bluuebunny/crossref_metadata_2025_split huggingface dataset:
Refractive surgery in anisometropic adult patients induce plastic changes in primary visual cortex
Authors: Vuori Elisa , Vanni Simo , Henriksson Linda , Tervo Timo M. T. , Holopainen Juha M. 
Published: April 2011

Abstract.Purpose:  To prospectively study the effect of refractive surgery in the primary visual cortex of adult anisometropic and isometropic myopic patients.Methods:  Two anisometropic and two isometropic myopic patients were examined with multifocal functional magnetic resonance imaging technique (mffMRI) before refractive surgery and at 3, 6, 9 and 12 months postoperatively. Two controls without refractive surgery were also examined with mffMRI in the beginning and in the end of the study. Anisometropic patients had only their more myopic eye operated to correct the anisometropia. The myopic isometropic patients had their both eyes operated.Results:  Operated anisometropic eyes showed 65% reduced amount of active voxels in foveal data at 12 months postoperatively compared with the preoperative situation. In unoperated anisometropic eyes, the corresponding value was 86% and in myopic patients and controls 31% and 1%, respectively. To confirm this finding, the number of activated voxels representing the innermost ring of the stimulus was also calculated, and an exactly similar phenomenon was encountered in the anisometropic patients. Both anisometropic patients improved the best‐spectacle‐corrected visual acuity in the operated eye after refractive surgery.Conclusion:  Our results suggest that plastic changes may take place in the primary visual cortex of anisometropic adult patients after refractive surgery.
ON COMPUTING GEODESICS IN BAUMSLAG–SOLITAR GROUPS
Authors: DIEKERT VOLKER , LAUN JÜRN 
Published: April 2011

We introduce the peak normal form for elements of the Baumslag–Solitar groups BS(p,q). This normal form is very close to the length-lexicographical normal form, but more symmetric. Both normal forms are geodesic. This means the normal form of an element u-1v yields the shortest path between u and v in the Cayley graph. For horocyclic elements the peak normal form and the length-lexicographical normal form coincide. The main result of this paper is that we can compute the peak normal form in polynomial time if p divides q. As consequence we can compute geodesic lengths in this case. In particular, this gives a partial answer to Question 1 in [4] For arbitrary p and q it is possible to compute the peak normal form (length-lexicographical normal form resp.) also the for elements in the horocyclic subgroup and, more generally, for elements which we call hills. This approach leads to a linear time reduction of the problem of computing geodesics to the problem of computing geodesics for Britton-reduced words where the t-sequence starts with t-1 and ends with t.
Automatic memory partitioning and scheduling for throughput and power optimization
Authors: Cong Jason , Jiang Wei , Liu Bin , Zou Yi 
Published: April 2011

Memory bottleneck has become a limiting factor in satisfying the explosive demands on performance and cost in modern embedded system design. Selected computation kernels for acceleration are usually captured by nest loops, which are optimized by state-of-the-art techniques like loop tiling and loop pipelining. However, memory bandwidth bottlenecks prevent designs from reaching optimal throughput with respect to available parallelism. In this paper we present an automatic memory partitioning technique which can efficiently improve throughput and reduce energy consumption of pipelined loop kernels for given throughput constraints and platform requirements. Also, our proposed algorithm can handle general array access beyond affine array references.
Our partition scheme consists of two steps. The first step considers cycle accurate scheduling information to meet the hard constraints on memory bandwidth requirements specifically for synchronized hardware designs. An ILP formulation is proposed to solve the memory partitioning and scheduling problem optimally for small designs, followed by a heuristic algorithm which is more scalable and equally effective for solving large scale problems. Experimental results show an average 6× throughput improvement on a set of real-world designs with moderate area increase (about 45% on average), given that less resource sharing opportunities exist with higher throughput in optimized designs. The second step further partitions the memory banks for reducing the dynamic power consumption of the final design. In contrast to previous approaches, our technique can statically compute memory access frequencies in polynomial time with little or no profiling. Experimental results show about 30% power reduction on the same set of benchmarks.
Hydrostatic Pressure Effect on the Fracture Toughness of Unidirectional (0-deg) Graphite/Epoxy Laminated Composites
Authors: Rhee Kyong Y. 
Published: April 2011

