Essential and Non-essential Work of Fracture of PI/SiO<Subscript>2</Subscript> Hybrid Thin Films

Essential work of fracture (EWF) analysis is used to study the effect of the silica doping level on fracture toughness of polyimide/silica (PI/SiO₂) hybrid films. By using double-edge-notched-tension (DENT) specimens with different ligament lengths, it seems that the introduction of silica additive can improve the specific essential work of fracture (w _e ) of PI thin films, but the specific non-essential work of fracture (βw _p ) will decease significantly as the silica doping level increasing from 1 to 5 wt.%, and even lower than that of neat PI. The failure process of the fracture is investigated with online scanning electron microscope (SEM) observation and the parameters of non-essential work of fracture, β and w _p , are calculated based on finite element (FE) method.     

In-plane and Out-of-plane Fracture Toughness of Physically Aged Polyesters as Assessed by the Essential Work of Fracture (EWF) Method

The in-plane (mode I) and out-of-plane (mode III) fracture behavior of amorphous (co)polyester sheets are studied as a function of physical aging using the essential work of fracture (EWF) concept. Physical aging was followed by measuring the yield stress and enthalpy relaxation of the (co)polyesters. The specific essential work of fracture term did not reflect the physical aging either in mode I or III loading. On the other hand, the corresponding values were closely matched indicating the mode III type loading turns into mode I for thin films. Aging was best manifested in the mode I – related EWF parameters, viz. specific yielding-related essential work and non-essential necking/tearing related work.     

Influence of femtolaser notch sharpening technique in the determination of essential work of fracture (EWF) parameters

The fracture behaviour of a 0.5 mm thick ethylene-propylene block copolymer is analyzed using the essential work of fracture method, using DENT-type specimens. The influence of three experimental parameters in the technique is evaluated: the effect of the notch sharpening technique, the use of a videoextensometer for monitoring the deformations and the measurement of ligament lengths before or after fracture tests were carried on.The results showed that the femtosecond pulsed laser ablation technique (femtolaser) produced sharp notches with no plastic deformation ahead of the notch tip, which yield smaller specific essential work of fracture (w_e) values than in the cases where the notches were sharpened with razor blades. The use of a videoextensometer has allowed removing the viscoelastic energy from the plastic work, with lower values of βw_p. The measurement of the ligament lengths before or after the test did not affect the results.     

The enriched space–time finite element method (EST) for simultaneous solution of fluid–structure interaction

The paper introduces a weighted residual‐based approach for the numerical investigation of the interaction of fluid flow and thin flexible structures. The presented method enables one to treat strongly coupled systems involving large structural motion and deformation of multiple‐flow‐immersed solid objects. The fluid flow is described by the incompressible Navier–Stokes equations. The current configuration of the thin structure of linear elastic material with non‐linear kinematics is mapped to the flow using the zero iso‐contour of an updated level set function. The formulation of fluid, structure and coupling conditions uniformly uses velocities as unknowns. The integration of the weak form is performed on a space–time finite element discretization of the domain. Interfacial constraints of the multi‐field problem are ensured by distributed Lagrange multipliers. The proposed formulation and discretization techniques lead to a monolithic algebraic system, well suited for strongly coupled fluid–structure systems. Embedding a thin structure into a flow results in non‐smooth fields for the fluid. Based on the concept of the extended finite element method, the space–time approximations of fluid pressure and velocity are properly enriched to capture weakly and strongly discontinuous solutions. This leads to the present enriched space–time (EST) method. Numerical examples of fluid–structure interaction show the eligibility of the developed numerical approach in order to describe the behavior of such coupled systems. The test cases demonstrate the application of the proposed technique to problems where mesh moving strategies often fail. Copyright © 2007 John Wiley & Sons, Ltd.     

The resistance of metakaolin (MK)–Portland cement (PC) concrete to the thaumasite-type of sulfate attack (TSA)––Programme of research and preliminary results

The thaumasite form of sulfate attack (TSA), is a deleterious physico-chemical attack of the calcium silicate hydrate (C–S–H), binding phase of concrete. Water:binder ratios (w:b), are known to control ingress of potentially deleterious ions by pore structure refinement at low (0.40) values. Equally, a physical–chemical barrier exists at about w:b ratio of 0.45 regardless of the binder type. The inclusion of ultra-fine pozzalans (e.g. metakaolin) in the binder has been shown to impart such properties through densification of the matrix and removal of calcium hydroxide.

