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排序方式: 共有280条查询结果,搜索用时 15 毫秒
91.
Elmar STRASSBURGER Steffen BAUER Gregor POPKO 《兵工学报(英文版)》2014,(2):226-238
Transparent armor consists of glass-polymer laminates in most cases. The formation and propagation of damage in the different glass layers has a strong influence on the ballistic resistance of such laminates. In order to clarify the course of events during projectile penetration, an experimental technique was developed, which allows visualizing the onset and propagation of damage in each single layer of the laminate. A telecentric objective lens was used together with a microsecond video camera that allows recording 100 frames at a maximum rate of I MHz in a backlit photography set-up. With this technique, the damage evolution could be visualized in glass laminates consisting of four glass layers with lateral dimensions 500 mm x 500 mm. Damage evolution was recorded during penetration of 7.62 mm AP projectiles with tungsten carbide core and a total mass of 11.1 g in the impact velocity range from 800 to 880 rn/s. In order to measure the deformation of single glass plates within the laminates, a piece of reflecting tape was attached to the corresponding glass plate, and photonic Doppler velocimetry (PDV) was applied. With the photonic Doppler velocimeter, an infrared laser is used to illuminate an object to be measured and the Doppler-shifted light is superimposed to a reference light beam at the detector. The simultaneous visualization and PDV measurement of the glass deformation allow determining the deformation at the time of the onset of fracture. The analysis of the experimental data was supported by numerical simulations, using the AUTODYN commercial hydro-code. 相似文献
92.
Ping Zhou Elmar Beeh Horst E. Friedrich 《Journal of Materials Engineering and Performance》2016,25(3):853-865
Magnesium alloys are promising materials for lightweight design in the automotive industry due to their high strength-to-mass ratio. This study aims to study the influence of tension-compression asymmetry on the radius of curvature and energy absorption capacity of AZ31B-O magnesium alloy sheets in bending. The mechanical properties were characterized using tension, compression, and three-point bending tests. The material exhibits significant tension-compression asymmetry in terms of strength and strain hardening rate due to extension twinning in compression. The compressive yield strength is much lower than the tensile yield strength, while the strain hardening rate is much higher in compression. Furthermore, the tension-compression asymmetry in terms of r value (Lankford value) was also observed. The r value in tension is much higher than that in compression. The bending results indicate that the AZ31B-O sheet can outperform steel and aluminum sheets in terms of specific energy absorption in bending mainly due to its low density. In addition, the AZ31B-O sheet was deformed with a larger radius of curvature than the steel and aluminum sheets, which brings a benefit to energy absorption capacity. Finally, finite element simulation for three-point bending was performed using LS-DYNA and the results confirmed that the larger radius of curvature of a magnesium specimen is mainly attributed to the high strain hardening rate in compression. 相似文献
93.
This article explores efficient climate policies in terms of investment streams into fossil and renewable energy technologies. The investment decisions maximise social welfare while observing a probabilistic guardrail for global mean temperature rise under uncertain technology and climate parameters. Such a guardrail constitutes a chance constraint, and the resulting optimisation problem is an instance of chance constrained programming, not stochastic programming as often employed. Our analysis of a model of economic growth and endogenous technological change, MIND, suggests that stringent mitigation strategies cannot guarantee a very high probability of limiting warming to 2 °C since preindustrial time under current uncertainty about climate sensitivity and climate response time scale. Achieving the 2 °C temperature target with a probability P? of 75% requires drastic carbon dioxide emission cuts. This holds true even though we have assumed an aggressive mitigation policy on other greenhouse gases from, e.g., the agricultural sector. The emission cuts are deeper than estimated from a deterministic calculation with climate sensitivity fixed at the P? quantile of its marginal probability distribution (3.6 °C). We show that earlier and cumulatively larger investments into the renewable sector are triggered by including uncertainty in the technology and climate response time scale parameters. This comes at an additional GWP loss of 0.3%, resulting in a total loss of 0.8% GWP for observing the chance constraint. We obtained those results with a new numerical scheme to implement constrained welfare optimisation under uncertainty as a chance constrained programming problem in standard optimisation software such as GAMS. The scheme is able to incorporate multivariate non-factorial probability measures such as given by the joint distribution of climate sensitivity and response time. We demonstrate the scheme for the case of a four-dimensional parameter space capturing uncertainty about climate and technology. 相似文献
94.
95.