Previous studies show that hydrostatic pressure affects elastic modulus, strength, and strain of fiber-reinforced composites. However, no work has been done to understand the compressive fracture behavior of laminated composites under hydrostatic pressure. This work is an experimental study of the hydrostatic pressure effects on the compressive fracture behavior of fiber-reinforced composites. Compressive fracture tests were conducted under various hydrostatic pressures from 0.1 MN/m2 (atmospheric pressure) to 200 MN/mi2 (2 Kbar). Test specimens used were unidirectional (0-deg) graphite/epoxy laminated composites with a single delamination at the center of specimen. First, compression load-displacement curves were determined as a function of hydrostatic pressure and delamination length. Second, the compressive fracture toughness was determined from the compliance method based on the load-displacement curves as a function of hydrostatic pressure. The results showed that compliance, fracture load, and fracture toughness were significantly influenced by hydrostatic pressure. The fracture load increases as hydrostatic pressure increases. The compliance decreases linearly with increasing pressure. Particularly, fracture toughness increased from 3.45 N-mm/mm2 to 4.53 N-mm/mm2 (32% increase) as hydrostatic pressure increased from 0.1 MN/mm2 to 200 MN/mm2. Apparently the increase of fracture toughness is due to the suppression of local delamination with increasing hydrostatic pressure.
Design and Manufacture of an Aerostatic Spindle Bearing System with Carbon Fiber-Epoxy Composites
Authors: Lee Dai Gil , Choi Jin Kyung 
Published: April 2011

Recently, aerostatic bearing systems are widely used in various fields in which high precision and high speed are required because of their advantages such as low friction and low heat generation. However, the aerostatic bearing systems have low radial and axial stiffnesses and poor damping properties due to low viscosity of air, and consequently have a low starting value of whirl vibration if heavy metallic spindles are employed. As the carbon fiber-epoxy composite material has excellent properties for structures, owing to its high specific modulus, high damping and low thermal expansion, the vibrational and thermal characteristics as well as the starting value of the whirl of spindles will be improved if the proper design and manufacturing methods for the composite spindle are developed. In this paper, a high speed aerostatic spindle was designed and manufactured using high modulus carbon fiber-epoxy composite material for the first time. The design method of the composite spindle and joining method of the composite spindle to the steel sleeves for aerostatic bearing mounting were developed. Using the developed aerostatic spindle bearing system, its static and dynamic characteristics were compared with those of a comparable stainless steel aerostatic spindle.
Matrix Cracking Effect on Delamination Growth in Composite Laminates Induced by a Spherical Indenter
Authors: Liu Sheng , Chang Fu-Kuo 
Published: April 2011

An investigation was performed to study the mechanics of delamination growth in fiber-reinforced laminated composites resulting from transverse concentrated loads. The effect of matrix cracking on delamination propagation was the primary focus of the study. For simplicity's sake, only cross-ply laminates were investigated. An analytical model was proposed for determining the response of the laminates containing a matrix crack-induced delamination and subjected to a concentrated load through a spherical indenter. Two types of matrix cracks were considered in the model: a surfacebending crack and an internal shear crack. The model consists of three parts: a stress analysis, a contact analysis, and a failure analysis. A nonlinear three-dimensional finite element model based on an updated Lagrange formulation was developed for calculating stresses and strains in composites. An augmented Lagrangian method was utilized to simulate the interfacial contact condition of the embedded delamination and the indentation between the composite and the indenter. Fracture mechanics was applied to predict the delamination growth in three dimensions. In order to verify the model, results of the calculations from the analysis were compared with the existing solutions and test data available in the literature. Experiments were also conducted to further substantiate the model and the predictions from the analysis. Overall, the predictions agreed well with the existing solutions and the test data.
Ultrasonic Detection of Damage in CFRPs
Authors: Kaczmarek H. 
Published: April 2011

A good knowledge on damage processes in CFRPs can be given by developing non-destructive testing. In order to improve ultrasonic capabilities to detect matrix cracking development, a water tank has been mounted directly on a servo-controlled machine and an automatic scanning by points has been investigated. More accurate testing procedures have been implemented including mechanical improvements, amplitude measurement accuracy, resetting points under loading, comparative evaluation of the attenuation between undamaged and damaged state. A comparison between X-ray and ultrasonic results has been made. Another aim was to control hidden damage due to impact and especially to measure the size and shape of delaminations at each interply location using an improved pulse echo method. Hence, conventional equipment has been modified in order to explore the material by thickness segments. A new procedure has been added to a classical equipment providing in a single scan multiple C-scan images of a coupon with a very limited amount of data. The main purpose of this paper is to describe the methods used and to discuss the original procedures that have been performed. The new capabilities of an improved ultrasonic set-up are going to be examined mainly in relation to crack-density measurement, matrix cracking development under tension and ply by ply delamination sizing and location. With these features, an ultrasonic set-up can find its application in many aeronautical fields: improving composites, moisture influence on damage, detecting of damage resulting from impact, structures lifetime.
Predicting Scaling Effect on the Notched Strength of Prepreg and Fiber Tow-Placed Laminated Composites
Authors: Shahid Iqbal , Sun Hsien-Tang , Chang Fu-Kuo 
Published: April 2011

