Revisiting classical concepts of Linear Elastic Fracture Mechanics - Part I: The closing ‘mathematical’ crack in an infinite plate and the respective Stress Intensity Factors

This is the first part of a short three-paper series, aiming to revisit some classical concepts of Linear Elastic Fracture Mechanics. The motive of this first paper is to highlight some controversial issues, related to the un­natu­ral overlapping of the lips of a ‘mathematical’ crack in an in­fin­­­ite plate load­ed by specific combinations of principal stresses at in­finity (predicted by the clas­si­c­al solu­tion of the respective first fundamental problem), and the closely as­so­ciated issue of negative mode-I Stress Intensity Factor. The problem is con­­­front­ed by superimposing to the first funda­mental problem of Lin­ear Elastic Frac­ture Mechanics for an in­fin­ite cracked plate (with stress-free crack lips) an ‘in­­verse’ mixed fund­amental problem. This superposition provides naturally ac­­­­­­­­ceptable stress and displacement fields, prohibiting overlapping of the lips (by means of contact stresses generated along the crack lips, which force the over­lapped lips back to naturally accepted position) and, also, non-negative mode-I Stress Intensity Factors. The solu­tions of this first paper form the basis for the next two papers of the series, dealing with the respective prob­lems in fi­­n­ite do­­mains (recall, for example, the cracked Brazil­ian disc con­fig­u­ra­tion) weak­­ened by artificial notches (rather than ‘math­e­mat­ical’ cracks), by far more interesting for practical engineer­ing ap­pli­­ca­tions

Numerical simulation of bone screw induced pretension: The cases of under-tapping and conical profile

Even though screw induced pretension impacts the holding strength of bone screws, its implementation into the numerical simulation of the pullout phenomenon remains a problem with no apparent solution. The present study aims at developing a new methodology to simulate screw induced pretension for the cases of: a) cylindrical screws inserted with under-tapping and b) conical screws. For this purpose pullout was studied experimentally using synthetic bone and then simulated numerically. Synthetic bone failure was simulated using a bilinear cohesive zone material model. Pretension generation was simulated by allowing the screw to expand inside a hole with smaller dimensions or different shape than the screw itself. The finite element models developed here were validated against experimental results and then utilized to investigate the impact of under-tapping and conical angle. The results indicated that pretension can indeed increase a screw's pullout force but only up to a certain degree. Under-tapping increased cylindrical screws' pullout force up to 12%, 15% and 17% for synthetic bones of density equal to 0.08 g/cc, 0.16 g/cc and 0.28 g/cc respectively. Inserting a conical screw into a cylindrical hole increased pullout force up to 11%. In any case an optimum level of screw induced pretension exists

The Network of Influences That Shape the Drawing and Thinking of Fifth Grade Children in Three Different Cultures: New York, U.S., Molaos, Greece, and Wadie Adwumakase, Ghana

Using an ecological systems approach, this qualitative study examined how continuously evolving, personal living experiences and the ideologies and attitudes of their material, folk, and school culture come to be (re) presented in the construction of images and meaning in children’s artwork. The research was conducted with three groups of fifth-grade students facilitated by the art teacher at their schools in three different countries: United States, Greece, and Ghana. Data in the form of a set of autobiographical drawings from observation, memory, and imagination with written commentary were created by each participant and supported with responses to questionnaires and correspondences from teachers and parents. The sets of drawings were analyzed in terms of how the drawings reflect the children’s (a) artistic expression as mediated by their interaction with local and media influences and (b) sense of self, agency, or purpose. The findings strongly suggest that style, details, content, and media use assumed a dominant role within the drawings. Furthermore, these results were reflected differently in the drawings of the cohort from each country. Having considered the set of drawings each child made as a network of enterprise emphasizes the active role the children played in the production of the artwork, involving their choices of theme and content, the media images incorporated, and the means by which a task was adapted to suit their interests. However, the results also show that the specific skills—drawing from observation, memory, and imagination—required by the four drawing tasks had a tempering effect on their creative output, leading to the conclusion that the children’s limited drawing experience constrained their ability to express themselves in pictorial representation with fluency. In view of these findings, lesson suggestions are designed to develop drawing skills across drawing modes in a rhizomatic manner of thinking. Suggestions for future research address exploring the evolution of children’s identity and sense of agency in the world through artistic expression; the role of the environment in which children draw as an embodied and embedded experience in a physical and sociocultural world; and further research into how and why children use images to communicate

Design of Bridges Against Seismic Faulting : Methodology and Applications

This paper presents a methodology for design of bridge–foundation systems against seismic faulting. The problem is decoupled in two steps. Step 1 deals with the response of a single bridge pier and its foundation subjected to faulting–induced deformation ; Step 2 deals with the detailed model of the superstructure, which is subjected to differential displacements computed in Step 1. We analyze typical viaduct and underpass bridges, founded on piles or caisson foundations. Piled foundations are found to be vulnerable to faulting– induced deformation. While end–bearing piles cannot really sustain any appreciable bedrock offset, floating piles may perform better, especially if combined with hinged pile–to–cap connections. Statically–determinate superstructures are shown to be less sensitive to faulting– induced differential displacements and rotations. Finally, an application of the method is shown for a major bridge, demonstrating the feasibility of design against seismic faulting

