Differences Between Mode I and Mode II Crack Propagation

Although mode I and mode II crack propagation show many similarities, in particular when analysed by linear elastic fracture mechanics, they differ significantly in the micro-structural behaviour. These differences, which are clearly noticeable in the fracture surface morphology, lead to fundamental differences in the macroscopic behaviour. Thus, mode II crack expansion under remote loading, appears to obey micro-structural scaling laws, implying that the dimensions of the process region stay essentially constant during crack expansion, rather than increasing with crack length. Therefore, expanding mode II cracks can almost reach the Rayleigh velocity, and actually also intersonic velocities. An expanding mode I crack, on the other hand, seems to obey continuum scaling laws, implying that the dimensions of the process region increase in proportion to crack length, leading to self-similar crack expansion at a velocity significantly below the Rayleigh speed and dependent on the remote load.     

MORPHOLOGICAL CHARACTERISTICS OF FREE BENDS

1　ＩＮＴＲＯＤＵＣＴＩＯＮ　Ａｍｏｎｇｔｈｅｄｉｖｅｒｓｉｔｙｏｆｅｘｉｓｔｉｎｇｒｉｖｅｒｃｈａｎｎｅｌｐｒｏｃｅｓｓｅｓ,ｍｅａｎｄｅｒｉｎｇｉｓｔｈｅｍｏｓｔｃｏｍｍｏｎａｎｄｆｒｅｑｕｅｎｔｏｎｅ.Ｉｔｉｓｔｙｐｉｃａｌｌｙｔｈｅｃｏｍｍｏｎ?..     

EISCAT雷达观测到的极区电离层等离子体反常对流

综合分析EISCAT雷达与卫星当地测量数据,并利用磁层磁场模式对磁力线进行追踪,研究了发生在极光椭圆朝极盖边界附近电离层中,一例反常的背离太阳流动的强等离子体对流事件,及相关的太阳风-磁层-电离层耦合过程.结果表明,磁暴期间IMFBz指向南时观测到这一反常高速对流,及其相应的等离子体性态特征,很可能是向阳侧磁层顶磁重联过程在电离层中的印记.     

Treatment of ground-motion predictive relationships for the reference seismic hazard map of Italy

In the framework of the 2004 reference seismic hazard map of Italy the amplitude of the strong-motion (expressed in terms of Peak Horizontal Acceleration with 10% probability of non-exceedence in 50 years, referred to average hard ground conditions) was computed using different predictive relationships. Equations derived in Italy and in Europe from strong-motion data, as well as a set of weak and strong-motion based empirical predictive relationships were employed in a logic tree procedure, in order to capture the epistemic uncertainty affecting ground-motion attenuation. This article describes the adjustments and conversions required to eliminate the incompatibilities amongst the relations. Particularly significant are distance conversions and style-of-faulting adjustments, as well as the problems related to the use of regional relations, such as the selection of a reference depth, the quantification of random variability and the strong-motion prediction. Moreover, a regional attenuation relationship specific for volcanic areas was also employed, allowing a more realistic evaluation of seismic hazard, as confirmed by the attenuation of macroseismic intensities.     

Influence of source region properties on scaling relations forM<0 events

Excavation induced seismic events with moment magnitudesM<0 are examined in an attempt to determine the role geology, excavation geometry, and stress have on scaling relations. Correlations are established based on accurate measurements of excavation geometry and methodology, stress regime, rock mass structure, local tectonics, and seismic locations. Scaling relations incorporated seismic moments and source radii obtained by spectral analysis, accounting for source, propagation, and site effects, and using Madariaga's dynamic circular fault model. Observations suggest that the interaction of stresses with pre-existing fractures, fracture complexity and depth of events are the main factors influencing source characteristics and scaling behaviour. Self-similar relationships were found for events at similar depths or for weakly structured rock masses with reduced clamping stresses, whereas a non-similar behaviour was found for events with increasing depth or for heavily fractured zones under stress confinement. Additionally, the scaling behaviour for combined data sets tended to mask the non-similar trends. Overall, depth and fracture complexity, initially thought as second order effects, appear to significantly influence source characteristics of seismic events withM<0 and consequently favour a non-similar earthquake generation process.     

中尺度模式中不同对流参数化方案的比较试验

MM5模式中有多种积云参数化方案可供选择,粗细网格采用不同的积云参数化方案对降水预报有一定的影响。采用常用的四种积云参数化方案,粗细网格进行不同配置,对辽东半岛大暴雨过程进行试验发现：细网格信息可以通过嵌套边界向外层传递,影响外层预报结果。细网格选取GR方案比选取AK、BM、KF方案模拟效果好很多;细网格参数化方案不变,粗网格取不同的参数化方案,对细网格的模拟结果差别不大。但选取一个好的参数化方案对细网格的模拟结果会稍有改进。     

A spatio-temporal analysis of academic performance at the Basic Education Certificate Examination in Ghana

Over the last decade, Ghana has more than tripled investment in its basic education system. Consequently, the country has made huge educational gains, primarily in providing universal access to basic education. However, many stakeholders are worried that academic performance is lagging because of disproportional attention to accessing basic education. Discussion of these concerns is hampered by ongoing disagreement about the true trajectory of academic performance at the basic education level and the widespread nature of students' lagging academic performance. In part, this disagreement stems from the failure of empirical studies to comprehensively examine trends in academic performance standards at the basic education level by concurrently considering a geographical and longitudinal perspective. Thus, this study examines the spatio-temporal trends of academic performance at the junior high school level since 2009 by using multilevel growth curve modeling, spatial statistics, and district-level longitudinal data. Results reveal 3 statistically distinct trajectories of academic performance: erratic, accelerating, and decelerating changes. Results also show that rural–urban gaps explain 31% of the performance trajectories, a trend which is expected to persist in the long term. In addition, we find extreme variations in academic performance within rural areas. Given the varying trajectories and geographical variability in academic performance, we suggest a localized approach to addressing challenges of low academic achievement at the basic education level in Ghana.