Seismic reliability assessment of classical columns subjected to near-fault ground motions

A methodology for the performance-based seismic risk assessment of classical columns is presented. Despite their apparent instability, classical columns are, in general, earthquake resistant, as proven from the fact that many classical monuments have survived many strong earthquakes over the centuries. Nevertheless, the quantitative assessment of their reliability and the understanding of their dynamic behavior are not easy, because of the fundamental nonlinear character and the sensitivity of their response. In this paper, a seismic risk assessment is performed for a multidrum column using Monte Carlo simulation with synthetic ground motions. The ground motions adopted contain a high- and low-frequency component, combining the stochastic method, and a simple analytical pulse model to simulate the directivity pulse contained in near source ground motions. The deterministic model for the numerical analysis of the system is three-dimensional and is based on the Discrete Element Method. Fragility curves are produced conditional on magnitude and distance from the fault and also on scalar intensity measures for two engineering demand parameters, one concerning the intensity of the response during the ground shaking and the other the residual deformation of the column. Three performance levels are assigned to each engineering demand parameter. Fragility analysis demonstrated some of the salient features of these spinal systems under near-fault seismic excitations, as for example, their decreased vulnerability for very strong earthquakes of magnitude 7 or larger. The analysis provides useful results regarding the seismic reliability of classical monuments and decision making during restoration process

Objekt-Manipulation und Steuerung der Greifkraft durch Verwendung von Taktilen Sensoren

This dissertation describes a new type of tactile sensor and an improved version of the dynamic tactile sensing approach that can provide a regularly updated and accurate estimate of minimum applied forces for use in the control of gripper manipulation. The pre-slip sensing algorithm is proposed and implemented into two-finger robot gripper. An algorithm that can discriminate between types of contact surface and recognize objects at the contact stage is also proposed. A technique for recognizing objects using tactile sensor arrays, and a method based on the quadric surface parameter for classifying grasped objects is described. Tactile arrays can recognize surface types on contact, making it possible for a tactile system to recognize translation, rotation, and scaling of an object independently.Diese Dissertation beschreibt eine neue Art von taktilen Sensoren und einen verbesserten Ansatz zur dynamischen Erfassung von taktilen daten, der in regelmäßigen Zeitabständen eine genaue Bewertung der minimalen Greifkraft liefert, die zur Steuerung des Greifers nötig ist. Ein Berechnungsverfahren zur Voraussage des Schlupfs, das in einen Zwei-Finger-Greifarm eines Roboters eingebaut wurde, wird vorgestellt. Auch ein Algorithmus zur Unterscheidung von verschiedenen Oberflächenarten und zur Erkennung von Objektformen bei der Berührung wird vorgestellt. Ein Verfahren zur Objekterkennung mit Hilfe einer Matrix aus taktilen Sensoren und eine Methode zur Klassifikation ergriffener Objekte, basierend auf den Daten einer rechteckigen Oberfläche, werden beschrieben. Mit Hilfe dieser Matrix können unter schiedliche Arten von Oberflächen bei Berührung erkannt werden, was es für das Tastsystem möglich macht, Verschiebung, Drehung und Größe eines Objektes unabhängig voneinander zu erkennen

A New Framework Based on a Discrete Element Method to Model the Fracture Behavior for Brittle Polycrystalline Materials

This work aims to develop and implement a linear elastic grain-level micromechanical model based on the discrete element method using bonded contacts and an improved fracture criteria to capture both intergranular and transgranular microcrack initiation and evolution in polycrystalline ceramics materials. Gaining a better understanding of the underlying mechanics and micromechanics of the fracture process of brittle polycrystalline materials will aid in high performance material design. Continuum mechanics approaches cannot accurately simulate the crack propagation during fracture due to the discontinuous nature of the problem. In this work we distinguish between predominately intergranular failure (along the grain boundaries) versus predominately transgranular failure (across the grains) based on grain orientation and microstructural parameters to describe the contact interfaces and present the first approach at fracturing discrete elements. Specifically, the influence of grain boundary strength and stiffness on the fracture behavior of an idealized ceramic material is studied under three different loading conditions: uniaxial compression, brazilian, and four-point bending. Digital representations of the sample microstructures for the test cases are composed of hexagonal, prismatic, honeycomb-packed grains represented by rigid, discrete elements. The principle of virtual work is used to develop a microscale fracture criteria for brittle polycrystalline materials for tensile, shear, torsional and rolling modes of intergranular motion. The interactions between discrete elements within each grain are governed by traction displacement relationships

Behavior of a sandy clay under vertical impact of geometric shapes

Sandy clay response under vertical impact of cone, plate, and plane geometric shape

Elastic impact of a pendulum on frictional surface

Ankara : The Department of Mechanical Engineering and the Graduate School of Engineering and Science of Bilkent University, 2012.Thesis (Master's) -- Bilkent University, 2012.Includes bibliographical refences.Constrained impacts with friction frequently exist in mechanical systems such as robotic arms, hard disk drives and other mechanisms. Such discontinuous contacts, if not designed and analysed properly, can lead to malfunctions. In particular, for the analysis of problems that involve eccentric collisions and reversal of friction force, use of stereomechanical impact theory with coefficient of restitution can produce paradoxical energy increase. Alternatively, continuum models, which provide more detailed analysis for such problems, can be used, however they are computationally tedious. Instead, here, contact is described by compliant elements with friction and applied to a physical pendulum. In this thesis, impact-momentum relations for general three-dimensional free collisions are modified for a pendulum which exemplifies an impact with friction and constraint. Inclusion of tangential compliance to model enables the model to demonstrate tangential force reversals and their transition between stick and slip, which is demonstrated using a sphere and a slender rod obliquely colliding with a rough massive plane. Use of compliant elements to describe impact by a planar pendulum produces differences in the behavior of a constrained system compared with free impacts. For instance, in free collisions an impact that starts with an initial sticking, is always followed by sliding. However, in a pendulum if the contact begins by sticking, it continues to stick throughout the duration of impact. Another difference appears when contact starts with an initial sliding. In free impact, sliding is followed by sticking and sliding, then the body rebounds unless the collision is inelastic. However, in the constrained case wedging of the pendulum is observed if initial angle of collision is below a critical value for a specified friction coefficient.Birlik, Seyit CanM.S

Dynamic Constitutive Equation For A Syntactic Foam Under Multi-Axial Stress State

Eco-Core is a fire resistant structural core material that was developed at North Carolina A&T State University in 2003. During the last 10 years, mechanical, fracture and fatigue properties as well as resistance to sea water were established for Eco-Core. A design methodology for Eco-Core in sandwich structures was provided. The objective of this research was to develop a dynamic constitutive equation for Eco-Core in a multi-axial stress state that is valid for both static and dynamic loadings, and then demonstrate the model to be used in a commercial code to solve real life problems. A special tri-axial loading and strain measurement test fixture was developed and used in static and dynamic tests. The material was tested at strain rates ranged from 3 10 /s to 3500/s. Analysis of the results showed that the net-deviatoric stress is independent of lateral stress. Based on the test results and a multi-variable regression analysis, a two part constitutive equation was developed. The first part was for the linear response and the second part was for the non-linear response that covers translation, crushing and densification of micro bubbles in the Eco-Core. The equation was validated by independent experiments and simulation by LS-DYNA. The dynamic energy absorption capability for Eco-Core was found to be superior compared to commercial materials such as PVC foam, Rohacell foam and Balsa wood