Journal of Engineering Mechanics

The effects of boundary conditions on the critical load level and the corresponding deflection mode shape of sandwich panels with a “soft” core due to in-plane loads are presented. The study is conducted using a closed-form high-order linearized buckling analysis that includes the influence of the transverse flexibility of the core as well as of the localized effects on the overall sandwich panel behavior, and allows the use of different boundary conditions for the upper and lower skin at the same section. The panel construction is general and consists of two skins (not necessarily identical), metallic or composite-laminated symmetric, and a soft core made of foam or a low-strength honeycomb. The closed-form high-order analysis yields the general buckling behavior of the structure, which means that the solutions obtained allow for interaction between the skins and the core. The solutions are general and are not based on separation of the buckling response on several types of uncoupled buckling modes, such as overall buckling, skins wrinkling, etc., as commonly used in the literature. The numerical scheme consists of finite differences to approximate the governing equations of the closed-form high-order formulation and to transform the set of linearized governing differential equations into an eigenvalue problem that is solved using the deflated iterative Arnoldi procedure. The influence of a general type of boundary conditions, including different conditions throughout the height of the same section and nonidentical conditions at the upper and lower skin, as well as of the core properties, on the buckling behavior of the sandwich panels is considered. The discrepancy between the Timoshenko-Reissner model and the present formulation is discussed. In particular, a partial fixity phenomenon due to the existence of the pinned boundary conditions, i.e., simply supported conditions, at the upper and lower skins at the edge is demonstrated. It is shown that the core properties affect the buckling loads and the corresponding modes of the panel in such a way that the structures with identical boundary conditions but with different cores may undergo different types of buckling such as overall and local as well as interactive loss of stability. The effect of an edge concentrated moment, induced by a couple and exerted on the skins only is also studied.

A wrinkling model for sandwich panels in compression is developed with the assumption of a continuous isotropic linear elastic core. Wrinkling stresses for the three modes, defined in a well-known book by H. G. Allen, are expressed by a unified, single expression. The expression depends linearly on a case parameter η, defined to specify the three cases of wrinkling: η = 0 for single-sided face wrinkling (case 1), η = 1 for in-phase wrinkling (case 2), and η = −1 for out-of-phase wrinkling (case 3). It is shown that the stresses in all three cases are almost identical for short wavelength wrinkling and can be expressed by a single simplified analytical expression; however, they may differ significantly in moderate and long wavelength wrinkling, and for these cases they are given by simplified analytical expressions. It is proved that the in-phase wrinkling stress is the lowest among the three cases. Based on the analysis conducted, limitations of the commonly used Winkler and two-parameter models are discussed. Finally, engineering design procedures are recommended for the wrinkling effect in sandwich panels under compression.

The prediction of crack growth is studied by the manifold method. The manifold method is a new numerical method proposed by Shi. This method provides a unified framework for solving problems dealing with both continuums and jointed materials. It can be considered as a generalized finite-element method and discontinuous deformation analysis. One of the most innovative features of the method is that it employs both a physical mesh and a mathematical mesh to formulate the physical problem. The physical mesh is dictated by the physical boundary of a problem, while the mathematical mesh is dictated by the computational consideration. These two meshes are interrelated through the application of weighting functions. In this study, a local mesh refinement and auto-remeshing schemes are proposed to extend the manifold method. The proposed model is first verified by comparing the numerical results with the benchmark solutions, and the results show satisfactory accuracy. The crack growth problems and the stress distributions are then investigated. The manifold method is proposed as an attractively new numerical technique for fracture mechanics analysis.

The transient elastodynamic full-field response and the dynamic stress intensity factor of a semi-infinite interface crack lying between dissimilar anisotropic media subjected to a dynamic body force are investigated. At t = 0, a concentrated antiplane dynamic point loading is suddenly applied at Medium 1. The total wave field is due to the effect of this point loading and the scattering of the incident wave by the interface crack. We introduce a linear coordinate transformation that can transform the anisotropic interface crack problem to the isotropic case. The relationship between the problem of anisotropic material and the corresponding isotropic problem is established for shear stresses and displacement in a Cartesian coordinate system. Exact transient closed-form solutions for stresses and stress intensity factors are obtained. Numerical results for the time history of stresses and stress intensity factors during the transient process are discussed in detail.

