Engineering Mechanics

Vibration and symmetry-breaking of boron-nitride nanotubes

by Rajib Chowdhury

Nanotechnology, 2010

The unique features of axial, torsional, transverse and radial breathing vibrations are captured for armchair and... more The unique features of axial, torsional, transverse and radial breathing vibrations are captured for armchair and zigzag singlewalled boron nitride nanotubes (BNNTs) based on molecular mechanics simulations and continuum mechanics theories. Equivalent Young's modulus 1TPa and shear modulus 0.4TPa are obtained independent of the chirality of BNNTs. In particular, a distorted optimized structure is observed for the first time for BNNTs with sufficiently large diameter and length. It is found that the deformed structures result in the behaviours of BNNTs deviating from those of classical columns/beams. Such symmetry breaking could also exert significant impacts on the structural instability (buckling) and the electronic properties of BNNTs that are sensitive to the structural symmetry.

Vibration of ZnO nanotubes: a molecular mechanics approach

by Rajib Chowdhury

Applied Physics A: Materials Science & Processing, 2010

We investigate the vibrational properties of two kinds of single-wall ZnO nanotubes. The simulations are carried out... more We investigate the vibrational properties of two kinds of single-wall ZnO nanotubes. The simulations are carried out for three types of zigzag nanotubes (5,0), (8,0), (10,0) and armchair nanotubes (3,3), (4,4), (6,6). The natural frequencies are determined by means of the molecular mechanics approach with the universal force field potential. The first four natural frequencies are obtained for length/diameter ratio of about 5–20. The vibration modes associated with these frequencies have been computed. Closed-form analytical expressions have been derived using the continuum shell theory for the physical explanations of the simulations results. We observe that the natural frequencies decrease as the aspect ratios increase. The results follow similar trends with results of previous studies for carbon nanotubes (CNT). However, the magnitudes of the frequencies are lower from the corresponding CNT counterparts, indicating that ZnO nanotubes are comparatively less stiff.

Nonlinear aspects of energy dissipation in wood-panel joints

by Sara Casciati

Casciati S. (2007). “Nonlinear aspects of energy dissipation in wood-panel joints”. Earthquake Engineering and Engineering Vibration, 6(3), 259-268. ISSN: 1671-3664.

DATA E LUOGO DI PUBBLICAZIONE: September 2007; Springer, New York, NY 10013, USA.

ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled... more ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled from wood panels. If they are too weak, local failures may occur, resulting in performance that is significantly below expectations. If they are too resistant, the joints may be unable to dissipate energy during vibrations, thus possibly initiating a fast progressive failure. This paper re-processes and re-elaborates the results of shaking table tests previously carried out by the author and other co-workers. The goal is to assess the feasibility of a joint which is able to dissipate energy during vibration, without degrading the connection performance.

KEYWORDS: dynamic excitation - energy dissipation - wood panels joint - monitoring system - shaking table tests

Fatigue characterization of a Cu-based shape memory alloy

by Sara Casciati

Casciati F., Casciati S., and Faravelli L. (2007). “Fatigue characterization of a Cu-based shape memory alloy”. Proceedings of the Estonian Academy of Sciences – Physics Mathematics, 56(2), 207-217. ISSN: 1406-0086.

DATE AND PLACE OF PUBBLICATION: June 2007; Estonian Academy Publishers, Tallinn 10143, Estonia.

ABSTRACT: Applications of a Cu-based shape memory alloy in monuments retrofitting were conceived, designed, and... more ABSTRACT: Applications of a Cu-based shape memory alloy in monuments retrofitting were conceived, designed, and tested. The polycrystalline nature of the material requires a preliminary discussion in order to allow the extension of properties which are well known for monocrystalline specimens. Furthermore, one of the main drawbacks of the investigated alloy is its brittleness. Therefore, fatigue tests were carried out and the results of those conducted under cycles of torsion loading–unloading are reported. In particular, the identification of the response range, which is most suitable for structural engineering applications, is pursued.

KEYWORDS: compression tests, fatigue tests, shape memory alloy, tension tests, torsion tests, thermal treatment.

Random imperfection fields to model the size effect in laboratory wood specimens

by Sara Casciati

Casciati S. and Domaneschi M. (2007). “Random imperfection fields to model the size effect in laboratory wood specimens”. Structural Safety, 29(4), 308-321. ISSN: 0167-4730.

DATA E LUOGO DI PUBBLICAZIONE: October 2007; Elsevier Science Bv, 1000 AE Amsterdam, Netherlands.

