蜂窝夹层结构弯曲疲劳寿命预测研究

复合材料夹层板兼具轻质高承载特性,但也面临多样的失效模式,深入研究其性能与失效机理,对确保结构安全与效能至关重要.针对一种玻璃纤维/氰酸酯织物层压面板和Nomex蜂窝复合的夹层板,采用特殊设计的试验夹具,开展三点弯曲静力和疲劳试验,分析该夹层板的弯—剪承载特性,观察其疲劳损伤和失效现象;同时,根据夹层板承力特点,对受面内力的面板和受面外剪切的芯材分别建立疲劳损伤模型,借助夹层板等效有限元模型,预测夹层板弯曲疲劳寿命.结果表明:所推荐的预测方法能够合理地反映夹层板弯曲疲劳损伤过程,疲劳寿命预测结果略为保守,与实测寿命误差属于可接受范围.     

UVCS Observations and Modeling of Streamers

We present results derived from the analysis of an equatorial streamer structure as observed by the UVCS instrument aboard SOHO. From observations of the H I Lyα and Lyβ lines we infer the density and temperature of the plasma. We develop a preliminary axisymmetric, magnetostatic model of the corona which includes the effects of gas pressure gradients on the magnetic structure. We infer a coronal plasma β > 1 in the closed field regions and near the cusp of the streamer. We add to the model a parallel velocity field assuming mass flux conservation along magnetic flux tubes. We then compute the Lyα emissivity and the line-of-sight integrals to obtain images of Lyα intensity, taking into account projection effects and Doppler dimming. The images we obtain from this preliminary model are in good general agreement with the UVCS observations, both qualitatively and quantitatively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Intrinsic time scale for reconnection on the dayside magnetopause

Recently much attention has been focused on the transient behavior of the magnetopause in response to pressure pulses and southward fluctuations of the interplanetary magnetic field. We examine the motion of the magnetopause behind the foreshock and conclude that this motion is affected by foreshock pressure variations but not by fluctuations in the direction of the magnetic field. Neither magnetopause erosion nor flux transfer event occurrence is controlled by the foreshock. On the contrary, flux transfer events occur at times of steady IMF and thier quasi-periodic behavior is controlled by the magnetopause or the magnetosphere and is not driven by the external boundary conditions. Since flux transfer events are clearly due to reconnection, this observation implies that the IMF must be southward some time perhaps as long as 7 minutes before flux transfer begins.     

Lyapunov stability designs for switching regulator/low-pass filtersystem

The second method of Lyapunov is employed to determine quantitatively accurate criteria for asymptotic stability of the fourth-order switching-regulator/low-pass filter system. Modeling the regulator as a linearized negative resistance, this analysis provides alternative stability relationships to those found in the literature and also reveals the limitations of the Routh stability criterion. Design examples are included which ensure system stability and reduce the amplitude of the filter resonant peak (audio-susceptibility constraint)