cytokines and chemokines as well as a plethora of growth factors. These signals are produced by, and affect behaviour of, cells including fibroblasts, fibrocytes, keratinocytes, endothelial cells and inflammatory cells resulting in significant cellular phenotypic and functional plasticity, as well as controlling composition and remodelling of structural proteins including collagen and fibronectin. The interdependent relationship between angiogenesis and fibroplasia relies on dynamic reciprocity between cellular components, matrix proteins and bioactive molecules. Unbalanced regulation of any one component can have significant consequences resulting in delayed healing, chronic wounds or abnormal scar formation. Greater understanding of angiogenic and fibroplastic mechanisms underlying chronic wound pathogenesis has identified novel therapeutic targets and enabled development of improved treatment strategies including topical growth factors and skin substitutes.