Cavitation during deformation of semicrystalline polymers

Cavitation phenomenon is observed during deformation in many semicrystalline polymers above their glass transition temperature. Numerous voids (cavities) both nanometer and micrometer size are formed inside amorphous phase between lamellae during deformation of a polymer. The cavitation is observed only in tension, never during compression or shearing. Most often used methods of voids detection are: microscopies (SEM, TEM, AFM and light microscopy), small angle X-ray scattering and measurements of density. Usually the voids are detected close to yielding or at yielding, strongly suggesting that yielding is often caused by cavitation. However, there is a competition between two processes: breaking of amorphous phase leading to cavitation and plastic deformation of lamellar crystals. Which process occurs first depends on the relation between compliances of those two phases. If the crystals are weak and defected their deformation occurs (mostly by chain slips mechanism) without cavitation. If the crystals in a polymer are thick and more perfect then the barrier for their deformation, represented by shear yielding stress, is increased and the cavitation sets in first and yielding is determined by the stress needed for cavitation. Further deformation involves deformation of crystals due to rapid local change of stress around voids. The influence of different morphological factors: crystal thickness, crystallinity degree, arrangement of crystalline elements (e.g. in spherulites), morphology of amorphous phase (free volume, entanglements, tie molecules) were analyzed. Experimental factors, such as temperature of deformation and rate of deformation influence remarkably the formation of cavities. Cavitation is generated at points where a high local triaxial state of stress is developed. Triaxiality of stress can be amplified by a notch, even very mild notch with large radius of curvature stimulates generation of cavities. Evolution of nano-cavities into micro-cavities and change of their shapes with increasing deformation were evidenced by SAXS. Initially voids are oriented perpendicularly to deformation direction, however, with increasing elongation they become oriented along deformation direction. Stress whitening is visual sign of cavitation and is caused be light scattering either by microvoids or by assemblies of nanovoids.     

Effects of porosity,dry unit weight,cement content and void/cement ratio on unconfined compressive strength of roof tile waste-silty soil mixtures

One of the conventional ways to improve the mechanical behavior of soils is to mix them with cementing agents such as cement, lime and fly ash. Recently, introduction to alternative materials or sub-products that can be adopted to improve the soil strength is of paramount importance. Therefore, the present study aims to investigate the effects of porosity (η), dry unit weight (γ_d) of molding, cement content (C) and porosity/volumetric cement content ratio (η/C_iv) or void/cement ratio on the unconfined compressive strength (q_u or UCS) of silty soil–roof tile waste (RT) mixtures. Soil samples are molded into four different dry unit weights (i.e. 13 kN/m³, 13.67 kN/m³, 14.33 kN/m³ and 15 kN/m³) using 3%, 6% and 9% cement and 5%, 15% and 30% RT. The results show that with the addition of cement, the strength of the RT–soil mixtures increases in a linear manner. On the other hand, the addition of RT decreases q_u of the samples at a constant percentage of cement, and the decrease in porosity can increase q_u. A dosage equation is derived from the experimental data using the porosity/volumetric cement content ratio (η/C_iv) where the control variables are the moisture content, crushed tile content, cement content and porosity.     

A new mathematical model for integrating all incidence matrices in multi-dimensional cellular manufacturing system

In the past several years, many studies have been carried out on cellular manufacturing based on a two-dimensional machine–part incidence matrix. Since workers have important role in doing jobs on machines, assignment of workers to cells becomes a crucial factor for fully utilization of cellular manufacturing systems. In this paper, an attempt is made to solve cell formation problem and minimize the number of voids and exceptional elements in a three dimensional (cubic) machine–part–worker incidence matrix. The proposed mathematical model captures the capability of workers in doing different jobs. To demonstrate the effectiveness of the proposed model, the solution of some test problems is compared with the literature method.     

Ground penetrating radar response from voids: A demonstration using a simple model

A simple Ricker wavelet model can be used to illustrate some fundamental properties of the ground penetrating radar (GPR) response from both air- and water-filled voids. Reflections from the top and the bottom of a void overlap significantly, and generate one common characteristic of a void response: a “bright spot”, analogous to the “bright spot” observed in seismic exploration. For time delays equal to about half the pulsewidth, the reflected wavelets superimpose to yield maximum reflection amplitudes. The top reflected wavelet becomes completely separated from the bottom reflected wavelet when the void time delay exceeds twice the wavelet pulsewidth. The two wavelet reflections can be individually identified at earlier time delays, approximately equal to the wavelet pulsewidth. This is still substantial and explains why it is difficult to use GPR to infer void thicknesses, especially for air-filled voids.     

Biodegradable polyurethane composite scaffolds containing Bioglass® for bone tissue engineering

Five types of solid and porous polyurethane composites containing 5–20 wt.% of Bioglass® inclusions were synthesized. Porous structures were fabricated by polymer coagulation combined with the salt-particle leaching method. In-vitro bioactivity tests in simulated body fluid (SBF) were carried out and the marker of bioactivity, e.g. formation of surface hydroxyapatite or calcium phosphate layers upon immersion in SBF, was investigated. The chemical and physical properties of the solid and porous composites before and after immersion in SBF were evaluated using different techniques: Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). Moreover the surface structure and microstructure of the composites was characterised by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), respectively. Mercury intrusion porosimetry, SEM and microtomography (μCT) were used to determine pore size distribution and porosity. The fabricated foams exhibited porosity >70% with open pores of 100–400 μm in size and pore walls containing numerous micropores of <10 μm. This pore structure satisfies the requirements for bone tissue engineering applications. The effects of Bioglass® addition on microstructure, mechanical properties and bioactivity of polyurethane scaffolds were evaluated. It was found that composite foams showed a higher storage modulus than neat polyurethane foams. The high bioactivity of composite scaffolds was confirmed by the rapid formation of hydroxyapatite on the foam surfaces upon immersion in SBF.     

