Efforts have been made to characterize the mechanical properties of carbon nanotubes. Previous work has concentrated on the tubes' longitudinal properties, and studies of their radial properties have lagged behind. We have used a scanning probe microscope with an indentation/scratch function to investigate the radial compression of multiwalled carbon nanotubes under an asymmetric stress. In particular, we have determined the radial compressive elastic modulus at different compression levels and have estimated the compressive strength to be well beyond 5.3 GPa.

a b s t r a c tBased on the pressure-state-response conceptual model, we constructed the urban ecological security evaluation index system for 13 cities in the Beijing-Tianjin-Hebei metropolitan region from 2003 to 2014, where integrated development has occurred up to the national strategic level. The entropy-weight method was used to calculate the index weights to eliminate subjectivity. After the perfection and detailing of the increased half-ladder membership function and the confirmation of the membership standard of urban ecological security evaluation, the fuzzy synthetic evaluation method was used to calculate, analyze and evaluate the ranks of urban ecological security from 2003 to 2012. The overall regional ecological security of the Beijing-Tianjin-Hebei metropolitan region increased during those 10 years. However, as expected for the obvious increase in the ecological state security level, neither the ecological pressure security nor the response security level significantly changed. In addition, urban social and economic indicators contributed more to the ecological security assessment than did the natural resource indicators and environmental indicators.

Urban parks improve quality of life by providing an array of cultural and natural ecosystem services, such as maintaining biodiversity, reducing air pollution, and benefiting residents' physical and mental states. Although government invests a large amount of capital into park management, parks are still underused in many cases. In this study, the association and relative importance of socio-demographic, environmental, and individual factors were investigated with regards to their impact on citizens' park visitation. Data was collected through cross-sectional door-to-door questionnaires and online databases, and was analyzed using a hierarchical regression model. Results showed that physical exercise (27.4% of collected reports) and rest and relaxation (26.7%) were the two most widespread reasons for park use. When asked about constraints of park visitation, time limitations were reported as being the most constraining factor. However, our quantitative analysis found no significant correlation. Socio-demographic, environmental, and individual variables explained 1%, 20% and 26% (adjusted R 2 ) of the total variance in frequency of park visitation between participants, respectively. A citizen's neighborhood greenspace, housing price, and distance from the nearest park were negatively correlated with park use. Among individual factors, a participant's reported affinity for park visits, time spent in their residential greenspace, and number of children under seven were positively correlated with park visitation, with a decreasingbcoefficient. This research highlights the predominant contribution of attitude over accessibility factors when it comes to park visitation. Furthermore, the data indicates that accessibility factors function differently for frequent, infrequent, and moderate park users and provision with parks within 1000 m of their home is recommended. The findings have implications for park management and future research.