Optical method for measuring slow crack growth in cementitious materials

The double-torsion (DT) test is commonly used to calculate slow or subcritical crack velocities in (quasi-)brittle engineering materials directly from the measured load relaxation of notched DT-specimens. In cementitious materials a significant part of the recorded load relaxation in the DT-test may be due to specimen creep deformation, and this would then lead to overestimated crack velocities. In this paper we describe a method to optically measure slow crack growth in cementitious materials by carrying out DT-tests under the optical microscope or inside the environmental SEM. Crack tip detection is facilitated by digital image correlation of the time-lapse microscope recordings. DT-tests at 10% relative humidity in hardened cement paste (with w/c-ratio of 0.4, 0.5 and 0.6) showed that optically measured crack velocities were significantly lower than those calculated from the DT-specimen relaxation. In many experiments the subcritical crack growth rapidly stopped, while an ongoing specimen load relaxation was recorded. At 90% relative humidity, load-relaxation in the DT-test was much stronger than at 10% relative humidity, because subcritical cracking and creep-induced relaxation both increase with moisture conten

Observations of nitrogen isotope fractionation in deeply embedded protostars

(Abridged) The terrestrial planets, comets, and meteorites are significantly enriched in 15N compared to the Sun and Jupiter. While the solar and jovian nitrogen isotope ratio is believed to represent the composition of the protosolar nebula, a still unidentified process has caused 15N-enrichment in the solids. Several mechanisms have been proposed to explain the variations, including chemical fractionation. However, observational results that constrain the fractionation models are scarce. While there is evidence of 15N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases. Our aim is to measure the 14N/15N ratio around three nearby, embedded low-to-intermediate-mass protostars. Isotopologues of HCN and HNC were used to probe the 14N/15N ratio. A selection of H13CN, HC15N, HN13C, and H15NC transitions was observed with the APEX telescope. The 14N/15N ratios were derived from the integrated intensities assuming a standard 12C/13C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting. Two sources, IRAS 16293A and R CrA IRS7B, show 15N-enrichment by a factor of around 1.5-2.5 in both HCN and HNC with respect to the solar composition. Solar composition cannot be excluded for the third source, OMC-3 MMS6. Furthermore, there are indications of a trend toward increasing 14N/15N ratios with increasing outer envelope temperature. The enhanced 15N abundances in HCN and HNC found in two Class~0 sources (14N/15N of 160-290) and the tentative trend toward a temperature-dependent 14N/15N ratio are consistent with the chemical fractionation scenario, but 14N/15N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 13 figure

Effect of Thermal Gradients on the Electromigration Lifetime in Power Electronics

The combined effects of electromigration and thermomigration are studied. Significantly shorter electromigration lifetimes are observed in the presence of a temperature gradient. This cannot be explained by thermomigration only, but is attributed to the effect of temperature gradient on electromigration-induced failures

Residual stresses in thermite welded rails: significance of additional forging

The aluminothermic welding (ATW) process is the most commonly used welding process for welding rails (track) in the field. The large amount of weld metal added in the ATW process may result in a wide uneven surface zone on the rail head, which may, in rare cases, lead to irregularities in wear and plastic deformation due to high dynamic wheel-rail forces as wheels pass. The present paper studies the introduction of additional forging to the ATW process, intended to reduce the width of the zone affected by the heat input, while not creating a more detrimental residual stress field. Simulations using a novel thermo-mechanical FE model of the ATW process show that addition of a forging pressure leads to a somewhat smaller width of the zone affected by heat. This is also found in a metallurgical examination, showing that this zone (weld metal and heat-affected zone) is fully pearlitic. Only marginal differences are found in the residual stress field when additional forging is applied. In both cases, large tensile residual stresses are found in the rail web at the weld. Additional forging may increase the risk of hot cracking due to an increase in plastic strains within the welded area

Thyroid Function after Subtotal Thyroidectomy in Patients with Graves' Hyperthyroidism

Background. Subtotal thyroidectomy is a surgical procedure, in which the surgeon leaves a small thyroid remnant in situ to preserve thyroid function, thereby preventing lifelong thyroid hormone supplementation therapy. Aim. To evaluate thyroid function after subtotal thyroidectomy for Graves' hyperthyroidism. Subjects and Methods. We retrospectively reviewed the medical records of all patients (n = 62) who underwent subtotal thyroidectomy for recurrent Graves' hyperthyroidism between 1992 and 2008 in our hospital. Thyroid function was defined according to plasma TSH and free T4 values. Results. Median followup after operation was 54.6 months (range 2.1–204.2 months). Only 6% of patients were euthyroid after surgery. The majority of patients (84%) became hypothyroid, whereas 10% of patients had persistent or recurrent hyperthyroidism. Permanent recurrent laryngeal nerve palsy and permanent hypocalcaemia were noted in 1.6% and 3.2% of patients, respectively. Conclusion. In our series, subtotal thyroidectomy for Graves' hyperthyroidism was associated with a high risk of postoperative hypothyroidism and a smaller, but significant, risk of persistent hyperthyroidism. Our data suggest that subtotal thyroidectomy seems to provide very little advantage over total thyroidectomy in terms of postoperative thyroid function

An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry

Aims: To investigate the chemical relations between complex organics based on their spatial distributions and excitation conditions in the low-mass young stellar objects IRAS 16293-2422 A and B. Methods: Interferometric observations with the Submillimeter Array have been performed at 5''x3'' resolution revealing emission lines of HNCO, CH3CN, CH2CO, CH3CHO and C2H5OH. Rotational temperatures are determined from rotational diagrams when a sufficient number of lines are detected. Results: Compact emission is detected for all species studied here. For HNCO and CH3CN it mostly arises from source A, CH2CO and C2H5OH have comparable strength for both sources and CH3CHO arises exclusively from source B. HNCO, CH3CN and CH3CHO have rotational temperatures >200 K. The (u,v)-visibility data reveal that HNCO also has extended cold emission. Conclusions: The abundances of the molecules studied here are very similar within factors of a few to those found in high-mass YSOs. Thus the chemistry between high- and low-mass objects appears to be independent of luminosity and cloud mass. Bigger abundance differences are seen between the A and B source. The HNCO abundance relative to CH3OH is ~4 times higher toward A, which may be due to a higher initial OCN- ice abundances in source A compared to B. Furthermore, not all oxygen-bearing species are co-existent. The different spatial behavior of CH2CO and C2H5OH compared with CH3CHO suggests that hydrogenation reactions on grain-surfaces are not sufficient to explain the observed gas phase abundances. Selective destruction of CH3CHO may result in the anti-coincidence of these species in source A. These results illustrate the power of interferometric compared with single dish data in terms of testing chemical models.Comment: 11 pages, 15 figures, accepeted by A&

Nonlinear elastic response of thermally damaged consolidated granular media

The mechanical properties of consolidated granular media are strongly affected by large temperature changes which induce the development and localization of stresses, leading in turn to damage, e.g., cracking. In this work, we study the evolution of linear and nonlinear elasticity parameters when increasing the temperature of the thermal loading process. We prove the existence of a link between linear and nonlinear elasticity properties. We show that the change of the nonlinear elasticity parameters with the increase in the thermal loading is larger at the lower temperatures than the corresponding change for the linear parameters, suggesting that nonlinear elasticity can be exploited for early thermal damage detection and characterization in consolidated granular media. We finally show the influence of grain size upon the thermal damage evolution with the loading temperature and how this evolution is mirrored by the nonlinear elasticity parameter