ABSTRACT. The composite nature of a wood continuum prevents one from extrapolating the results of laboratory tests on... more ABSTRACT. The composite nature of a wood continuum prevents one from extrapolating the results of laboratory tests on standard wood specimens to structural elements of significant size. Therefore, these elements are usually tested under standardized loading conditions in order to detect a sort of average material behaviour.
In this paper, the initial step consists, instead, of testing the material specimens. The extension of the results to structural elements is then pursued by introducing a random field, or, in a discretized model, a random array of imperfections.
The calibration of the suitable spatial distribution of the imperfections is then investigated by a mixed experimental–numerical approach, for a reference beam. The analyses on the relative finite elements model are iterated to match the response of the full scale laboratory tests.

KEYWORDS: Biaxial tests; Finite element model; Imperfections; Laboratory tests; Random field; Wood specimens

Cohesive Crack Propagation in a Random Elastic Medium

by Sara Casciati

Bruggi M., Casciati S., and Faravelli L. (2008). “Cohesive crack propagation in a random elastic medium”. Probabilistic Engineering Mechanics, 23(1), 23-35. ISSN: 0266-8920.

DATA E LUOGO DI PUBBLICAZIONE: January 2008; Elsevier Sci Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT. The issue of generating non-Gaussian, multivariate and correlated random fields, while preserving the... more ABSTRACT. The issue of generating non-Gaussian, multivariate and correlated random fields, while preserving the internal auto-correlation structure of each single-parameter field, is discussed with reference to the problem of cohesive crack propagation. Three different fields are introduced to model the spatial variability of the Young modulus, the tensile strength of the material, and the fracture energy, respectively. Within a finite-element context, the crack-propagation phenomenon is analyzed by coupling a Monte Carlo simulation scheme with an iterative solution algorithm based on a truly-mixed variational formulation which is derived from the Hellinger–Reissner principle. The selected approach presents the advantage of exploiting the finite-element technology without the need to introduce additional modes to model the displacement discontinuity along the crack boundaries. Furthermore, the accuracy of the stress estimate pursued by the truly-mixed approach is highly desirable, the direction of crack propagation being determined on the basis of the principal stress criterion. The numerical example of a plain concrete beam with initial crack under a three-point bending test is considered. The statistics of the response is analyzed in terms of peak load and load–mid deflection curves, in order to investigate the effects of the uncertainties on both the carrying capacity and the post-peak behaviour. A sensitivity analysis is preliminarily performed and its results emphasize the negative effects of not accounting for the auto-correlation structure of each random field. A probabilistic method is then applied to enforce the auto-correlation without significantly altering the target marginal distributions. The novelty of the proposed approach with respect to other methods found in the literature consists of not requiring the a priori knowledge of the global correlation structure of the multivariate random field.

KEYWORDS: Multivariate non-Gaussian random fields; Auto-correlation; Cohesive crack propagation; Truly-mixed finite element method; Monte Carlo simulations

Structural components in shape memory alloy for localized energy dissipation

by Sara Casciati

Casciati S. and Faravelli L. (2008). “Structural components in shape memory alloy for localized energy dissipation”. Computers & Structures, 86(3-5), 330-339. ISSN: 0045-7949.

DATE AND PLACE OF PUBBLICATION: February 2008; Pergamon–Elsevier Science Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT: Cu-based shape memory alloys (SMA) offer their feature of superelastic hysteresis at sustainable costs, and... more ABSTRACT: Cu-based shape memory alloys (SMA) offer their feature of superelastic hysteresis at sustainable costs, and for a window of temperatures applicable to most of civil engineering applications. Structural components can easily be obtained in the form of wires or plates. Their insertion in standard structures produces devices for local energy dissipation under dynamic loading. Several solutions are experimentally investigated throughout the paper.

KEYWORDS: Dissipative devices; Hysteresis; Shaking table test; Shape memory alloys; Superelasticity; Training

Experimental and numerical studies toward the implementation of shape memory alloy ties in masonry structures

by Sara Casciati

Casciati S. and Hamdaoui K. (2008). “Experimental and numerical studies toward the implementation of shape memory alloy ties in masonry structures”. Smart Structures and Systems, 4(2), 153-169. ISSN: 1738-1584.

DATA E LUOGO DI PUBBLICAZIONE: March 2008; Techno-Press, Daejeon 305-600, South Korea.

ABSTRACT: The use of pre-tensioned shape memory alloy (SMA) wires to retrofit historic masonry structures is... more ABSTRACT: The use of pre-tensioned shape memory alloy (SMA) wires to retrofit historic masonry structures is investigated. A small wall, serving as a prototype masonry specimen, is constructed to undergo a series of shaking table tests. It is first studied in its original state, and its dynamic characteristics (in terms of modal frequencies) are extracted from the recorded signals. The results are then compared with those obtained when an increasing number of couples of pre-stressed SMA wires are introduced in the specimen to link the bricks together. A threedimensional finite element model of the specimen is developed and calibrated according to the modal parameters identified from each experimental test (with and without SMA wires). The calibration process is conducted by enhancing the masonry mechanical behaviour. The results and the effectiveness of the approach are presented.