The effect of scaling on the notched strength and response of composite plates was investigated. Unidirectional prepreg and tow-placed laminated composites, both of which were made of the same fibers and resin system, were considered. In order to predict the scaling effect, the local damage accumulative inside the notched composites was considered. A finite element analysis based on an accumulative damage model previously proposed was developed for analyzing the response and tensile failure of composite laminates containing a slit or a circular hole of various sizes. The model consists of a constitutive equation which relates the damaged material to an accumulated damage state; and damage accumulation criteria which predict the state of damage as a function of the applied stresses. The free-edge induced damage was not considered in the model. The effect of scaling on the notched strength of laminated composites was evaluated by comparing the results of the calculations from the analysis with the test data generated at Stanford University and the Boeing Company. The predictions agreed with the test data very well for the notch sizes ranging from as small as 0.25 inch up to 2.5 inches. It was found that the larger the notch size, the more accumulated damage was predicted. Hence, the scaling effect on the notched composite strength is strongly related to the extent of damage accumulated inside the composites. The model also shows that the fiber interaction zone affects significantly the residual strength of notched composites. The higher fiber interaction zone tends to reduce fiber tensile strength, but to increase tensile notched strength. With the same material system, although the fiber tensile strength of unidirectional tow-placed composites was lower than prepreg composites, tow-placed composites exhibited higher notched strength than did the prepreg composites.
The Continuous Cuing Process for Thermoset Polymer Composites. Part 1: Modeling and Demonstration
Authors: Kim Cheol , Teng Hong , Tucker Charles L. , White Scott R. 
Published: April 2011

This paper presents a new manufacturing method, continuous curing, that is specifically engineered for extremely thick parts made from thermosetting polymer matrix compounds. In this process, matrix curing takes place continuously as the precursor material is being laid down. The energy for curing is obtained from the exothermic curing reaction through controlled heat release; once started, the process is self-sustaining. Thermal spiking, an unavoidable feature of autoclave curing, can be completely eliminated in continuous curing by manipulating the process parameters. In addition, manufacturing times are reduced by an order of magnitude or more. A simple analytical model is presented first to show the inherent features of continuous curing. Next, a detailed thermo-chemical process model is developed and numerical results are obtained for a glass/polyester composite. The process is demonstrated by curing a 100 mm thick graphite/epoxy lamina. Finally, thermal spiking behavior and manufacturing times are compared for conventional autoclave curing and continuous curing.
Effects of Hydrostatic Pressure on the Compressive Properties of Laminated, 0° Unidirectional, Graphite Fiber/Epoxy Matrix Thick-Composite
Authors: Rhee K. Y. , Pae K. D. 
Published: April 2011

Compression tests were performed on laminated, 0° continuous, unidirectional, graphite fiber/epoxy matrix thick-composites at various superposed hydrostatic pressures to 4 kbar (400 MPa) at room temperature. Cylindrical samples were made by laminating continuous filament prepreg tapes (3M, SP-319) with fibers oriented along the direction of the cylinder axis. The samples were cured at 135°C for 4 hours in a platen press and post-cured at 135°C for 4 hours in a vacuum oven. The samples contained 60% fiber by volume. The tests were conducted in a high pressure apparatus which was capable of containing pressures to 7 kbar (700 MPa) and of operating at temperatures between −100 and 300°C. The test results show that the compressive stress-strain curves undergo significant changes with increasing pressure. The stress-strain curves show perfectly linear elastic behavior at lower pressure range but exhibit curvature toward the end of the curves at higher pressure range. The compressive modulus obtained from the stress-strain curves increases bilinearly with increasing pressure with a discontinuity located at ∼2 kbar. This continuity has been associated with the low temperature secondary glass transition which is being shifted to room temperature from sub-zero temperature by the applied pressure. The modulus increases from 23.84 GPa at atmospheric pressure to 26.60 GPa at 4 kbar. The increase of the modulus at high pressure is due to finite compressive deformation of matrix by high pressure. Both fracture strength and strain increase linearly with pressure. The fracture strength increases from 376 MPa at atmospheric pressure to 607 MPa at 4 kbar. One of the important findings of our experimental study is that the compressive properties of the thick-composite are far less than the tensile properties of prepreg tapes. It was apparent that fracture of samples occurred by end-crushing due to a combination of fiber buckling, delamination of plies in the transverse direction, and kinking at all pressure levels. High pressure makes it more difficult for the fracture of the samples to be initiated; thereby the fracture strength increases with increasing pressure.
Thermal Model for Three-Dimensional Irregular Shaped Pultruded Fiberglass Composites
Authors: Chachad Y. R. , Roux J. A. , Vaughan J. G. , Arafat E. S. 
Published: April 2011

