Micromechanisms of fracture and strengthening in free-standing Pt-aluminide bond coats under tensile loading

Free-standing Pt-aluminide (PtAl) bond coats exhibit a linear stress strain response under tensile loading and undergo brittle cleavage fracture at temperatures below the brittle-to-ductile transition temperature (BDTT). Above the BDTT, these coatings show yielding and fail in a ductile manner. In this paper, the various micromechanisms affecting the tensile fracture stress (FS) below the BDTT and yield strength (YS) above the BDTT in a PtAl bond coat have been ascertained and quantified at various temperatures. The micromechanisms have been identified by carrying out microtensile testing of stand-alone PtAl coating specimens containing different levels of Pt at temperatures between room temperature and 1100 degrees C and correlation of the corresponding fracture mechanisms with the deformation substructure in the coating. An important aspect of the influence of Pt on the tensile behavior, slip characteristics, FS/YS and BDTT in the PtAl coating has also been examined. The addition of Pt enhances the FS of the coating by Pt solid solution strengthening and imparts a concomitant increase in fracture toughness and yet causes a significant increase in the BDTT of the coating. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

A fractional order proportional integral controller for path following and trajectory tracking of miniature air vehicles

In this paper, a fractional order proportional-integral controller is developed for a miniature air vehicle for rectilinear path following and trajectory tracking. The controller is implemented by constructing a vector field surrounding the path to be followed, which is then used to generate course commands for the miniature air vehicle. The fractional order proportional-integral controller is simulated using the fundamentals of fractional calculus, and the results for this controller are compared with those obtained for a proportional controller and a proportional integral controller. In order to analyze the performance of the controllers, four performance metrics, namely (maximum) overshoot, control effort, settling time and integral of the timed absolute error cost, have been selected. A comparison of the nominal as well as the robust performances of these controllers indicates that the fractional order proportional-integral controller exhibits the best performance in terms of ITAE while showing comparable performances in all other aspects.

Characterizing Frictional Contact Loading via Isochromatics

Isochromatic patterns in the vicinity of frictional contacts furnish vital clues for characterizing friction. Though friction effects are evident in a diametrally loaded circular disk, three-point loading provides better results towards highlighting friction. In this paper, a new method of characterizing friction at loading contacts using photoelastic isochromatics patterns is presented. Location of isotropic points (IPs) formed in three-point and four-point loadings of circular disk is used as a main tool to quantify the friction component using theoretical analysis. Bifurcation of isochromatic fringe loops near the distributed loads is explained by the presence of anti-symmetric Hertzian shear traction in addition to Hertzian normal traction. The classical solution by Flamant for point load at the edge of half plane is used to derive stresses in circular disk for all required loading configurations. A semicircualr ring under three-point loading is examined using photoelasticity to understand the isochromatics pattern theoretically by considering normal and shear traction components at loaded regions.

Asymptotic expansions for wavenumbers in orthotropic fluid-filled circular cylindrical shells for intermediate fluid loading

Coupled wavenumbers in infinite fluid-filled isotropic and orthotropic cylindrical shells are considered. Using the Donnell-Mushtari (DM) theory for thin shells, compact and elegant asymptotic expansions for the wavenumbers are found at an intermediate fluid loading for both the coupled rigid-duct modes (''fluid-originated'') and the coupled structural wavenumbers (''structure-originated modes'') over the entire frequency range where DM theory is valid. The coupled rigid-duct expansions are found to be valid for O(1) orthotropy and for all circumferential orders, whereas the coupled structural wavenumber expansions are valid for small orthotropy and for low circumferential orders. These two above results are then used to derive the expansions for a set of multiple complex roots that display a locking behavior at this intermediate fluid-loading. The expansions are matched with the numerical solutions of the coupled dispersion relation and the match is found to be good over most of the frequency range. (C) 2014 Acoustical Society of America.

Effect of adsorbate loading on selectivity during adsorption of C/C and C/C n-alkane binary mixtures in silicalite

Adsorption experiments of mixtures of long chain alkanes into silicalite under liquid phase conditions show selectivity inversion and azeotrope formation. These effects are due to the subtle interplay between the size of the adsorbed molecules and pore topology of the adsorbent. In this study, the selective uptake of lighter component during liquid phase adsorption of C/C and C/C n-alkane binary mixtures in the zeolite silicalite is understood through configurational bias grand-canonical Monte Carlo molecular simulation technique and a coarse-grained siting analysis. The simulations are conducted under conditions of low and intermediate levels of loading. The siting pattern of the adsorbates inside the zeolite pores explain the selectivity as seen in experiments.

