Proper Loading Conditions Research Articles

Structural health building response induced by earthquakes: Material softening and recovery

AbstractUnder proper loading conditions, micro‐to‐nanoscale heterogeneities (ie, the bond system) that are commonly found within the materials of a system can coalesce until causing macroscopic alterations of the system properties. The bond system is responsible for atypical and invariant‐scale nonlinear elastic processes in granular media, from laboratory‐tested materials (mm) to the Earth's crust (km). The unusual observed behavior involves slow recovery, or relaxation, of the elastic properties after dynamic loading. Several models have been designed to explain non‐linear elasticity, although their physics is still partially unknown. Here, we show that recovery processes are also observed at intermediary scales (m) in civil engineering structures, and that they might be related to structural health due to the healing of cracks. For Japanese buildings subjected to earthquakes, we observe rapid co‐seismic reductions of their resonance frequency, followed by fascinating recoveries over different time scales. For the first time, slow recovery is presented in buildings over short times (ie, seconds) for a single earthquake; over intermediate times (ie, months) for a sequence of aftershocks; and over long times (ie, years) for a series of earthquakes. This study focuses on the analysis of long‐term recovery and recovery during aftershocks, in order to detect permanent damage. By comparing two buildings with different damage levels after the 2011 Tohoku earthquake, we show how relaxation models can characterize the level of cracking caused by damaging events. Our results demonstrate that nonlinear elasticity, combined with existing monitoring techniques, can be a powerful approach for damage detection and damage characterization.

A feasibility study of continuous grain refinement of sheet metal

Various severe plastic deformation (SPD) techniques have been used to develop ultrafine-grained (UFG) materials to enhance material properties. Equal channel angular extrusion/pressing (ECAE/ECAP) is an effective technique to severely deform material and refine grains, however, the drawback is that a continuous process on sheet metal is confined due to the process mechanism. Equal channel angular drawing (ECAD), a continuous strip drawing process, has limitations in achieving grain refinement because drawing is a stretching phenomenon contrary to an extrusion or pressing process. In the present work, experiments were conducted on interstitial-free (IF) steel to demonstrate that grain refinement of thin sheets can be achieved through pack extrusion; and while there was shear deformation, ECAD failed to produce UFG microstructure. The finite element analysis showed the sheet metals have distinctive stress-strain states after ECAE and ECAD. The feasibility of combining ECAE and ECAD to approach the stress-strain state of the sheet metal in pack extrusion for grain refinement was investigated. Simulation results showed that with proper loading conditions, continuous grain refinement of thin metal strips is sufficiently possible.

Negative Poisson's ratio in cubic materials along principal directions

This report employed molecular statics simulation and density‐functional‐theory calculation to study the Poisson's ratios of face‐centered‐cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings.

Finite Element Analysis of Human Tactile Sensing to Differentiate Thin Foils through Comparison Between Vertical & Angled Loads

Due to increasing research demand on human tactile sensations for robot-human interfaces and new tactile sensors, this research studies human tactile sensing to differentiate the thicknesses of two extremely thin foils that are made of Cu and stainless steel (SUS). We performed finite element analysis (FEA) on the fingertip's 3D elastic model to improve previous simulations through error analyses, better selection of nodal points, and proper loading conditions. We obtained the optimum mesh size to achieve numerical error below 4%. We also compared the von Mises (VM) stress of Cu with one of the SUS foils under different loading states to monitor the tc/ts ratio calculated from the thicknesses of the Cu and SUS foils that cause identical VM stress. The simulated result shows that the ratio becomes considerably large when thicknesses tc (Cu) and ts (SUS) are calculated from the differences of the VM stress between the angular and vertical loads. Consequently, the difference between the twitch motion and the datum provides the best ratio, tc/ts = 1.6, which resembles the results of psychophysical experiments.

Deformation and stability of a pinned shallow arch constrained by a rigid plate and loaded by a concentrated moment

This paper studies the behavior of a pinned half-sine arch, with a center rigid constraint plate, under a static concentrated moment. Under proper loading conditions, the arch will be in contact with the constraint plate at discrete points. This type of configurations is referred to as the contact equilibrium configuration. Geometric restrictions on the deformation of the arch at the contact point are derived. Then, the method of mode expansion is used to solve the force equilibrium equations together with the geometric restrictions for the equilibrium configuration. Due to the restrictions on the deformation of the arch imposed by the constraint plate, the classical potential energy method cannot be directly applied to determine the stability of the contact equilibrium configuration. A modified potential energy method is proposed for overcoming this problem. With the proposed method, the effects of the magnitude and location of the applied moment on the deformation and stability of the arch are investigated thoroughly. We find that, in the presence of the constraint plate, the arch possesses more complicated deformation patterns. Finally, experiments are conducted to validate the theoretical results.

P-MNS-01 EVALUATION OF NANOMETER SCALE MECHANICAL PROPERTIES OF EXTREMELY THIN DIAMOND-LIKE CARBON (DLC) FILMS(Micro/Nanosystem Science and Technology,Technical Program of Poster Session)

Nanometer scale mechanical properties of extremely thin DLC films deposited by filtered cathodic vacuum arc (FCVA) and electron cyclotron resonance chemical vapor deposition (ECR-CVD) methods were evaluated. DLC films, whose target thicknesses are 100nm, 5nm, and 2nm were, deposited on Si (100) substrates by FCVA and ECR-CVD method. Nanoindentation hardness and nanowear resistance of these DLC films are investigated by AFM (Atomic force microscopy). Nanoindentation hardness of 100-nm-thick DLC films deposited by FCVA and ECR-CVD is 57GPa and 25GPa, respectively. The nanowear resistance of 5nm- or 2nm-thick DLC films can be evaluated by AFM at proper load conditions. Then nanowear test reveal the excellent wear resistance of the FCVA-DLC films.

A generalization of the timoshenko beam model for coupled vibration analysis of thin‐walled beams

AbstractA generalization of the Timoshenko beam model is presented which accounts for the influence of the shear strains, due to non‐uniform bending and torsion, on the flexural‐torsional vibrations of thin‐walled cores with open or closed cross‐section. The axial displacement field incorporates the torsion secondary warping as well as the warping terms depending on the shear resultants. It is shown that exact solutions for the interior domain problem can be obtained under proper load conditions. Moreover, a discrete model for the free‐vibration analysis is derived by adopting a linear interpolation of the unknown functions and a reduced integration in order to avoid locking phenomena. Various applications are developed, including the case of the coupled vibrations of a shear‐core‐steel‐frame building.