Multiple film cracking in film/substrate systems with residual stresses and unidirectional loading

Multiple film cracking in film/substrate systems is analyzed in the present study. Specifically, the experimental measurements of multiple cracking of SiO _x films of various thicknesses on polyethylene terephthalate substrates are analyzed. The system is subjected to both residual stresses and unidirectional tensile loading. Considering a three- dimensional geometry, an analytical model is developed to derive the stress distribution in the system, and the film-cracking problem is analyzed using both the strength and the energy criteria. Compared to the strength criterion, the energy criterion shows better agreement with the measurements of the crack density versus applied strain relation.     

Telomerase activity in female and male rat germ cells undergoing meiosis and in early embryos

Telomerase is a ribonucleoprotein that synthesizes telomeric DNA at the ends of eukaryotic chromosomes. It has been hypothesized that telomerase activity is necessary for cellular immortalization and that telomerase activity is present in cells of germline origin. The objective of the present study was to determine the level of telomerase activity in the following rat cells: 1) oocytes from follicles at different stages of development, 2) spermatogenic cells, and 3) early embryos. Telomerase activity was quantitated using a recently developed, sensitive polymerase chain reaction-based assay and a human kidney cell line (293) as a standard. Telomerase activity was found in oocytes from early antral and preovulatory follicles, as well as in ovulated oocytes. The level of enzyme activity in early antral and preovulatory follicles was comparable to that of the 293 cells, while levels in ovulated oocytes were 50-fold lower. Telomerase activity was present in even lower levels in pachytene spermatocytes and round spermatids, and no telomerase activity was detected in spermatozoa from either the caput or the cauda epididymis. After fertilization, telomerase activity was present in 4-cell embryos. Telomerase activity was also detected in several rat somatic tissues. These data demonstrate that telomerase activity is present in germ cells at several stages of differentiation, with the exception of spermatozoa, and suggest that telomerase activity may be important during meiosis. The high levels of telomerase activity in individual oocytes may serve as a marker for monitoring the effects of hormonal agents, aging, and toxins on oocyte quality.     

Bacteremia and fungemia in patients with advanced human immunodeficiency virus (HIV) infection in Taiwan

To understand the etiology and clinical outcome of bacterial and fungal sepsis in patients with advanced human immunodeficiency virus (HIV) infection in Taiwan, we conducted a prospective study of nonmycobacterial bacteremia and fungemia in HIV-infected patients with fever who were admitted to a university hospital in Taiwan during a 42-month period. Of 210 patients, 41 (19.5%) had a total of 52 episodes of sepsis due to nonmycobacterial bacteria or fungi, or both (15.5% of 336 episodes of fever). All but one patient had acquired immunodeficiency syndrome (AIDS), and the mean CD4 lymphocyte count was 29/microL (range, 0-321/microL). A total of 57 pathogens (39 bacteria and 18 fungi) were isolated from blood; polymicrobial sepsis due to both bacteria and fungi occurred in four episodes. Nontyphoid Salmonella (NTS) was the most common cause of community-acquired bacteremia (24/30, 80%). Staphylococcus aureus bacteremia was diagnosed in three episodes while Streptococcus pneumoniae bacteremia was found in only one. Cryptococcus neoformans was the most common cause of fungemia and was responsible for 12 episodes, while fungemia due to Penicillium marneffei and Histoplasma capsulatum, two emerging fungi in Taiwan, were diagnosed in four cases and one case, respectively. Nine episodes, eight of bacteremia and one of candidemia, were nosocomial. The overall in-hospital mortality was 29%, and nosocomial sepsis was associated with a higher mortality rate (56%, p = 0.02). The mean duration of survival after recovery from initial sepsis was 426 days. We conclude that NTS bacteremia was the most common cause of sepsis in patients with advanced HIV infection in Taiwan and clinicians caring for such patients should watch for emerging fungal infections. Nosocomial sepsis was associated with a high mortality rate. The mean survival duration after recovery from sepsis of our patients was short.     

Evaluations of residual axial stresses and interfacial friction in Nicalon fibre-reinforced macro-defect-free cement composites

The residual axial stress and the interfacial frictional stress of Nicalon fibre-reinforced macro-defect-free cement composites have been evaluated using the indentation technique. In these composites, both the residual axial stress in the fibre and the interfacial bonding are relatively high compared to other ceramic composites. Owing to the high residual axial stress, Poisson's effect can be ignored in analysing the stress-displacement relation of the fibre, and the interfacial frictional stress along the sliding length can be assumed to be a constant during indentation loading and unloading. Because of the strong interfacial bonding, consideration of the upper bound of the sliding zone length during unloading is essential. The sliding zone length during unloading is limited by the debond length at peak loading. The theoretical predictions of stress-displacement relations are in excellent agreement with the experimental results.     

Some considerations of asymmetric cracking in an applied-moment double cantilever beam

In situ observations of crack propagation in applied-moment double cantilever beam specimens have been used to obtain the R-curve behaviour of Si₃N₄/50% BN–50% Al₂O₃ laminated composites, in which the BN–Al₂O₃ layers function as weak interphases. The crack plane and the crack direction were, respectively, normal and parallel to the plane of the laminated layers. During crack propagation, both delamination and crack deviation from the centreline of the specimen occurred. A deviated crack resulted in an uneven moment of inertia in the two beams of the specimen. For a non-laminated material, a deviated crack would become unstable, such that the crack would propagate towards the beam with the smaller moment of inertia. It was found in the present study that delamination in a laminated composite can stabilize the propagation of a deviated crack. The stabilization of a deviated crack with delamination was due to a decrease in the inequality in the moment of inertia of the two beams compared to that without delamination.     

Matrix cracking with frictional bridging fibres in continuous fibre ceramic composites

Interfacial debonding and matrix cracking due to residual axial stresses have been analysed for unidirectional fibre-reinforced ceramic composites. The analytical solutions for the crack-opening displacement, the axial displacement of the composite due to interfacial debonding, and the critical residual axial stress for matrix cracking have been obtained. The solutions were then compared with those for tensile loading in the fibre direction. Three issues related to Part I, i.e. the effective fracture toughness of the composite, the critical loading stress for matrix cracking in the presence of residual stresses, and the debonded fibre length due to loading, were also addressed in the present study.     

Minimum-Variance and Maximum-Likelihood Deconvolution for Noncausal Channel Models

This paper extends the previous works of Mendel and his students on the subject of deconvolution from causal channel (wavelet) models to noncausal channel models. Noncausal wavelets occur, for example, in seismic data processing when a land vibrator is used to excite the Earth. Minimum-variance and maximum-likelihood deconvolution algorithms are developed herein for symmetrical and/or nonsymmetrical time-invariant wavelets that are excited by stationary and/or nonstationary white noise inputs. Minimum-variance deconvolution algorithms for a noncausal wavelet turn out to be quite different than those for a causal wavelet; however, maximum-likelihood deconvolution algorithms for a noncausal wavelet, which involve event detection and amplitude restoration, are essentially the same as those for a causal wavelet. Examples are provided that illustrate the performance of the different deconvolution algorithms.