Fiber-reinforced plastic tubes can be used as permanent formwork and external reinforcement for concrete columns. Failure of such columns is often accompanied by audible noise from cracking of concrete and shifting and settling of aggregates, snapping of the inner layers of the jacket, and ultimately fiber fracture. Since the state of the concrete core is not readily visible from the outside, nondestructive evaluation tools such as acoustic emission (AE) can help assess its structural integrity. The objective of this research was to investigate the applicability of AE technique to hybrid columns, and to correlate the AE parameters to the state of stress in concrete. The AE signature of hybrid columns was studied on over 40 specimens with different lengths, cross sections, jacket types, and jacket thicknesses. The study showed that (1) rate of change of cumulative AE counts with respect to the applied load correlates well with the degree of damage sustained by the concrete core; (2) longer specimens, thicker jackets, and unbonded tubes display higher AE activities; and (3) the Kaiser effect is not present in hybrid columns.

Characterization of the tensile behavior of unreinforced and fiber-reinforced cement and concrete is delicate because of strain localization, which introduces a strong heterogeneity of the strain throughout the specimen and complicates the interpretation of test data. An experimental technique to distribute damage is analyzed in the framework of continuum damage mechanics by introducing internal variables to model the degradation mechanisms taking place in this configuration. A mechanism-based formulation is used to derive the state potential and the kinetic laws. Comparisons are performed for cement matrices with different volume fractions and orientations of short fibers. The paper shows the shortcomings of this experimental technique, though it is still possible to use it to compare the responses of plain and fiber-reinforced concrete.

This paper presents a method for the failure analysis of structural glass components of buildings. Structural glass is generally prestressed by tempering. In the method, residual stresses are first computed by simulation of this tempering process of soda-lime-silica glass plates using the finite-element method. The model includes both stress and structural relaxations. The edge effects of tempering are modeled. Then, the failure strength of annealed glass is obtained by a statistical analysis of tests performed on small specimens. Loading rate effects are also taken into account. Computational results are associated with the statistical approach for the failure prediction of large annealed and tempered glass plates. This prediction method is validated by experimental results from four-point bending tests up to failure.

A micromechanical model is proposed for determining the overall linear elastic mechanical properties of simple-texture brick masonry. The model, originally developed for long-fiber composites, relies on the exact solution due to Eshelby and describes brickwork as a mortar matrix with insertions of elliptical cylinder-shaped bricks. Macroscopic elastic constants are derived from the mechanical properties of the constituent materials and phase volume ratios. Conformity of the suggested model to real brickwork behavior has been verified by performing uniaxial compression tests on masonry panels composed of fired bricks and mud mortar. Composite masonry panels of varying phase percentages were then constructed and tested by replacing several of the fired bricks with mud bricks. Comparison of experimental results with theoretical predictions demonstrates that the model is suitable even in the presence of strongly differentiated phases, and is moreover able to predict different behavior as a function of phase concentration. The model fits experimental results more closely than the micromechanical models previously reported in the literature.

The evaluation of strength reduction of RC structures subjected to mechanical damage process and chemical attack is carried out, with regard to concrete deterioration and steel corrosion. A coupled environmental-mechanical damage model, developed as an extension of that previously published is presented. Two independent scalar mechanical damage parameters are introduced, each of them representing the degradation mechanisms occurring under tensile and compressive stress conditions. The stiffness recovery upon loading reversal, which is manifest when passing from tension into compression, is fully captured by the proposed model. The environmental damage is strongly related to the diffusion process, as well as to the evolution of the chemical reaction between pollutant and cementitious constituents. An enhanced local method is proposed to regularize the problem of nonobjectivity of the finite-element solution due to the strong strain softening behavior of concrete material. The splitting test of a concrete specimen and a static analysis of an RC frame subjected to mechanical loads and chemical attacks are carried out, and the damage evolution is analyzed in detail.

