Thermodynamic approach to rheological modeling and simulations at the configuration space level of description

The so-called matrix model is a general thermodynamic framework for microrheological modeling. This model has already been proven to be applicable for a wide class of systems, in particular to models formulated at the configuration tensor level of description. For models formulated at the configuration space level of description a matrix formulation is readily obtained, but for the subsequent analysis one still needs an explicit solution of the configuration space distribution functions. In the present paper we describe an approach in which this problem is solved by combining the matrix model with a Lagrangian simulation method in configuration space developed recently by Szeri and Leal. The result is a consistent and unified formulation of stress tensor expressions, including the stress averaging, and the evolution equations. This formulation is also suited for numerical simulations. In this way, the range of applicability of the matrix model is extended substantially. In order to clarify the principles of the method and some aspects of its implementation, a simple example is discussed in some detail

Fracture in high performance fibre reinforced concrete pavement materials

An innovative pavement system known as Ultra Thin Continuously Reinforced Concrete Pavement (UTCRCP) was recently developed in South Africa. The technology is currently being implemented on some major routes in the country. The system consists of a high performance fibre reinforced concrete pavement slab with a nominal thickness of approximately 50 mm. The material has a significant post crack stress capacity compared to plain concrete. Current design methods for UTCRCP are based on conventional linear elastic concrete pavement design methodology, which does not take into account post crack behaviour. Questions can be raised with regards to the suitability of conventional approaches for the design of this high performance material. The hypothesis of the study is that the accuracy of design models for UTCRCP can benefit from the adoption of fracture mechanics concepts. The experimental framework for this study includes fracture experiments under both monotonic and cyclic loading, on specimens of different sizes and geometries and produced from several mix designs. The aim is to quantify size effect in the high performance fibre reinforced concrete material, to determine fracture mechanics material parameters from monotonic tests, and to investigate the fatigue behaviour of the material. As part of the study a method is developed to obtain the full work of fracture from three point bending tests by means of extrapolation of the load-displacement tail. This allows the specific fracture energy (Gƒ) of the material to be determined. An adjusted tensile splitting test method is developed to determine the tensile strength (ƒτ) of the material. The values of Gƒ and ƒτ are used in the definition of a fracture mechanics based cohesive softening function. The final shape of the softening function combines a crack tip singularity with an exponential tail. The cohesive crack model is implemented in finite element methods to numerically simulate the fracture behaviour observed in the experiments. The numerical simulation provides reliable results for the different mixes, specimen sizes and geometries and predicts the size effect to occur. Fracture mechanics based models for the prediction of the fatigue performance of the material are proposed. The predictive performance of the models is compared against a model representing the conventional design approach. It is concluded that the findings of the study support the thesis that design methods for UTCRCP can benefit from the adoption of fracture mechanics concepts. This conclusion is mainly based on the following findings from the study: The high performance fibre reinforced concrete material was found to be subject to significant size effect. As a consequence the MOR parameter will not yield reliable predictions of the flexural capacity of full size pavement structures, In contrast to the MOR parameter, the fracture mechanics damage models developed as part of this study do provide reliable predictions of the flexural behaviour of the material, The fatigue model developed based on fracture mechanics concepts, though not necessarily more precise, is more accurate. Thesis (PhD)--University of Pretoria, 2011.Civil EngineeringPhDUnrestricte

Over-focused? The relation between patients’ inclination for conscious control and single- and dual-task motor performance after stroke

Background: Many stroke patients are inclined to consciously control their movements. This is thought to negatively affect patients’ motor performance, as it disrupts movement automaticity. However, it has also been argued that conscious control may sometimes benefit motor performance, depending on the task or patientś motor or cognitive capacity. Aim: To assess whether stroke patients’ inclination for conscious control is associated with motor performance, and explore whether the putative association differs as a function of task (single- vs dual) or patientś motor and cognitive capacity. Methods: Univariate and multivariate linear regression analysis were used to assess associations between patients’ disposition to conscious control (i.e., Conscious Motor Processing subscale of Movement-Specific Reinvestment Scale; MSRS-CMP) and single-task (Timed-up-and-go test; TuG) and motor dual-task costs (TuG while tone counting; motor DTC%). We determined whether these associations were influenced by patients’ walking speed (i.e., 10-m-walk test) and cognitive capacity (i.e., working memory, attention, executive function). Results: Seventy-eight clinical stroke patients (<6 months post-stroke) participated. Patients’ conscious control inclination was not associated with single-task TuG performance. However, patients with a strong inclination for conscious control showed higher motor DTC%. These associations were irrespective of patients’ motor and cognitive abilities. Conclusion: Patients’ disposition for conscious control was not associated with single task motor performance, but was associated with higher motor dual task costs, regardless of patients’ motor or cognitive abilities. Clinical relevance: Therapists should be aware that patients’ conscious control inclination can influence their dual-task performance while moving. Longitudinal studies are required to test whether reducing patients’ disposition for conscious control would improve dual-tasking post-stroke