A small-scale experimental design programme to establish the potential resistance of metakaolin–Portland cement (MK–PC), blended concrete to the thaumasite-type of attack is ongoing. Results are presented for concrete incorporating dolomitic limestone aggregate and with 0% and 7% metakaolin replacement of a high-C₃A PC binder at w:b ratios to 0.40 and 0.46. Exposure to three environments containing a sulfatic clay, sulfate solution and water has produced evidence of deleterious reactions within these samples. Visual data are quantified by a wear rating and supported by compressive strength and expansion values for up to 280 days of exposure. These results confirm the importance of low water binder ratios in the resistance of ion ingress and give an early indication of the desirable durability-enhancing properties of MK replacement of PC. Finally, the results also show that the two sulfatic environments of exposure produce marked differences in the degradation modes, which may be important in the methodologies used to determine TSA in the laboratory with reference to field observations.     

Fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends and the effect of compatibilization on their properties

Attempts have been made to study the fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends. The effect of compatibilization on their properties was also studied. EWF tests were conducted to measure the essential specific work of fracture (w_e) and non-essential specific work of fracture (βw_p). The ESCR of the samples was investigated using a special modified tensile creep test under an aggressive environment (sunflower oil). It was found that EWF methodology could be applied to uncompatibilized and compatibilized HIPS/PE blends as well as HIPS. The essential specific work of fracture of compatibilized HIPS/PE blends was higher than uncompatibilized HIPS/PE blends and pure HIPS, while its non-essential work of fracture was higher than uncompatibilized blends and lower than pure HIPS. The results also showed that the ESCR of HIPS decreases with incorporation of PE, but an effective compatibilization of this blend increases its ESCR even higher than pure HIPS. The different properties of compatibilized and uncompatibilized blends and HIPS, in EWF and ESCR tests, were attributed to the different mechanisms of fracture in these materials. The different mechanisms of fracture were justified using morphological studies performed on fracture surfaces of each sample. SEM images showed that there is a reasonable correlation between mechanisms of fracture and microstructure of the samples.     

On the preconditioned conjugate gradient method for solving (A – λB)X =0

Classical iterative methods when applied to the partial solution of the generalized eigenvalue problem Ax =λ Bx , may yield very poor convergence rates particularly when ill-conditioned problems are considered. In this paper the preconditioned conjugate gradient (CG) method via the minimization of the Rayleigh quotient and the reverse power method is employed for the partial eigenproblem. The triangular splitting preconditioners employed are obtained from an incomplete Choleski factorization and a partial Evans preconditioner. This approach can dramatically improve the convergence rate of the basic CG method and is applicable to any symmetric eigenproblem in which one of the matrices A , B is positive definite. Because of the renewed interest in CG techniques for FE work on microprocessors and parallel computers, it is believed that this improved approach to the generalized eigenvalue problem is likely to be very promising.     

The M1 Energy‐efficiency building Plus – Conclusion of monitoring / Das M1 Energieeffizienzhaus Plus – Abschluss des Monitorings

The M1 energy‐efficiency building Plus was to render practical proof that the advantages of solid construction such as carrying capacity, fire protection and sound protection could be connected to state‐of‐the‐art and future energy efficiency standards in the scope of a pilot project. With simple and thought‐through planning details and a coordinated system technology, energy plus solid houses are no longer merely visions. The M1 project mostly focuses on the claim to economic efficiency and saleability of the product, points out new possibilities for implementation to consumers, planners and executing companies. The house is to document, that the means available now are sufficient to construct a solid building that produces more energy than it consumes. In the technical term of construction physics, the building is targeted at a negative final energy and primary energy consumption. The data presented in this article from the monitoring of the last two years show that a plus energy target can be achieved by “nearly” conventionally built solid buildings. For both years, the M1 pus energy house reached an excess of final and primary energy.     

Preparation of gold nanosheets using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers via photoreduction

Gold nanosheets having single crystalline structure were successfully synthesized using the bulk phase mixture of HAuCl₄ and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers through the irradiation of a glow lamp for 5 days. When the molar ratio of propylene oxide to ethylene oxide block units in the block copolymer is about 1.75, mostly gold nanosheets were obtained. Gold nanosheets with an average width of 8 and 5 μm were obtained from the when the molar ratio of gold salt to the ethylene oxide units in the block copolymer were 1/80 and 1/160, respectively.     