To mimic the adhesive effects of gecko toes, artificial surfaces have been manufactured recently using polydimethylsiloxanes (PDMS). However, the effects of repeated contacts on the adhesive properties remain largely unexplored. In this paper we report on the effect of repeated pull‐off force measurements on the adhesion behavior of PDMS (polymer kit Sylgard 184, Dow Corning) tested with a borosilicate glass probe. A decrease in pull‐off force with increase in number of test cycles is found until a plateau is reached. The initial value and the rate of change in pull‐off force strongly depend on the sample preparation procedure, including curing time and cross‐linking. It is proposed that the behavior is due to steady coverage of the probe with free oligomers. The results are crucial for developing reusable, durable, and residue‐free bioinspired adhesives. 相似文献
96.
It is known that in historical buildings as well as new buildings, facade claddings consisting of marble panels, are prone to damage patterns such as softening, crack formation, and bowing. The degree of damage is apparently dependent on the location of the building, the external climatic conditions, and the type of fastening of the facade panels. In this article, it is examined by means of nonstationary thermal simulation, to which extent the temperature fields, which develop in the facade panel, have an effect on the damage of the panel. It is shown that the diurnal and annual amplitude of the panel temperature induced by solar flux, external air temperature fluctuations, and long-wave input radiation and irradiation have a substantial influence on the damage process. On the example of the simulation of the thermal behavior of a facade panel for the building locations in Athens, Greece, Vienna, Austria, and Helsinki, Finland, it is shown that the observed increase of the susceptibility of marble facades to damages can be explained with the increase of the geographical latitude through the differences in the external climatic conditions. It is strongly recommended that architects and restorers install specific marble facade panels (like carrara marble) in close thermal contact with the wall behind, since this reduces the susceptibility of the facade to damage. 相似文献
97.
Stereoscopic 3D technology gives visual content creators a new dimension of design when creating images and movies. While useful for conveying emotion, laying emphasis on certain parts of the scene, or guiding the viewer's attention, editing stereo content is a challenging task. Not respecting comfort zones or adding incorrect depth cues, for example depth inversion, leads to a poor viewing experience. In this paper, we present a solution for editing stereoscopic content that allows an artist to impose disparity constraints and removes resulting depth conflicts using an optimization scheme. Using our approach, an artist only needs to focus on important high‐level indications that are automatically made consistent with the entire scene while avoiding contradictory depth cues and respecting viewer comfort. 相似文献
98.
Piotr Didyk Elmar Eisemann Tobias Ritschel Karol Myszkowski Hans‐Peter Seidel 《Computer Graphics Forum》2010,29(2):713-722
High‐refresh‐rate displays (e. g., 120 Hz) have recently become available on the consumer market and quickly gain on popularity. One of their aims is to reduce the perceived blur created by moving objects that are tracked by the human eye. However, an improvement is only achieved if the video stream is produced at the same high refresh rate (i. e. 120 Hz). Some devices, such as LCD TVs, solve this problem by converting low‐refresh‐rate content (i. e. 50 Hz PAL) into a higher temporal resolution (i. e. 200 Hz) based on two‐dimensional optical flow. In our approach, we will show how rendered three‐dimensional images produced by recent graphics hardware can be up‐sampled more efficiently resulting in higher quality at the same time. Our algorithm relies on several perceptual findings and preserves the naturalness of the original sequence. A psychophysical study validates our approach and illustrates that temporally up‐sampled video streams are preferred over the standard low‐rate input by the majority of users. We show that our solution improves task performance on high‐refresh‐rate displays. 相似文献
99.
Thomas Schlich 《Ethik in der Medizin》1999,11(2):79-88
Zusammenfassung. Die aktuelle Diskussion um den Hirntod ist von historischen Argumenten gepr?gt. Die erste zentrale Frage lautet, ob die Einführung
des Hirntodkriteriums einen Bruch mit der Vergangenheit bedeutet oder nicht. Zweitens geht es darum, ob der Hirntod zum Zweck
der Organgewinnung eingeführt wurde oder nicht. Diese historischen Fragestellungen werden jedoch in der Hirntoddiskussion
nicht mit angemessenen historischen Mitteln bearbeitet. Das Beispiel der Hirntoddiskussion zeigt die Allgegenwart geschichtlicher
Argumente in der Diskussion ethischer Probleme und die Bedeutung eines Qualit?tsstandards für solche Aussagen über die Vergangenheit.
相似文献
100.