Orientation, Size, and Temperature Dependent Ductile Brittle Transition in NiAl Nanowire under Tensile Loading - A Molecular Dynamics Study

In the present paper, thermo-mechanical response of B2-NiAl nanowire along the < 100 >, < 110 >, and < 111 > orientations has been studied using molecular dynamics simulations. Nanowire with cross-sectional dimensions of similar to 20x20 angstrom(2), similar to 25x25 angstrom(2), and similar to 30x30 angstrom(2) and temperature range of 10 K-900 K has been considered. A Combined effect of size, orientation, and temperature on the stress-strain behavior under uniaxial tensile loading has been presented. It has been observed that < 111 > oriented NiAl nanowire that is energetically most stable gives highest yield stress which further reduces with < 110 > and < 100 > orientations. A remarkable ductile brittle transition (DBT) with an increase in temperature has also been reported for all the orientations considered in the present study. The DBT observed for the nanowire has also been compared with the reported DBT of bulk B2-NiAl obtained from experiments. Alternate technique has also been proposed to increase the toughness of a given material especially at lower temperature regions, i.e. below DBT.

Capturability of Augmented Pure Proportional Navigation Guidance Against Time-Varying Target Maneuvers

This paper proposes a variation of the pure proportional navigation guidance law, called augmented pure proportional navigation, to account for target maneuvers, in a realistic nonlinear engagement geometry, and presents its capturability analysis. These results are in contrast to most work in the literature on augmented proportional navigation laws that consider a linearized geometry imposed upon the true proportional navigation guidance law. Because pure proportional navigation guidance law is closer to a realistic implementation of proportional navigation than true proportional navigation law, and any engagement process is predominantly nonlinear, the results obtained in this paper are more realistic than any available in the literature. Sufficient conditions on speed ratio, navigation gain, and augmentation parameter for capturability, and boundedness of lateral acceleration, against targets executing piecewise continuous maneuvers with time, are obtained. Further, based on a priori knowledge of the maximum maneuver capability of the target, a significant simplification of the guidance law is proposed in this paper. The proposed guidance law is also shown to require a shorter time of interception than standard pure proportional navigation and augmented proportional navigation. To remove chattering in the interceptor maneuver at the end phase of the engagement, a hybrid guidance law using augmented pure proportional navigation and pure proportional navigation is also proposed. Finally, the guaranteed capture zones of standard and augmented pure proportional navigation guidance laws against maneuvering targets are analyzed and compared in the normalized relative velocity space. It is shown that the guaranteed capture zone expands significantly when augmented pure proportional navigation is used instead of pure proportional navigation. Simulation results are given to support the theoretical findings.

Analysis of cracked and un-cracked semicircular rings under symmetric loading

Edge cracked specimens have been widely utilized for fracture testing. Edge cracked semicircular disk (ECSD) specimen has now been well characterized with regard to its form factor and weight function. This paper presents a modified semicircular ring version of this specimen to enhance the form factor in general while retaining other desirable features. The efficacy of the modified design is proved by combining theory of elasticity solutions with finite element results to arrive at the optimum design geometry. New insights emerging from this work are used to theoretically re-examine the arch-tension and the four-point bend specimens. (C) 2014 Elsevier Ltd. All rights reserved.

An experimental study on loading rate effect on acoustic emission based b-values related to reinforced concrete fracture

This article reports on analysis of fracture processes in reinforced concrete (RC) beams with acoustic emission (AE) technique. An emphasis was given to study the effect of loading rate on variation in AE based b-values with the development of cracks in RC structures. RC beams of length 3.2 m were tested under load control at a rate of 4 kN/s, 5 kN/s and 6 kN/s and the b-value analysis available in seismology was used to study the fracture process in RC structures. Moreover, the b-value is related to the strain in steel to assess the damage state. It is observed that when the loading rate is higher, quick cracking development lead to rapid fluctuations and drops in the b-values. Also it is observed that concrete behaves relatively more brittle at higher loading rates (or at higher strain rates). The average b-values are lower as a few but larger amplitudes of AE events occur in contrast to more number of low amplitude AE events occur at low loading rates (or at low strain rates). (C) 2014 Elsevier Ltd. All rights reserved.

