The dynamic creep behavior of a filled poly(dimethylsiloxane) elastomer was studied under cyclic stress. The stress level was chosen such that the increase in the internal temperature was small and that microcracks were not observed. This work has demonstrated that cyclic stress in combination with high temperature accelerates the degradation of the elastomer. The results suggest that because of the applied force, breaks in the load-bearing chains of the network occur. These breaks, while relieving the mechanical stress, create highly reactive ionic fragments. It is believed that because of the subsequent reactions of the ionic fragments, changes in the specific gravity, storage modulus, effective crosslink density, and length of the sample (creep) are observed. The observed decrease in the storage modulus is thought to occur because of the reaction of the ionic fragments with moisture, which results in the formation of silanol chain ends that reduce the effective crosslink density. The results also show that contrary to the prediction of the Boltzmann's Superposition Principle, the rate of creep is greatly enhanced when the sample is subjected to a sinusoidally varying dynamic load as compared to a comparable static load. The polymer weight loss was found to be linear with time and strongly dependent on the level of applied dynamic and static force. In addition, the weight loss and rate of creep were also found to be strongly dependent upon temperature. 相似文献
Software quality assurance (SQA) audits are becoming a standard practice in corporations seeking to enter contractual relations with other organizations, or just evaluating their own state of readiness. This paper describes one approach to conducting SQA audits. Excerpts from reports of actual audits conducted on several organizations engaged in software development are used to show how an audit is conducted and what can be achieved through the auditing process. The paper is not meant to be a definitive answer or guide to doing SQA audits. Rather, it serves as an annotated case study of how SQA audits can be conducted with an emphasis on data gathering and reporting.Dr Paul W. Oman is an associate professor of computer science at the University of Idaho, and an independent software consultant who specializes in software analysis. He is the director of the Software Engineering Test Lab within the UI College of Engineering, where he conducts research in software development methods, CASE tools, and software maintainability issues. Oman has published over 50 articles and reports on software tools, software development, and computer science education. He has a PhD in computer science and is a member of the IEEE, IEEE Computer Society, and ACM. 相似文献
This study was designed to determine metabolic and physical performance responses to ingestion of pre-exercise meals with different macronutrient and fiber profiles. Twelve physically active subjects (6 males and 6 females) were used to investigate the metabolic and physical performance consequences of consuming pre-exercise meals consisting of oat, corn, or wheat cereals. A fasting trial served as the control, and all subjects received each treatment in a Latin-square design. Blood samples were drawn before and 85 min after meal ingestion, during 90 min of cycling exercise (60% VO2peak), after a 6.4 km performance ride, and during 60 min of recovery. Expired air samples were collected to determine nutrient utilization. Resting carbohydrate oxidation rates and plasma insulin concentrations after oat ingestion were less than after wheat, and corn and wheat ingestion, respectively (P < 0.05). During exercise, the change in plasma glucose from pre-exercise was greater after consuming wheat and corn compared with oat (P < 0.05), and it was inversely related to pre-exercise plasma insulin concentration (r = -0.55, P = 0.0001). Plasma free fatty acid concentrations were inversely related to plasma lactate concentrations (r = -0.58, P = 0.0001). Free fatty acid concentrations and fat oxidation were greater in fasting trials than all others, but performance ride times did not differ among treatments. Plasma branched-chain amino acid concentrations resembled their respective meal profiles throughout exercise, the performance ride, and recovery. These results indicate that pre-exercise meal composition can influence glucose homeostasis during early exercise and plasma branched-chain amino acid concentrations over a substantial range of metabolic demands. 相似文献
Mother–daughter dyads from incestuous vs nonclinical families were compared in terms of (1) family life stress, (2) problem-solving appraisal, (3) direct coping behaviors, (4) mother–daughter communication, and (5) family adaptation and cohesion. 15 mother–daughter pairs from an incestuous family involving an adult male caretaker were matched on several variables to 16 mother–daughter pairs from a nonclinical family. The results revealed that mothers from the incest group (compared with the nonclinical group) reported greater amounts of family life stress, had more negative perceptions of their problem-solving appraisal, and reported more communication difficulties. The daughters from the incest group appeared to be quite similar to the nonclinical group daughters, although they did acknowledge more family stress. Implications of the results for therapeutic interventions as well as future research were discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
A probabilistic risk assessment (PRA) model, developed for the Thermal Protection System (TPS) of the space shuttle orbiter and presented in the previous paper, is used as a management tool to identify root-cause, organizational factors of the various failure modes. The objective is to set priorities in the process of resource allocation to minimize the risk of accident caused by the failure of the TPS. Starting with the technical characteristics of the system and the inputs of the risk assessment model, the approach is to identify the human decisions and actions and the key organizational factors that influence the risk.
Among the management factors that affect the reliability of the TPS are time pressures that have occurred in the past, liability concerns and conflicts among contractors, the low status of the tile work and material technicians among maintenance personnel, the absence of priorities in tile testing, and under-recognized couplings among subsystems (such as the external tank insulation as a source of debris that may hit the tiles). It is shown here how using the PRA results to set priorities in the maintenance of the tiles can allow reduction of the overall risk, and how critical zones of debris sources can be identified on the surface of the external tank and the solid rocket booster. It was found, for instance, that detecting and fixing loose tiles in the most risk-critical areas and securing insulation by up to 80%, and securing the insulation of external systems in specified areas could reduce the TPS risk by about 75%. 相似文献
We have demonstrated feasibility to form silicon-on-insulator (SOI) substrates using plasma immersion ion implantation (PIII)
for both separation by implantation of oxygen and ion-cut. This high throughput technique can substantially lower the high
cost of SOI substrates due to the simpler implanter design as well as ease of maintenance. For separation by plasma implantation
of oxygen wafers, secondary ion mass spectrometry analysis and cross-sectional transmission electron micrographs show continuous
buried oxide formation under a single-crystal silicon overlayer with sharp Si/SiO2 interfaces after oxygen plasma implantation and high-temperature (1300°C) annealing. Ion-cut SOI wafer fabrication technique
is implemented for the first time using PIII. The hydrogen plasma can be optimized so that only one ion species is dominant
in concentration and there are minimal effects by other residual ions on the ion-cut process. The physical mechanism of hydrogen
induced silicon surface layer cleavage has been investigated. An ideal gas law model of the microcavity internal pressure
combined with a two-dimensional finite element fracture mechanics model is used to approximate the fracture driving force
which is sufficient to overcome the silicon fracture resistance. 相似文献