KEYWORDS: masonry; numerical modelling; shaking table tests; shape memory alloys; system identification.

FINITE ELEMENT ANALYSIS OF A HIGHLY PARALLEL ROBOTIC SURFACE

by Chris Salisbury

Published in the proceedings of the global IMECE2011 conference.

A  novel  three-dimensional  robotic  surface  is  devised  and more A  novel  three-dimensional  robotic  surface  is  devised  and
implemented  using  triangular  modules.  Each  module  is
connected  by  a  passive  sixbar  mechanism  that  mimics  the
constraints of a spherical joint at each triangle intersection. The
finite element method (FEM) is applied to the static loading of
this  device  using  linear  three  dimensional  (6  degrees  of
freedom) beam elements to calculate the cartesian displacement
and force and the angular displacement and torque at each joint.
In this way, the traditional methods of finding joint forces and
torques  are  completely  bypassed.  An  effiecient  algorithm  is
developed  to  linearly  combine  local  stiffness  matrices  into  a
full structural stiffness matrix for the easy application of loads.
This is then decomposed back into the local matrices to easily
obtain joint variables used in the design and open-loop control
of the surface.

DYNAMIC FINITE ELEMENT ANALYSIS OF A HIGHLY PARALLEL ROBOTIC SURFACE

by Chris Salisbury

Published in the proceedings of the SMASIS2011 conference.

A  novel  three-dimensional  robotic  surface  is
devised  using ... more A  novel  three-dimensional  robotic  surface  is
devised  using  triangular  modules  connected  by
revolute  joints  that  mimic  the  constraints  of  a
spherical  joint  at  each  triangle  intersection.  The
finite  element  method  (FEM)  is  applied  to  the
dynamic  loading  of  this  device  using  three
dimensional    (6    degrees    of    freedom)    beam
elements  to  not  only  calculate  the  cartesian
displacement  and  force,  but  also  the  angular
displacement and torque at each joint. In this way,
the traditional methods of finding joint forces and
torques  are  completely  bypassed.  An  effiecient
algorithm  is  developed  to  linearly  combine  local
mass  and  stiffness  matrices  into  a  full  structural
stiffness  matrix  for  the  easy  application  of  loads.
An  analysis  of  optimal  dynamic  joint  forces  is
carried  out  in  Simulink®  with  the  use  of  an
algebraic Ricatti equation.

Statistical approach to a SHM benchmark problem

by Sara Casciati

Casciati S. (2010). “Statistical approach to a SHM benchmark problem”. Smart Structures and Systems, 6(1), 17-27. ISSN: 1738-1584.

DATE AND PLACE OF PUBLICATION: January 2010; Techno-Press, Daejeon 305-600, South Korea.

ABSTRACT: The approach to damage detection and localization adopted in this paper is based on a statistical comparison... more ABSTRACT: The approach to damage detection and localization adopted in this paper is based on a statistical comparison of models built from the response time histories collected at different stages during the structure lifetime. Some of these time histories are known to have been recorded when the structural system was undamaged. The consistency of the models associated to two different stages, both undamaged, is first recognized. By contrast, the method detects the discrepancies between the models from measurements collected for a damaged situation and for the undamaged reference situation. The damage detection and localization is pursued by a comparison of the SSE (sum of the squared errors) histograms. The validity of the proposed approach is tested by applying it to the analytical benchmark problem developed by the ASCE Task Group on Structural Health Monitoring (SHM). In the paper, the results of the benchmark studies are presented and the performance of the method is discussed.

KEYWORDS: damage detection; damage localization; regression analysis; structural health monitoring; sum of the squared errors.

Response Surface Models to Detect and Localize Distributed Cracks in a Complex Continuum

by Sara Casciati

Casciati S. (2010). “Response surface models to detect and localize distributed cracks in a complex continuum”. Journal of Engineering Mechanics – ASCE, 136(9), 1131-1142. ISSN: 0733-9399.

DATE AND PLACE OF PUBLICATION: September 2010; ASCE – American Society of Civil Engineers, Reston, VA 20191-4400, USA.

ABSTRACT:Linear response surface
RS models are used to represent the relationship between samples of response time... more ABSTRACT:Linear response surface
RS models are used to represent the relationship between samples of response time histories measured by sensors placed across a structure. Different structural states of a general time-variant system are considered for short intervals capturing a linearized model of each state. Within this framework, the error associated with each RS model is sensitive to a modification of the structural state. A method that relates the changes of the statistical characterization of the error to the occurrence of a structural modification is developed for damage detection. The localization of damage is then pursued by identifying the largest discrepancies resulting from the comparison between the statistics of the sum of the squares of the error obtained at each sensor location. The generality of the method is shown by applying it to the experimental data of a realistic structure, which is representative of a continuous body affected by distributed cracking.

