Producing PLGA fine particles containing high magnetite nanoparticles by using the electrospray technique

Loading magnetite (Fe₃O₄) nanoparticles (NPs), extensively using magnetic agents in magnetic resonance imaging (MRI) and drug delivery in a matrix of polymeric fine particles (FPs), can optimize not only the delivery of these diagnostic and therapeutic agents but also the design of multifunctional drugs. In an effort to use a new method for producing high magnetite loaded polymeric particles, oleic acid (OA) capped magnetite NPs were synthesized and loaded into biocompatible and biodegradable FPs of poly lactic-co-glycolic acid (PLGA) by using the electrospray (ES) technique; and the effect of voltage, flow rate and magnetite content on the morphology, size, size distribution, uniformity and magnetic properties of fabricated magnetic FPs (MFPs) were studied. Results of SEM images and calculations showed that solution flow rate is a major factor in ES and the particle size of magnetite loaded PLGA FPs increases considerably as the flow rate increases. Particle size did not change considerably due to an increase in voltage; however, particle uniformity first increased and then decreased due to an increase in flow rate or voltage. High magnetite content of 72% was achieved for magnetite loaded PLGA FPs and an increase in the magnetite content resulted in an increase in the saturation magnetization of magnetite loaded PLGA FPs; though, their sphericity decreased.     

Diffusivity and micro-hardness of blended cement materials exposed to external sulfate attack

Many degradation processes in cement based materials include the diffusion of one or more chemical species into concrete and consequent chemical reactions which alter the chemical and physical nature of the microstructure. External sulfate attack is mostly described by a coupled diffusion-reaction mechanism which leads to the decomposition of hardened cement constituents and cracking of the paste. This paper discusses the significance of diffusion properties and chemical changes in external sulfate attack in blended cement based composites. A method based on Particle Induced X-ray Emission (PIXE) was developed to measure the diffusion properties in a non-destructive test method. Quantitative Energy Dispersive Spectrometry (EDS) and micro-hardness technique were also used to study the chemical and mechanical changes from sulfate attack. Diffusion coefficients and rates of reaction were determined for paste and mortar mixtures, showing higher diffusion rates and lower hardness values in mortar compared to paste for control mixtures. Partial replacement of cement with fly ash improved the transport properties and reduced the level of damage in exposure to sulfate attack.     

Die Köche und der Brei

Ohne Zusammenfassung     

Modeling of tension stiffening in reinforced cement composites: Part II. Simulations versus experimental results

This is the second part of a two-part paper involving a numerical model for simulations of tensile behaviour of reinforced cement-based composites. The model simulates the tensile stress strain response of a brittle matrix composite, tension stiffening effect of cracked matrix, and crack spacing evolution in tension members. The paper presents the simulations of four independent experimental results obtained from literature: steel reinforced concrete, concrete reinforced with steel and glass fiber reinforced plastic (GFRP), alkali resistant (AR) glass textile reinforced concrete and AR glass fabric reinforced cement pastes. The first and third experiments had complete input information for the simulations, and the predicted responses compare quite well to the experimental results. The second and last experiments did not have complete input data but, the properties can be estimated from other sources or by means of back calculations. The predicted responses reasonably agreed with the experimental results.     

Investigation of aqueous stability of taxol in different release media

In the present study, the aqueous stability of taxol in different aqueous media and immiscible aqueous/organic systems at 37?°C was investigated. The aqueous media included phosphate buffered saline (PBS) and PBS containing 10% methanol, 10% ethanol, 10% hydroxypropyl β-cyclodextrin (HP-βCD), 1% sodium citrate and 1% Tween 80. The immiscible systems consisted of PBS/octanol, PBS/dichloromethane, PBS/chloroform and PBS/ethyl acetate. The concentrations of taxol and related derivatives in each of the media were determined through the high-performance liquid chromatography assay. Results showed that hydrolysis and epimerization were two major types of degradation for taxol in the aqueous media starting from the initial hours of contact (6 hours). Addition of Tween 80 to PBS moderately increased the aqueous stability of taxol. As well, using PBS containing 10% HP-βCD inhibited the taxol hydrolysis, while epimerization still in process. In the case of immiscible systems, except for PBS/ethyl acetate system, no evidences of taxol hydrolysis were observed. Meanwhile, epimerization of taxol in PBS/dichloromethane and PBS/chloroform systems underwent due to the ability of C–Cl bonds to form hydrogen bonding with the hydroxyl group of C7 of taxol.     

