Modeling of crack initiation, propagation and coalescence in rocks

One of the most successful criteria proposed so far to describe the initiation and propagation of cracks under quasi-static loading in rock-like materials is a stress-based criterion developed by Bobet (Fracture coalescence in rock materials: experimental observations and numerical predictions. Sc. D, Thesis, Massachusetts Institute of Technology, 1997) which is embedded in FROCK, a Displacement Discontinuity code that was developed by the rock mechanics group at MIT. Even though the predictions obtained with this criterion generally correspond to the experimental results, there are cases in which the quasi-static crack propagation results obtained with FROCK are not satisfactory. For this reason, a qualitative study using the Finite Element code, ABAQUS, was conducted to analyze stress-, strain- and energy-based criteria used for modeling crack development. Based on the ABAQUS relative quantitative analysis, it was found that the strain- and stress-based criteria may be more appropriate than the energy-based criterion to model quasi-static crack development. Thus, a strain-based and a normal stress-dependent criterion were implemented in FROCK. The cracking patterns obtained with these proposed criteria were compared with those obtained using Bobet’s original stress-based criterion and with experimental observations made in molded gypsum specimens. The proposed strain-based criterion implemented in FROCK appeared to yield better results than Bobet’s stress-based criterion. The influence of the friction angle ( $\upvarphi $ φ ) on the cracking patterns was studied with the proposed normal stress-dependent criterion and showed that friction angles closer to $0^{\circ }$ 0 ° yielded the best results, which may indicate that, at least for the microscale, the critical shear stress at which rock fails does not depend upon the normal stresses applied.     

Age-related deficits in prospective memory: The influence of task complexity.

Younger and older Ss were asked to perform an action whenever target words occurred during a short-term memory task. The difficulty of this prospective memory task was manipulated by varying the delay preceding the occurrence of a target event and by varying the number of different target events. Age-related performance differences emerged when there were several different target events but not when there was 1 target event presented several times. Age-related performance differences, when they occurred, were associated with poorer retrospective memory for the target events. The results were interpreted in terms of a componential analysis of prospective memory, which assumes both similarities and differences between prospective and retrospective memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Aging and prospective memory: The influence of increased task demands at encoding and retrieval.

A feature of prospective memory tasks is that they tend to be embedded into other background activities. Two experiments examined how the demands of these background activities affect age differences in prospective memory. The first experiment showed that increasing the demands of the background activities (by adding a digit-monitoring task) significantly reduced prospective memory performance. Planned comparisons revealed that age differences in prospective memory were reliable only in the more demanding background condition. The second experiment revealed significant prospective memory declines when the demands were selectively increased at encoding for both younger and older adults. When the demands were selectively increased at retrieval, older adults were particularly affected. The authors propose a model that relies on both automatic retrieval processes and working memory resources to explain prospective memory remembering. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Decision Analysis Applied to Tunnel Exploration Planning. I: Principles and Case Study

Exploration planning is a process of decision making under uncertainty. This paper, Part I of two dealing with the exploration problem, briefly reviews the decision analytical procedure for tunnel exploration, and provides practical techniques to do so. Specifically, this paper presents an approach by which the effect of additional exploration can be assessed before actually committing to exploration, through a process of so-called virtual exploration. This is practically done with an easy-to-use software package developed in Visual Basic for Applications in Microsoft Excel, called the Decision Aids for Tunnel Exploration. Part II will describe the uncertainties that enter into the tunnel exploration decision procedure, and will provide practical techniques to asses the effects of these uncertainties on the exploration decision making.     

Risk analysis during tunnel construction using Bayesian Networks: Porto Metro case study

This paper presents a methodology to systematically assess and manage the risks associated with tunnel construction. The methodology consists of combining a geologic prediction model that allows one to predict geology ahead of the tunnel construction, with a construction strategy decision model that allows one to choose amongst different construction strategies the one that leads to minimum risk. This model used tunnel boring machine performance data to relate to and predict geology. Both models are based on Bayesian Networks because of their ability to combine domain knowledge with data, encode dependencies among variables, and their ability to learn causal relationships. The combined geologic prediction-construction strategy decision model was applied to a case, the Porto Metro, in Portugal. The results of the geologic prediction model were in good agreement with the observed geology, and the results of the construction strategy decision support model were in good agreement with the construction methods used. Very significant is the ability of the model to predict changes in geology and consequently required changes in construction strategy. This risk assessment methodology provides a powerful tool with which planners and engineers can systematically assess and mitigate the inherent risks associated with tunnel construction.     