Fracture precursor phenomena in marble specimens under uniaxial compression by means of Acoustic Emission data

Marble specimens are subjected to a specially designed stepwise load­­ing protocol, in an attempt to detect fracture precursor phenomena taking advantage of Acoustic Emission (AE) data. The analysis is carried out in terms of the number of acoustic hits recorded and the time evolution of the improved b-value (Ib-value), the cumulative energy of the acoustic signals and the F-fun­c­tion. During the stage of in­creasing load, intense acoustic activity is detected as the cor­responding stress reaches the transitional phase from the linear to the nonlinear mechanical re­sponse of the material. When the stress is stabilized at levels exceeding 95% of the material’s compressive strength, the acous­tic activ­ity is drastically reduced. During the first seconds of the stress stabil­iz­ation stage the reduction follows an exponential law. Special attention is paid to the phases, where the occurrence of AE hits shows a strong increase. During these phases acoustic signals of low frequency and high RA are re­corded, in­di­cating that the micro-cracking process is of shear rather than of opening mode

Innovative Experimental Techniques in the Service of Restoration of Stone Monuments - Part I: the Experimental Set up

AbstractThe mechanical response of marble epistyles interconnected to each other by means of metallic connectors when subjected to shear loading is studied experimentally. The study is motivated by the need of the scientific group of the Athenian Acropolis to substitute damaged ancient connections by new ones. In this direction “I”-shaped titanium connectors are placed in the grooves sculptured by ancient stonemasons in the marble blocks and the empty space is then filled by a suitable cement-based material. Guided by the experience gathered from the inspection of failed connections, which clearly indicates that failure starts at the interior of the “titanium-mortar-marble” complex, along the material interfaces, an experimental protocol was improvised, aiming at pumping data from the interior of the interconnected epistyles. For this to be accomplished innovative sensing techniques like pressure stimulated currents, digital image correlation and acoustic emission were used in conjunction with traditional ones. In the first part of this short two-paper series the experimental set-up, the materials and the specimens’ geometry are described

Wind farm control to meet grid requirements

This research aims to design a wind farm controller to trade off the grid requirements against the operators need to for optimal utilisation of their assets by using the Power Adjusting Controller (PAC). The PAC was developed for the 5MW Supergen Exemplar wind turbine [1] and allows the wind farm operator to manipulate the power outcome of each individual wind turbine, and consequently alter the power output of the wind farm. The PAC can be used to either reduce or increase the power output of a wind turbine, whilst keeping its operation within a safe operational region

Revisiting classical concepts of Linear Elastic Fracture Mechanics - Part I: The closing ‘mathematical’ crack in an infinite plate and the respective Stress Intensity Factors

This is the first part of a short three-paper series, aiming to revisit some classical concepts of Linear Elastic Fracture Mechanics. The motive of this first paper is to highlight some controversial issues, related to the un­natu­ral overlapping of the lips of a ‘mathematical’ crack in an in­fin­­­ite plate load­ed by specific combinations of principal stresses at in­finity (predicted by the clas­si­c­al solu­tion of the respective first fundamental problem), and the closely as­so­ciated issue of negative mode-I Stress Intensity Factor. The problem is con­­­front­ed by superimposing to the first funda­mental problem of Lin­ear Elastic Frac­ture Mechanics for an in­fin­ite cracked plate (with stress-free crack lips) an ‘in­­verse’ mixed fund­amental problem. This superposition provides naturally ac­­­­­­­­ceptable stress and displacement fields, prohibiting overlapping of the lips (by means of contact stresses generated along the crack lips, which force the over­lapped lips back to naturally accepted position) and, also, non-negative mode-I Stress Intensity Factors. The solu­tions of this first paper form the basis for the next two papers of the series, dealing with the respective prob­lems in fi­­n­ite do­­mains (recall, for example, the cracked Brazil­ian disc con­fig­u­ra­tion) weak­­ened by artificial notches (rather than ‘math­e­mat­ical’ cracks), by far more interesting for practical engineer­ing ap­pli­­ca­tions

The critical influence of some “tiny” geometrical details on the stress field in a Brazilian Disc with a central notch of finite width and length

The role of some geometrical characteristics of the notches ma­chined in circular discs, in order to determine the mode-I fracture tough­ness of brittle materials, is discussed. The study is implemented both analyti­cally and numerically. For the analytic study advantage is taken of a recently intro­duced solution for the stress- and displacement-fields developed in a finite disc with a central notch of finite width and length and rounded corners. The vari­ation of the stresses along strategic loci and the deformation of the peri­me­ter of the notch obtained analytically are used for the calibration/validation of a flexible nu­mer­ical model, which is then used for a parametric investiga­tion of the role of geometrical features of the notched disc (thickness of the disc, length and width of the notch, radius of the rounded corners of the notch). It is con­cluded that the role of the width of the notch is of critical im­port­ance. Both the ana­lytic and the numerical studies indicate definitely that ignoring the ac­curate geo­metric shape of the notch leads to erroneous results concerning the actual stress field around the crown of the notch. Therefore, it is possible that misleading values of the fracture toughness of the material of the disc may be obtained