The principle of virtual displacements and the principle of virtual forces in the case of large deformations

Summary The principle of virtual displacements is known to be valid for an arbitrary choice of the stress and conjugate strain quantities. Contrarily the existence of the principle of virtual forces depends on this choice. In the present note the conditions for both principles to exist are established.     

On error estimates and adaptivity in elastoplastic solids: Applications to the numerical simulation of strain localization in classical and Cosserat continua

The a posteriori error estimates based on the post-processing approach are introduced for elastoplastic solids. The standard energy norm error estimate established for linear elliptic problems is generalized here to account for the presence of internal variables through the norm associated with the complementary free energy. This is known to represent a natural metric for the class of elastoplastic problems of evolution. In addition, the intrinsic dissipation functional is utilized as a basis for a complementary a posteriori error estimates. A posteriori error estimates and adaptive refinement techniques are applied to the finite element analysis of a strain localization problem. As a model problem, the constitutive equations describing a generalization of standard J₂-elastoplasticity within the Cosserat continuum are used to overcome serious limitations exhibited by classical continuum models in the post-instability region. The proposed a posteriori error estimates are appropriately modified to account for the Cosserat continuum model and linked with adaptive techniques in order to simulate strain localization problems. Superior behaviour of the Cosserat continuum model in comparison to the classical continuum model is demonstrated through the finite element simulation of the localization in a plane strain tensile test for an elastopiastic softening material, resulting in convergent solutions with an h-refinement and almost uniform error distribution in all considered error norms.     

The Gibson-Ashby model for additively manufactured metal lattice materials: Its theoretical basis,limitations and new insights from remedies

The Gibson-Ashby (G-A) model has been instrumental in the design of additively manufactured (AM-ed) metal lattice materials or mechanical metamaterials. The first part of this work reviews the proposition and formulation of the G-A model and emphasizes that the G-A model is only applicable to low-density lattice materials with strut length-to-diameter ratios greater than 5. The second part evaluates the applicability of the G-A model to AM-ed metal lattice materials and reveals the fundamental disconnections between them. The third part assesses the deformation mechanisms of AM-ed metal lattices in relation to their strut length-to-diameter ratios and identifies that AM-ed metal lattices deform by concurrent bending, stretching, and shear, rather than just stretching or bending considered by the G-A model. Consequently, mechanical property models coupling stretching, bending and shear deformation mechanisms are developed for various lattice materials, which show high congruence with experimental data. The last part discusses new insights obtained from these remedies into the design of strong and stiff metal lattices. In particular, we recommend that the use of inclined struts be avoided.     

Biomimetism and bioinspiration as tools for the design of innovative materials and systems

Materials found in nature combine many inspiring properties such as sophistication, miniaturization, hierarchical organizations, hybridation, resistance and adaptability. Elucidating the basic components and building principles selected by evolution to propose more reliable, efficient and environment-respecting materials requires a multidisciplinary approach.     

Quantum, classical, and multi-scale simulation of silica–water interaction: molecules, clusters, and extended systems

Over the past 6 years, we have engaged in a multi-faceted computational investigation of water–silica interactions at the fundamental physical and chemical level. This effort has necessitated development and implementation of simulation methods including high-accuracy quantum mechanical approaches, classical molecular dynamics, finite element techniques, and multi-scale modeling. We have found that water and silica can interact via either hydration or hydroxylation. Depending on physical conditions, the former process can be weak ( < 0.2 eV) or strong (near 1.0 eV). Compared to hydration, the latter process yields much larger energy gains (2–3 eV/water). Some hydroxylated silica systems can accept more water molecules and undergo further hydroxylation. We have also studied the role of external stress, effects of finite silica system size, different numbers of water molecules, and temperature dependences.     

Synthesis and wear of Al based,free standing functionally gradient materials: effects of different reinforcements

Abstract

In the present study, three aluminium based functionally gradient materials (FGMs), reinforced with different ceramic particulates (silicon carbide, aluminium oxide, and titanium carbide), were successfully synthesised using the innovative gradient slurry disintegration and deposition (GSDD) technique. The results for Al/SiC and Al/Al₂O₃ revealed, in common, an increase in the weight percentage of reinforcement along the direction of deposition, to result in an increase in porosity and microhardness. However, for Al/TiC, the reverse trend was observed, with porosity and microhardness decreasing with increasing distance from the base of the ingot. The porosity levels for Al/TiC were also found to be significantly lower than those ofthe other two FGMs. Thermomechanical analysis of the FGMs showed thatthe average coefficient of thermal expansion of the high reinforcement end was reduced, as compared to the high aluminium end. Sliding wear test results also revealed that the high reinforcement end was more wear resistant than the high aluminium end, except for the case of Al/Al₂O₃. An attempt is made to interrelate the effects of different types of particulates, with microstructural development, microhardness and wear rate results obtained in the present study.     

