Introduction to the special issue on recent developments of peridynamics

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Analysis and optimization of cracked composite laminates

This thesis deals with three inter-related topics. The first topic is concerned with the solution of a cracked transversely isotropic—orthotrOpic composite laminate of finite thickness. The transversely isotropic sublaminate which is sandwiched between orthotropic outer sublarninates contains a central crack or an array of periodically distributed cracks which are perpendicular to the interfaces between the two media. When the crack is wholly within the sublaminate, solutions are obtained for the stress intensity factors and the crack-induced interfacial stresses in three loading modes. When the crack tips touch the interfaces, the stress singularities are no longer the usual square-root type but are determined by the mechanical properties of the media. In this case, solutions are obtained for stress singularities and the corresponding stress intensity factors. The degenerate case when the outer sublarninates are also isotropic but dissimilar from the central sublaminate is also solved. The second topic concerns the application of the above fracture mechanics solutions to crack problems of laminates composed of unidirectional fibre-reinforced composites. In view of the fact that unidirectional fibre-reinforced composites are prone to transverse cracking and that laminates made from unidirectional plies are prone to delamination, a cracked [(il9),,2 /(900)n1 ]s symmetric laminate is studied with a view to examining the mutual constraining effect of plies on transverse cracking and the role of transverse cracks in causing delamination. The fracture mechanics framework is used to reveal the mechanisms behind the enhancement of the socalled in situ strengths of unidirectional laminae in multidirectional laminates. When the tips of a transverse crack touch the interfaces, the effect of the properties of the constraining sublarninates on the stress singularities and stress intensities at the tips of the crack is investigated. The third topic is concerned with two types of optimum strength design of composites laminates. First, for a fibre-reinforced antisymmetric [(:l:0°)n2 / (90°)n1 / (2120),,2] angle-ply laminate, the design variables of the laminate, viz. the ply angle 0 and relative ply thickness, are chosen in such a way as to minimize the stress intensity factor at the crack tip in the (900),“ lamina without exceeding the interfacial maximum principal tensile stress. Secondly, based upon the extensive fracture mechanics analysis (from topics one and two), a set of in situ strength parameters for unidirectional laminae in a multidirectional laminate is proposed. The in situ strength parameters take into account the influence of adjacent laminae and thickness of a particular lamina upon its transverse tensile and in-plane shear strengths when it is used in a multidirectional laminate. These strength parameters are then employed to calculate a stress norm which determines how close the stress state in the lamina is to its failure state. The stress norm is incorporated into the formalism of an optimization problem in order to enhance the load bearing capacity of multidirectional laminates

A bridging law and its application to the analysis of toughness of carbon nanotube-reinforced composites and pull-out of fibres grafted with nanotubes

Bridging laws are essential in predicting the mechanical behaviour of conventional short-fibre-reinforced composites and the emerging nanofibre-reinforced composites. In this paper, we first review some studies on the toughness of carbon nanotube-reinforced composites that is induced by the pull-out of the nanotubes from the matrix, and on the development of the corresponding bridging laws. A close examination of the available bridging laws for carbon nanotubes reveals that some fundamental issues need to be further addressed. We propose a simple nonlinear and smooth bridging law to describe the pull-out force-displacement behaviour of carbon nanotubes from a matrix. This law contains only two material parameters, reflects the basic features of the pull-out experiments, and is easy to use. We then use this bridging law to calculate the fracture toughness of carbon nanotube-reinforced nanocomposites and predict the pull-out force-displacement response of conventional short fibres that are grafted with carbon nanotubes. Some parametric studies are conducted to reveal the influence of various parameters at the nano- and micro-scale on these properties.China Scholarship Council; Australian Research Council via Discovery Project Grant [DP130103958]; National Natural Science Foundation of China [11232001, 11521202]SCI(E)[email protected]; [email protected],SI361-3738

Loss Cone Refilling Rates in Galactic Nuclei

A gap in phase space is opened up by a binary supermassive black hole as it ejects stars in a galactic nucleus. This gap must be refilled before the single black hole that subsequently forms can disrupt or accrete stars. We compute loss cone refilling rates for a sample of elliptical galaxies as a function of the mass ratio of the binary. Refilling times are of order 10 Gyr or longer in bright elliptical galaxies. Tidal flaring rates in these galaxies might be much lower than predicted using time-independent models.Comment: 4 page

Overexpression of an isoform of AML1 in acute leukemia and its potential role in leukemogenesis

AML1/RUNX1 is a critical transcription factor in hematopoietic cell differentiation and proliferation. From the _AML1_ gene, at least three isoforms, _AML1a_, _AML1b_ and _AML1c_, are produced through alternative splicing. AML1a interferes with the function of AML1b/1c, which are often called AML1. In the current study, we found a higher expression level of _AML1a_ in ALL patients in comparison to the controls. Additionally, AML1a represses transcription from promotor of macrophage-colony simulating factor receptor (M-CSFR) mediated by AML1b, indicating that AML1a antagonized the effect of AML1b. In order to investigate the role of _AML1a_ in hematopoiesis and leukemogenesis _in vivo_, bone marrow mononuclear cells (BMMNCs) from mice were transduced with AML1a and transplanted into lethally irradiated mice, which develop lymphoblastic leukemia after transplantation. Taken together, these results indicate that overexpression of AML1a may be an important contributing factor to leukemogenesis