Effects of Overload and Underload on Internal Strains/Stresses and Crack Closure during Fatigue-Crack Propagation

The overload and/or underload occurring during constant-amplitude fatiguecrack growth result in the retardation and/or acceleration in the crack-growth rate, making it difficult to predict the crack-propagation behavior and fatigue lifetime. Although there have been numerous investigations to account for these transient crackgrowth behavior, the phenomena are still not completely understood. Neutron and X-ray diffraction, and electric-potential measurements were employed to investigate these transient crack-growth micromechanisms; gain a thorough understanding of the crack-tip deformation and fracture behaviors under applied loads; and establish a quantitative relationship between the crack-tip-driving force and crack-growth behavior. Five different fatigue-crack-growth experiments (i.e., fatigued, tensile overloaded, compressive underloaded, tensile overloaded-compressive underloaded, and compressive underloaded-tensile overloaded) were performed to observe these transient crack-growth behaviors. The development of internal-strain distributions during variable-amplitude loadings, and the resultant residual-stress distributions around a crack tip were examined using neutron diffraction. The effects of a single tensile overload on fatigue-crack growth were focused on probing the crack-growth-retardation micromechanisms. Neutron diffraction and polychromatic X-ray microdiffraction showed high dislocation densities and considerable crystallographic tilts near the crack tip immediately after the overload. The interactions between the overload-induced plastic zone and newly-developed fatigue plastic zone, and their influences on the evolution of residual-strain profiles are discussed. Neutron-diffraction and electric-potential measurements provide in-situ observation of the crack-opening/closing processes and internal-stress distributions in the vicinity of the crack tip during real-time fatigue-crack propagation following a tensile overload. Immediately after applying a tensile overload, the crack-tip became blunt and the large compressive residual stresses were developed around the crack tip. In the retardation period after the tensile overloading, the combined effects of the cracktip blunting at an overload point and compressive-residual stresses accompanying the crack closure induced the stress concentration at a blunting region until a maximum crack-arrest load was reached. Then, the stress concentration was transferred from the blunting region to actual crack-tip position with gradual crack opening, requiring a higher applied load. This observation of the stress-transfer phenomenon significantly promotes the fundamental understanding of overload-retardation phenomena. The postoverload crack-growth rates were normalized with the effective-stress-intensity-factor range, which suggests that it can be considered as the fatigue-crack-tip-driving force

Transistor Degradations in Very Large-Scale-Integrated CMOS Technologies

The historical evolution of hot carrier degradation mechanisms and their physical models are reviewed and an energy-driven hot carrier aging model is verified that can reproduce 62-nm-gate-long hot carrier degradation of transistors through consistent aging-parameter extractions for circuit simulation. A long-term hot carrier-resistant circuit design can be realized via optimal driver strength controls. The central role of the V GS ratio is emphasized during practical case studies on CMOS inverter chains and a dynamic random access memory (DRAM) word-line circuit. Negative bias temperature instability (NBTI) mechanisms are also reviewed and implemented in a hydrogen reaction-diffusion (R-D) framework. The R-D simulation reproduces time-dependent NBTI degradations interpreted into interface trap generation, Δ N it with a proper power-law dependency on time. The experimental evidence of pre-existing hydrogen-induced Si–H bond breakage is also proven by the quantifying R-D simulation. From this analysis, a low-pressure end-of-line (EOL) anneal can reduce the saturation level of NBTI degradation, which is believed to be caused by the outward diffusion of hydrogen from the gate regions and therefore prevents further breakage of Si–H bonds in the silicon-oxide interfaces

Integrating spatial and spectral information for automatic feature identification in high -resolution remotely sensed images

This research used image objects, instead of pixels, as the basic unit of analysis in high-resolution imagery. Thus, not only spectral radiance and texture were used in the analysis, but also spatial context. Furthermore, the automated identification of attributed objects is potentially useful for integrating remote sensing with a vector-based GIS.;A study area in Morgantown, WV was chosen as a site for the development and testing of automated feature extraction methods with high-resolution data. In the first stage of the analysis, edges were identified using texture. Experiments with simulated data indicated that a linear operator identified curved and sharp edges more accurately than square shaped operators. Areas with edges that formed a closed boundary were used to delineate sub-patches. In the region growing step, the similarities of all adjacent subpatches were examined using a multivariate Hotelling T2 test that draws on the classes\u27 covariance matrices. Sub-patches that were not sufficiently dissimilar were merged to form image patches.;Patches were then classified into seven classes: Building, Road, Forest, Lawn, Shadowed Vegetation, Water, and Shadow. Six classification methods were compared: the pixel-based ISODATA and maximum likelihood approaches, field-based ECHO, and region based maximum likelihood using patch means, a divergence index, and patch probability density functions (pdfs). Classification with the divergence index showed the lowest accuracy, a kappa index of 0.254. The highest accuracy, 0.783, was obtained from classification using the patch pdf. This classification also produced a visually pleasing product, with well-delineated objects and without the distracting salt-and-pepper effect of isolated misclassified pixels. The accuracies of classification with patch mean, pixel based maximum likelihood, ISODATA and ECHO were 0.735, 0.687, 0.610, and 0.605, respectively.;Spatial context was used to generate aggregate land cover information. An Urbanized Rate Index, defined based on the percentage of Building and Road area within a local window, was used to segment the image. Five summary landcover classes were identified from the Urbanized Rate segmentation and the image object classification: High Urbanized Rate and large building sizes, Intermediate Urbanized Rate and intermediate building sizes, Low urbanized rate and small building sizes, Forest, and Water