Life estimation of shafts using vibration based fatigue analysis

Many rotating machinery components fail due to fatigue when subjected to continuous fluctuating stresses. Hence, estimation of fatigue crack initiation life is essential to avoid catastrophic failure. Effective vibration based fatigue life analysis requires measurement of accurate time varying signal. In this study, experimentally observed fatigue lives of rotating shaft, for three different notch configurations, are compared with fatigue lives estimated using two approaches based on an acquired vibration signal. The first one is time domain approach (based on Rainflow cycle counting) while the second one is frequency domain approach (based on power spectral density moments). In the frequency domain approach, fatigue life is estimated using the narrow-band approximation and Dirlik’s empirical solution. The performance of two approaches in estimating fatigue life for the same signal length taken at different time intervals from the total signal acquired is also discussed. In addition, experimental uncertainty analysis is performed and discussed in this study. A good correlation is found between the estimated fatigue life using Dirlik’s rainflow range probability density function and experimental life. Therefore, this study concludes that the Dirlik’s approach can be considered as preferable method for estimating fatigue life of rotating shaft.     

Severity Estimation of Cracked Shaft Vibrations within Fluid Film Bearings

The equations of motion, with four degrees of freedom, taking into consideration the flexibility, damping and cross coupling of the fluid film bearings are derived for a cracked Jeffcott rotor supported on fluid film bearings.

Dimensionless equations are developed for dynamic radial load, dynamic pressure developed in the fluid film bearings and coefficient of dissipation considering the journal vibrations in two harmonics; bearing fluid film stiffness and damping coefficients. These are applied to a cracked Jeffcott rotor supported on different types of bearings, i.e., cylindrical journal bearings, offset cylindrical bearings, tilting pad journal bearings and three-lobe bearings. Based on the allowable dynamic pressure developed in the fluid bearings, the severity of cracked shaft and allowable crack depths are estimated in this study. Measurement of dynamic pressure and dissipation for monitoring the crack growth is suggested. However, 2x vibration is the best indicator of cracks in the shafts.     

Optimum short-time polynomial regression for signal analysis

Sādhanā - We propose a short-time polynomial regression (STPR) for time-varying signal analysis. The advantage of using polynomials is that the notion of a spectrum is not needed and the...     

Electrical properties of SiO2/TiO2 high-k gate dielectric stack

The trapping/detrapping behavior of charge carriers in ultrathin SiO₂/TiO₂ stacked gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Titanium tetrakis iso-propoxides (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO₂ films at low temperature (<200 °C) on strained-Si/relaxed-Si_0.8Ge_0.2 heterolayers by plasma-enhanced chemical vapor deposition (PECVD) in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. Stress-induced leakage current (SILC) through SiO₂/TiO₂ stacked gate dielectric is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of TiO₂ layer. The increase in the gate current density observed during CVS from room temperature up to 125 ^oC has been analyzed and modeled considering both the buildup of charges in the layer as well as the SILC contribution. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of 10⁻¹⁹ cm² as compared to 10⁻¹⁶ cm² in SiO₂ has been observed. A temperature-dependent trap generation rate and defects have also been investigated using time-dependent current density variation during CVS. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating high-k stacked layers. SILC generation kinetics, i.e. defect generation probability under different injected fluences for various high-constant stress voltages in both polarities have been studied. An empirical relation between trap generation probability and applied stress voltage for various injected fluences has been developed.     

Highly conducting transparent nanocrystalline Cd1−xSnxS thin film synthesized by RF magnetron sputtering and studies on its optical, electrical and field emission properties

Transparent conducting Cd_1−xSn_xS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates for various tin concentrations in the films. X-ray diffraction studies showed broadening of peaks due to smaller crystal size of the Cd_1−xSn_xS films, and SEM micrographs showed fine particles with average size of 100 nm. Sn concentration in the films was varied from 0% to 12.6% as determined from energy-dispersive X-ray analysis. The room-temperature electrical conductivity was found to vary from 8.086 to 939.7 S cm⁻¹ and corresponding activation energy varied from 0.226 to 0.076 eV. The optimum Sn concentration for obtaining maximum conductivity was found to be 9.3%. The corresponding electrical conductivity was found to be 939.7 S cm⁻¹, and the mobility 49.7 cm² V⁻¹ s⁻¹. Hall measurement showed very high carrier concentrations in the films lying in the range of 8.0218×10¹⁸–1.7225×10²⁰ cm⁻³. The conducting Cd_1−xSn_xS thin films also showed good field emission properties with a turn on field 4.74–7.86 V μm⁻¹ with variation of electrode distance 60–100 μm. UV–Vis–NIR spectrophotometric studies of the films showed not needed the optical band gap energy increased from 2.62 to 2.80 eV with increase of Sn concentration in the range 0–12.6%. The optical band gap was Burstein–Moss shifted, and the corresponding carrier concentration obtained from the shift also well matched with that obtained from Hall measurement.     

Thickness dependent electronic structure of ultra-thin tetrahedral amorphous carbon (ta-C) films

Microstructural properties of ultrathin (1-10 nm) tetrahedral amorphous carbon (ta-C) films are investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray Photoelectron Spectroscopy, Raman spectroscopy and Atomic Force Microscopy (AFM). The CK-edge NEXAFS spectra of 1 nm ta-C films provided evidence of surface defects (C―H bonds) which rapidly diminish with increasing film thickness. A critical thickness for stabilization of largely sp³ matrix structure distorted by sp² sites is observed via the change of π*C═C peak behavior. Meanwhile, an increase in the film thickness promotes an enhancement in sp³ content, the film roughness remains nearly constant as probed by spectroscopic techniques and AFM, respectively. The effect of thickness on local bonding states of ultrathin ta-C films proves to be the limiting factor for their potential use in magnetic and optical storage devices.     

Hybrid orientation technology and strain engineering for ultra-high speed MOSFETs

We report here RF MOSFET performance in sub-45-nm hybrid orientation CMOS technology. Based on the combination of hybrid orientation technology (HOT) and process-induced local strain engineering, MOSFET RF performance is investigated using CAD (TCAD) technology. Transistor optimization on (100) substrate via silicon nitride (Si ₃ N ₄ ) cap layer thickness for n-MOSFETs, Ge mole fraction optimization for p-MOSFETs on (110) substrates and channel length scaling have resulted in record RF performance, viz. the cut-off frequency, ${f_{\rm T}}$ .