Modeling and optimizing of factors affecting erosion–corrosion of AA6063–(TiC/Al2O3) hybrid composites by experimental design method

In this article, modeling and optimizing of factors affecting erosion–corrosion wear of aluminum alloy A6063 reinforced with (Al₂O₃/TiC) particles have been determined by experimental design method. The erosion–corrosion wear characteristics and mechanism of AA6063–(TiC/Al₂O₃) with experimental parameters namely; type and concentration of corrosive media in the slurry, erosion speed and time have been investigated. Two models for reinforced and unreinforced alloys were applied to describe the influences of these factors on the erosion behavior of alloys. The erosion–corrosion mechanisms of the AA6063–(TiC/Al₂O₃) were dominated by particles erosion wear in alkaline slurry, and by the interaction of particle erosion wear and medium corrosion in acidic slurry. The results of experimental work are coinciding with that of calculated ones confirming the successful modelization.     

Effect of hydrophilic–hydrophobic balance on biocompatibility of poly(methyl methacrylate) (PMMA)–hydroxyapatite (HA) composites

The purpose of this study is to put in evidence the correlation between hydrophilic/hydrophobic balance and biocompatibility of PMMA–HA composites, in order to select the best composites for futures clinical applications. For this purpose, PMMA–HA cements with different compositions were prepared and static contact angle measurements, water absorption and gingival fibroblasts cell culture were performed and discussed.     

Fracture behaviour of 2D-weaved, silica–silica continuous fibre-reinforced, ceramic–matrix composites (CFCCs)

Significantly improved fracture resistance (in terms of fracture toughness and fracture energy) can be imparted to monolithic ceramics by adopting composite design methodology based on fibre reinforcement technology. The present paper describes the fracture behaviour of one such fibre-reinforced material, namely the silica–silica based continuous fibre-reinforced, ceramic–matrix composite (CFCC) in two orthogonal notch orientations of crack divider and crack arrester orientations. Different fracture resistance parameters have been evaluated to provide a quantitative treatment of the observed fracture behaviour. From this study, it has been concluded that the overall fracture resistance of the CFCC is best reflected by total fracture energy release rate (J_c), which parameter encompasses most of the fracture events/processes. The J_c values of the composite are found to be more than an order of magnitude higher than the energy values corresponding to the plane strain fracture toughness (J_KQ, derived from K_Ic, the plane strain fracture toughness) and >200% higher than elastic–plastic fracture toughness (J_Ic). Apart from this, the composite is found to exhibit high degree of anisotropy in the fracture resistance and also, a significant variation in the relative degree of shear component with crack extension.     

Optimizing the design of a moderator-based neutron detector for a flat response curve in the 2–14 MeV energy range

The efficiency of a neutron detector with boron trifluoride proportional tubes embedded in a polyethylene moderator was simulated with a Monte Carlo program. A moderator structure where the detector had uniform sensitivity for neutrons from 2 to 14 MeV was determined by simulation. A counter was built based on the simulation results. The counter's efficiencies were calibrated with an Am–Be source and an accelerator that served as a D–D and D–T neutron source. Experimental neutron efficiencies of these sources are approximately uniform. The simulated model was validated by the consistent results between the calculated and experimental data.     

A modification of the Petrov–Galerkin method for the transient convection–diffusion equation

A variation of the Petrov–Galerkin method of solution of a partial differential equation is presented in which the weight function applied to the time derivative term of the transient convection–diffusion equation is different from the weight function applied to the special derivatives. This allows for the formulation of fourth-order explicit and centred difference schemes. Comparison with analytic solutions show that these methods are able to capture steep wave fronts. The ability of the explicit method to capture wave fronts increases as the amount of convective transport increases.     

Fracture behavior and mechanical properties of Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) alloy at room temperature

The microstructure, mechanical properties and fracture behavior of gravity die cast Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) (WNG421) alloy are studied at room temperature in different thermal conditions, including as-cast, solution-treated and different aging-treated (both isothermal and two-step aging) conditions. The results indicate that WNG421 alloy shows different behaviors of crack initiation and propagation in different thermal conditions during tensile test at room temperature. After pre-aged at 200 °C for 5 h, the hardness of WNG421 alloy first reduces and then increases when secondary aged at 250 °C (two-step aging). The peak hardness and corresponding tensile strength of the two-step aged alloy both increases compared with those in 250 °C isothermal peak-aged condition. Tensile strength of WNG421 alloy at room temperature in low temperature (200 °C) isothermal peak-aged condition is much higher than that in high temperature (250 °C) isothermal peak-aged condition.     