Theories of TL systems: failures, successes, conflicts, trends: insights into possible future materials and techniques

In this paper some of the many theoretical models dealing with characteristics of TL materials are discussed. Special attention is given to some of the models dealing with the effects of ionisation density, that is Modified Track Structure Theory (MTST) and Microdosimetric Track Structure Theory (MTT) for the calculation of Heavy Charged Particle relative TL efficiencies, as well as solid-state models based on conduction band/valence band theory. Failures, successes, conflicts and trends are highlighted as well as a peek into future avenues of research for dosimetric TL materials.     

Studies of the oxidation stability,mechanical characteristics of materials based on max phases of the Ti-Al-(C,N) systems,and of the possibility of their use as tool bonds and materials for polishing

Thermogravimetry and differential thermal analysis have been used to study the resistance to the air oxidation of high-density samples of Ti₃AlC₂, Ti₂AlC and Ti₂Al(C_1?x N _x ) solid solutions. It has been shown that the Ti₃AlC₂ samples are more stable than Ti₂AlC and Ti₂Al(C_1?x N _x ) solid solutions and as the nitrogen content of the solid solution increases to x = 0.75, the oxidation resistance decreases. The following characteristics have been exhibited by the material containing 89 wt % Ti₃AlC₂ (the rest being Al₂O₃ and TiC) having density 99% of theoretical: bending strength R _bm = 500 MPa, compressive strength R _cm = 700 MPa, fracture toughness K _Ic = 10.2 MPa·m^0.5, hardness HRA = 70 GPa, H _V = 4.6 GPa, Young modulus = 149.4 ± 28.7 GPa. After sintering with diamonds or cBN (50 wt %) at 5.5–7.7 GPa and 1350–1960°C for 0.07–1.0 h the Ti₃AlC₂ MAX phase decomposes to form TiC and TiAl or TiB₂ and a thin layer of Al₄C₃ forms at the interface with diamond. The Al₄C₃ decomposition in a composite material due to the interaction with the air moisture results in the crack initiation along the diamond perimeter, which brings about the material fracture in 1–2 weeks. It has been found that the Ti₃AlC₂ powder is efficient for polishing natural and synthetic jewelry crystals and competitive in polishing efficiency and quality with ACM 2/1 grade diamond.     

Discoloration mechanism,structures and recent applications of thermochromic materials via different methods: A review

Thermochromic material is a kind of smart material whose color will vary as the result of the phase transition caused by the temperature change. The characteristics of thermochromic materials are the memory functions to the temperature, having great potential applications in aerospace, military, anti-counterfeiting technology, construction and other fields. In recent years, many kinds of thermochromic materials have been prepared by different methods and their discoloration mechanisms are various according to published literatures. In this paper, the classification, discoloration mechanism, preparation methods, application fields and development trend of thermochromic materials are reviewed.     

Interface crack problem of functionally graded piezoelectric materials: effects of the position of electromechanical impact and gradient

This paper analyzes the transient response of the dynamic stress intensity factor for an interfacial crack of a functionally graded piezoelectric material (FGPM) coated on the surface of a homogeneous piezoelectric substrate. Different from previous analyses, this study mainly considers a realistic situation when electromechanical loadings are suddenly applied at the material surface. Obtained results are compared with those when the crack surfaces are directly loaded by the same impacts. By using the integral transform method, the problem is reduced to solving two singular integral equations. It is found that dynamic stress intensity factors are significantly amplified and reduced depending on the negative and positive gradient for electromechanical impacts at the material surface. Positive or negative electric impact also decreases or increases the overshoot of the dynamic stress intensity factor. It is suggested that designing an FGPM with a positive gradient index is safer than a negative gradient index.     

Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

Investigation of Zr-incorporated mesoporous titania materials via nonsurfactant templated sol-gel route: Synthesis, characterization and stability

The synthesis of Zr-incorporated mesoporous titania materials was achieved by a HCl-catalyzed nonsurfactant templated sol-gel process, followed by solvent extraction to remove the urea template. The materials were characterized by FT-IR, X-ray energy dispersive spectroscopy (XEDS), nitrogen adsorption-desorption measurement (BET), powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Zirconia has been successfully incorporated into the skeleton of titania and the observed Zr/Ti compositions are close to the designed Zr/Ti molar ratios in the feeds. The content of zirconia has significant effects on the mesoporosity and the pore parameters as well as the thermal and hydrothermal stabilities of the materials. Anatase crystal structures were formed in the materials with low Zr/Ti molar ratios. As the Zr/Ti ratio is increased, the content of anatase structures decreases and the material becomes essentially amorphous at the Zr/Ti ratio of 1/1. The materials have narrowly distributed average pore diameters in the range of 2.5–5.1 nm and disordered pore or channel structures. The incorporation of Zr increases the thermal stability but decreases the hydrothermal stability of the materials.     

Adoption and implementation of group technology classification and coding systems: insights from seven case studies

Results are reported from an empirical study of classification and coding (CC) system usage among manufacturers. The investigation, selection, justification, implementation and operation of CC systems by six user firms are presented in case study form. A case history of a former CC system user is also presented. User characteristics and experiences are compared and analysed across the seven cases. The paper concludes with managerial and research implications regarding CC system selection, justification, implementation and usage.     

Categorization and trend of materials science research from Science Citation Index (SCI) database: A case study of ceramics,metallurgy, and polymer subfields

This article offers information on the characteristics and number of materials research articles indexed in the Science Citation Index (SCI) database in the year of 2004. 22,843 articles in full-text forms from 169 journals from the materials field (which included ceramics, metallurgy, and polymer journals) were retrieved from the SCI database and exported to EndNote software. The retrieved articles were carefully analyzed by eight scientists and experts in those subfields and categorized using SPSS into eight different categories, being (1) New materials, (2) Materials characterizations, (3) Materials improvement, (4) New process and/or process improvement, (5) Mathematical and theoretical models and/or computer simulations, (6) Novel and comprehensive explanations, (7) Testing conditions, and (8) Comparative studies, whose definitions were clearly indicated. The results were then considered in terms of the percentage of the number of articles in each materials subfield, country of corresponding author, and number of authors. The overall results suggested that, most materials articles published in 2004 were focused on new process and process improvement (27%), while materials characterizations (23%) and testing conditions (12%) took the 2nd and 3rd places, especially for the ceramics and polymer articles. The highest numbers of articles in the ceramics and polymer subfields were focused on new processes and/or process improvement, and those for the metallurgy subfield were on materials characterization. In the SCI database, the largest number of materials articles was authored from Asian scientists although the majority of the materials journals were run by editors from Europe in North America/Canada continents. There was no coherent relationship between the authors’ and editors’ affiliations. China, Japan and the United States of America (USA) were shown to be the top three countries which had the highest publication numbers in the materials field. Japan had the highest publication numbers in the ceramics subfield while China possessed most publications in polymer and metallurgy subfields. However, when considering the journal impact factors, the leading positions of the countries changed. The results from this work could assist materials scientists to select suitable international journals in relevant association with the contents of their to-publish works. Finally, it was noted that most material research articles were written by 3–4 authorships.     

高温自生压力法制备基于天然产物的碳及其复合材料

高温自生压力法(RAPET)是一种新型、简单而又高效的无机材料制备方法。以含水天然产物如甘薯、香菜和野藤等为前驱物,经冷冻干燥其骨架结构得以维持,然后通过RAPET法在700℃碳化,制得了保持天然物原有微观骨架结构的碳材料;以液态油脂花生油为原料,经RAPET法直接制得微米碳球;粘胶纤维和脱脂棉经RAPET碳化后能维持其原有纤维形貌;用钛酸丁酯(TBOT)和正硅酸四乙酯(TEOS)浸渍的脱脂棉和粘胶纤维,经RAPET法一步合成了C/TiO2和C/SiO2复合材料。利用扫描电子显微镜(SEM)观察了产物的形貌,以比表面积及孔径分布测试仪对产物的比表面积和孔结构进行了表征,通过X射线衍射(XRD)研究了所得C/TiO2和C/SiO2复合材料的晶相特征。结果显示,所得产物比表面积从4m2/g到405m2/g不等,C/TiO2中TiO2晶相为锐钛矿型。     