Archipelago of the Negev Desert A Temporal/Collective Plan for Beer Sheva,Israel

Known locally as the ‘non-city’, Beer Sheva in southern Israel is made up of segregated communities with no central core. Rafi Segal proposes a way of creating connectivity while accepting the city's lack of centre and optimising on its beautiful desert landscape and Bedouin inheritance. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of the Strain Rate and Microstructure on Damage Growth in Aluminum

Materials used in soldier protective structures, such as armor, vehicles and civil infrastructures, are being improved for performance in extreme dynamic environments. Nanocrystalline metals show significant promise in the design of these structures with superior strengths attributed to the dislocation-based and grain-boundary-based processes as compared to their polycrystalline counterparts. An optimization of these materials, however, requires a fundamental understanding of damage evolution at the atomic level. Accordingly, atomistic molecular dynamics simulations are performed using an embedded-atom method (EAM) potential on three nano-crystalline aluminum atom systems, one a Voronoi-based nano-crystalline system with an average grain size of 10 nm, and the other two single crystals. These simulations are performed under the condition of uniaxial expansion at several strain rates ranging from 10⁶s^-1 to 10¹⁰s^-1. Results for the effective stress are discussed with the aim of establishing the role of the strain rate and microstructure on the evolution of the plastic strain and void volume fraction and the eventual loss of stress carrying capability of the atom systems.     

Effect of geometry on void formation in commercial electroplating of thin strips to copper

Three plating trials comprised of six samples were performed to evaluate the ability to attach thin strips of varying cross section to a copper substrate via commercial nickel electroplating in a nickel sulfamate bath. Nickel plated to the top, bottom, and sides of the nickel strips, as well as to the substrate. A significant void formed beneath rectangular nickel strips for all geometries studied, including strip widths of 500 µm to 5000 µm and gap thicknesses from 100 µm to 1500 µm. This is due to the starvation of ions when two regions of growing grains impinge and entrap a volume of electrolyte, surrounding it completely by deposited nickel. Impingement often occurs just past the edge of the strip which causes the void width to be greater than the strip width and form seams at the edge of the void. The rate of plating is greatest in areas where sharp corners exist, due to higher current density. More plating reaches under the strip as the width decreases and/or gap thickness increases. Thus, the void size decreases with decreasing aspect ratio, defined as the strip width over gap thickness. The use of a cylindrical strip produces sound plating with no voids. This is because the lack of corners allows impingement to occur first beneath the center of the circular cross section, so liquid is never surrounded. The results of this study demonstrate how to minimize or avoid void formation in commercial electroplating of suspended strips which is of great importance in the installation of sensor strips in the coating layer of continuous casting molds.     

孔洞对双晶TiAl合金断裂行为影响的声发射响应

本文基于分子动力学方法,对含孔洞的双晶TiAl合金试样进行了单轴拉伸模拟,在纳米尺度下研究了材料变形和断裂过程中的缺陷演化行为及其声发射响应。研究发现：孔洞大小和位置对材料的弹性模量影响较小,屈服强度随孔洞尺寸的增大而降低;进入塑性变形后,孪晶界对孔洞边缘连续发射的位错有阻碍作用,使晶体强度增加;达到屈服应力时,含晶界孔洞的试样更容易产生稳定的位错结构,阻碍其他位错运动,从而提高了晶体强度;通过对拉伸过程中的声发射信号进行分析,发现声发射信号主要来源于晶格振动,并且具有较大的功率值范围和较低的中值频率;位错滑移的声发射信号表现出宽频域的特点,位错增殖和位错塞积的声发射信号表现出低功率的特点;裂纹扩展的声发射信号属于突发型信号,表现为高频率、高功率的特征。     

Phase transformation and oxidation behavior of Pt-modified MCrAlY coatings

The effects of platinum (Pt) layer on phase transformation and oxidation behavior of MCrAlY (M = Co, Ni) coatings were investigated by isothermal and cyclic oxidation tests at temperature of 1100 °C. Pt layer of about 5 μm thickness was deposited on MAR M247-DS superalloy by an electroplating method, and then the MCrAlY layers were coated with air plasma spray (APS) and vacuum plasma spray (VPS) methods. APS process creates internal oxides and interface voids, whereas the VPS process shows fewer voids in the interface between the MCrAlY layer and the superalloy used as substrate. The oxides and voids formed in the APS process deteriorate the oxidation resistance, indicating that the oxides and voids are transferred to Al₂O₃ compounds during the oxidation tests. The Pt layer shows a strong affinity with Ni element and a weak affinity with Co and Cr elements, resulting in the formation of β-(Ni,Pt)Al phase. The Pt pre-coated layer forces down for the diffusion of soluble elements to get the reduced deplete zone in the superalloy. The VPS process enhances the oxidation resistance of the MCrAlY coating and shows better bonding strength, owing to the generation of homogenous microstructure in the coating and less processing defects.