Journal of Engineering Mechanics

In this paper, we study the deformation and stability of a pinned buckled beam under a point force. The buckled beam is constrained by a tensionless elastic foundation, which is flat before deformation. From static analysis, we found a total of five different deformation patterns: (1) noncontact, (2) full contact, (3) one-sided contact, (4) isolated contact in the middle, and (5) two-sided contact. For a specified set of parameters, there may coexist multiple equilibria. To predict the response of the buckled beam foundation system as the point force moves from one end to the other, we have to determine the stability of these equilibrium configurations. To achieve this, a vibration method is adopted to calculate the natural frequencies of the system, taking into account the slight variation of the contact range between the buckled beam and the tensionless foundation during vibration. It is concluded that among all five deformation patterns, Deformations 1, 2, 3, and 4 may become stable for certain loading parameters. In the extreme case in which the foundation is rigid, on the other hand, only Deformations 1 and 3 are stable.

An innovative design for a semiactive variable stiffness (SAVS) device is presented in this paper. This beamlike device is capable of altering its stiffness in a smooth manner between minimum and maximum levels by using the variations of moment of inertia of an area as it rotates around a normal axis passing through its centroid. Analytical expressions for the stiffness of the proposed device have been derived. As an application of the SAVS device in engineering, a semiactive tuned vibration absorber (SATVA) is developed that is capable of real-time retuning and operates effectively in broadband frequency excitations. A single-degree-of-freedom (SDOF) system coupled with the SATVA is considered. The excitation force is assumed to arise from a rotating unbalance whose frequency varies with a constant acceleration and then reaches a steady-state operating condition. The absorber stiffness is varied to tune the absorber’s natural frequency to the forcing frequency in real time. The effectiveness of SATVA is evaluated by comparing the system’s responses with those of the system with passive vibration absorber. The results show the salient features of the proposed SATVA in transient response reduction compared with the traditional passive vibration absorber.

The vortex gripper is a recently developed pneumatic noncontact handling device that takes advantage of air-swirling flow to cause upward lifting force and that thereby can pick up and hold a work piece placed underneath without any contact. It is applicable where, e.g., in the semiconductor wafer manufacturing process, contact should be avoided during handling and moving in order to minimize damage to a work piece. For the purpose of a full understanding of the mechanism of the vortex gripper, a computational fluid dynamics (CFD) study was conducted in this paper, and at the same time, experimental work was carried out to measure the pressure distribution on the upper surface of the work piece. First, three turbulence models were used for simulation and verified by comparison with the experimental pressure distribution. It is known that the Reynolds stress transport model (RSTM) can reproduce the real distribution better. Then, on the basis of the experimental and numerical result of RSTM, an insight into the vortex gripper and its flow phenomena, including flow structure, spatial velocity, and pressure distributions, and an investigation into the influence of clearance variation was given.

This paper presents a three-dimensional (3D) micromechanical finite-element (FE) network model for predicting elastic damage behavior of the idealized stone-based materials. Stone-based composite materials have multiphase structures: an aggregate (or stone) skeleton, a binding medium, fillers, and air voids. Numerical simulation of the micromechanical behavior of the idealized stone-based materials was accomplished by using a microframe element network model that incorporated the mechanical load transfer between adjacent particles. The elastic stiffness matrix of this special element was obtained from an approximate elastic stress-strain analysis of straight cement between particle pairs. A damage-coupled microframe element was then formulated with bilinear damage laws, including elastic and softening behavior based on the equivalent fracture release energy. Indirect tension and compression simulations were conducted with developed FE models on the idealized digital samples of the stone-based materials. These simulations predicted the internal microdamage distribution and global fracture behavior of these samples, which qualitatively agree with the laboratory observations. The results indicate that the developed FE models have the capability to predict the typical loading-related damage behavior observed from the stone-based materials.

Moving forces are a common loading pattern for flexible beams, found in many applications in both civil and mechanical engineering. These forces give rise to a transient response, the nature of which depends on the time variation of the amplitude of the force and its position along the beam. In addition to the possibility of numerical evaluation, closed form solutions of the beam response are beneficial for their simplicity of use, and because they allow an understanding of the system behavior. On the other hand, these prove to be rather complicated in most cases, and only a limited number of cases are available in the literature. This paper studies the simple but common case of a supported beam loaded by a force with sinusoidal time variation moving at a constant speed. Simple equations are presented for the approximated responses at and away from resonance, and their accuracy is discussed. Transient frequency response functions are also shown. Finally, as an example, the results are applied to an evaluation of the response of a beam footbridge to the action of a walker, and compared to code specifications.

The growth of viscoelastic cracks in hot-mix asphalt (HMA) concrete mixtures was simulated by using a disk-shaped compaction test. Modeling techniques, including the interconversion and continuous spectrum methods, the harmony search algorithm, and the state-variable approach, substantially enhanced the computational efficiency of the generalized J-integral. The generalized J-integral was used for determining the viscoelastic crack growth parameter on the basis of the fracture mechanics under cyclic loading conditions. This paper shows that the parameter can be used for simulating the viscoelastic crack growth in HMA concrete mixtures even under conditions of different loading magnitudes and frequencies.

This paper presents findings from a study directed at identifying key properties of ingredient materials that are influential on the early age cracking behavior of concrete, with an emphasis on the effects of aggregate size, aggregate morphologies, and water-cement ratio (w/c). Fracture energy ($G_F$) was determined using a wedge-split test configuration for concrete samples at the age of 12 h. Based on image analysis, three signature morphologies of aggregate particles, i.e., the angularity, surface texture, and surface area, were quantitatively determined in terms of developed angularity index (AI), surface texture (ST) index, and surface area (SA) measurement, respectively. The high consistency between $G_F$ and aggregate SA of the concrete samples suggests that the interfacial transition zone (ITZ) at the cement paste-aggregate interface is the critical location that primarily accommodates the 12 h cracking of concrete. The critical role of ITZ in the early age cracking of concrete was further confirmed by its microstructural and chemical features under scanning electron microscopy/energy dispersive X-ray spectroscopy.