Microstructural study: an aid to determination of failure mechanism in nickel base superalloy blades

Cast nickel base superalloys are extensively used for high temperature gas turbine blade applications. The elevated temperature properties in these alloys are optimized through engineered microstructure, which is a combination of (a) γ-solid solution of Ni with elements such as W, Mo, Cr, Ta, Re etc, (b) γ′ precipitates, and (c) dispersed carbides. Over the years, the demand for high engine efficiency has resulted in the development of new generation superalloys with improved elevated temperature properties, especially creep resistance. These superalloys are highly alloyed with solid-solution strengthening elements and hence, impose greater technological challenges in fabrication. Due to high alloy contents, these alloys are prone to formation of detrimental phases such as the topologically close packed (TCP) phases. These phases may appear in the microstructure during the blade fabrication stages or precipitate out during exposure to high temperature and stress. In addition, the γ′ precipitates can change in morphology, shape and size during applications resulting in deterioration in high temperature mechanical properties, in general. These unfavorable microstructural changes often lead to premature failure in gas turbine engines. While analyzing these failures, the microstructural study provides important information in identifying whether the blades had faulty microstructure to start with or the abnormalities observed have resulted during exposure to service conditions. This in turn can be related to engine operating conditions. In the present paper, this has been demonstrated through analysis of two service failure cases wherein the high pressure turbine blades had failed in flight leading to aircraft accidents.     

On stability of NiTi wire during thermo-mechanical cycling

The use of NiTi wire as thermal actuator involves repeated thermal cycling through the transformation range under a constant or fluctuating load. The stability of the material under such conditions has been a concern for the past many years. Experimental results show that for a given alloy composition, the repetitive functional behaviour of NiTi wire is largely dependent on the processing schedule/parameters and the stress-strain regime of thermo-mechanical cycling (TMC). Among the various processing parameters, retained cold work in the material and the shape memory annealing temperature/time have significant influence. It has been shown in the present study that for a stable functional behaviour, the material needs to be tailored through judicious selection of these parameters. Study also shows that, after processing, the material requires an additional stabilization treatment for ensuring minimal variation in the repetitive functional response upon TMC.     

A secure image encryption scheme based on three different chaotic maps

Multimedia Tools and Applications - In the current decade, chaos based image encryption has distinctly captured a remarkable position in multimedia data security. In this paper, a hybrid chaos...     

Removal of fluoride from aqueous solution by polypyrrole/Fe3O4 magnetic nanocomposite

Polypyrrole (PPy)/Fe₃O₄ magnetic nanocomposite as a novel adsorbent was prepared via in-situ polymerization of pyrrole (Py) monomer using FeCl₃ oxidant in aqueous medium in which Fe₃O₄ nanoparticles were suspended. The adsorbent was characterized by Attenuated Total Reflectance Fourier transform infrared spectroscope (ATR-FTIR), Brunauer–Emmet–Teller (BET) method, field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscope (HR-TEM), X-ray photoelectron spectroscope (XPS) and X-ray diffraction (XRD). Magnetic property of the adsorbent was measured by electron spin resonance (ESR). Subsequently, the ability of the adsorbent to remove fluoride ions from aqueous solution was demonstrated in a batch sorption mode. Results reveal that the adsorption is rapid and that the adsorbent has high affinity for fluoride, which depends on temperature, solution pH and adsorbent dose. From equilibrium modelling, the equilibrium data is well described by Freundlich and Langmuir–Freundlich isotherms while the adsorption kinetics is described by the pseudo-second-order model. Thermodynamic parameters confirm the spontaneity and endothermic nature of the fluoride adsorption. Meanwhile, the fluoride adsorption proceeds by an ion exchange mechanism.     