Adaptive modeling of damage growth using a coupled FEM/BEM approach

We propose a coupled boundary element method (BEM) and a finite element method (FEM) for modelling localized damage growth in structures. BEM offers the flexibility of modelling large domains efficiently, while the non‐linear damage growth is accurately accounted by a local FEM mesh. An integral‐type nonlocal continuum damage mechanics with adapting FEM mesh is used to model multiple damage zones and follow their propagation in the structure. Strong form coupling, BEM hosted, is achieved using Lagrange multipliers. Because the non‐linearity is isolated in the FEM part of the system of equations, the system size is reduced using Schur complement approach, then the solution is obtained by a monolithic Newton method that is used to solve both domains simultaneously. The coupled BEM/FEM approach is verified by a set of convergence studies, where the reference solution is obtained by a fine FEM. In addition, the method is applied to multiple fractures growth benchmark problems and shows good agreement with the literature. Copyright © 2015 John Wiley & Sons, Ltd.     

Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.     

Comparative evaluation of early age toughness parameters in fiber reinforced concrete

Early age strength development is a major consideration for design and construction processes such as the shotcrete mixtures used for tunneling applications. Adding the fibers to high strength concrete helps in resisting potential early age thermal and shrinkage cracking in addition to maintaining long-term strength. The post cracking tensile strength is one of the critical safety parameters to insure a safe level of ground support. Results of several bending tests on early-age fiber reinforced concrete are presented as load–deflection responses. A strain softening response is used to model the behavior of different types of fiber reinforced concrete and simulate the experimental flexural response. Closed form equations for moment–curvature response of a rectangular beam in conjunction with crack localization rules are utilized. As a result, the stress distribution that considers a shifting neutral axis can be simulated which provides a more accurate representation of the residual strength of the fiber cement composites. The analysis is performed to evaluate effects of age and fiber type on back calculated tensile stress strain response, along with experimental and simulated flexural load–deflection curves. The back-calculated tensile post cracking strengths are compared and correlated with the corresponding parameters used by ASTM, JCI, and RILEM methods and scale factors for the elastic methods are proposed which are in-line with the current fib Model Code. Caution must be exercised in application of results from the standard test methods due to the overestimation of the residual strength parameters that are based on elastic approaches.     

Development of Fabric Constitutive Behavior for Use in Modeling Engine Fan Blade-Out Events

The development of a robust and reliable material model for fabrics used to prevent fan blade-out events in propulsion engines has significant importance in the design of fan-containment systems. Currently, Kevlar is the only fabric approved by the Federal Aviation Administration to be used in fan-containment systems. However, very little work has been done in building a mechanistic-based material behavior model, especially one that can be used to quantify the behavior of Kevlar when subjected to high-velocity projectiles. Experimental static and high strain rate tensile tests have been conducted at Arizona State University to obtain the material properties of Kevlar fabric. In this paper we discuss the development and verification of a constitutive model for dry fabrics for use in an explicit finite-element program. Results from laboratory tests such as tension tests including high-strain rate tests, picture frame shear tests, and friction tests yield most of the material properties needed to define a constitutive model. The material model is incorporated in the LS-DYNA commercial program as a user-defined subroutine. The validation of the model is carried out by numerically simulating actual ballistic tests conducted at NASA-GRC and fan blade out tests conducted at Honeywell Aerospace (Propulsion Engines).     

Geometrical and mechanical aspects of fabric bonding and pullout in cement composites

Fabric reinforced cement composites are a new class of cementitious materials with enhanced tensile strength and ductility. The reinforcing mechanisms of 2-D fabric structures in cement matrix are studied using a fabric pullout model based on nonlinear finite difference method. Three main aspects of the composite are evaluated: nonlinear bond slip characteristic at interface; slack in longitudinal warp yarns, and mechanical anchorage provided by cross yarn junctions. Parametric studies of these key parameters indicate that an increase in the interfacial bond strength directly increases the pullout strength. Grid structures offering mechanical anchorage at cross yarn junctions can substantially increase the pullout resistance. Presence of slack in the yarn geometry causes an apparently weaker and more compliant pullout response. The model was calibrated using a variety of test data on the experimental pullout response of AR-Glass specimens, manufactured by different techniques to investigate the relative force contribution from bond at interface and from cross yarn junctions of alkaline resistant glass fabric reinforced cement composites.