4,347 research outputs found
Fracture energy of graphite from microstructure-informed lattice model
Full text linkGraphite remains a key structural material in the nuclear industry, the integrity assessment of which in demanding reactor environments is critical for safe operation of plant. Fracture of graphite is preceded by growth and coalescence of distributed micro-cracks within a process zone, classifying it as a quasi-brittle material alongside cement-based and ceramic materials. The evolution of micro-crack population to failure is well represented by discrete lattice models, e.g. (Wang and Mora 2008). Here, a recently developed 3D lattice (Jivkov and Yates 2012), with elastic spring elements and brittle-damage behaviour is used to generate microstructure representative models of two graphite grades at a representative meso length scale. Micro-cracks are represented by spring failures and the macroscopic damage results from their collective behaviour. Presented results capture a transition from graceful, plastic-like failure at lower porosities, with energy dissipation via micro-cracking, to glass-like behaviour with negligible energy dissipation at higher porosities. The results are in good agreement with experimental data. Thus, the proposed methodology can calculate fracture energy from the stress-strain curve, or formulate cohesive and damage evolution laws for continuum models, based exclusively on microstructural features
The Treatment of Ties in AP Correlation
No full textThe Kendall tau and AP correlation coefficients are very commonly use to compare two rankings over the same set of items. Even though Kendall tau was originally defined assuming that there are no ties in the rankings, two alternative versions were soon developed to account for ties in two different scenarios: measure the accuracy of an observer with respect to a true and objective ranking, and measure the agreement between two observers in the absence of a true ranking. These two variants prove useful in cases where ties are possible in either ranking, and may indeed result in very different scores. AP correlation was devised to incorporate a top-heaviness component into Kendall tau, penalizing more heavily if differences occur between items at the top of the rankings, making it a very compelling coefficient in Information Retrieval settings. However, the treatment of ties in AP correlation remains an open problem. In this paper we fill this gap, providing closed analytical formulations of AP correlation under the two scenarios of ties contemplated in Kendall tau. In addition,we developed an R package that implements these coefficients.Best Short Paper Accepted author manuscriptMultimedia ComputingWeb Information System
Delamination Analysis of A Class of AP-PLY Composite Laminates
No full textA recently developed fiber placement architecture, AP-PLY, has been shown to give significantly improved damage tolerance characteristics of composite structures. The behavior of delaminations resulting from low speed impact damage is of particular concern. Major attention has been paid to expand current knowledge on the delamination response of simple AP-PLY composite structure and move towards in-depth understanding of the failure mechanisms behind the damage tolerance. This thesis presents the approaches to predict delamination onset and analyze delamination growth, in support of the search of the optimum woven pattern for AP-PLY composite laminates. The recovered interlaminar stress between layers combined with the maximum stress criterion determined the delamination onset of simple AP-PLY composite laminate under out-of-plane loads. 2D finite element models with cohesive elements inserted in the interfaces of woven layers have been built to evaluate the delamination initiation and propagation in the different woven patterns of simple AP-PLY composite beams. The parameters of the woven pattern, such as the woven angle, the number of woven plies, the number of straight filled plies, and the location of the woven patterns in through the thickness direction, were investigated and shown to have a significant effect on delamination creation and growth. An energy method based on beam theory was proposed to analyze the strain energy release rate (SERR) of an existing crack in an AP-PLY beam structure. The developed analytical method was implemented in isotropic materials and the obtained SERR of a crack was validated by reference results and finite element solutions. The general behavior of crack growth on the left or right crack tip was evaluated and basic trends leading to crack propagation on one side of the crack were established. A correction factor was introduced to improve the accuracy of the SERR of a small crack through the numerical calculation. The singularity of crack tip caused by dissimilar materials was investigated and was found that the inclusion of the singularity effect could increase the accuracy for small cracks. It has been shown that the neutral axis needs to be relocated to decouple the bending and membrane behavior of unsymmetrical composite laminates, thus to meet the requirement of minimizing the strain energy of the delaminated beam to calculate the SERR of a delaminated composite beam. The calculated SERR of a crack in a composite beam has been verified by comparing with a finite element model. The woven plies in AP-PLY composite laminate altered the layup and two conventional laminates with different stacking sequences were identified in an AP-PLY composite laminate based on the assumption that the resin areas were ignored. A step by step approach was developed to obtain the SERR of a crack that goes across different materials. The analytical SERR determined when two materials are used in sequence, sets the stage for optimization of AP-PLY composite laminates without taking account of the effect of the resin area. The procedure of optimization of simple AP-PLY pattern was proposed and industry may benefit for many applications. An equivalent stiffness approach was used to model regions containing resin pockets and straight or inclined composite layers. A series of three point bending tests was carried out where the failure process and loading capacity were evaluated. The methodology, procedure of optimization, philosophy outlined in this thesis might also be applied to the more complicated fully woven AP-PLY composite laminates. The work in this thesis contributes to the understanding of the behavior of AP-PLY composite laminates with delaminations
