Journal of Engineering Mechanics

Modeling of the evolution of distributed damage such as microcracking, void formation, and softening frictional slip necessitates strain-softening constitutive models. The nonlocal continuum concept has emerged as an effective means for regularizing the boundary value problems with strain softening, capturing the size effects and avoiding spurious localization that gives rise to pathological mesh sensitivity in numerical computations. A great variety of nonlocal models have appeared during the last two decades. This paper reviews the progress in the nonlocal models of integral type, and discusses their physical justifications, advantages, and numerical applications.

The reinforcing elements of multilayer elastomeric isolation bearings, which are normally steel plates, are replaced by a fiber reinforcement. In contrast to the steel reinforcement which is assumed to be rigid, the fiber reinforcement is flexible in extension. This paper presents the theoretical approach to analyze the bending stiffness of the fiber-reinforced circular isolators. The elastomer is assumed to be incompressible and pressure dominant. The stiffness formula is derived. The influence of fiber flexibility on the mechanical properties of fiber-reinforced circular isolators under bending moment, such as the pressure in the elastomer, the stress in the reinforcement, and the bonding shear stress between the elstomer and the reinforcement, are presented.

Discontinuous deformation analysis (DDA) is a discrete element method that was developed for computing large deformation in fractured rock masses. In this paper, we present an alternative derivation of the basic theory of DDA for planar (two-dimensional) problems, and we address the accuracy of the DDA method for sliding blocks using sensitivity analyses. Results of analyses with different parameters show that the residual and relative errors in the displacement of a frictional sliding block are controlled by the perturbation in the initial time steps of the simulation. In addition, we expose a systematic error induced by the DDA penalty formulation. Overall, the initial perturbation of the solution decreases with decreasing friction angle and increasing contact penalty and, counterintuitively, decreases with increasing time step size. All analyses show that the long runout behavior of the system trends toward the analytic solution, independent of the initial perturbation. The resulting precision of the long runout simulation is more than sufficient for all problems of engineering interest.

The paper addresses the problem of unsteady flow in an elastic pipe, starting from rest and tending towards a turbulent steady state. Careful experiments, involving uncommonly long, smooth-walled conduits, indicate that the hydraulic performance of an elastic pipe is controlled by its finite speed of response to a change in boundary conditions. The actually occurring, pulsatile flow is well described by a water-hammer analysis of the establishment process, but disagrees significantly with theoretical predictions based on assumed conduit rigidity. Flow accelerations, both temporal and convective, are shown to cause a significant increase in the values of the lower-critical Reynolds number of laminar-to-turbulent transition.

The collapse of cylindrical shells under external pressure is known to be controlled by global elastic buckling, material failure, or a combination thereof. In the case of composites, delamination is another factor affecting the stiffness and stability of the structural component. Thin inner delaminated layers are expected to locally buckle inwards, under hoop compression, reducing the effective thickness of the ring wall. This may lead to a premature collapse of composite rings. The problem is numerically treated and parametrically studied. A Fourier series-based finite-element model is formulated for delaminated composite tubular cross sections. A special “chord length” procedure is developed to enable the convergence of the local buckling solution in quasi-static nonlinear analysis. Parametric studies are conducted to assess the influence of delamination length, depth, location, out-of-roundness imperfection, and ring layup on the pressure levels at delamination buckling and collapse.

Discrete element methods are important tools for the investigation of the mechanics of granular materials. In two dimensions, the reliability of these numerical approaches can be explored using physical tests on rod assemblies. This work highlights the importance of representing the actual distribution of rod shapes and surface friction in numerical simulations. The sensitivity of the response of hexagonally packed rods to minor changes in particle geometry and friction is investigated using a combination of laboratory tests and discrete element simulations. Laboratory test results highlight the influence of small variations in rod geometry on the global response, with the peak friction angle decreasing significantly as the standard deviation of the rod size distribution increased. Small changes in rod shape are also seen to be important. The numerical simulations indicate that the peak friction angle decreases as the standard deviation of the distribution of particle surface friction increases. This paper illustrates the way in which laboratory tests and numerical simulations can be used in a complementary manner to better understand the micromechanics of the response of granular materials.

Aerodynamic forces on bridges are commonly separated into static, self-excited, and buffeting force components. By delving into the relationships among force descriptors for static, self-excited, and buffeting components, novel perspectives are developed to unveil the subtle underlying complexities in modeling aerodynamic forces. Formulations for airfoil sections and those based on quasi-steady theory are both considered. The time domain modeling of unsteady aerodynamic forces including their frequency-dependent characteristics and spanwise correlation is presented, which are often neglected in current time domain analyses due to modeling difficulty. A nonlinear aerodynamic force model is proposed to take into account the nonlinear dependence of the aerodynamic forces on the effective angle of incidence. The nonlinear aerodynamics may become increasingly critical when the aerodynamic characteristics of innovative bridge deck designs, with attractive aerodynamic performance, exhibit significant sensitivity with respect to the effective angle of incidence and with the increases in the bridge span. Clearly, in these cases one may be pushing the envelope of the current linear aerodynamics which has successfully served thus far. The synergistic review of the writers’ recent work in bridge aerodynamics presented here, in light of the current state-of-the-art in this field, may serve as a building block for developing new analysis tools and frameworks for the accurate prediction of the response of long span bridges under strong wind excitation.

Seven-wire steel strands are widely used in various types of prestressed concrete structures. When a strand is subjected to a tensile force, the traveling time of the stress wave will be affected due to the elongation of the strand together with the changes in wave velocities. In this paper, the Wigner-Ville transform technique was used to analyze the measured stress waves in order to identify the arrival time of each frequency component. A numerical calculation considering the elastic waveguide theory and the acoustoelastic effect was conducted. A commonly used 12.7-mm (1/2 in.) diameter seven-wire prestressing strand (Grade 270) was tested. Tensile force up to 142 kN (32 kips) was applied to the strand. The results indicate that evaluation of the tensile force in the strand can be accomplished by measuring the traveling time of a single-frequency component of the propagating stress wave. The Wigner-Ville transform technique can efficiently identify the arrival time of each frequency component of the stress-wave signals with reasonable accuracy. The numerical and experimental results correlate well with each other. Results of this study present a technique that can provide an efficient nondestructive measurement of stress retension levels in long post-tensioned steel strands.

This paper is concerned with the numerical simulation of hot metal forming, especially superplastic forming. In this first part, a complete thermo-viscoplastic formulation at finite strains is derived and a unified stress update algorithm for thermo-elastoplastic and thermo-elasto-viscoplastic constitutive equations is obtained. The resulting unified implicit algorithm is both efficient and very inexpensive. A staggered scheme is used for the global resolution of the thermomechanical problem. We will also describe the basis of our thermomechanical frictional contact model. Discussion of computational aspects and efficiencies will be assessed in the second part of this work.

In this paper we will apply the theoretical framework developed in Part I to various metal forming processes. These numerical simulations are exposed showing the capability of the formulation to simulate finite anisothermal deformation of solids and thermal field evolution due to viscoplastic and frictional heating.

Cyclic load tests on shear panels of low-yield alloy of aluminum (3003-O) were performed to determine the onset and effect of inelastic web buckling on load-deformation behavior. Yielding of shear panels of aluminum can be used as a means to dissipate energy through hysteresis provided strength deterioration due to inelastic buckling is controlled. Gerard’s formulation for inelastic buckling, as reported in 1948, was found to be in excellent agreement with experimental results and can be used to predict the onset of inelastic shear buckling and to design shear panels so that inelastic buckling does not occur at strains below the design requirements.