Computer simulation of the manufacturing of pultruded composite materials has been limited strictly to one or two-dimensional modeling of simple flat or circular shaped composites. This research presents the modeling of unsteady-state temperature and degree of cure distributions for the manufacturing of fiberglass-epoxy composite materials with irregular cartesian geometries in three-dimensions. The model is capable of predicting temperature and degree of cure distributions for composites with cartesian shapes in three dimensions, and temperature profiles in pultrusion dies without the aid of predetermined temperature values used as die wall boundary conditions. One of the benefits of this model is in designing the heating section of pultruder machines. Using a differential scanning calorimeter (DSC) the chemical kinetic parameters for Shell EPON 9420 epoxy resin were obtained. A finite difference control volume technique was utilized in the development of the numerical model for solving the governing energy and species equation used in modeling the entire heating section of the pultruder. The combinations of pull speed, fiber volume, and die temperature profiles can be modeled very economically in manufacturing composites for very specific needs. Since this research is not limited in terms of predetermined temperature values, it can be easily tailored to predict a multitude of temperature profiles suited for a pultrusion process. This research is also important because it provides realistic modeling of irregular cross-sectional geometries.
Determination of Stress Gradients in Fibrous Composites by X-Ray Diffraction from Fillers
Authors: Prinz B. , Schnack E. 
Published: April 2011

The method of X-ray diffraction from crystalline filler particles has been applied to fibrous composites. The stress/strain transfer behaviour between the particles and the surrounding matrix is described by the X-ray elasticity constants (XEC). These constants have been determined by experiment as well as by means of theoretical models. Above all for the first time, complete quantitative XEC are given for CdO fillers which have turned out to be the most suitable filler material. The exact knowledge of the XEC of particle/laminate systems formed the foundation to apply the method to analyse interlaminar stress concentrations. For the first time, stress gradient measurements even on notched laminates have been successfully carried out. Free edge stress distributions obtained by experiments are in good agreement with results from finite element simulations.
Tensile Strength Testing of GRP Pipes at Elevated Temperatures in Aggressive Offshore Environments
Authors: Hale J. M. , Gibson A. G. , Speake S. D. 
Published: April 2011

A programme of tests has been conducted to characterize the strength reduction of two GRP composite materials in the form of filament wound pipes as a function of temperature and testing environment. This complements previous work on the same materials using coupon specimens [1]. The GRP materials and testing environments were selected to be of maximum relevance to the offshore industry. The materials were E-glass fibre reinforcement in matrices of two epoxy resin systems. The testing environments were sea water and sea water with dissolved CO2 and H2S gasses. The temperature range tested was ambient to 150°C. The tests were performed on specimens of commercial filament wound pipe loaded in pure axial tension. A special purpose testing facility was developed for this work, and this is described briefly. Results are presented for the strength of both materials as a function of temperature when tested dry and a quantitative discrepancy noted with the previous results obtained using coupon specimens. Results of further tests conducted to investigate this phenomenon are presented and a suggested explanation is proposed. Further results are presented for the strengths of the materials in the test environments which are shown to be in qualitative agreement with the previous coupon tests. It is shown that the axial strength of GRP pipes is significantly reduced at elevated temperatures, that the degree of this reduction is dependent on the matrix material and is magnified by sea water absorption and that dissolved CO2 and H2S have little effect.
Strain Rate Effects on the Transverse Compressive and Shear Behavior of Unidirectional Composites
Authors: Hsiao H. M. , Daniel I. M. , Cordes R. D. 
Published: April 2011

Methods for dynamic characterization of composite materials were extended and applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for dynamic characterization of composite materials in compression and shear at strain rates up to 1800 sO. Strain rates below 10 s5l were generated using a servohydraulic testing machine. Strain rates between 10 sol and 300 sol were generated using the drop tower apparatus. Strain rates above 500 s-l were generated using the Split Hopkinson Pressure Bar. Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the transverse direction were characterized. Off-axis (15°, 300, 450 and 600) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from 10-4 s-1 (quasi-static) up to 1800 s-1, were recorded. The 90-degree properties, which are governed by the matrix,show an increase in modulus and strength over the static values but no significant change in ultimate strain. The stress-strain curve stiffens as the strain rate increases. This stiffening behavior is very significant in the nonlinear region for strain rates between 10-4 s-I and 1 s51. For strain rates above 1 s-1, the stress-strain behavior continues this stiffening trend until it is almost linear at a strain rate of 1800 s-1. The shear stress-strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly as the strain rate increases.