The paper examines different failure modes when concrete materials are subjected to triaxial load scenarios in uniaxial and confined compression, in direct tension, as well as in shear. The different failure modes are studied with the aid of a novel concrete model that describes the hardening/softening behavior of concrete in tension, low-confined compression, that captures the transition from brittle to ductile failure with increasing confinement and that expands the triangular deviatoric section in tension to the circular one in high confined compression. The nonassociated plasticity formulation is based on a smooth three-invariant loading function that is C¹-continuous in stress-space except for the vertex at the point of equitriaxial tension. The paper concludes with an in-depth discussion of localization properties of concrete materials under tension, compression, and shear.

A model has been developed for the prediction of wrinkling during the compression molding process for composites with thick cross sections. The onset of wrinkling is modeled as a localized bifurcation model, which produces additional solutions in the form of wrinkles in a single layer or group of neighboring layers. Average mechanical, thermal, and cure properties of the composite material needed for simulation of compression molding processes are developed.

The effects of nonlinear viscous damping on the dynamic response of single-degree-of-freedom (SDOF) structural systems are analyzed. This kind of damping characterizes a special class of fluid viscous dampers recently utilized in the field of vibration control as base-isolation devices or viscoelastic elements included in steel braces of framed structures. The analytical relationship adopted to reproduce the mechanical behavior of the fluid viscous dampers is a fractional power-law of the velocity, the exponent of which ranges between 0.1 and 0.2. This function had been previously calibrated on the results of a special experimental survey carried out at the University of Florence. The dynamics of the classical linear-viscous SDOF oscillator is herein reformulated on the basis of the above-mentioned fractional viscous damping (FrVD) relationship. In particular, the transient and steady-state responses are examined in both free and forced vibration conditions. The magnification and transmissibility factors are analytically determined for different damping levels. Moreover, the relation between the viscous damping coefficient and the frequency ratio (i.e., the ratio of the dynamic load to the oscillator frequencies) is defined. The diagrams describing these functions provide direct correlations between the damping as well as the elastic properties of the system and the frequency content of the dynamic action.

The buckling problem of angle-ply laminated long hollow cylinders under axial compression is investigated on the basis of 3D elasticity considerations. The study is based on a recursive and successive approximation method and the introduction of a periodic complex displacement field. By this approach, the composite cylinders may be composed of a number of anisotropic layers, each of which may have different material properties, thicknesses, and laminate profile. The eigenvalue problem, from which the buckling load is found, always has a dimension of 3 × 3, regardless of the number of the layers. The buckling of either symmetric or antisymmetric angle-ply laminated cylinders under axial compression is investigated through a parametric study. The results are presented in graphic form for cylinders having various thicknesses and layups.

Subsurface flow and salinity patterns in a tidal marsh plain determine the character and health of the wetland ecosystem. The presence of a flow field with both saturated and unsaturated regions requires that the governing equations and the associated equations of state transition smoothly between the two regions. The fluid density in the marsh plain is not constant; it responds to changes in both pressure head and salinity. These considerations lead to the development of the marsh plain equations, the coupled field equations for subsurface flow and salinity transport in the continuous saturated/unsaturated flow domain of a tidal wetland.

In this paper a finite-element analysis on ductile fracture in two-dimensional quasi-static state is performed by using the local approach concept in continuum damage mechanics. An isotropic damage model based on the generalized concept of effective stress is proposed. Crack propagation is achieved by removing critically damaged elements. The finite-element approximation of a largely deforming body based on the incremental total Lagrangian concept is carried out. As numerical examples, the mesh size sensitivity analysis and the simulation of the shearing mode failure in plane strain state are carried out to verify the present formulation qualitatively. For an edge cracked plate under plane stress state, load-displacement curves and successively fractured shapes are shown. It can be concluded that the proposed model may be stated as a reasonable tool to explain ductile fracture initiation and crack propagation.

A damage model for the concrete-steel interface is proposed in this paper that can be used to represent the constitutive law of the lumped interface model. Both the normal and tangential damage factors are defined as well as their evolution equations are proposed. The coupling between these two damage factors is studied. From the comparison with experimental results, it is seen that the proposed damage model provides a realistic description of the interface behavior and its underlying mechanism, and is suitable for the simulation of the tangential traction-slip relationship of the concrete-steel interface under monotonic loading.