KEYWORDS: Dynamics; Vibration; Structural response; Statistics; Damage; Monitoring; Masonry; Cracking

Dynamic behavior of a masonry civic belfry under operational conditions

by Sara Casciati

Casciati S. and Al-Saleh R. (2010). “Dynamic behavior of a masonry civic belfry under operational conditions”. Acta Mechanica, 215(1-4), 211-224. ISSN: 0001-5970.

DATE AND PLACE OF PUBLICATION: December 2010; Springer Wien, A-1201 Wien, Austria.

ABSTRACT: Slender structures, such as towers, are characterized by a high sensitivity to dynamic excitation. As a... more ABSTRACT: Slender structures, such as towers, are characterized by a high sensitivity to dynamic excitation. As a consequence, meaningful information about their behavior under operational conditions can be obtained by monitoring their response to ambient vibrations. Furthermore, significant stresses could be induced to the ancient masonry walls when the dynamic forces due to the swinging of a bell are acting. To assess the structural conditions of a case study representative of such type of structures and to plan an adequate retrofit, numerical analyses are carried out on a model whose modal parameters are calibrated based on the elaboration of the results from the ambient vibrations tests. In particular, full time histories analyses are performed using as input either the signal recorded while the bell was hit by a hammer, or the numerically calculated dynamic forces that would be produced by the actual swinging of the bell. The first set of analyses aims to investigate the capability of an equivalent linear elastic model to capture the actual dynamic response of the structure. The second set of analyses provides an evaluation of the tower response when dynamic loads of higher intensity and likely occurrence are considered.

Mechanics of Sn-4Ag-0.5 Cu solder joints in a ball grid array assembly during reflow and temperature cycles

by Nazri Kamsah

Analytical and Experimental Comparisons of Electromechanical Vibration Response of a Piezoelectric Bimorph Beam for Power Harvesting

by Mikail Lumentut

Special Issue (In Press)

Two-dimensional modeling to compute plastic zone in front of compact tension sample of a multiphase material

by Nazri Kamsah

Computational Materials Science, Volume 47, Issue 2, December 2009, Pages 482-490

Authors: Yunan Prawoto, Roslinda Idris, Nazri Kamsah, Nasir Tamin

Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai, Johor, Malaysia

More and more components subjected to dynamic high stress loading are heat treated to get multiphase microstructure as... more More and more components subjected to dynamic high stress loading are heat treated to get multiphase microstructure as their final products, instead of conventional tempered martensite. It is gaining popularity due its manufacturing benefits, including the elimination of the tempering process, and its better fatigue performance. Multiphase of ferrite and martensite benefits its fatigue performance by its high toughness yet maintains the high tensile properties due to controlled microstructure. The plastic zone size in front of the crack is the key to fatigue performance. In this research, a multiphase (polygonal ferrite and martensite) microstructure was developed. Manipulation method to use commercial software to calculate the plastic zone size in front of a crack is presented.
Two-dimensional modeling of squares having variation of ferrite fraction was used as local models. The shape of the ferrites was derived from actual microstructure of multiphase material. These local models were put in front of the CT specimen in a global model. The analysis result shows the variation of the plastic zone sizes as the ferrite fraction varies and saturates at about 65% ferrite fraction.

Sailboat Mast Design

by Natalie Baddour

Alexandre Bergeron's 4th year thesis project

The following is meant to be a review of sailboat mast design methods and their practical application. In the first... more The following is meant to be a review of sailboat mast design methods and their practical application. In the first portion, the Euler‐Bernoulli beam theory, the P‐Δ method, Skene’s method and the Nordic Boat Standard are summarised and evaluated as design methods for masts. It was found that the Nordic Boat Standard represents the state of the art available in the public domain. In the second portion, the practical case of the Ultimate 20 racing yacht is reviewed. The Ultimate 20 has shown a tendency for the mast failing by buckling at a stress concentration on the bottom of the mast, and several solutions have been brought forward to solve this issue. In order to quantitatively establish which configuration is best, this boat’s mast was modelled according to Euler‐Bernoulli beam theory and practically tested using strain gauges. The results of both the mathematical model and the practical testing show consistent results and trends. It was found that the best single solution to reduce the chances of mast failure was the addition of baby stays.

Optimization of Speed Control Hump Profiles

by Caglar Baslamisli

Robust Control of Anti-Lock Brake System

by Caglar Baslamisli

Handling Stability Improvement Through Robust Active Front Steering and Active Differential Control

by Caglar Baslamisli