Effects of Microstructure and Loading on Fracture of Sn-3.8Ag-0.7Cu Joints on Cu Substrates with ENIG Surface Finish

When dropped, electronic packages often undergo failure by propagation of an interfacial crack in solder joints under a combination of tensile and shear loading. Hence, it is crucial to understand and predict the fracture behavior of solder joints under mixed-mode high-rate loading conditions. In this work, the effects of the loading conditions (strain rate and loading angle) and microstructure interfacial intermetallic compound (IMC) morphology and solder yield strength] on the mixed-mode fracture toughness of Sn-3.8 wt.%Ag-0.7 wt.%Cu solder joints sandwiched between two Cu substrates with electroless nickel immersion gold (ENIG) metallization have been studied, and compared with the fracture behavior of joints attached to bare Cu. Irrespective of the surface finish, the fracture toughness of the solder joints decreased monotonically with strain rate and mode-mixity, both resulting in increased fracture proportion through the interfacial IMC layer. Furthermore, the proportion of crack propagation through the interfacial IMC layer increased with increase in the thickness and the roughness of the interfacial IMC layer and the yield strength of the solder, resulting in a decrease in the fracture toughness of the joint. However, under most conditions, solder joints with ENIG finish showed higher resistance to fracture than joints attached directly to Cu substrates without ENIG metallization. Based on the experimental observations, a fracture mechanism map is constructed correlating the yield strength of the solder, the morphology and thickness of the interfacial IMC, and the fracture mechanisms as well as the fracture toughness values for different solder joints under mode I loading.

Study the effect of electrical and mechanical shock loading on liberation and milling characteristics of mineral materials

Milling is an energy intensive process and it is considered as one of the most energy inefficient processes. Electrical and mechanical shock loading can be used to develop a pre-treatment methodology to enhance energy efficiency of comminution and liberation of minerals. Coal and Banded Hematite Jasper (BHJ) Iron ores samples were taken for the study to know the effect of shock loading. These samples were exposed to 5 electric shocks of 300 kV using an electric shock loading device. A diaphragmless shock tube was used to produce 3 and 6 compressed air shocks of Mach number 2.12 to treat the coal and Iron ore samples. Microscopic, comminution and liberation studies were carried out to compare the effectiveness of these approaches. It was found that electric shock loading can comminute the coal samples more effectively and increases the yield of carbon by 40% at 1.6 gm/cc density over the untreated coal samples. Mechanical shock loading showed improved milling performance for both the materials and 12.90% and 8.1% reduction in the D-80 of the particles was observed during grinding for treated samples of coal and iron, respectively. Liberation of minerals in BHJ Iron ore was found unaffected due to low intensity of the mechanical shock waves and non conductivity of minerals. Compressed air based shock loading is easier to operate than electrical shock loading and it needs to be explored further to improve the energy efficacy of comminution. (C) 2014 Elsevier Ltd. All rights reserved.

A smart approach to achieve an exceptionally high loading of metal nanoparticles supported by functionalized extended frameworks for efficient catalysis

The problem associated with metal nanoparticle (NP) agglomeration when trying to achieve a high loading amount has been solved by a new method of functionalization of MOFs' pores with terminal alkyne moieties. The alkynophilicity of the Au3+ ions has been utilized successfully for an exceptionally high loading (similar to 50 wt%) of Au-NPs on supported functionalized MOFs.

Effect of mass loading on ionic polymer metal composite actuators and sensors

Ionic polymer metal composites (IPMC) actuator for flapping insect scale wing is advantageous due to its low mass, high deflection and simple actuation mechanism. Some of the factors that affect the actuation of IPMC are the amount of hydration in the polymer membrane and the environmental conditions such as temperature, humidity etc. In structural design, the attachment of wing on the IPMC actuators is an important concern as the attached wing increases the mass of actuators thereby affecting the parameters like displacement, stiffness and resonant frequencies. Such IPMC actuators have to produce sufficient actuation force and frequency to lift and flap the attached wing. Therefore, it is relevant to study the influence of attachment of wing on the actuator parameters (displacement, resonant frequency, block force and stiffness) and performance of the actuators. This paper is divided into two parts; the first part deals with the modeling of the IPMC actuators for its effect on the level of water uptake and temperature using energy based method. The modeling method adapted is validated with the experimental procedure used to actuate the IPMC. The second part deals with the experimental analysis of IPMC actuation at dry, wet and in water conditions. The effect of end mass loading on the performance of 20 Hz, high frequency actuator (HFA) and 8.7 Hz, low frequency IPMC actuators (LFA) and sensors is studied. The IPMC actuators are attached with IPMC flapping wing at its free end and performance analysis on the attached wing is also carried out.