When some critical condition is reached at a material point of a solid body, a localized strain starts developing which makes the strain field discontinuous and highly accelerates local damaging of material. The present paper addresses this kind of strain localization in spatial geometrically exact beams. Here we propose a new beam finite element formulation which accounts for softening of material by applying the embedded strong strain discontinuity technology. The formulation is essentially an extension of the original strain-based formulation, upgraded such to allow for detecting the onset of strain localization and to introduce additional equations for evaluating singular strain peaks and jumps of displacements and rotations at the localized section in further deformation. The consistency condition that the equilibrium and the constitutive stress-resultants are equal is shown to be naturally suited for the implementation into the discontinuous formulation. The condition for the onset of strain localization at a beam crosssection is here related to the loss of uniqueness of the beam cross-sectional constitutive equations. If the condition for a unique inverse is violated, two solutions are possible for cross-sectional strains. In a subsequent deformation, one of the two solutions follows the softening regime of material. The discontinuous increments in strains, displacements and rotations at the softening cross-section are obtained from the equations of the structure supplemented by the consistency conditions of the softening cross-section.
COBISS.SI-ID: 7933537
The paper discusses a complex model for a nonstationary planar thermal analysis of expandable intumescent coatings. Following the existing one-dimensional models, we develop novel and improved equations for the two-dimensional thermal analysis of intumescent coatings. A progressive expansion due to chemical reactions, phase changes, and the time and temperature-dependent thermal properties of the coating are considered. In the heating process, the coating may locally experience virgin, intumesced, or charred phases, and their transition with time. The rate of the density loss due to the pyrolysis reaction is described with the Arrhenius equation. The thickness of the coating is assumed to increase enormously during the pyrolysis. Consequently, the energy and mass equilibrium equations are formulated with respect to both the deformed and undeformed configuration. Since most of material properties of commercial products are not given by manufacturers, an innovative procedure is proposed to determine the time-dependent thermal conductivities and remaining fundamental properties of the coating from the set of measured temperatures. This, together with the two-dimensional formulation of the thermal equations with respect to the undeformed configuration, makes the present model unique and appropriate for the thermal analyses of an arbitrary steel cross section protected with intumescent coatings.
COBISS.SI-ID: 8134497
Rheological behavior of wood under uniaxial compression along and perpendicular to the grain in constant environment was examined. Tests with constant deformation rate until failure and stress relaxation tests with constant deformation applied stepwise were carried out. The experimental results of stress relaxation showed nonlinear material behavior over time that got more prominent under high deformation levels. Considerable amount of stress relaxed during applying the deformation. Wood experienced greater stress relaxation along the grain than perpendicular to it. Three rheological models for orthotropic material were calibrated to the experimentally determined stress-time curves in longitudinal and transverse directions simultaneously. Small deformation levels assuming linear strains were accounted for in the models. Required elastic material parameters were determined from the tests with constant deformation rate. A model including the highest number of viscoelastic material parameters was the most successful in predicting stress relaxation of wood under stepwise deformation. Modeling indicated that wood behavior was very close to linear viscoelastic in relaxation under small deformation. The obtained material parameters made the model suitable for predicting rheological behavior of wood comprehensively, under sustained deformation or load in constant conditions.
COBISS.SI-ID: 8317281
This paper presents an exact model for studying the global buckling of concrete-filled steel tubular (CFST) columns with compliant interfaces between the concrete core and steel tube. This model is then used to evaluate exact critical buckling loads and modes of CFST columns. The results prove that interface compliance can considerably reduce the critical buckling loads of CFST columns. A good agreement between analytical and experimental buckling loads is obtained if at least one among longitudinal and radial interfacial stiffnesses is high. The parametric study reveals that buckling loads of CFST columns are very much affected by the interfacial stiffness and boundary conditions.
COBISS.SI-ID: 1537569475
In this paper, a new velocity-based finite element approach for non-linear dynamics of beam-like structures is introduced. In contrast to standard approaches we here base the formulation on velocities and angular velocities expressed in the most suitable basis regarding standard approximation and interpolation techniques. The additivity of angular velocities in local frame description brings several benefits, such as trivial discretization and update procedure for the primary unknowns and improved stability properties of the time integrator. On the other hand, different initial orientations of elements connected together lead to nodal angular velocities that are expressed in different frames and cannot be directly equalized. The compatibility of angular velocities over the finite element boundaries thus needs to be solved. We avoid introducing constraint equations and additional degrees of freedom and introduce a computationally cheap solution instead.
COBISS.SI-ID: 8151137