A probabilistic model for cleavage fracture with a length scale--parameter estimation and predictions of stationary crack experiments

This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 °C. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.     

Automatic coding of dialogue acts in collaboration protocols

Although protocol analysis can be an important tool for researchers to investigate the process of collaboration and communication, the use of this method of analysis can be time consuming. Hence, an automatic coding procedure for coding dialogue acts was developed. This procedure helps to determine the communicative function of messages in online discussions by recognizing discourse markers and cue phrases in the utterances. Five main communicative functions are distinguished: argumentative, responsive, informative, elicitative, and imperative. A total of 29 different dialogue acts are specified and recognized automatically in collaboration protocols. The reliability of the automatic coding procedure was determined by comparing automatically coded dialogue acts to hand-coded dialogue acts by a human rater. The validity of the automatic coding procedure was examined using three different types of analyses. First, an examination of group differences was used (dialogue acts used by female versus male students). Ideally, the coding procedure should be able to distinguish between groups who are likely to communicate differently. Second, to examine the validity of the automatic coding procedure through examination of experimental intervention, the results of the automatic coding procedure of students, with access to a tool that visualizes the degree of participation of each student, were compared to students who did not have access to this tool. Finally, the validity of the automatic coding procedure of dialogue acts was examined using correlation analyses. Results of the automatic coding procedure of dialogue acts of utterances (form) were related to results of a manual coding procedure of the collaborative activities to which the utterances refer (content). The analyses presented in this paper indicate promising results concerning the reliability and validity of the automatic coding procedure for dialogue acts. However, limitations of the procedure were also found and discussed.     

On the mechanism of human stefin B folding: II. Folding from GuHCl unfolded, TFE denatured, acid denatured, and acid intermediate states

In this report, we demonstrate the utility of interleukin-2 (IL-2) stimulation of spleen cell cultures and bivariate flow cytometry in the analysis and purification of the C57BL/6J mouse Y Chromosome. We determined that the DNA content of the C57BL/6J Y Chromosome is approximately 94.7 Mb, making it similar in size to human Chromosome 16 and significantly larger than previous estimates. In addition, we describe the bulk isolation of mouse Y Chromosomes and demonstrate enrichment of the isolated material using a fluorescence in situ hybridization strategy. We detail the construction of two small insert Y Chromosome-specific libraries, ideal for sampling Y Chromosome sequences. From these libraries 1566 clones were analyzed. We provide a detailed characterization of 103 clones, generating nearly 50 kb of sequence. For 30 of these clones, we identify regions of homology to known Y chromosomal sequences, confirming the enrichment of the sorted DNA. From the remaining characterized clones, we describe the development of 15 male-specific PCR assays and 19 male-female PCR assays potentially originating from the pseudoautosomal region or other areas of X-Y or autosome-Y homology.     

On the effect of treating poly(acrylic acid) with argon and tetrafluoromethane plasmas: Kinetics and degradation mechanism

Poly(acrylic acid) (PAAc) films were treated with either an argon or a tetrafluoromethane (CF₄) plasma and subsequently analyzed with X-ray photoelectron spectroscopy (XPS). PAAc films were decarboxylated during both types of plasma treatments. In addition, during the CF₄ plasma treatment, the PAAc films became fluorinated. The plasma phase during the argon plasma treatment of PAAc films was investigated with optical emission spectroscopy. It was shown that during this plasma treatment carbon dioxide, water, and possibly hydrogen were liberated from the PAAc surface. By covering the surface of PAAc films with different materials (lithium fluoride, UV fused silica, and glass) during the plasma treatment, it was possible to differentiate between photochemically induced and particle-induced changes of the surface. This method was used to show that decarboxylation during the argon plasma treatment was caused by vacuum UV radiation (wavelength < 150 nm) and the decarboxylation/fluorination during the CF₄ plasma treatment was induced by reactive fluorine-containing species from the plasma phase. Furthermore, during both processes, etching of the PAAc surface occurred. Based on these mechanisms, kinetic models were derived that could be used to describe the measured kinetic data adequately. © 1994 John Wiley & Sons, Inc.     

Surface plasmon resonance evaluation of various aminoglycoside-RNA hairpin interactions reveals low degree of selectivity

Aminoglycoside antibiotics, which are able to selectively bind to RNA, are considered to be an important lead in RNA-targeting drug discovery. In this study, surface plasmon resonance (SPR) was employed to explore the interaction of aminoglycosides with known tobramycin-binding RNA hairpins (aptamers) and an unrelated RNA hairpin. It was established that aminoglycosides have multiple interactions with RNA hairpins. Unexpectedly, the different hairpins showed comparable affinity for a set of related aminoglycosides. The observed absence of selectivity presents an extra hurdle in the discovery of novel aminoglycosides as specific drugs that target defined RNA hairpins.     

Group awareness tools: It’s what you do with it that matters

This study examined the effect of using a group awareness tool on online collaboration. Furthermore, we examined whether the effect of using a group awareness tool on online collaboration is mediated by group awareness (i.e., students’ awareness of their group members’ levels of participation). To answer these questions, we determined how often and how long 107 secondary education students used the Participation-tool (PT), a group awareness tool designed to visualize group members’ relative contribution to the online collaborative process. Our analyses show that duration of PT use (how long students displayed the tool on their screens) significantly predicted group members’ participation in the online dialogue, their participation when writing collaborative texts, equality of participation within the group, and coordination and regulation of activities in the relational space (i.e., discussing the collaboration process with group members). No effect of using the PT on group performance was found. Mediation analyses showed that the effect of using the PT is only partially mediated by group awareness: an indirect effect of using the PT, via enhanced awareness of participation, on student participation during chat discussions and the collaborative writing process was found.     

Second Home Owner Perceptions of Their Environmental Impacts: The Case of Hartbeespoort

Second home tourism studies have found that there are environmental concerns which arise through the development and continued use of a second home. The majority of these studies, however, have been conducted in the Global North, and no research of this kind has yet been conducted in South Africa. This study, therefore, aims to investigate the environmental implications of second home tourism in Hartbeespoort, South Africa, with special reference to the perceptions second home owners hold with regards to their impacts on the environment. The perception held by majority of second home owner respondents is that neither they, nor their households, impact upon the environment. This perception brings the environmental sustainability of second home tourism in South Africa into question.     

Numerical implementation of a J_2- and J_3-dependent plasticity model based on a spectral decomposition of the stress deviator

A new plasticity model with a yield criterion that depends on the second and third invariants of the stress deviator is proposed. The model is intended to bridge the gap between von Mises’ and Tresca’s yield criteria. An associative flow rule is employed. The proposed model contains one new non-dimensional key material parameter, that quantifies the relative difference in yield strength between uniaxial tension and pure shear. The yield surface is smooth and convex. Material strain hardening can be ascertained by a standard uniaxial tensile test, whereas the new material parameter can be determined by a test in pure shear. A fully implicit backward Euler method is developed and presented for the integration of stresses with a tangent operator consistent with the stress updating scheme. The stress updating method utilizes a spectral decomposition of the deviatoric stress tensor, which leads to a stable and robust updating scheme for a yield surface that exhibits strong and rapidly changing curvature in the synoptic plane. The proposed constitutive theory is implemented in a finite element program, and the influence of the new material parameter is demonstrated in two numerical examples.