The Eulerian–Lagrangian method of fundamental solutions for two-dimensional unsteady Burgers’ equations

The Eulerian–Lagrangian method of fundamental solutions is proposed to solve the two-dimensional unsteady Burgers’ equations. Through the Eulerian–Lagrangian technique, the quasi-linear Burgers’ equations can be converted to the characteristic diffusion equations. The method of fundamental solutions is then adopted to solve the diffusion equation through the diffusion fundamental solution; in the meantime the convective term in the Burgers’ equations is retrieved by the back-tracking scheme along the characteristics. The proposed numerical scheme is free from mesh generation and numerical integration and is a truly meshless method. Two-dimensional Burgers’ equations of one and two unknown variables with and without considering the disturbance of noisy data are analyzed. The numerical results are compared very well with the analytical solutions as well as the results by other numerical schemes. By observing these comparisons, the proposed meshless numerical scheme is convinced to be an accurate, stable and simple method for the solutions of the Burgers’ equations with irregular domain even using very coarse collocating points.     

Mechanics of three-dimensional braided structures for composite materials – Part II: prediction of the elastic moduli

In the first part of this series of paper [Z.X. Zang, R. Postle, Mechanics of three-dimensional braided structures for composite materials – Part I: fabric structure and fibre volume fraction, Comp. Struct. 49 (2000) 451–459], it was demonstrated that the braiding angles and fibre volume fraction can be represented as functions of the normalized pitch length introduced as a key parameter of three-dimensional braided reinforced composite materials. In the present paper, the models for the prediction of the tensile and shear moduli of three-dimensional braided composites are established by numerical simulation and mathematical modelling. Three-dimensional braided preforms are produced from the material system comprising glass/polypropylene and their moduli are measured. The results predicted from the braided composite models are supported by the experimental data.     

Reduced temperature (22–100 °C) ageing of an Mg–Zn alloy

Heat treatment at intermediate temperatures (70–100 °C) doubles the age hardening response of the binary Mg–2.8 at.% Zn alloy when compared to the conventional T6 heat treatment and ambient temperature ageing. The maximal hardening produced at 70 °C is associated with the highest number density of the homogeneously distributed precipitates. At least six different types of coherent and semicoherent precipitates were simultaneously present in the microstructure aged at 70 °C: [0 0 01]_Mg rods and laths, Guinier Preston (GP) zones, GP1 zones {0 0 0 1}_Mg plates and prismatic precipitates containing 19–26 at.% Zn. Artificially aged alloy is strengthened mainly by sparsely distributed and precipitates and occasional GP zones. Strengthening in the naturally aged condition is produced by the combination of GP1 zones, prismatic precipitates and clusters containing about 20 Zn atoms.     

Evaluation by extended Hückel method on the hardness of the B–C–N materials

Hard materials, e.g. diamond and cubic boron nitride (c-BN), are widely applied to improve the lifetime and the performance of many kinds of cutting and forming tools. These materials are usually used at high temperature, so the study of stability on these materials at high temperature is very important. However, diamond is a low resistance to the oxidation, it should be replaced with the boron-based hard materials. Recently, boron–carbon–nitrogen (B–C–N) ternary materials are expected to possess a high hardness, a high thermal stability at high temperature. We estimated the hardness and the stability of B–C–N materials at high temperature by the extended Hückel method. The extended Hückel method is one of the molecular orbital calculations and needs the cluster model of materials for the calculation. The cluster model of B–C–N materials was regarded as a zinc blende structure. In the present work, we used two physical quantities, i.e. a cohesive energy and an energy fluctuation, as a measure of hardness and stability of materials. The cohesive energy indicates the coherence of bonds between atoms. The energy fluctuation shows the reactivity of materials. Hardness, structure, solid-state properties and reactivity of materials can be estimated from these physical quantities. When the composition of B–C–N materials was boron: 25 at.%, nitrogen: 25 at.% and carbon: 50 at.%, the cohesive energy was the lowest. This result implies B–C–N ternary materials are not harder than c-BN and/or diamond. Cubic-BN was the lowest energy fluctuation of B–C–N materials, and the energy fluctuation increased as increasing of carbon atom. The reactivity of B–C–N materials was high at a high temperature with an increase of carbon atoms. These results imply that B–C–N materials are not suitable for the hard cutting materials.