Multi-scale interaction potentials ( F − r ) for describing fracture of brittle disordered materials like cement and concrete

Fracture processes in brittle disordered materials like many geo-materials (rock, ice, concrete, cement, etc.) are a trade off between local stress concentrations caused by the heterogeneity of such materials, and local strength. At those locations where the ratio between stress and strength exceeds a critical threshold value, cracking may initiate. Depending on the size of the cracks they can be arrested by stronger and stiffer elements in the structure of the material, or they will propagate and become critical. Critical cracks lead to localisation of deformations and to softening. In currently popular cohesive crack models still some continuum ideas remain, namely the notion of stress, whereas the localisation of deformations is handeled correctly by means of displacements. During softening the macro-crack traverses the specimen's cross-section, thereby gradually decreasing the effective load-carrying area. This growth process is affected both by structure (specimen) size and boundary conditions, and a better description of softening may be achieved by using load and displacement as state variables. In this paper, a new method of modelling fracture is proposed by using fracture potentials (F − r relations) at various observation scales, from atomistic and molecular to macroscopic. The virtual material can be interpreted as being built up from spherical elements; the fracture potential describes the interactions between the spheres. Since the spherical elements interact at their contacts-points only, a force-separation law (F-r) suffices. Size/scale effects are dealt with directly in the F-r relation; size/scale effects on strength are merely a special point in the entire description and do not require a separate la

Size effect on strength and fracture energy for numerical concrete with realistic aggregate shapes

Fracture of concrete at the scale of the aggregate structure (or smaller) is a complicated process. Simple simulation models may be of help in understanding fracture in more detail, provided that the material structure is incorporated in as much detail as possible. A combined approach using computed tomography and image processing allows us to model concrete close to reality. The shape of the aggregates is included in a 3D beam lattice model for fracture. Fracture of concrete beams is simulated under 3-point bending with different sizes, aggregate densities and aggregates shapes, focusing on the size effect on structural strength and fracture energ

On flow properties, fibre distribution, fibre orientation and flexural behaviour of FRC

For improving the mechanical properties of fibre reinforced concrete one can either increase the fibre content, use hybrid fibre systems, or one can attempt to align fibres in the direction of stress. In this paper, it is attempted to use the flow-properties of the fresh (self-compacting) concrete to change the fibre distribution and orientation. Using a single mixture of fibre reinforced concrete, containing 3% of 30mm long straight steel fibres, the fibre distribution and orientation was determined in three different parts of a ‘U-shaped specimen' where the concrete could flow in three different directions. The fibre distribution and orientation was determined from a CT-scan. Flexural tests show that the mechanical behaviour depends on the fibre distribution and orientation, which can be affected by changing the viscosity of the fresh mixtur

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

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