Role Of Prestress Research Articles

The role of prestress and its optimization in cable domes design

For their lightweight, versatile forms and architectural impact, cable-strut structural systems have been widely used as large span roofs of arenas, stadiums and open squares. In this paper, the matrix theory for pin-jointed trusses is firstly recalled. Then, through an elementary cable dome, the role of prestress is outlined and commented. Finally, a special optimization procedure, based on genetic algorithms, allows a thorough comparison between classical structural schemes, whose bearing capacity is due to the mechanical stiffness, and prestressed structures, whose bearing capacity is also due to the geometrical stiffness provided by the prestress.

Computational Insights Into Cytoskeletal Rheology

Cytoskeletal rheology is critical in the interaction of cells with their environment. It influences cell migration, mechanotransduction, and the ability of the cell to transmit externally applied forces. Models for cell mechanics have progressed considerably in recent years, and are now capable of simulating the effects of thermal fluctuations, cross-linking, and stresses generated by motor activity, all thought to be important determinants of cytoskeletal viscoelastic behavior. At the same time, model systems such as reconstituted actin gels have been used to gain insight into the factors that give rise to the unique cytoskeletal behavior. Of interest in this regard are: (i) the unique power-law rheology observed in the linear stress regime, (ii) nonlinear strain stiffening, (iii) fluidization that occurs above some critical level of stress, beyond which the modulus falls precipitously, and (iv) the role of prestress on cell stiffness. Here we present results from recent models that address each of these phenomena. We show through modeling that (i) thermal fluctuations of actin filaments are important only at low levels of prestress and small values of the ratio of persistence length to distance between cross-links, (ii) power-law rheology appears to emerge as a consequence of motor-induced prestress and cross-links, (iii) in the strain-stiffening regime, prestress is the primary determinant of network modulus, (iv) cross-link rupture rather than unfolding accounts for network remodeling and stress relaxation behavior, (v) motor activity generates prestress, but also has a strong influence on network morphology and the formation of stress fibers. [Support from the Sumsung Scholarship Program (to TYK) and the NIH (GM076689) is gratefully acknowledged.]

Variational formulation of pre-stressed solid–fluid mixture theory, with an application to wave phenomena

Fluid saturated porous media are modelled by the theory of mixtures and the placement maps of the solid and of the fluid are considered. The momentum balance equations are derived in the framework of a variational approach: We take an action functional and two families of variations and assume that the sum of the virtual work of the external forces and the variation of such an action along each variation are zero. Constitutive equations for the two Cauchy stress tensors and for the interaction force are derived taking into account a general state of pre-stress for the solid and for the fluid species. Governing equations are therefore formulated, however, for the sake of simplicity, only the case of pure initial pressure is further investigated. The propagation of bulk (transversal and longitudinal) waves and the influence of pre-stress are studied: In particular, stability analyses are carried out starting from dispersion relations and the role of pre-stress is investigated. Finally, a numerical example is established for a given state of pre-stress, deriving the phase velocities and the attenuation coefficients of transversal and longitudinal waves.

A novel boundary element approach to time-harmonic dynamics of incremental nonlinear elasticity: The role of pre-stress on structural vibrations and dynamic shear banding

A new boundary element technique is developed to analyze two-dimensional, time-harmonic, small-amplitude vibrations, superimposed upon a homogeneously pre-stressed, orthotropic and incompressible elastic solid. New expressions for the Green’s functions for incremental applied tractions are obtained, in which ‘static’ and ‘dynamic’ contributions are uncoupled. The dynamic contributions are regular, whereas the static terms are strongly singular and are solved in closed-form expressions, particularly useful for numerical calculations. As a consequence of the static/dynamic de-coupling, these expressions turn out to be useful also for quasi-static deformation. The formulation is tested for different boundary value problems. These include a problem with certain boundary conditions investigated by Ryzhak in the static case, for which an analytic solution is proposed here in the time-harmonic regime. The effect of pre-stress is shown to strongly influence the vibrational response of elastic structures. It is shown that natural frequencies strongly decrease when pre-stress approaches quasi-static bifurcation loads and wave propagation speeds tend to vanish when the boundary of ellipticity is approached. Near this boundary (but still within the elliptic region) we observe a focussing of vibrations along plane waves parallel to the shear bands.

Central role of prestress in stress propagation in airway smooth muscle cells

Central role of prestress in stress propagation in airway smooth muscle cells

The Role of Prestress and Architecture of the Cytoskeleton and Deformability of Cytoskeletal Filaments in Mechanics of Adherent Cells: a Quantitative Analysis

Mechanical properties of adherent cells were investigated using methods of engineering mechanics. The cytoskeleton (CSK) was modeled as a filamentous network and key mechanisms and corresponding molecular structures which determine cell elastic behavior were identified. Three models of the CSK were considered: open-cell foam networks, prestressed cable nets, and a tensegrity model as a special case of the latter. For each model, the modulus of elasticity (i.e. an index of resistance to small deformation) was given as a function of mechanical and geometrical properties of CSK filaments whose values were determined from the data in the literature. Quantitative predictions for the elastic modulus were compared with data obtained previously from mechanical tests on adherent cells. The open-cell foam model yielded the elastic modulus (103–104Pa) which was consistent with measurements which apply a large compressive stress to the cell. This suggests that bending of CSK filaments is the key mechanism for resisting large compression. The prestressed cable net and tensegrity model yielded much lower elastic moduli (101–102Pa) which were consistent with values determined from equilibrium measurements at low applied stress. This suggests that CSK prestress and architecture are the primary determinants of the cell elastic response. The tensegrity model revealed the possibility that buckling of microtubules of the CSK also contributed to cell elasticity.

A first estimation of prestress in so-called circularly fibered osteonic lamellae

The existence and role of prestress in the various hierarchical structures of long bone are long standing questions. In this study, the prestress and associated strain that exist in a component of human bone microstructure, circularly fibered osteonic lamella, are estimated. Such estimates allow the formulation of hypotheses on prestress formation and lamellar stiffness. Dimensional measurements were obtained for eight fully calcified lamellae. These dimensions, before isolation from the surrounding alternate osteon and after strain relief by isolation and axial sectioning, furnish data upon which a geometric lamellar model is constructed. A material model is based on the most likely hypothesis as to lamellar structure. This geometric-material model allows estimation of the preexisting strain. The largest strains occur in shear circumferential–axial and normal axial strain directions, averaging 0.08 and 0.05, respectively. The geometric-material model expresses prestress in terms of as yet unknown elastic moduli. The average prestress magnitude is the largest in shear circumferential–axial direction, compensating for alternate osteon weakness in this direction. The estimated axial prestress confirms long hypothesized alternate osteon precompression, which impedes fractures of areas of collagen bundles transverse to the osteon axis at low stresses. The results of the model support the formulation of the following biological hypotheses: (a) lamellar prestress occurs at a supra-molecular level, namely through collagen bundles which are themselves likely to be prestressed; (b) collagen bundles oblique to the lamellar axis are responsible for shear prestress; (c) prestress ranges up to 0.11 GPa; and (d) the lamella is less stiff than alternate osteon.