THE EPIDERMIS AND CYCLIC AMP

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73687/1/j.1365-2133.1974.tb06390.x.pd

Crackling noise in three-point bending of heterogeneous materials

We study the crackling noise emerging during single crack propagation in a specimen under three-point bending conditions. Computer simulations are carried out in the framework of a discrete element model where the specimen is discretized in terms of convex polygons and cohesive elements are represented by beams. Computer simulations revealed that fracture proceeds in bursts whose size and waiting time distributions have a power law functional form with an exponential cutoff. Controlling the degree of brittleness of the sample by the amount of disorder, we obtain a scaling form for the characteristic quantities of crackling noise of quasi-brittle materials. Analyzing the spatial structure of damage we show that ahead of the crack tip a process zone is formed as a random sequence of broken and intact mesoscopic elements. We characterize the statistics of the shrinking and expanding steps of the process zone and determine the damage profile in the vicinity of the crack tip.Comment: 11 pages, 15 figure

Physical aspects of fracture scaling and size effect

ISSN:0376-9429ISSN:1573-267

Continuous motor sequence learning: cortical efficiency gains accompanied by striatal functional reorganization.

The acquisition and generation of action sequences constitute essential elements of purposeful human behavior. However, there is still considerable debate on how experience-driven changes related to skill learning are expressed at the neural systems level. The current functional magnetic resonance imaging (fMRI) study focused on changes in the neural representation of continuous movement sequences as learning evolved. Behavioral and neural manifestations of nonvisual motor practice were studied both within the time frame of a single scanning session, as well as after several days of extended practice. Based on detailed behavioral recordings which enabled the continuous characterization of the ongoing learning process at the single subject level, sequence-specific decreases in activation throughout a learning-related network of cortical areas were identified. Furthermore, the spatial layout of this cortical network remained largely unchanged after extensive practice, although further decreases in activation levels could be observed as learning progressed. In contrast, the posterior part of the left putamen showed increased activation levels when an extensively trained sequence needed to be recalled. Overall, these findings imply that continuous motor sequence learning is mainly associated with more efficient processing in a network of consistently recruited cortical areas, together with co-occurring activation pattern changes at the subcortical level

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A solution to the parameter-identification conundrum: multi-scale interaction potentials

Softening is a structural property, not a material property. Any material will show softening, but in this paper the focus is primarily on cement and concrete, which show this property very clearly owing to their coarse heterogeneity (relative to common laboratory-scale specimen sizes). A new model approach is presented, based on pair-potentials describing the interaction between two neighbouring particles at any desired size/scale level. Because of the resemblance with a particle model an equivalent lattice can be constructed. The pair-potential is then the behavioral law of a single lattice element. This relation between force and displacement depends on the size of the considered lattice element as well as on the rotational stiffness at the nodes, which not only depends on the flexibility of the global lattice to which the element is connected but also on the flexural stiffness of the considered element itself. The potential $$F-r$$ F − r relation is a structural property that can be directly measured in physical experiments, thereby solving size effects and boundary effect