Perturbation model for composite materials applied to analysis of cracks
Full text linkThe behaviour of the stress intensity factor is investigated for long plane cracks with one tip interacting with a region of graded material properties. The material outside the region is considered to be homogeneous elastic. The analysis is based on assumed small differences in stiffness in the entire body. The linear extent of the body is assumed to be large compared with that of the graded region. The crack tip, including the graded region, is assumed embedded in a square-root singular stress field. The stress intensity factor for an arbitrarily shaped region is given by a singular integral. Solutions are presented for rectangular regions with elastic gradient parallel to the crack plane. The limiting case of infinite strip is solved analytically, leading to a very simple expression. Further, a fundamental case of material properties variation is considered, allowing the solution for an arbitrary variation to be represented by Fourier’s series expansion. An interesting feature of the solution is that the stress intensity factor remains finite and does neither vanish nor become unbounded as for the cases where modulus of elasticity posses jumps. A numerical study of the fundamental strip case, with finite variations of material properties, performed using the finite element method is communicated in brief. The analytical solution is compared with the numerical results and is shown to have a surprisingly large range of validity. If an error of 5% is tolerated, modulus of elasticity in the strip may drop by around 40% or increase with around 60%
Structure function analysis of blazars AP Librae and 3c279
No full textHighest Honors in AstronomyBlazars AP Librae and 3c279 are analyzed for microvariability using a technique known as structure function analysis. AP Librae was observed in April
of 2005 and 3c279 was observed in April of 2007. The data for AP Librae was
previously reduced by Andrew Collazzi and the author reduced the data for
3c279. Both sets of data were reduced using Robert Knop's data reduction
program.
The author ran structure function analysis on both sets of data. Structure function analysis is a statistical analysis run on data that is suppose to
nd timescales of variability, periodicity, and the noise type of data. Previous analysis of AP Librae confirmed mircrovariability, which also shows up in
the structure function of AP Librae. Blazar 3c279 was much quieter than AP
Librae and showed no microvariability durning any of the nights.College of Arts and ScienceDepartment of Physics and Astronom
Deformation-promoted nucleation of corrosion cracks: State, problems, and perspectives
Full text linkOne model for corrosion crack initiation and propagation in metals is reviewed. The model incorporates three physical processes operating at the metal-environment interface – material dissolution, passive film formation and surface straining. The dissolution triggers surface advance; the passivation restrains the access of the environment to bare metal; the deformation causes for passivity breakdown. An essential advantage of the model is that it allows for formulating the crack development as a mechanical moving boundary problem, without the need for a specific crack growth criterion. The problem is solved using a finite elements and a moving boundary tracking procedure. The simulations demonstrate how cracks form and grow in a single continuous course. Key results for plane cracks nucleating from surface pits in an elastic-plastic material body under low-cycle fatigue load are presented. The developed cracks morphology is found independent of the initial pit size. Plasticity is found to influence the curvature at the tip of the nucleated corrosion cracks, but not its width. Mathematical and finite element analyses of stationary cracks with appropriate geometry are involved to explain the behaviour predicted by the model. The most important evolution length parameter, the width of the corrosion crack, is found to depend on the size constraints of the tracking procedure. This limits the validity of the model to the crack nucleation stage. It is concluded that the model is deficient for determining all length scales of a developed crack observed in reality. Physical processes to be considered in an advanced model are proposed and discussed
Perturbation model for composite materials applied to analysis of cracks
No full textThe behaviour of the stress intensity factor is investigated for long plane cracks with one tip interacting with a region of graded material properties. The material outside the region is considered to be homogeneous elastic. The analysis is based on assumed small differences in stiffness in the entire body. The linear extent of the body is assumed to be large compared with that of the graded region. The crack tip, including the graded region, is assumed embedded in a square-root singular stress field. The stress intensity factor for an arbitrarily shaped region is given by a singular integral. Solutions are presented for rectangular regions with elastic gradient parallel to the crack plane. The limiting case of infinite strip is solved analytically, leading to a very simple expression. Further, a fundamental case of material properties variation is considered, allowing the solution for an arbitrary variation to be represented by Fourier’s series expansion. An interesting feature of the solution is that the stress intensity factor remains finite and does neither vanish nor become unbounded as for the cases where modulus of elasticity posses jumps. A numerical study of the fundamental strip case, with finite variations of material properties, performed using the finite element method is communicated in brief. The analytical solution is compared with the numerical results and is shown to have a surprisingly large range of validity. If an error of 5% is tolerated, modulus of elasticity in the strip may drop by around 40% or increase with around 60%
AP-based wireless intrusion detection systems
No full textThis thesis was scanned from the print manuscript for digital preservation and is copyright the author.
Researchers can access this thesis by asking their local university, institution or public library to
make a request on their behalf. Monash staff and postgraduate students can use the link in the References field
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