RESUMO
This paper presents the characterization of a TIG welding process carried out by means of an arc welding power supply able to provide dc or pulsed current. The arc welding power supply is based on resonant power converters and an FPGA-based control circuit. Dc and multiple pulsed operations up to 1 kHz with different pulse widths have been tested. The operation of the proposed welding power supply has been compared to that of a high-quality commercial welding machine. Regarding performance, the investigated electrical parameters are: power factor, power conversion efficiency and the energy consumption of the process. The radiography and mechanical properties of the welds have been examined. The mechanical properties of the welded joints characterized through tensile tests are the yield stress, tensile strength and the strain under maximum stress. In addition, the impact properties of the joints were determined through Charpy tests and the curves relating energy absorbed and temperature were obtained. The results show an improved performance of the proposed arc welding power supply over the commercial counterpart, with higher efficiency and power factor, as well as lower energy consumption. The yield stress and tensile strength results indicate that the welded plates using pulsed modes with the proposed power supply are comparable to the reference weld performed with dc operation using the commercial welder. Remarkably, it was observed that the ductility of the welded plates using pulsed modes with the proposed power supply outperforms those of the reference weld carried out with dc arc using the commercial welder.
RESUMO
One of the biggest challenges in facilitating the installation of concrete is the development of fibre-reinforced concrete. Although nowadays fibre reinforced concrete is relatively common, it is still necessary to deepen in the study on its behaviour, especially regarding its fatigue behaviour. This paper proposes a new methodology to analyse the bending fatigue behaviour of notched test specimens. From these tests, it was possible to verify that, despite carrying out the tests with load control, the presence of fibres extends the fatigue life of the concrete after cracking. This effect is of great importance since during the extra lifetime with the cracked concrete, the damage to the concrete will be evident and the corresponding maintenance measures can be carried out. Regarding the analysis of the results, in addition to obtaining a traditional S-N curve, two new criteria have been applied, namely energy and notch growth. From these two new approaches, it was possible to determine critical energy values that can be used as predictive indicators of the collapse of the element. Moreover, from the notch growth analysis, it was possible to determine crack growth rate as a function of the stress conditions for the concrete and the specific geometry. From the comparison among the results obtained from the different tests, a limit cracking index of 0.05 mm can be defined.
RESUMO
The chemical composition of mollusk shells offers information about environmental conditions present during the lifespan of the organism. Shells found in geological deposits and in many archeological sites can help to reconstruct past climatic conditions. For example, a correlation has been found between seawater temperature and the amount of some substituent elements (e.g., magnesium, strontium) in the biogenerated calcium carbonate matrix of the shell, although it is very species-specific. Here we propose the use laser-induced breakdown spectroscopy (LIBS) to estimate Mg/Ca ratios in modern specimens of the common limpet Patella vulgata. An automated setup was used to obtain a sequence of Mg/Ca ratios across a sampling path that could be compared with the seawater temperatures recorded during the organism's lifespan. Results using four shells collected in different months of the year showed a direct relationship between the Mg/Ca ratios and the seawater temperature, although the sequences also revealed small-scale (short-term) variability and an irregular growth rate. Nevertheless, it was possible to infer the season of capture and the minimum and maximum seawater temperatures from the LIBS sequences. This fact, along with the reduction in sampling and measurement time compared with other spectrometric techniques (such as inductively coupled plasma mass spectrometry [ICP-MS]), makes LIBS useful in paleoclimatic studies.