On downstream transmissions in EPON Protocol over Coax (EPoC): An analysis of Coax framing approaches and other relevant considerations

We present different mechanisms for downstream transmissions in the coax segment of Ethernet Passive Optical Network (EPON) Protocol over Coax (EPoC). EPoC is the transparent extension of EPON over a cable operator’s Hybrid Fiber-Coax network. For managing and controlling such a hybrid network, a network operator prefers to have a unified scheduling, management, and quality of service environment that includes both the optical and coax portions of the network. In EPoC, this is achieved by extending the EPON Medium Access Control to run over the coax physical layer, to have a centralized end-to-end network control from the cable head-end to the end users premises. In this paper, we focus on the downstream transmissions in EPoC. We study three different framing approaches for downstream coax frames based on how sub-carriers in an orthogonal frequency division multiplexed symbol are modulated. We discuss the merits and demerits of each approach and then compare them based on their control overheads and the maximum average data transmission rates each of them can achieve. We analyze how different parameters such as modulation profile, symbol duration, number of sub-carriers and length of resource blocks affect the data rates and the performance of downstream transmissions. We present simulation results to examine the implications of these factors on packet-level performance, such as delay. The results indicate that dynamic and hybrid framing approaches tend to perform better than static approaches, when traffic and usage pattern are identical to those in real-world scenarios. Finally, we outline the important engineering and research problems in this area which can be topics of future research.     

Multi-channel Fusion Based Adaptive Gait Trajectory Generation Using Wearable Sensors

This paper presents a method to regenerate lower limb joint angle trajectories during gait cycle by judging human intention using wearable sensor system. Myoelectric signals from user are used to detect the intention of gait initiation and gait phases. Multi-channel redundant fusion technique is implemented to obtain a robust stride time and gait phase calculation algorithm. Joint trajectories corresponding to particular gait events and phases are regenerated using a Radial basis neural network. The network is trained with joint angle data measured by Inertial Measurement Unit (IMU) from users with varying anthropomorphic features. Generated trajectory is adaptive to anthropomorphic as well as gait velocity variation. Contribution of this paper is in development of a wearable sensor system, multi-channel redundant fusion to calculate stride time and an adaptive gait trajectory generation algorithm. The proposed method of trajectory generation is used to regenerate lower limb joint motion in sagittal plane for wearable robotic devices like prosthesis and active lower limb exoskeleton.     

Minimizing axial dispersion in narrow packed column using superhydrophobic wall

The scope of minimizing dispersion in narrow packed column using superhydrophobic (SH) wall is assessed experimentally with implications in analytical techniques such as liquid chromatography. The study includes devising a packed column (7-19 mm) with lotus leaf pasted on the inner wall and establishing a gravity driven flow through it. The flow dispersion is characterized based on the residence time distribution study of the column. The results are compared against similar flow through smooth packed column. Experimental results reveal the influence of two factors: column diameter as well as the wall features, superhydrophobic or smooth. For similar surface features, the axial dispersion reduces with decrease in column diameter due to the increase in voidage, which leads to plug flow. For the same diameter, between smooth and superhydrophobic, effects of slip in the latter reduce the dispersion significantly. Thus, the introduction of superhydrophobic narrow columns can play a crucial role in minimizing dispersion in analytical techniques.     

Synthesis and Characterization of Quenched and Crystalline Phases: Q-Carbon,Q-BN,Diamond and Phase-Pure c-BN

We report the synthesis and characterization of quenched (Q-carbon and Q-BN) and crystalline (diamond and c-BN) phases using a non-equilibrium technique. These phases are formed as a result of the melting and subsequent quenching of amorphous carbon and nanocrystalline h-BN in a super undercooled state by using high-power nanosecond laser pulses. Pulsed laser annealing also leads to the formation of nanoneedles, microneedles and single-crystal thin films of diamond and c-BN. This formation is dependent on the nucleation and growth times, which are controlled by laser energy density and thermal conductivities of substrate and as-deposited thin film. The diamond nuclei present in the Q-carbon structure (~ 80% sp³) can also be grown to larger sizes using the equilibrium hot filament chemical vapor deposition process. The texture of diamond and c-BN crystals is 〈111〉 under epitaxial growth and 〈110〉 under rapid unseeded crystallization. Our nanosecond laser processing opens up a roadmap to the fabrication of novel phases on heat-sensitive substrates.