Magnetic separation of the second kind: Magnetogravimetric, magnetohydrostatic, and magnetohydrodynamic separations

Magnetogravimetric, magnetohydrostatic, and magnetohydrodynamic separation techniques can be classified as magnetic separations of the second kind. Magnetic separation of the first kind (ordinary magnetic separation) relies on the inherent magnetic susceptibility of the material to be separated. When the medium of separation rather than the separated particles is made magnetizable, a new system of gravity separations can result (magnetic separation of the second kind). In magnetogravimetry, a colloidal solution of a ferro- or ferrimagnetic substance (magnetic fluid) acts as the separation medium. Magnetohydrostatic separations are conducted in an aqueous solution (or melt) of a strongly paramagnetic salt. Magnetohydrodynamics applies the Faraday effect (mutual orthogonality of the force thrust, electric, and magnetic fields) on suspended conducting minerals in an electrolytic solution placed in crossed electric and magnetic fields. The first technique was pioneered mainly in the United States, while the last two techniques were pioneered by Bunin and Andres in the Soviet Union and introduced to the West by Andres. The principles underlying the three separation techniques will be discussed.     

Synthesis of high payload nanohydrogels for the ecapsulation of hydrophilic molecules via inverse miniemulsion polymerization: caffeine as a case study

Abstract

The association of an active principle with a nanocarrier is known to improve its stability and protect it from external factors. Nevertheless, loading of nanoparticles with highly hydrophilic substances like caffeine remains a tricky issue. In the present study, inverse miniemulsion systems were successfully coupled to UV radiation to synthesize polymeric nanohydrogels for drug delivery. The proper choice of the continuous and dispersed phase chemical composition led to the entrapment of active principle into the miniemulsion droplets. Our confinement-based strategy enabled unprecedented caffeine encapsulation efficiency inside 100-nm particles. Dimensional, thermal, and spectroscopic characterizations were carried out to investigate both unloaded and loaded nanohydrogels. Furthermore, in vitro release studies evaluated caffeine release kinetics from nanohydrogels by means of dialysis tests. It was demonstrated that controlled and sustained release occurred within the first 50?hours. Experimental data were found to fit the Higuchi model suggesting that the active principle release is diffusion controlled.     

Freeway management systems and motor vehicle crashes: a case study of Phoenix, Arizona.

This paper measures the impact of a freeway management system (FMS) on the incidence of reported motor vehicle crashes in Phoenix, AZ. Using a fixed effects negative binomial regression model, I find that the FMS reduces the frequency of crashes involving property damage only, possible injury, and minor injury by 25, 30, and 21%, respectively. I find no evidence for an effect on the frequency of major injury crashes or fatal crashes, although such accidents account for less than 5% of the total crashes in Phoenix. Classifying the data by the type of crash rather than by severity, I find that the frequencies of rearend crashes and sideswipe crashes are reduced by 25 and 37%, respectively, and I find no evidence of an effect on single vehicle crashes. The results are robust to many different model specifications, including a variety of functional forms, covariates, and data. A conservative estimate of the annual crash benefits of the FMS in Phoenix ranges from $4.8-13.2 million, depending on various assumptions about the value of pain and suffering and about the extent of crash underreporting. These annual crash benefits far outweigh the $1.6 million in annual operating and maintenance costs of the Phoenix FMS, and they offset considerably the approximately $47 million invested to date in the design and construction of the system.     

Measuring the evolutionary stability of software systems: case studies of Linux and FreeBSD

In software evolution, stability is defined as the ability of a module to remain largely unchanged when faced with newer requirements and/or changes in the environment. Although stability is an important long-term design characteristic for hardware systems, it has not been studied deeply for software systems. Stability is directly related to software evolvability and maintainability; and it affects the software evolution process. A model based on software version differences is presented to measure the evolutionary stability of software modules. This model represents and normalises two distances: the source code and the structure distances, between two versions of an evolving software module. As a case study based on this model, the evolutionary stability of Linux and FreeBSD modules is compared. The results of the study shows that the evolutionary stability of a software module in Linux and FreeBSD depends more on its function type and less on the system environment.