Artificial neural networks for predicting the maximum surface settlement caused by EPB shield tunneling

Numerous empirical and analytical relations exist between shield tunnel characteristics and surface and subsurface deformation. Also, 2-D and 3-D numerical analyses have been applied to such tunneling problems. Similar but substantially fewer approaches have been developed for earth pressure balance (EPB) tunneling. In the Bangkok MRTA project, data on ground deformation and shield operation were collected. The tunnel sizes are practically identical and the subsurface conditions over long distances are comparable, which allow one to establish relationships between ground characteristics and EPB – operation on the one hand, and surface deformations on the other hand. After using the information to identify which ground- and EPB-characteristic have the greatest influence on ground movements, an approach based on artificial neural networks (ANN) was used to develop predictive relations. Since the method has the ability to map input to output patterns, ANN enable one to map all influencing parameters to surface settlements. Combining the extensive computerized database and the knowledge of what influences the surface settlements, ANN can become a useful predictive method. This paper attempts to evaluate the potential as well as the limitations of ANN for predicting surface settlements caused by EPB shield tunneling and to develop optimal neural network models for this objective.     

Impact of intake water temperatures on reticular temperatures of lactating dairy cows

Automatic temperature recording may allow early detection of disease, estrus, heat stress, and the onset of calving. The phase IV Cattle Temperature Monitoring System (MaGiiX Inc., Post Falls, ID) utilizes a passive bolus equipped with a temperature sensor, a stationary panel reader to query the bolus, and software to collect, analyze, and display data. One potential limitation to collection of reticular temperatures is the effect of water temperature and consumption on recorded temperatures. Two replicated 3 × 3 Latin square experiments were conducted at the Purdue Dairy Research and Education Center to assess the impact of water intake on reticular temperatures using the Cattle Temperature Monitoring System. Nine high-producing, mid-lactation, second-parity cows with low somatic cell counts were selected. Before administering a water treatment, access to feed and water was restricted for at least 2 h. Baseline reticular temperatures were established from measurements before water intake. In experiment 1, treatments were 25.2 kg of hot water (34.3°C ± 1.0), warm water (18.2°C ± 0.4), or cold water (7.6°C ± 0.4). In experiment 2, treatments were 18.9 kg of body-temperature water (38.9°C ± 0.2), cold water (5.1°C ± 0.4), or control (no water). Following water intake, reticular temperatures were collected for 3 h. In experiment 1, an initial dramatic decrease in reticular temperature was observed followed by a gradual increase toward baseline. Least squares means for maximum drop in temperature were 8.5 ± 0.5, 6.9 ± 0.5, and 2.2 ± 0.5°C for cold, warm, and hot water treatments, respectively. Yet at 3 h, reticular temperatures did not return to the baseline. In experiment 2, control cows remained within the baseline confidence interval through the observation period, and cows receiving body temperature water experienced an initial decrease in temperature (0.4 ± 0.2°C) with a return to within the baseline confidence interval within 15 min. Cows receiving cold water did not return to within the baseline confidence interval after a large decrease of 9.2 ± 0.2°C during the 3-h observational period. Moreover, a regression analysis of continued ascent in temperatures predicted that temperatures would return to baseline within 3.5 h. These results demonstrate that, when cows consume large quantities of cold water, the effect of water intake is sizable and sustained. The value of reticular temperatures for daily monitoring in a production setting hinges largely on the implications of this impact.     

Multiscale modeling in computational biomechanics

Biomechanics is broadly defined as the scientific discipline that investigates the effects of forces acting on and within biological structures. The realm of biomechanics includes the circulatory and respiratory systems, tissue mechanics and mechanotransduction, and the musculoskeletal system and motor control. As in many other biological phenomena, many spatial scales are crossed by biomechanics research: intracellular, multicellular, and extracellular matrices; and tissue, organ, and multiorgan systems. It is well established that the effect of forces at higher scales influence behavior at lower scales and that lower-scale properties influence higher-scale response. However, computational methods that incorporate these interactions in biomechanics are relatively rare. In general, computational models that include representation of multiple spatial or temporal scales are loosely defined as multiscale. The fact that multiscale modeling is not well defined lends the term to a variety of scenarios within the computational physiology community. In biomechanics, multiscale modeling may mean establishing a hierarchical link between the spatial and temporal scales, while the output of a larger-scale system is passed through a finely detailed representation at a lower scale (e.g., body-level movement simulations that provide net joint loading for tissue-level stress analysis). In reality, multiscale modeling may require more intricate representation of interactions among scales. A concurrent simulation strategy is inevitable to adequately represent nonlinear associations that have been known for decades [1].     

A hybridized displacement discontinuity and indirect boundary element method to model fracture propagation

In mechanical modelling of fracture propagation, complications arise from the stress concentrations at the fracture tips and nonlinear responses caused by opening/closing of fractures, by nonlinear constitutive relations of fracture surfaces sliding on each other, and by fracture propagation. The hybridized Displacement Discontinuity and Indirect Boundary Element Method described in this paper avoids problems associated with other numerical methods when analyzing fracture propagation. The method, which includes analytical influence functions and thus makes numerical integration unnecessary, is described in the first part of this paper. In the second part a number of examples are given in which a variety of fracture propagation problems in two dimensions are modelled with the hybridized method. These examples include classical problems in which tension is applied to cracked plates but also others where shearing is applied. Comparisons with solutions obtained by other authors are shown to be satisfactory.